design and production of nanoparticles formulated from nano-emulsion

Journal of Controlled Release 128 (2008) 185–199

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Journal of Controlled Release

j ourna l homepage: www.e lsev ie r.com/ locate / jconre l

Review

Design and production of nanoparticles formulated from nano-emulsiontemplates—A review

Nicolas Anton a,b, Jean-Pierre Benoit a,c,⁎, Patrick Saulnier a

a Inserm U646, Ingénierie de la vectorisation particulaire, 10 rue A. Boquel, F-49100 Angers, France; University of Angers, F-49100 Angers, Franceb ESPCI, Laboratoire Colloïdes et Matériaux Divisés, ParisTech, 10 rue Vauquelin, Paris, F-75005 France; CNRS, UMR 7612, Paris, F-75005 France; Université Pierre et Marie Curie-Paris 6,Paris, F-75005 Francec École pratique des hautes études (EPHE), 12 rue Cuvier, F-75005 Paris, France

⁎ Corresponding author. Inserm U646, Ingénierie de lE-mail addresses: [email protected] (N. Anton),

0168-3659/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jconrel.2008.02.007

A B S T R A C T
A R T I C L E I N F O
Article history:
A considerable number of n Received 3 December 2007Accepted 11 February 2008Available online 23 February 2008
Keywords:Nano-emulsionNanoparticleNanocapsuleColloidal carrierHigh-energy methodLow-energy methodDrug deliverySolvent displacementPhase inversion temperaturePIT method

anoparticle formulation methods are based on nano-emulsion templates, whichin turn are generated in various ways. It must therefore be taken into account that active principles and drugsencapsulated in nanoparticles can potentially be affected by these nano-emulsion formulation processes.Such potential differences may include drug sensitivity to temperature, high-shear devices, or even contactwith organic solvents. Likewise, nano-emulsion formulation processes must be chosen in function of theselected therapeutic goals of the nano-carrier suspension and its administration route. This requires thenanoparticle formulation processes (and thus the nano-emulsion formation methods) to be more adapted tothe nature of the encapsulated drugs, as well as to the chosen route of administration. Offering acomprehensive review, this paper proposes a link between nano-emulsion formulation methods andnanoparticle generation, while at the same time bearing in mind the above-mentioned parameters for activemolecule encapsulation. The first part will deal with the nano-emulsion template through the differentformulation methods, i.e. high energy methods on the one hand, and low-energy ones (essentiallyspontaneous emulsification and the phase inversion temperature (PIT) method) on the other. This will befollowed by a review of the different families of nanoparticles (i.e. polymeric or lipid nanospheres andnanocapsules) highlighting the links (or potential links) between these nanoparticles and the different nano-emulsion formulation methods upon which they are based.

© 2008 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1862. The great stability of nano-emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1863. High-energy emulsification methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

3.1. Devices and processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1873.2. The choice of surfactants, monomers, aqueous and oily phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1883.3. On the potentialities, advantages and disadvantages of high-energy methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

4. Low-energy emulsification methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1884.1. Spontaneous nano-emulsification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

4.1.1. The diffusion mechanism and diffusion path theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1884.1.2. The emulsion inversion point (EIP) method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

4.2. Phase inversion temperature (PIT) method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1905. The generation of nanoparticles from the nano-emulsion template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

5.1. On nanoparticle definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1925.2. Polymeric nanospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

5.2.1. ‘in situ’ polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1925.2.2. Formulations with preformed polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

5.3. Solid lipid nanoparticles (SLNs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

a vectorisation particulaire, 10 rue A. Boquel, F-49100 Angers, France; University of Angers, F-49100 Angers, [email protected] (J.-P. Benoit), [email protected] (P. Saulnier).

l rights reserved.

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http://dx.doi.org/10.1016/j.jconrel.2008.02.007

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186 N. Anton et al. / Journal of Controlled Release 128 (2008) 185–199

5.4. Nanocapsules (NC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1945.4.1. Polymeric nanocapsules: ‘in situ’ interfacial polymer synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1945.4.2. Polymeric nanocapsules: nanoprecipitation of preformed polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1955.4.3. Lipid nanocapsules (LNC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

1. Introduction

Over the past fewdecades, extensive research has been done on thestudy of nanoparticle generating processes. Owing to the variety of theapplication fields of such colloidal objects (from nanomedicine, drugdelivery and cosmetics, to printing ink or petroleum sciences...), and asexisting nanoparticles are now innumerable, a thorough knowledge ofthe formulating processes (and their potentialities) is essential inorder to achieve the given purposes and needs for research. Likewise,in that most nanoparticle formulations are effectively based onnanometric-scaled emulsions, so-called nano-emulsions, the studyof nanoparticle formulation has to include knowledge of nano-emulsion formation governing phenomena. Nano-emulsions arenanometric-sized emulsions, typically exhibiting diameters of up to500 nm. Nano-emulsions are also frequently known as miniemul-sions, fine-dispersed emulsions, submicron emulsions and so forth,but are all characterized by a great stability in suspension due totheir very small size, essentially the consequence of significant stericstabilization between droplets, which goes to explain why theOstwald ripening is the only adapted droplet destabilization process(detailed below). It follows therefore that nano-emulsion systemscan be regarded as a template for nanoparticle generation, even ifthese two steps can often be combined into one. Therefore, theinnumerable variants of nanoparticle formulation are mainly basedon three different groups of methods for the generation of nano-emulsions, i.e. high-energy methods, the low-energy spontaneousemulsification method, and the low-energy phase inversion tem-perature (PIT) method.

The different kinds, or morphologies, of the nanoparticlesgenerated, can be broken down into polymeric nanospheres, solidlipid nanoparticles (SLNs), or polymeric or lipid nanocapsules. Thelinks between nano-emulsion formulation processes and nanoparticlemorphology are neither obvious nor systematic and should be tackledwith particular detachment: Such is the purpose of the current review.Indeed, by establishing a link between the formulation of nano-emulsions and nanoparticle generation, our intention has been tohighlight the extent to which experimental processes can be adaptedto given specifications.

In the first part, the nano-emulsion template is presented by athorough description of the mechanisms and phenomena governingits formation, including a comprehensive review of the differentexisting methods. Special attention has been given to the ‘low-energy’processes, since they constitute a privileged way to prevent thepotential degradation of encapsulated molecules during processingand are also important for, (among others) energy yields in the case ofindustrial scale-up.

In the second part, nanoparticle formulation processes arereviewed with regard to the place of the nano-emulsion generationmethods (amongst others) that they are based on. Furthermore, theirpotential adaptation to other nano-emulsion forming methods isdiscussed and their potential significance highlighted. Accordingly, thechoice of the energy-type method, the use of organic solvent in theformulation, the choice of the polymer according to its biocompat-ibility or biodegradability, even the choice of an in situ synthesis duringnanoparticle generation, as well as the use of preformed polymers andfinally the choice of nanoparticle morphology, all of these parametersmust be thoroughly considered and closely adapted to the therapeuticobjectives.

In the case of polymers in situ synthesized with polymericnanospheres of nanocapsule creation, the potential interactionsbetween drugs (or active molecules to be encapsulated) and thepolymers being formed must be systematically considered. Indeed,covalent bonds may be established between drugs and polymers, andthus their potential mutual reactivity must be taken into account inthe choice of monomers and nanoparticle-generating systems.

The last remark concerns nano-emulsion and nanoparticle char-acterization. Given the numerous papers and reviews (e.g. [1–3])where this aspect has been covered in great detail, we have not dealtwith it in the present review. The droplet size distribution is essentiallydisclosed by dynamic light scattering (DLS), but also by transmissionelectronic microscopy (TEM) coupled with negative staining, or cryo-TEM, freeze-fracturing followed by replication plus TEM, or capillaryhydrodynamic fractionation (CHDF), etc. More detailed information onsurface particle characterization may be obtained by surface potentialcharacterization (such as [1–3] potential) and an indication of thenanoparticle surface morphology may be highlighted by specificapproximations of electrophoreticmodels (e.g. soft particlemodel [4]).Finally, small-angle neutron scattering (SANS) or small-angle X-rayscattering (SAXS) may be useful for investigating the internalmorphology of such colloidal objects.

2. The great stability of nano-emulsions

The main particularity of nano-emulsions, making them primecandidates for nanoparticle engineering, is their great stabilityof dropletsuspension. A kinetic stability that lasts for months, stability againstdilution or even against temperature changes, totally unlike the(thermodynamically stable) microemulsions. Emulsions are thermo-dynamically unstable systems, due to the free energy of emulsionformation (ΔGf) greater than zero. The large positive interfacial energyterm (λΔA) outweighs the entropy of droplet formation (TΔSf), alsopositive. The terms λ and ΔA respectively represent the surface tensionand the surface area gained with emulsification. Emulsion instability istherefore induced by the positive sign of ΔGf (Eq. (1)).

DGf ¼ gDA� TDSf ð1Þ

Accordingly, the physical destabilization of emulsions is related to thespontaneous trend towards a minimal interfacial area between thetwo immiscible phases. Therefore, a minimization of interfacial area isattained by two mechanisms: (i) Flocculation followed mostly bycoalescence, and (ii) Ostwald ripening.

In nano-emulsion systems, flocculation is naturally prevented bysteric stabilization, essentially due to the sub-micrometric dropletsize. In short [5–7], when interfacial droplet layers overlap, stericrepulsion occurs,from two main origins. The first one is the unfavor-able mixing of the stabilizing chain of the adsorbed layer, depending onthe interfacial density, interfacial layer thickness δ, and Flory–Hugginsparameter χ1,2 (which reflects the interactions between the interfaciallayer and solvent). The second one is the reduction of the configura-tional entropy, due to the bending stress of the chains, which occurswhen inter-droplet distance h becomes lower than δ.

Generally, the sum of the energies of interaction UT adopts a typicalshape of systems wherein molecules repel and particles attract eachother, showing a weak minimum, around h=2δ, and a very rapidincrease below this value (see Fig. 1 for illustration). The depth of the

Fig. 1. Diagram of the influence of emulsion droplet radius on steric stabilization.

187N. Anton et al. / Journal of Controlled Release 128 (2008) 185–199

minimum |U0| will induce predispositions for coagulate in the colloidalsuspension, that is to say, it is intimately linked to the stability of thesuspension. |U0| is shown to be dependent on the particle radius r, theHamaker constant A, and the adsorbed layer thickness δ, with the resultthat the higher the δ/r ratio, the lower the value of |U0|. Now, in the caseof nano-emulsion droplets, δ/r becomes very high in comparison withmacro-emulsions, which in the end totally inhibits its ability to coa-gulate. On the other hand, it is worth noting that the small droplet sizesalso induce stabilization against sedimentation or creaming, in so far asthe droplets are solely under the influence of the Brownian motion.

Taking all this into account, the destabilization of nano-emulsionsis due only to a mass transfer phenomenon between the dropletsthrough the bulk phase, well described in the literature [8] as Ostwaldripening in emulsions. At theorigin of this destabilization process, thedifferences, however slight, of the droplet radius induce differences inchemical potential of the material within the drops. The reduction offree energy in the emulsionwill result in the decrease of the interfacialarea, and therefore in the growth of the bigger emulsion droplets atthe expense of the smaller ones. The dispersed phase migratesthrough the bulk from the smaller droplets to the bigger ones, owingto the higher solubility in the bulk of the smaller droplets. Ostwaldripening is initiated and will increase throughout the process. As anillustration and under the assumption that only one componentcomposes the dispersed phase, the solubility, C(r), of the dispersedmaterial throughout the dispersion medium is expressed as a functionof the droplet radius r, from the Kelvin equation [9], Eq. (2),

C rð Þ ¼ Clexp2gMqRTr

� �ð2Þ

where C∞ is the bulk solubility of the dispersed phase, M its molarmass, and ρ its density. In most studies, the follow-up of Ostwaldripening as the temporal evolution of the droplet diameter stillremains well fitted, even under the approximations involved in Eq. (2).In addition, the literature provides a number of theories dealing withcalculations of the rate of ripening, such as the most famous (andcomplete) given by Lifshitz and Slezov [10,11] andWagner [12], the so-called LSW theory. Besides the consideration of Eq. (2), the diffusion ofdispersed materials through the continuous medium is assumed to bediffusion-controlled, i.e. crossing the interface with ease. Details onLSW theories are fully developed and discussed in the literature[13,14,8] leading to the commonly used expression of the ageing rate,ω, in Eq. (3),

x ¼ dr3cdt

¼ 8DClgM9q2RT

ð3Þ

where rc is the critical radius of the system at any given time, at thefrontier between the growth and decrease of the droplets. Conse-

quently, Ostwald ripening is reflected by a linear relationship betweenthe cube radius and time.

In processes involved in nanoparticle engineering, i.e. for multi-component emulsion droplets, by addingmonomer, polymer, or simplysurfactant or co-surfactant, the above approximation is surpassed. Therate of ripening can be reduced by several orders ofmagnitudewhen theadditive has a substantially lower solubility in the bulk phase than themain component of the droplet. This phenomenon has been widelystudied [15–22], since it appears to be an efficient method to reduce theOstwald ripening rate, even when using small amounts of additives. Inshort, it is explained by the difference of solubility in the continuousphase between the dispersed phase noted (1) and the additive (2), lesssoluble in this example. The first step remains similar to the ripeningwithout additives, since only the component (1) diffuses from thesmaller to the larger droplets, due to thehigher chemical potential of thematerials within the smaller drops. Gradually, the chemical potential inthe larger droplets increases due to the presence of the component (2),until the diffusion process of (1) is stopped. Equilibrium is reachedbetween the two opposing effects and the limiting process becomes thediffusion of the less soluble additive (2), significantly reducing theripening rate and the nano-emulsion destabilization.

A final remark, which may be of importance here, concerns theinfluence on the nano-emulsion destabilization of layer density andstructure in the interfacial zone. Indeed, up to now it has beenconsidered that Ostwald ripening is a diffusion-controlled process, butthis assumption does not take into account the fact that surfactants,polymeric emulsifiers or stabilizers can create a thick steric barrier atthe droplet interface [23,24]. As a consequence, the diffusion of theinner material of the droplets may be slowed down, reducing theripening rate.. The substantial difference in stability between nano-emulsions and nanocapsules for instance, appears essentially fromsuch details.

3. High-energy emulsification methods

In this section, we will consider emulsification methods involvinghigh (mechanical) energy used in the formation of nano-emulsion,that is to say, the use of devices to force the creation of huge interfacialareas. Nano-emulsion generation is very commonly performed withsuch high-energy emulsification methods, particularly exploited innano-emulsion polymerization [1,2]. The formation of such nano-metric-scaled droplets is governed by directly controllable formula-tion parameters such as the quantity of energy, amount of surfactantand nature of the components, unlike the low-energy methods(presented in the following sections), governed by the intrinsicphysicochemical properties and behavior of the systems. It followstherefore that high-energy methods present natural predispositionsto preserve the formation processes of nano-emulsions droplets,against even the slightest potential modifications of the formulationlike the addition of monomer, initiator, surfactant, etc.

3.1. Devices and processes

The mechanical processes generating nanometric emulsionsinclude, as a first step of the drop creation, the deformation anddisruption of macrometric initial droplets, followed by the surfactantadsorption at their interface to insure the steric stabilization discussedabove. The challenge of these mechanical nano-emulsificationmethods is to combine these two steps, in order to allow and optimizenano-emulsion generation. Three main groups of devices are used inthe literature: The rotor/stator devices, which appear in the firstarticles on nano-emulsions, and then high-efficiency devices, includ-ing ultrasound generators and high-pressure homogenizers.

Rotor/stator type apparatuses, such as Omni-mixerpsy® or Ultra-turraxpsy®, do not provide a good dispersion in terms of droplet sizeand monodispersity [25] in comparison with the nano-emulsions


generated by thetwo others kinds of devices (and also with the low-energy methods). Indeed, the energy provided is mostly dissipated,generating heat and being wasted in viscous friction [26,27]. There-fore, the additional free energy ΔGf necessary to create the hugeinterfacial area of nano-emulsions is not obtained.

Nano-emulsions generated by sonifiers are generally attributed toa mechanism of cavitation [28,29], but are not as yet understood wellenough. The ultrasoundwaves in liquidmacroscopic dispersion, resultin a succession of mechanical depressions and compressions,generating cavitation bubbles, which tend irremediably to implode.Subsequently, this shock provides sufficient energy locally to increaseΔA corresponding to nanometric-scaled droplets. Efficiency of nano-emulsification by sonication (considered as the final size of the nano-emulsion droplets as well as the time needed to attain this asymptoticsize), depends both on the composition of the emulsion and the powerdevice. Indeed, the addition of surfactants and/or monomers has beenshown an important parameter to efficiently reduce droplet sizes [30].Sonication is thus the most popular way to produce nano-emulsionsand nanoparticles for research purposes. It does not, however, appearpractical for use on an industrial scale, for which high-pressure [31]devices (and low-energy methods) are often preferred.

High-pressure homogenizers, generally Microfluidizer or Manton–Gaulin devices, are designed in order to forcemacro-emulsions to passthrough narrow gaps, by imposing high pressures. The fluidaccelerates dramatically,reaching, in the microchannels of Microflui-dizers [2] for instance, a velocity of around 300 m·s−1. As a result,shear, impact and cavitation forces are applied on very small volumesand generate nano-scaled nano-emulsion droplets (closely related tothe phenomena involved in the use of sonifiers).

3.2. The choice of surfactants, monomers, aqueous and oily phases

The nature and amount of the surfactant, monomer or hydrophobeused in the formulation completely determine the size distribution,structure and stability of the resulting nano-emulsions and nanopar-ticles. Thus, the different components are chosen in function of theformulation strategies undertaken. For instance, Landfester [32–36]presented from nano-emulsions (by sonication), (i) the formulation ofinorganic particles by playing on thephysicochemical properties ofmolten salt droplets, (ii) the formulation of polymeric nanospheres byin situ polymer synthesis within nano-emulsion droplets, (iii) thecombination of both these types of technology to provide hybridnanoparticles, and finally (iv) the use of oil as a hydrophobe togenerate core-shell nanocapsules, by polymer-specific synthesis andsegregation to the oil/water region [35], or by interfacial nanopreci-pitation [36]. As regards high-energy methods for generatingnanoparticles, the literature extensively reports comparisons betweenthe different devices, hydrophobes, surfactants and monomers, forinstance in Ref. [1]. In the current paper, by describing the variousreported strategies for generating nanoparticles and nanocapsules, wedraw a parallel between high-energy technologies and those involvingonly low-energymethods. Our purpose here is thus to propose furtherinsights into the possible transpositions from high-energy to low-energy nanoparticle-generating methods.

3.3. On the potentialities, advantages and disadvantages of high-energymethods

In general, high-energy nano-emulsification methods present agood potential for polymeric nanoparticle generation, since theformulation parameters are directly controllable. Thus the additionof monomers, initiators, or encapsulating molecules appears not toinfluence the emulsification process, governed by the high shearprocesses. If anything, it may be and additional molecules to beencapsulated,monomers, initiators, or stabilizing agents that interferewith the emulsification process, unlike for the low-energy methods in

which nano-emulsification is totally governed by the physicochemicalbehaviors of the surfactants. However, when the purpose of the ex-periment is the encapsulation of fragile molecules such as peptides,proteins, ornucleic acid, often encountered inpharmaceutical ormedicalresearch, high-energy methods may give rise to drug degradation,denaturation or activity loss during processing. Moreover, in the case ofan industrial scale-up, it is of importance to consider the energetic yield,which is incomparable between high and low-energy methods [37,7].This is especially true for sonication, since during the emulsificationprocess, only thenear-volume of the sonifier nip is affected by ultrasonicwaves, and for high volumes, a weak additional mechanical stirring isneeded to homogenize the sizes and generate nano-emulsions. Inconcrete terms, the emulsification time (i.e. energy) to providehomogeneous nano-emulsions increases in function of the volume tonano-emulsify, which is fundamentally not the case for all low-energymethods.

4. Low-energy emulsification methods

Let us nowmove on to nano-emulsification methods, involving onlya low quantity of applied energy to generate nano-emulsions. Nano-metric-scaled emulsion droplets may be obtained by diverting theintrinsic physicochemical properties of the surfactants, co-surfactantsand excipients composing the formulation Two groups of methods areproposed in the literature and developed below: (i) The first onedescribes emulsification as a spontaneous phenomenon [38–47], whichuses the rapid diffusion of water-soluble solvent, solubilized first in theorganic phase, moving towards the aqueous one when the two phasesare mixed. Among the works on spontaneous emulsification, the lite-rature emphasizes the solvent displacement method [4–51], also calledtheOuzo effect [52],which consists in nano-emulsion formulation due tothe specific and very rapid diffusion of an organic solvent (e.g. acetone,ethanol...) from the oily phase to the aqueous one. In theory,the spontaneous nano-emulsification process can provide as much oil-in-water as water-in-oil nano-emulsions, but the majority of thereported studies concern o/w generation. (ii) Secondly, the so-calledphase inversion temperature (PIT) method [53–64], which uses thespecific properties of polyethoxylated surfactants to modify theirpartitioning coefficient as a function of the temperature, and leads tothe creation of bicontinuous systems when the temperature is close tothe PIT, broken-up to generate nano-emulsions. Practically, it leads to o/w nano-emulsions.

4.1. Spontaneous nano-emulsification

4.1.1. The diffusion mechanism and diffusion path theoryThis section describes the main principles of spontaneous emulsi-

fication, underlining the governing phenomena and mechanisms andconsidering the suitability of this method for nanoparticle generation.It is interesting to note first that the spontaneous features of suchphenomena are simply the results of the initial non-equilibrium statesof the two bulk liquids when they are brought into contact withoutstirring. It is only under specific conditions that spontaneous emul-sification occurs and in some cases, nanometric-scaled droplets aregenerated. The spontaneous emulsification process itself increasesentropy and thus decreases the Gibbs free energy of the system [65].

The establishment of phase diagrams, as well as video-microscopyexperiments is essential for evidencing the conditions related tospontaneous emulsification [45]. Evolution of the system is basicallypromoted by diffusion of a solute into the phase inwhich it has greatersolubility. Thus, spontaneous emulsification behaviors can potentiallybe predicted by following the diffusion pathway within the phasediagram.. The different cases are described below, and of course, thistheory has been widely supported by experiments reported in theliterature. The video-microscopy technique was introduced to observethe behavior of the liquid at the interface of the two immiscible phases

Fig. 2. Diffusion path in a typical water/alcohol/oil system. Segment (1–2), diffusionpath of the aqueous phase. Segment (3–4), diffusion path of the oily phase. Segment (2–3), interfacial local equilibrium. (a) Case where no spontaneous emulsification occurs.(b) Spontaneous emulsification occurs when the diffusion path crosses the two-phaseequilibrium region, segment (1'–2).


brought into contact without stirring. Thus, the regions of sponta-neous emulsification may be disclosed, as well as their respectivelocation towards the interface.

The source of energy of spontaneous emulsification reportedlystemmed mainly from interfacial turbulences, closely related to thesurface tension gradient induced by the diffusion of solutes betweentwo phases. Likewise, the interfaces are subject to capillary wavesfrom thermal origins, gradually amplified as the surface tensiondecreases [45]. The drops are created as a result of sufficiently largeinterfacial corrugations, similar to the dynamic behavior of thefrontier between microemulsion and the bulk phase in multi-phaseequilibrium systems, i.e. a continuous coalescence and break-off ofemulsion droplets [66]. Such a phenomenon has been called disper-sion, spontaneously increasing the entropy and decreasing the Gibbsfree energy of the system. The other (complementary) spontaneousemulsification mechanism, known as condensation, is also assumed tobe intimately linked to the fluctuation of the interfacial amphiphileconcentration. Owing to theregion of local supersaturation (over-concentration of surfactant at interface) induced by the diffusionprocess, spontaneous interfacial expansion takes place, resulting inthe nucleation and growth of drops. These conditions appearanalogous to the system behavior in the two-phase microemulsionregions, for instance, where drops are continuously nucleated, grow,by similar spontaneous phenomena and disappear by coalescing(maintaining the two-phase equilibrium).

The theoretical mechanism was proposed by Ruschak and Miller[67], and has since been broadly corroborated by experiments and bythe literature. These authors have put forward their theory from thesolution of the diffusion equations for thesemi-infinite phase, andunder some assumptions detailed below. The variation of compositionin each phase (aqueous and oily) is directly represented on the ternaryphase diagram by straight lines, from the semi-infinite reservoirs, tothe interfacial concentration. Such a schematic representation of theevolution of the concentration within each phase is called a ‘diffusionpath’. This model, however, is based on the following assumptions, (i)that non-equilibrium phases are brought into contact, and eventuallysome species should diffuse into the opposite phase, (ii) that for ‘semiinfinite’ phases, the theory is limited to time, for which someproportions of both contacted phases retain their initial composition,(iii) that the diffusion coefficients of all components are equal in aphase, which is precisely the condition for representing the diffusionpaths as straight lines, and finally (iv) that the interface presents alocal equilibrium. Thus, spontaneous emulsification only depends onthe diffusion path with regards to the equilibrium phase diagram. Onthe other hand, stirring the two phases brought into contact has noinfluence on the ownmechanism of spontaneous emulsification, eventhough it increases the rate of emulsification by increasing theinterfacial area A.

The first illustration is provided by the study of the water/alcohol/oil ternary system, presented in Fig. 2 inspired from Ref. [45], where apure water {w} phase (point 1) is brought into contact with an alcoholplusoil {a+o} phase (point 4). The local equilibrium at the interface isshown via the dotted segment (2–3) at the frontier of the two-phaseequilibrium region (in the phase diagram). Depending on the initialcomposition and on the nature of the alcohol, the location of theinterfacial equilibrium appears to condition spontaneous emulsifica-tion, as illustrated by the difference between the Fig. 2a and b. Actually,it allows the diffusion path to cross the two-phase microemulsionequilibrium region (i.e. spontaneous emulsification (SE) region). Thismay indicate that the maximum intensity of spontaneous emulsifica-tion is not necessarily near the interface, but at the maximum depthwithin the SE region. Hence, the deeper the diffusion path within theSE region, the higher the interfacial turbulences and dispersion,diffusion or condensation phenomena. It is therefore easy to imagine abridge with the droplet size of the forming emulsion intimately linkedto the intensity of the spontaneous phenomenon. Nano-emulsion

droplets appear to be formed in this way, with the use of a highquantity of diffusing solvent in the oily phase [48–51]. Bouchemal et al.[51], for instance, proposed a study on the optimization of the solventdisplacement method formulating nano-emulsions for cosmetic andpharmaceutical applications, inwhich the overall solvent/oil ratio wasaround 0.01. Likewise, these authors disclosed the important influenceon the nano-emulsificationprocess, of oil viscosity, surfactant HLB, andthe nature of the solvent (also as a function of its toxic potential) andmiscibility with water.

Of course, in all these optimized systems for nano-emulsion (andnanoparticle) formulation by such spontaneous emulsification meth-ods, the systems are more complex than the three-component modeldescribed above. In fact, this model only accounts for the dropletformation, nonetheless unstable and highly subject to destabilizationafter formation (even the nanometric-scaled droplets). Therefore,after creation, the newly formed interfaces have to be stabilized bysurfactant adsorption. Hence, the initial phase diagrams are modifiedaccordingly, and a more complex diffusion path has to be consideredbetween the different phases potentially formed in the interfacialregion. It is to be noted that the presence of liquid crystalline (LC)phases are acknowledged as playing a decisive role in thesespontaneous-forming formulations. Two examples presenting thespontaneous emulsification of such quaternary systems are proposedin Fig. 3, for both surfactants having a negative and positive Winsor Rratio. R is defined as the ratio between the inter-molecular interac-tions per unit interfacial area, surfactant–oil/surfactant–water [68].

In the case shown in Fig. 3a, of the rather hydrophilic surfactant(Rb1), the formation of the LC phase in the semi-infinite aqueous one(point 1) appears in equilibrium with a pure aqueous sub-phase,

Fig. 3. Diffusion path in a typical water/alcohol+surfactant/oil system (see details in thetext).


segment (2–3), before establishing the local interface equilibriumwiththe oil-rich phase, segment (4–5). The diffusion path is simpler andpresents the spontaneous emulsification of hydrophilic droplets in theoil segment (5–5'). Symmetric phenomena are also conceivable in thecase where the rather lipophilic surfactant (RN1) is used, presented inFig. 3b, where an isotropic phase is generally formed in the surfactant/oil-rich region. Subsequently, spontaneous emulsification of oilydroplets in water arises within the segment (1'–2).

The study of nano-emulsion formation using this method implies athorough establishment of the phase diagram to disclose the potentialfeasibility diagram and optimization. In this context, it follows that thepotential influence of additional components (like monomers andpolymers, whether or not they are neutral in the formulation), need tobe investigated, both on the phase diagrams and on the diffusionpathway. This may, to some extent imply restrictions in terms of easeof handling, modifying and adapting the nanoparticle formulation tothe given needs. However, the generation of nanocapsules andnanospheres by nanoprecipitation or in situ polymerization, fromnano-emulsions using the solvent displacementmethod, has provideda great number of examples developed below, e.g. the works of Fessiet al. [69–73].

4.1.2. The emulsion inversion point (EIP) methodAnother spontaneous emulsification method, known as the emul-

sion inversion pointmethod, has been reported in numerous works. Ata constant temperature, it consists in diverting the intrinsic features ofthermodynamically stable microemulsions D or Liquid crystals LC tobe nano-structured by a progressive dilutionwithwater or oil, in orderto create thermodynamically unstable but kinetically stable, respec-tively direct or inverse nano-emulsions [74–82]. In fact, these authorsexplain that by slightly changing the water or oil proportion within

established microemulsion structures, the surfactant hydration ispotentially changed, as well as their affinity for the aqueous phase, andthus instabilities are created in the microemulsion network, resultingin its break-up into nano-emulsions. The addition of water {w}, forinstance, in oil plus surfactant {o+s} continuousmedium [78–80] givesrise to weak oil/water interfacial curvature fluctuations, therebyinducing the system to fall into the thermodynamically favorablestate of nano-emulsion at this time. Of course after formation, otherdestabilizing mechanisms affect the nano-emulsion, such as Ostwaldripening, mentioned in the previous sections.

Once again, in order to determine suitable conditions for generatingnano-emulsion, the equilibrium phase diagram needs to be carefullystudied and the phases analyzed and characterized. This time, it is thedilution pathway which indicates the best conditions to form nano-emulsions, i.e. the conditions for which the emulsion droplets formedare the smallest. Themain results appear to show that nanometric-sizedemulsion droplets are formed when the whole phase to be dispersedappears solubilizedin the bicontinuous system in the phase diagram[59,60,80]. These phases are reported to be either bicontinuousmicroemulsion, or lamellar liquid crystals Lα. To finish, regarding theformulation of nanoparticles from nano-emulsions, this method is verysimilar to thatof solvent diffusion, owing to the fact that theprocessmaybe very dependent and sensitive to phase behavior. Thus, adapting theprocess to some formulation specifications (addition of monomers,initiators, drugs to be encapsulated...) is likely to modify the phasediagram, and thus potentially disrupt the nano-emulsion formationprocess.

4.2. Phase inversion temperature (PIT) method

The phase inversion temperature (PIT) method is particularlyinteresting since it is an organic, solvent-free and low-energy method.The latter two experimental conditions are potentially the mostsuitable for application in the fields of nano-medicine, pharmaceuticalsciences and cosmetics, to prevent the drug to be encapsulated fromdegradation during processing. Likewise, since the process is relativelysimple and low-energy consuming, it allows easy industrial scale-up.The PITconceptwas introduced in the last decade by Shinoda and Saito[53,54], using the specific ability of surfactants, usually nonionic, (NS)such as polyethoxylated surfactants, tomodify their affinities forwaterand oil in function of the temperature, andtherefore toundergo a phaseinversion. Indeed, the so-called transitional emulsion phase inversionoccurs when, at fixed composition, the relative affinity of thesurfactant for the different phases is changed, resulting in a gradualmodification of the temperature. For example, an oil-in-water (o/w)emulsion is subjected to a phase inversion, giving rise to awater-in-oil(w/o) one, when the temperature rises. Within the transitional regionbetween macro-emulsions, i.e. for the temperatures at which thenonionic surfactants exhibit a similar affinity for the two immisciblephases, the ternary system shows an ultralow interfacial tension andcurvature, typically creating microemulsions, bicontinuous and nano-scaled systems [56,83–89]. Therefore, the PIT method consists insuddenly breaking-up the chosen bicontinuous microemulsion main-tained at the PIT, by a rapid cooling [59,62,64] or bya suddendilution inwater or oil [57,61,90,91]. Nano-emulsions are immediately generated.

These bi-continuous systems have been thoroughly and widelycharacterized by establishing phase diagrams at equilibriumand formu-lation maps under dynamic conditions. The influence of the formu-lation (electrolyte concentration, temperature...) and compositionparameters (surfactant amount orwater/oil weight ratio,WOR=100×water / (water+oil)), has been largely reported on the potentialities toformulate nanometric-scaled emulsion droplets [57,59,60,62–64,83,92].

During the emulsion inversion phenomena, the respective affinityof the NS for both immiscible phases is given by the differencebetween the chemical potentials of the surfactants in each phase.According to the physicochemical definition of De Donder, Eq. (4), the


surfactant affinity difference (SAD) is defined with Eq. (5), at thephysicochemical equilibrium, considering the activity coefficientsclose to the unity. It follows that the SAD is closely linked to the NSpartitioning coefficient.

Ai ¼ ABi þ RTln aiCið Þ ð4Þμi is the chemical potential of the NS in phase i, μi° are the standards, athe activity coefficients, and C the surfactant concentration.

SAD ¼ ABwater � ABoil ¼ RTlnCoil

Cwater

� �ð5Þ

In the case of ionic surfactants, the emulsion inversion corresponds tothe SAD=0, but this is not the case for nonionic surfactants and cor-responds to a given reference noted SADref. Hence, this deviation withregards to the optimum formulationwas defined with an adimensionalvariable [93–97], known as the ‘hydrophilic lipophilic deviation’ (HLD),given for ionic and nonionic surfactants, and for a hydrocarbon n-alcaneoily phase by the following Eqs. (6) and (7), respectively.

HLD ¼ SAD=RT ¼ rþ lnS� kACNþ tDT þ aA ð6Þ

HLD ¼ SAD� SADref� �

=RT

¼ a� EONþ bS� kACNþ tDT þ aA ð7Þ

whereEON is thenumberof ethyleneoxide groups forNS, S is theweightpercentage of electrolytes in the aqueous phase, ACN the amount ofcarbon numbers of the n-alcane composing the oily phase, ΔT thetemperature difference from the reference temperature (25 °C), A theweight percentage of alcohol potentially added (not necessary for thePIT method), σ, α, k, t parameters in function of the used surfactant, a aconstant given from the types of alcohol and surfactant, and finally b aconstant function of the nature of the added electrolytes.

Thus, the correlation between the HLD empirical expressions (6)and (7), and the SAD definition (5), gives the link between thetemperature variation and the amphiphile partitioning coefficient[98], and thereby the surfactant behavior regarding the water/oilinterface when using the PIT method. Hence, when NS is mainly usedfor generating nano-emulsions by the PIT method, formulation-composition maps are typically built, as reported in Fig. 4a. Underconstant stirring and for a fixed surfactant amount in the formulation,the emulsion gradually undergoes a phase inversion, as the HLD ischanged by temperature variation. According to the HDL variation, at aconstant WOR, the process is called transitional phase inversion.

Moreover, for the lowest and highest WOR, emulsion inversiondoes not occur, due to the excessively rich water and oil regions. Theemulsion morphology changes from the ‘normal’ to the ‘abnormal’emulsion types, to form simple or multiple emulsions, respectively in

Fig. 4. (a) Typical ‘formulation-composition map’ for water/nonionic surfactant/oil system, ssystem, HLD as a function of the composition, (b) for low surfactant amounts, and (c) for high

accordance with Bancroft's rule and not. The illustration is provided inFig. 4awith the transitions between (i) o/wandw/o/w, and (ii) w/o ando/w/o. In this case, even if the emulsion does not clearly exhibit a phaseinversion, conditions are still suitable to perform the PIT method,where particular microemulsion structures can form at HLD=0, thusalso leading to the generation of nano-emulsions, see for instance Ref.[99].

The study of emulsion inversion involving only the variation ofWOR at a constant HLD is known as catastrophic phase inversion, andhas been extensively studied [100–105]. It regards the transitions(horizontal pathways in Fig. 4a) between (i) w/o andw/o/w for HLDN0and (ii) o/w and o/w/o for HLDb0, therefore this phenomenon isbasically not included in the PIT method.

Fig. 4b and c show the corresponding equilibrium phase diagrams,exhibiting the different thermodynamic equilibriumsWinsor I to IV infunction of the temperature. Kahlweit-fish diagram [57,60,63,64,83]finally traces as well, such an evolution of the systemmorphology. Forinstance, in the case ofWOR=50, a rise in temperature crosses the fishbody in Fig. 4b, and crosses the caudal fin in Fig. 4c. According to thecomprehensive study proposed by Morales et al. [60], optimum con-ditions for nano-emulsion generation are closely linked to the ability ofthemicroemulsion, precisely at the PIT, to solubilize all thephases to bedispersed. Indeed in most cases, this corresponds to the Winsor II andIVmicroemulsion formation,when the system ismaintained at the PIT.In the basic cases of Fig. 4b and c, the nano-emulsionswill be generatedfrom the systems exhibiting W IV equilibrium microemulsions, orpotentially W IV+LC, at the PIT, essentially for systems with highersurfactant amounts, Fig. 4c. Finally, the process implied in the PITmethod of generating nano-emulsions, which suddenly breaks-up themicroemulsions, can essentially be considered as irreversible since thenano-suspension formed is kinetically stable. It appears to go beyondthe confines of the studied ternary system from the phase diagramsand formulation-compositionmaps, to create a kinetically stable nano-emulsion state. Of course, it should only be interpreted as a quasi-stable state, even if it is stable for months, and when achieved, thedestabilization will provide the phase equilibrium considered above.Establishing the phase diagram is a requisite preliminary study inorder to grasp and analyze the conditions suitable for nano-emulsionformulation.

In this context, the link between EIP and PIT methods is clarified,highlighting the latter (PIT) as the one providing suitable experi-mental conditions to attain the nanometric structuring of the ternarysystem (in function of the formulation variables), similar to structuresalready established with mixing the components using the EIPmethod at room temperature (see above).

Thus, the PIT method appears exclusively governed by the PEO-surfactant phase behaviorwith regards to the formulationvariables, andparticularly the temperature. In this context, it is totally conceivable to

howing the emulsion inversion zones. Typical equilibrium phase diagrams for the samesurfactant amounts. The frontier between both behaviors is roughly defined at 10 wt.%.


addneutral components to the formulation,which influence neither thesystem phase behavior, nor the forming domain of nano-emulsions. Forinstance, some formulations based on the PIT method [99,106–109],include the addition of neutral amphiphiles for the formulation (such asphospholipids), in order to play on the final nanoparticle properties andstructure: In these examples, the presence of phospholipids increasesthe lipid nanocapsule stability and acts as a framework on the final shellstructure.

To summarize, the phase inversion mechanisms of emulsions stabi-lized by PEO-surfactants, appear to establish close links between the NSpartitioning coefficients and the temperature variation. Furthermore,many works elucidate this solubilizing behavior of NS moving towardsthe aqueous phase as the consequence of the hydration state of thesurfactant ethylene oxide (EO) chains [110–117]. In concrete terms, thewater solubility and self association of NS are totally governed by thestructuring state of water molecules, associated by hydrogen bonds intoflickering clusters, wrapping the surfactant polar head. This well docu-mented, volumic surfactant behavior was transposed at the water/oilinterface, showing the analogy between cloud point and PIT [118,119],and more recently [120] a comprehensive study presented this mol-ecular behavior of structured water, within the inversion of emulsions.In these works, NS behavior appears related to a salting-out effect, itselfinduced by the formulation variables and by the temperature pathwayimposed on the system.

To conclude, the low-energy and solvent-free PIT method generallyappears relatively adaptable, easy to handle. The incorporation ofadditional molecules (to a certain extent of course) in most cases hasbeen shown to be neutral for the formulation, or only slightly influencethe global trends (weak PIT shift, weak modification of the nano-emulsion droplet size...), but not the global phenomena governing theprocess. It follows then that thepotentialityof thePITmethod togeneratenanoparticles at a low energetic cost, free from the toxicity of organicsolvent,with apotentially lowamountof surfactant (e.g. at 5wt.% in [90]),makes such a process essentially one of the most appealing methods.However, as presented below, the literature mainly reports formulationof nanoparticles based on nano-emulsion templates formed by high-energy methods and by the solvent displacement method. Transpositionand adaptation to the low-energy PIT method thus remains anecdotal.

5. The generation of nanoparticles from the nano-emulsiontemplate

5.1. On nanoparticle definition

Nanoparticles are frequently defined [121] as solid colloidalparticles ranging in size from 10 nm to 1 μm. Nanoparticles are builtfrommacromolecular and/ormolecular assemblies, inwhich the activeprinciple is dissolved, entrapped, encapsulated, or even adsorbed orattached to the external interface. Thus, one fundamental advantage ofnanoparticles with regard to other colloidal drug delivery systems(liposomes, niosomes, microemulsions etc.) and a fortiori to nano-emulsions, is their great kinetic stability and rigid morphology.

Therefore, nanoparticles can be divided into two main families:nanospheres, which have a homogeneous structure in the wholeparticle, andnanocapsules,which exhibit a typical core-shell structure.A main challenge of the formulation of nanoparticles isadapting thechoice of their own structure to the final aims of drug delivery:Biocompatibility of the polymer, physicochemical properties of thedrug, and therapeutic goals. Hence, the following sectionswill focus onpolymeric or lipid nanospheres and nanocapsules, thus presenting (i) ageneral overview on the different formulation methods, the strategiesundertaken and the process-governing phenomena, (ii) a few illustra-tions bywayof a non-exhaustive list of examples, and (iii) the extent towhich the high- and low-energymethods are used or could be used forsimilar results. Finally, the partitioning coefficient will govern thechoice of the formulation, and even the choice of particle morphology.

5.2. Polymeric nanospheres

5.2.1. ‘in situ’ polymerizationThe generation of polymeric nanospheres formed by in situ

polymerization is exclusively provided by the literature from nano-emulsions formed using high-energy methods. The chemical reactionsare principally described as the radical polymerization of droplets,monomers are generally used ashydrophobeand specific surfactants arechosen for generating nano-emulsions.Thus, the nano-emulsions arecomposed of pure monomer droplets surrounded by the adsorbed,stabilizing surfactants. Subsequently, polymerization starts in thedroplets themselves by the addition, in most cases, of the initiator inthe continuous phase, chosen for the hydrophobic phase in function ofits partial solubility. The initiation of droplet polymerization can also beUV-induced [122], or ultrasonically induced [123], or even enzyme-induced [124]. As regards the prevalent way for initiating the poly-merization process by adding initiator molecules, the widely acceptedmechanism [1,2] for successful nano-emulsion polymerization isdescribed as the droplet nucleation mechanism. It suggests that theradicals enter each one of the monomer droplets taken as individualreaction sites, and in that way, the particle number and size do notchange during polymerization. This is consistent with the trend, whichcorrelates the use of an oil-soluble initiator to the improvement in thenumber of nucleated droplets. Initiator molecules can also be includedfrom the start within the nano-emulsion droplets. Polymerization isthen started up by raising the temperature, e.g. in Ref. [125].Azobisisobutyronitrile (AIBN) is added in the oil prior to ultrasonication,whereas potassium persulfate (KPS) is subsequently added in theexternal phase for the same system. Inverse nano-emulsion polymer-ization is also conceivable [126–128], and hydrophilic components maythus be entrapped in the droplets.

This type of radical polymerization of nano-emulsions is frequentlychosen in the literature, and it is a non-negligible detail to consider,since it could also be aggressive for the potentially encapsulatedfragile molecules. Antonietti and Landlester [1] briefly reviewed theirworks dealing with non-radical polymerization in nano-emulsions,which include polyaddition [34,129], anionic polymerization [130], ormetal catalysed polymerization reactions [131]. For instance in[34,129], they made polyurethane latex nanospheres by direct nano-emulsification (sonication) of a mixture of the two lipophilic species:diisocyanate and activated diol, which react to eachother within thenano-emulsion droplets. The successful reaction is due to the fact that(i) the reactants have to exhibit low water solubility, (ii) polymeriza-tion kinetics is lower than emulsification time, (iii) the diisocyanatehas to show a higher reactivity with the other hydrophobe reactant(diol) than with the water that forms the continuous phase. Finally, anumber of examples of nano-emulsion polymerizations using suchpolymerization technology (radical or not) and high-energy methods,are reported, for instance in the literature reviewed in great detail byAntonietti and Landlester [1], or by Asua [2]. Such a listing is not ourpurpose in the current paper.

Does this nanosphere technology seem adaptable to low-energymethods? The literature provides few examples of nanosphereformulation by in situ polymerization with low-energy methods. Thereason is simple: The hydrophobic phase in high-energy processes isonly composed of the monomer itself, whereas in low-energymethods, it includes the oil. Theoretically speaking, adapting theformer to the latter requires similar behavior and interaction ofsurfactants/oil and surfactants/hydrophobic monomer, which appearsrelatively unlikely. Nevertheless, as proposed by Magdassi andSpernath [132], it is only when such conditions are met that it maybe conceivable to perform the PIT nano-emulsification method easily,even then only after thorough characterization of the systems. As aresult, monomer nano-emulsion droplets are formed and theinitiation process can be carried out until the formation of polymericnanospheres.


Finally, as reported in the works of Gallardo et al. [133], during theformulation process of nanocapsules using a reactive alkylcyanoacry-late monomer and a solvent diffusion method, nanospheres can begenerated and favored by the use of a protic solvent [134]. Theseobjects should in fact have a mixed oil/polymer morphology, a porousmatrix and should potentially be surrounded by a polymer capsule.

5.2.2. Formulations with preformed polymersThis section presents formulations in which the macromolecules

are dissolved in the phase to be dispersed (mainly organic solvent). Theprocess irremediably involves the removal of the organic (and volatile)solvent from the formulation and therefore polymer precipitationwithin the organic phase template. Removing the solvent can beperformedbyevaporation or diffusion shock. Themain difference fromthe previous process appears to be the fact that not only are syntheticpolymers used, but also natural macromolecules, such as chitosan,polysaccharides, alginate, gelatin etc, hence increasing their biocom-patibility with the potential therapeutic objectives. The second pointdiffering from the previous section is the number of examples in theliterature proposing the formulation of nanoparticles by low-energymethod, especially the solvent displacement method.

Let usfirst consider a few examples of nanosphere formulation usingclassical sonication methods. Polymeric nano-dispersion is createdsimply by dissolving the polymer in the organic phase, and by nano-emulsification using an appropriate method of sonication. Then, thepolymer-containing nano-droplets formed are gradually evaporated togenerate nanoparticles, as described in Ref. [135]. Kietzke et al. [136]proposed novel approaches working with semiconducting polymerblends. Likewise, Yang et al. [137] used chloroform to dissolve a main-chain liquid crystalline polymer (MCLCP), and after a 5 min-sonicationphase to establish the nano-emulsion, the solvent was removed byevaporation and a very stable suspension of nanoparticles was gen-erated.Today, numerous examples are provided in the field of drugdelivery and in the controlled release of lipophilic and hydrophiliccomponents [138] such as protein in PLGA nanospheres [139]. Perez etal. [140] proposed an original method for encapsulating plasmid DNAunder PLA-PEG nanoparticleswith a double emulsion-like structure (w/o/w), by a two-step sonication, followed by solvent (ethyl acetate/methylene chloride) evaporation in order to induce polymer precipita-tion. An alternative of their second sonication consists in a solventdiffusion which creates the final nanoparticles (i.e. spontaneousemulsification by solvent displacement), inducing polymer precipita-tion: This could constitute a bridge to the low-energy, nanoparticle-generating methods. We can also cite some works studying thephenomena of crystallization and undercooling of poly(ethyleneoxide) [141,142] within the nano-emulsion droplets. Finally, workingon the intrinsic solubility andmelting point (Tm) of a particular polymerwithout the use of an organic solvent, Quaglia et al. [143] reported amethod known as ‘melting/sonication’ (MS)which consists in the nano-emulsifation by sonication in water, of a fluid, non water-misciblecopolymer at TNTm. Cooling to room temperature then hardens thecopolymer. As a result, spherical and non-aggregated particles areformed, and depending on the macromolecule properties, particularstructures (i.e. core-shell...) can be adopted by the nanospheres.

Most nanosphere engineering using preformed polymersdescribed in the literature, is shown to be performed by low-energyspontaneous nano-emulsification, the so-called solvent displacementmethod described above. The general idea is to consider themacromolecules dissolved in organic solvent (plus possibly hydro-phobes, like oil), as neutral for the spontaneous nano-emulsificationprocess. Thus, the solvent diffusion towards the aqueous phase,generating nano-emulsions causes the polymer to precipitate uni-formly within the nano-emulsion template. Classical examples are theworks of Fessi et al. [144–147] or Leroux et al. [49,71,148]. Manydifferent polymers and organic solvents are commonly used. We canfind for instance the couples {poly(D,L-lactic acid)/ethyl acetate} in

[145], {Eudragitpsy® L/acetone, dimethyl sulfoxide, isopropyl alcohol,ethanol or ethyl lactate} in [146], {poly(D,L-lactic acid), Eudragitpsy® E,cellulose acetate phthalate, cellulose acetate trimelitate/ethyl acetate,2-butanone} in [49], or {PLGA, PLGA:poloxamer, PLGA:poloxamine/dichloromethane, methylene chloride} in [149–152].

As regards such formulation through the phase inversion tempera-turemethods, it is theprocess itself that seems ill adapted to the conceptof desolubilizing the polymer fromthenano-emulsion droplets to createnanoparticles. In fact, a non-volatile dispersed phase is generally used,and avoiding it does not appear physically possible. However, by sub-stituting oil with a volatile solvent, into which the polymer has beenintroduced beforehand, cf Ref. [132], the authors established nano-emulsion followed by solvent evaporation below the PIT to generatenanoparticles. To date, the literature does not provide other formula-tions of polymeric nanospheres using the PIT method. Likewise, in thatusing dissolved polymers involves the use of harmfulorganic solvents,the main advantage of PIT methods, that of being organic solvent-free,is lost: There appears to be no further interest to change the phaseinversion temperature method in such a way.

5.3. Solid lipid nanoparticles (SLNs)

Solid lipid nanoparticles [153–155] are commonly defined as nano-scaled lipid matrices, solid at physiological temperatures and stabilizedby surfactants. Owing to the choice of lipids (generally biocompatibleand biodegradable) and given that the particles are stable for years, thistype of nanoparticle appears to be a privileged, promising drug deliverysystem, especially for the parenteral method. However, the limits lie inthe fact that thedrugmolecules tobeencapsulatedgenerally have apoorsolubility in lipids, and are thus rapidly expelled after polymorphictransition [156]. Ongoing research is looking to increase encapsulationrates and lower these problems, using nanostructured lipid matrices[157] or lipid-drug conjugates [158]. Here, the formulation of SLNs isprovided by both high- and low-energy methods.

High-energy production methods of SLNs [159,160] for nano-emulsion generation may be similar to those described above forcopolymers, such as the melting/sonication method: It consists in (i)maintaining the lipid phase (plus potentially solubilized drug) 5–10 °Cabove its melting point, (ii) premixing it in aqueous surfactantsolution at the same temperature, (iii) nano-emulsifying the pre-emulsion using a high-energy method (high pressure homogenizer[153–155,159–161] or sonication [162–166]), and finally (iv) cooling itdown to room temperature to crystallize the lipids. Special care has tobe taken to avoid the lipid memory effect, making new crystallizationpossible [167]. These processes are reviewed in detail by Müller et al.[154]. These authors illustrate the advantages of such a protocol evenfor encapsulating thermo-sensitive drugs, since exposure to anincreased temperature is relatively short. For highly thermo-sensitiveor hydrophilic molecules, they propose an alternative method knownas the ‘cold homogenization technique’ [154].

Low-energy methods are also used for generating solid-matrixlipid nanospheres. These methods are mainly based on the formula-tion of microemulsions (still above the melting point), followed bywater dilution which induces a cooling of the system and lipidnano-particle precipitation [168–171]. This process can be compared to theEIP method (presented above) for nano-emulsion generation, bysubstituting oil for melt lipid. Additional cooling gives rise to lipidcrystallization and the generation of SLNs.

Other processes found in the literature are based on the solventdiffusion spontaneous emulsification method [172–178]. Like nano-emulsions, SLNSs are generated by the solvent displacement method,but again substituting oil for melt lipids, for instance, glycerylmonostearate,used in Ref. [172]. Of course, it means controling thetemperature of the organic phase solubilizing the lipid, typically 5–10 °C above the lipid melting point. The suspension of SLNs is thenquickly formed and the lipids crystallized after pouring this hot


organic solution into an aqueous one at room temperature. In Ref.[172], the authors solubilized glyceryl monostearate in a mixture ofethanol and acetone at 50 °C, before generating SLNs by dilution intoan acidic aqueous solution. The nanoparticles formed exhibiteddiameters from 50 to 500 nm, depending on the experimentalconditions (pH, drug encapsulated, etc.).

Thus, it is conceivable that when oil is added with the lipids in theformulation process at temperatures above the lipid melting point, acertain structuring may be induced in the particle, as it is the case inthe formulation of nanocapsules by solventdiffusion (see below).Therefore, the addition of oil will create a porous lipid matrix as withthe nanoparticle structure, in which the porosity will be controlled bythe amount of oil [178]. Such a particle appears to be a relatively goodcandidate for drug delivery, since it offers a fine arrangement of therelease profiles from the specifications. As a last remark, high-energymethods may be used to incorporate this oil, in view of controlling theparticle morphology.

The PIT method also appears easily adaptable to the process ofgenerating SLNs and to our knowledge, no work of this nature has yetbeen reported. Excipients should be chosen in function of theirphysicochemical properties: (i) to allow the system to undergo atransitional phase inversion, and (ii) to exhibit a sufficiently high PIT,compared to the lipid melting point. Hence, strong system character-ization and optimization has to be done for the finely dispersed SLNgeneration. Finally, it may also be possible to produce the above-mentioned porous lipid matrix using the solvent diffusion method bythe further incorporation of oil in the oily phase.

5.4. Nanocapsules (NC)

Let us nowmove on to the last category of nanoparticles formulatedfrom the nano-emulsion template covered in the present review:Nanocapsules (NC). It consists of colloidal objects exhibiting a core-shellstructure. The core acts as a liquid reservoir for drugs, mainly lipophilicsolvent (and usually oil) but also aqueous core NC, Both are describedbelow. The shell is generally made of polymers, preferentially bio-degradable, i.e. dense and rigid, even if on the nanometric scale, such aconcept is still being discussed. For the past 20 years, as reviewed in Ref.[3], Couvreur's team have been doing extensive research on nanocap-sules as drug carriers and on their therapeutic applications.

The advantages of such a structure are: Firstly, the high drugencapsulation efficiency due to the optimized drug solubility in thenanoparticle core and low polymer content compared to polymericnanospheres. Secondly, since the drug is ‘protected’within theNC core,tissue irritation at the administration site as well as the burst effect arelowered, and the drug itself remains protected against degradation.

Experimentally speaking, several methods are usually used toestablish such a core-shell structure on a nanometric scale. As fornanospheres, these include: In situpolymerization at thenano-emulsiondroplet interface, nanocapsule generation using preformedpolymer andthe generation of lipid nanocapsules. The NC morphology and theformulation strategy totally dependon the therapeutic objectives andonthe drug to encapsulate. Mainly, lipophilic and hydrophilic cores of theNC are distinguished, requiring the NC to be dispersed respectively inwater and in oil.Thus, for the parenteral administration route, it isevidently the aqueous dispersion medium which will be adopted, theaqueous-core NC dispersed in oil being an unsuitable system. Thereforeover the past few years, much effort has been made to formulateaqueous-core NC as a hydrophilic drug carrier, eventually with apolymerosome or vesicule-like structure, as presented below.

5.4.1. Polymeric nanocapsules: ‘in situ’ interfacial polymer synthesisThe first remark about this strategy of nanocapsule formulation is

the universality of the method with regard to the formulation of thenano-emulsion template. Since polymer synthesis is performed at aspecific location (the droplet interface), and after the formation of

nano-emulsion, this process may be considered independent from themethod chosen to generate nano-emulsions. Hence, regarding thechoice of monomers and the chemical reactions during polymeriza-tion, the nano-emulsion template can constitute the starting point ofthe process. Thus, all the methods used to formulate nano-emulsions,both low and high-energy, can be considered.

It is possible to introduce themonomers (i) in the continuous phase,reacting with thematerial constituting the droplets, (ii) vice versa in thedroplets, reacting with the continuous phase, or polymerizationinitiated by adding initiator in the continuous phase, and finally thecombination (iii) in both phases, reacting together to induce an inter-facial polycondensation. These three points are developed below. Incases (ii) and (iii), themonomers have to be included in the formulationprocess. However, the weak amounts generally added do not influencenano-emulsion formulation.

(i) In the first case, monomers can be added in the external phaseafter completing the nano-emulsion formulation. Thus, the monomeris freely soluble in the continuous medium and a reagent towards thedispersed phase itself. The interfactial polymerization reaction isinitiated at contact with the droplet and the morphology of thepolymer shell is in function of the initial quantity of the monomer. Forinstance the polymerization of oil-soluble alkylcyanoacrylate onaqueous droplets of inverted nano-emulsion, are part of a processinitiated by Lambert et al. [179–182] for the formulation of aqueous-core nanocapsules. Nanometric-scaled emulsion droplets are gener-ated using amechanicalmethod andmonomer isobutyl-cyanoacrylateis then added, a very reactive species towards the aqueous droplets.The reaction is catalyzed (and initiated) by the presence of nucleo-philes such as hydroxyl ions. Interfacial polymerization is completedwithin seconds andw/o nanocapsules are generated. The authors thenextract colloidal objects from the organic phase and perform a re-suspension in water. Various hydrophilic drugs have been encapsu-lated in this way, such as oligonucleotides [179–181,183] includingantisenses [182], or siRNA [184]. Overall a relatively high encapsulationefficacy has been observed. More recently, this process has even beenoptimized by Hillaireau et al. [185,186] by polymerizing onto a w/omicroemulsion template (surely similarly to water-loaded invertedmicelle suspension), instead of mechanically established nanometricaqueous dispersion.

Prime candidate monomers for performing such nanocapsuleformulation strategies appear to have both a good solubility in theexternal phase and a sufficient reactivity with the phase constituting thedispersed nano-emulsion droplets. The morphology of theresultingnanocapsules will be in function of the amount of monomer added aswell as the polymerization time. Thus, molecules other than alkylcya-noacrylates could be considered as candidates for nano-emulsion in situinterfacial polymer synthesis. Take for example diisocyanate lipophilicmolecules, inwhich the isocyanate functions are possibly hydrolizable byanelectrophile site-rich solvent suchaswater, leading toamine formation.Theamine, then reacts rapidlywithanothermonomermolecule, since it ismore reactive than water: The polycondensation reaction occurs exactlyat the interface. Our recent works [187] report the formation of aqueous-core nanocapsules, which work on the same principle, but throughdifferent formulation strategies, i.e. by low-energy (PIT method), andinterfacial nano-emulsion diisocyanate polycondensation.

However, such interfacial polycondensation is nothing other thanthe adaptation to nanometric dispersion of the interfacial polycon-densation reported for microcapsule generation, for instance by Penséet al. [188,189].

To summarize: When the monomer is added in the continuousphase (case (i)), interfacial polymerization does not appear dependenton the method chosen to generate the nano-emulsion templates,high- and low-energy, PIT method, spontaneous emulsification, etc.

(ii) In the second case, a great number of publications present theformulation of nanocapsules by interfacial polymerization, where themonomer is included in the dispersed nano-emulsion droplets. Let us


begin by the high-energy formulations of Tiarks et al. [35,32] (see also[190]), with the formulation of polymeric nanospheres: The hydro-phobe is now composed of a monomer, an initiator and oil, dispersedin water by sonication, generating nano-emulsions. Polymerization isinitiated via the temperature, the monomer precipitates in oil andthere is gradually segregation towards the water/oil interface, thusgenerating nanocapsules. In fact, this phenomenon has given rise tothe theory of droplets composed of binary mixture [191], and thepolymer/oil system can adopt several configurations (i.e. the polymercan engulf the oil totally, partially or not at all). Nanocapsule gen-eration requires the polymer to totally engulf the oil core after itssegregation. Such segregating behavior is governed by the respectiveinterfacial tensions between the three immiscible species.

As regards the solvent displacement method, (still) concerningalkylcyanoacrylate, a large number of papers have been reported,initiated by Al Khouri Fallouh et al. [192,193] and thereafter widelystudied and optimized [134,194–199]. These authors include alkyl-cyanoacrylate monomers within the (water miscible) organic solventplus oil phase, and polymerization is initiated along with theinstantaneous diffusion of the solvent to the aqueous bulk phase.Since both the generation of nano-emulsions and the interfacialpolymerization reaction are extremely fast, it is assumed that they areproduced simultaneously.

(iii) Finally, in the third case, the nano-emulsion is first preparedincluding a monomer within the oil droplets (for instance diisocya-nate) and the polycondensation reaction proceeds at the water/oilinterface. Thus again, it is possible to generatenano-emulsions usingboth high- and low-energy methods, as long as the inner monomer isincluded and without further reaction of the latter with the aqueousphase. The main challenge of these studies is to adapt the interfacialpolycondensation processes, well known for micrometric droplets[200–202], to a nanometric dispersion exhibiting a huge water/oilinterface, as well as to the characterization methods used, in order toensure that the polymer coating is successfully achieved.

For example, Takasu and Kawaguchi [203] propose the synthesis ofa polyurea shell, by coating nano-emulsion made of styrene in thehydrophobe phase (before generating latex core-shell nanoparticlesvia a second styrene polymerization). Nano-emulsions containingdiisocyanates (such as isophorone diisocyanate or tolylene-2,4-diisocyanate) are established by sonication and the polycondensationreaction begins subsequently, when the aqueous solution of diamine(isophorone diamine or hexamethylene diamine) is added to the o/wnano-emulsion. The reaction mixture is then slowly stirred for nomore than 2 h to complete the water-insoluble polyurea coating.

Montasser et al. [204–206] and Bouchemal et al. [72] have pro-vided some examples regarding such an in situ interfacial poly-condensation, for which the nano-emulsion template is generated bya low-energy method: Solvent displacement. The reaction is per-formed by a stepwise reaction between for instance, diisocyanatemolecules within oil droplets and activated diols, poly(ethyleneoxide), or even diamine, solubilized in the aqueous phase, in orderto generate, respectively, polyurethane or poly(ether urethane) shells,or polyurea. It is to be noted that the diols have to be ‘activated’ withthe help, for instance, of diazobicyclo, 2-2-2,octane molecules.

Finally about the PIT method, it appears conceivable to incorporatediisocyanate into the droplets in order to induce similar interfacialchemical reactions (as in the case (i)), but only when the selecteddiisocyanate exhibits low reactivity with the aqueous phase duringprocessing until completion of nano-emulsion generation.

5.4.2. Polymeric nanocapsules: nanoprecipitation of preformed polymersThe general protocol to specifically induce the precipitation of

preformed polymers at the droplet interface appears principally basedon the solvent displacement nano-emulsifying method. It is the onlyexperimental procedure which combines (i) the use of solventsefficient enough to solubilize the macromolecules, and (ii) its specific

displacement towards the bulk phase, during which the precipitatingpolymers will be deposed according to the profile of the nanometricoil droplets being formed. Earlier works on such a process wereproposed by Fessi et al. [69,70] using poly(D,L-lactic acid) (PLA) or poly(alkylcyanoacrylate) polymers and acetone as the displacing solvent.Of course, these pioneer works were followed by a large number ofvariants, applications and patents (e.g. [73]), using different polymerssuch as for instance poly(isobutyl-cyanoacrylate) [207], PLA, Eudra-gitpsy® E, poly(ε-caprolactone) (PCL) [208–210], poly(lactic-co-glycolic acid) (PLGA) [211] and so forth.

The first advantage thus appears to be the apparent lack ofpotential and aggressive drug-monomer interaction owing to theabsence of chemical reaction, in comparison with other methods inwhich the polymerization (interfacial or not) is in direct contact withthe drug. Secondly, as various types of polymers are used (see also [3]),this method appears relatively adaptable to given specific andtherapeutic purposes, etc. However, this low-energy process inex-orably implies the use of organic solvents and their specific drawbacksfor pharmaceutical applications.

Another method (somewhat anecdotal in comparisonwith the onedescribed above), presents the generation of polymeric nanocapsuleswith preformed polymers in two steps: (i) Firstly, by preparing thenano-emulsion template, whatever the (high- or low-energy) methodused and (ii) secondly, by coating itwith the polymer deposition on thewater/oil nano-emulsion surface. The polymers are added in thecontinuous phase (even after nano-emulsion is complete) and theirprecipitation onto the nano-emulsion droplets is induced by solventevaporation. For instance, Paiphansiri et al. [36] propose the encap-sulation of an antiseptic agent, a chorhexidine digluconate solution,within a w/o nano-emulsion generated by sonication. The polymersthey use, poly(methyl methacrylate) (PMMA), poly(methacrylate)(PMA) or PCL, are solubilzed in dichloromethane (DCM) graduallyadded in the continuous organic phase of the nano-emulsion. Next, thetemperature is maintained, with slow stirring, above boiling pointuntil the DCM has completely evaporated. The polymers precipitateonto the nano-emulsion water droplets, thus forming nanocapsules.The authors present such a mechanism of specific segregation on thedroplet interface, similar to their otherworks presented above (Section5.4.1) dealing with polymer synthesis and specific segregation andtherefore totally engulfing the nanocapsule core, depending solely onthe respective interfacial tension between the three species [35].

Finally, another strategy was developed by Prego et al. [212–214],reporting specific chitosan or PEG-chitosan nano-emulsion coating.Nano-emulsions are obtained by the low-energy solvent displacementmethod, and stabilized by lecithin. After eliminating the solvent, thenanocapsules are created by simple incubation with the polymer.These nanocapsules have been used as carriers for hydrophilicmolecules such as peptides (e.g. salmon calcitonin in Refs. [213,214]),simply by including the peptide within the organic solvent before it ispoured into the aqueous phase to diffuse and generate nano-emulsions. The peptides are entrappedwithin the o/w nano-emulsion,and the surrounding chitosan wall acts as a barrier, preventing exten-sive release.

To summarize, the formulation of nanocapsules using preformedpolymers are mainly performed (in the first case) by combining thesolvent displacement method and the specific nanoprecipitation ofpolymers onto the water/oil interface of the forming oil droplets. Theseobjects are exclusively o/w nanocapsules. A second strategy consists inestablishing first the nano-emulsion template, and then forcing thepolymer (solubilized in the continuous phase) to specifically precipitateto the nano-emulsion interface. Oily-core as well as aqueous-corenanocapsules can be formulated. Furthermore, since the two formula-tion steps can be dissociated, nano-emulsion formulation can beperformed whatever the method chosen. Thus, if the specifications arerespected, it is conceivable to include the solvent-free and low-energyPIT method for nano-emulsion generation.


5.4.3. Lipid nanocapsules (LNC)Lipid nanocapsules were introduced by Heurtault et al. [106–

108,215], and are commonly defined as exhibiting a core-shell structurecomposed of a liquid oily core and an amorphous surfactant shell. Theformulation process is in fact based on the PIT method plus thetemperature cycling treatment. Biocompatible excipients are chosen forthe formulation and for nanocapsule administration via a parenteralroute, mainly medium-chain triglycerides (caprilic triglycerides) asthe oil phase, a polyoxyethylene-660-12-hydroxy stearate as the PEOnonionic surfactant and MilliQpsy® water plus NaCl as the aqueousphase. An additional, somewhat neutral component, lecithin, wasintroduced in the formulation, and has been shown [216–218] to in-crease the nanocapsule stability significantly, creating a ‘framework’ inthe shell. Therefore, the own formulation process was actually es-tablished as a function of the physicochemical properties of suchadequate components. Since the temperature cycling process has beenshown [90] to increase the quality of the nano-emulsions (in terms ofsize and PDI) by increasing the surfactant amount at the water/oilinterface, the generation of nano-emulsions by the PIT method plustemperature cycling leads, in fact, to droplets exhibiting an importantquantity of surfactants in the interfacial region. In addition to thepresentcase, the fact that the final formulation is reached by a sudden dilutionwith water at temperature below the nonionic surfactant melting point(~30 °C), indicate that shell crystallization could occur. Finally, asdiscussed above concerning droplets stabilized with macromolecularassemblies or thick polymeric species [23,24], Ostwald ripening issignificantly reduced even more than for the simple nano-emulsions,and it is precisely the case for LNC. In view of all these results, theseparticular nano-emulsions, formulated by temperature cycling, aredenominated as lipid nanocapsules. Nevertheless, the structure of theseLNC being specific and typical of the formulating method and of theproperties of the used nonionic surfactants, it is hard to imagine theformulation of similar objects usingmethods other than the PITmethod.

6. Conclusions

The formulation of nanoparticulate drug carriers, based on nano-emulsion formulation, appears at first to require adaptation to thetherapeutic aims and specificity of the drug to encapsulate. That is tosay, both high- and low-energy nano-emulsion formulation methods,whether or not they include the use of organic solvents, have to beadapted according to the active molecule properties, i.e. sensitivity totemperature, high-shear devices, contact with organic solvents, etc.This review has presented the extent to which these nanoparticulatedrug carriers are generated using various nano-emulsion formulationprocesses. High- and low-energy methods used to establish nano-emulsions are presented in the first part. The second part proposes alink towards the generation of nanoparticles. The formulation of themain groups of nanoparticles, i.e. polymeric nanospheres, solid lipidnanoparticles and nanocapsules, are reviewed in the light of the nano-emulsion formation method as well as conceivable alternatives (notnecessarily reported in the literature). The global aim of this review isto provide the formulator with a broad basis on the adaptation, oreven the carrying out of a process, according to specific needs.

References

[1] M. Antonietti, K. Landfester, Polyreactions in miniemulsions, Prog. Polym. Sci. 27(2002) 689–757.

[2] J.M. Asua, Miniemulsion polymerization, Prog. Polym. Sci. 27 (2002) 1283–1346.[3] P. Couvreur, G. Barratt, E. Fattal, P. Legrand, C. Vauthier, Nanocapsule technology:

a review, Crit. Rev. Ther. Drug Carr. Syst. 19 (2) (2002) 99–134.[4] H. Ohshima, Electrophoresis of soft particles: analytic approximations, Electro-

phoresis 27 (2006) 526–533.[5] D.H. Napper, Polymeric Stabilisation of Colloidal Dispersions, Academic Press,

London, 1983.[6] T.F. Tadros, The Effect of Polymer on Dispersion Properties, Polymer adsorption

and colloid stability, 1982.

[7] T.F. Tadros, P. Izquierdo, J. Esquena, C. Solans, Formation and stability of nano-emulsions, Adv. Colloid Interface Sci. 108–109 (2004) 303–318.

[8] P. Taylor, Ostwald ripening in emulsions, Adv. Colloid Interface Sci. 75 (1998)107–163.

[9] L.M. Skinner, J.R. Sambles, Kelvin equation. Review, J. Aerosol Sci. 3 (1972) 199.[10] I.M. Lifshitz, V.V. Slezov, The kinetics of precipitation from supersaturated solid

solutions, J. Phys. Chem. Solids 19 (1961) 35.[11] I.M. Lifshitz, V.V. Slezov, Kinetics of diffusive decomposition of supersaturated

solid solutions, Soviet Physics J.E.T.P. 35 (1959) 331.[12] C. Wagner, Theorie der alterung von niederschlägen durch umlösen (ostwald-

reifung), Elektrochem. 65 (1961) 581–591.[13] M. Kahlweit, Ostwald ripening of precipitates, Adv. Colloid Interface Sci. 5 (1975)

1–35.[14] W.J. Dunning, Particle growth in suspensions, No. 38 in SCI Monograph, Academic

Press, London, 1973.[15] W.I. Higuchi, J. Misra, Physical degradation of emulsions via the molecular

diffusion route and the possible prevention thereof, J. Pharm. Sci. 51 (1962)459–466.

[16] R. Buscall, S.S. Davis, D.S. Potts, The effect of long-chain alkanes on the stability ofoil-in-water emulsions. the significance of ostwald ripening, Colloid Polym. Sci.257 (1979) 636–644.

[17] S.S. Davis, A. Smith, Emulsion Theory and Practice, Academic Press, London, 1973.[18] A. Smith, S.S. Davis, Proceedings: the role of molecular diffusion in the bulk

stability of o-w hydrocarbon emulsions, J. Pharm. Pharmacol. 25 (Suppl) (1973)117 pp.

[19] S.S. Davis, H.P. Round, T.S. Purewal, Ostwald ripening and the stability of emulsionsystems: an explanation for the effect of an added third component, J. ColloidInterface Sci. 80 (1981) 508–511.

[20] A.S. Kabalnov, A.V. Pertsov, E.D. Shchukin, Ostwald ripening in two-componentdisperse phase systems: application to emulsion stability, Colloid Surf. 24 (1987)19–32.

[21] A.S. Kabalnov, A.V. Pertsov, Y.D. Aprosin, E.D. Shchukin, Influence of nature andcomposition of disperse phase on stability of direct emulsions againsttranscondensation, Kolloidn. Zh. (English Translation) 46 (1985) 965–967.

[22] P. Taylor, R.H. Ottewill, Trends in Colloid and Interface Science VIII, Vol. 97 ofProgress in Colloid and Polymer Science, Ch. Ostwald ripening in O/Wminiemulsions formed by the dilution of O/W microemulsions, Springer, Berlin,1994, pp. 199–203.

[23] W.I. Higuchi, A.H. Goldberg, Mechanisms of interphase transport. ii. Theoreticalconsiderations and experimental evaluation of interfacially controlled transportin solubilized systems, J. Pharm. Sci. 58 (1969) 1342–1352.

[24] T. Yotsuyanagi, W.I. Higuchi, A.H. Ghanem, Theoretical treatment of diffusionaltransport into and through an oil–water emulsion with an interfacial barrier atthe oil–water interface, J. Pharm. Sci. 62 (1973) 40–43.

[25] B. Abismail, J.P. Canselier, A.M.Wilhelm, H. Delmas, C. Gourdon, Emulsification byultrasound: drop size distribution and stability, Ultrason. Sonochem. 6 (1999)75–83.

[26] P. Walstra, Principles of emulsion formation, Chem. Engng. Sci. 48 (1993) 333.[27] S.E. Friberg, S. Jones, Othmer Encyclopedia of Chemical Technology, Kroschwith,

J.I., 1994[28] C. Bondy, K. Söllner, On the mechanism of emulsification by ultrasonic waves,

Trans. Faraday Soc. 31 (1935) 835–842.[29] T.J. Mason, Industrial sonochemistry: potential and practicality, Ultrason

Sonochem. 30 (1992) 147–196.[30] O. Behrend, K. Ax, H. Schubert, Influence of continuous phase viscosity on

emulsification by ultrasound, Ultrason Sonochem. 7 (2000) 77–85.[31] C.J. Samer, F.J. Schork, The role of high shear in continuous miniemulsion

polymerization, Ind. Eng. Chem. Res. 38 (1999) 1801–1807.[32] K. Landfester, The generation of nanoparticles in miniemulsions, Adv. Mater. 13

(2001) 765–768.[33] M. Willert, R. Rothe, K. Landfester, M. Antonietti, Synthesis of inorganic and

metallic nanoparticles by miniemulsification of molten salts and metals, Chem.Mater. 13 (2001) 4681–4685.

[34] K. Landfester, F. Tiarks, H.-P. Hentze, M. Antonietti, Polyaddition in miniemul-sions: a new route to polymer dispersions, Macromol. Chem. Phys. 201 (2000)1–5.

[35] F. Tiarks, K. Landfester, M. Antonietti, Preparation of polymeric nanocapsules byminiemulsion polymerization, Langmuir 17 (2001) 908–918.

[36] U. Paiphansiri, P. Tangboriboonrat, K. Landfester, Ploymeric nanocapsulescontaining an antiseptic agent obtained by controlles nanoprecipitation ontowater-in-oil miniemulsion droplets, Macromol. Biosci. 6 (2006) 33–40.

[37] P. Walstra, P.E.A. Smoulders, Modern Aspects of Emulsion Science, The RoyalSociety of Chemistry, Cambridge, 1998.

[38] G. Quincke, Ueber emulsionbildung und den einfluss der galle bei der verdauung,Plüger Archiv für die Physiologie 19 (1879) 129–144 and references therein.

[39] J.T. Davies, D.A. Haydon, An investigation of droplet oscillation during masstransfer. ii. A dynamical investigation of oscillating spherical droplets, Proc. 2ndInt. Congr. Surf. Act. London, vol. 1, 1957, p. 417.

[40] P. Becher, Emulsions: Theory and Practice, Van Nostrand Reinhold, New York, 1966.[41] E.S.R. Gopal, Emulsion Science, Academic Press, New York, 1968.[42] M.J. Groves, video disposable diaper having an emulsion concentrate, Chem. Ind.

12 (1978) 417–423.[43] E. Rubin, C.J. Radke, Dynamic interfacial tension minima in finite systems, Chem.

Eng. Sci. 35 (1980) 1129–1138.[44] D.Z. Becher, Encyclopedia of Emulsion Technology, vol. 2, Marcel Dekker, New

York, 1985.


[45] C.A. Miller, Spontaneous emulsification produced by diffusion—a review, ColloidsSurf. 29 (1988) 89–102.

[46] M.S. El-Aasser, C.D. Lack, J.W. Vanderhoff, F.M. Fowkes, Miniemulsificationprocess—different form of spontaneous emulsification, Colloids Surf. 29 (1986)103–118.

[47] C.W. Pouton, Formulation of self-emulsifying drug delivery systems, Adv. DrugDeliv. Rev. 25 (1997) 47–58.

[48] Y. Kawashima, H. Yamamoto, H. Takeuchi, T. Hino, T. Niwa, Properties of a peptidecontaining dl-lactide/glycolide copolymer nanospheres prepared by novelemulsion solvent diffusion methods, Eur. J. Pharm. Biopharm. 45 (1998) 41–48.

[49] D. Quintanar-Guerrero, E. Allémann, H. Fessi, E. Doelker, Pseudolatex preparationusing a novel emulsion–diffusion process involving direct displacement ofpartially water-miscible solvents by distillation, Int. J. Pharm.188 (1999) 155–164.

[50] M. Trotta, M. Gallarate, F. Pattarino, S. Morel, Emulsions containing partiallywater-miscible solvents for the preparation of drug nanosuspensions, J. Control.Release 76 (2001) 119–128.

[51] K. Bouchemal, S. Briançon, E. Perrier, H. Fessi, Nano-emulsion formulation usingspontaneous emulsification: solvent, oil and surfactant optimisation, Int. J.Pharm. 280 (2004) 241–251.

[52] F. Ganachaud, J.L. Katz, Nanoparticles and nanocapsules created using the ouzoeffect: spontaneous emulsification as an alternaive to ultrasonic and high-sheardevices, Chem. Phys. Chem. 6 (2005) 209–216.

[53] K. Shinoda, H. Saito, The effect of temperature on the phase equilibria and thetypes of dispersions of the ternary system composed of water, cyclohexane, andnonionic surfactant, J. Colloid Interface Sci. 26 (1968) 70–74.

[54] K. Shinoda, H. Saito, The stability of o/w type emulsions as a function oftemperature and the hlb of emulsifiers: the emulsification by pit-method, J.Colloid Interface Sci. 30 (1969) 258–263.

[55] T. Forster, F. Schambil, H. Tesmann, Emulsification by the phase inversiontemperature method: the role of self-bodying agents and the influence of oilpolarity, Int. J. Cosmet Sci. 12 (1990) 217–227.

[56] T. Forster, F. Schambil, W. von Rybinski, Production of fine disperse and long-termstable oil-in-water emulsions by the phase inversion temperature method,J. Disp. Sci. Technol. 13 (1992) 183–193.

[57] T. Forster, W. von Rybinski, A. Wadle, Influence of microemulsion phases on thepreparationoffine-disperse emulsions, Adv. Colloid InterfaceSci. 58 (1995)119–149.

[58] A.J. Sing, A. Graciaa, J. Lachaise, P. Brochette, J.L. Salagers, Interactions andcoalescence of nano-droplets in translucent o/w emulsions, Colloids Surf. A 152(1999) 31–39.

[59] P. Izquierdo, J. Esquena, T.F. Tadros, C. Dederen, M.J. Garcia, N. Azemar, C. Solans,Formation and stability of nano-emulsions prepared using the phase inversiontemperature method, Langmuir 18 (2002) 26–30.

[60] D. Morales, J.M. Gutiérrez, M.J. García-Celma, Y.C. Solans, A study of the relationbetween bicontinuous microemulsions and oil/water nano-emulsion formation,Langmuir 19 (2003) 7196–7200.

[61] R. Pons, I. Carrera, J. Caelles, J. Rouch, P. Panizza, Formation and properties of mini-emulsions formed by microemulsions dilution, Adv. Colloid Interface Sci. 106(2003) 129–146.

[62] P. Izquierdo, J. Esquena, T.F. Tdros, J.C. Dederen, J. Feng, M.J. García-Delma, N.Azemar, C. Solans, Phase behavior and nano-emulsion formation by the phaseinversion temperature method, Langmuir 20 (2004) 6594–6598.

[63] J.L. Salager, A. Forgiarini, L. Marquez, A. Pena, M. Pizzino, P. Rodriguez, M. Rondon-Gonzalez, Using emulsion inversion in industrial processes, Adv. Colloid InterfaceSci. 108–109 (2004) 259–272.

[64] C. Solans, P. Izquierdo, J. Nolla, N. Azemar, M.J. Garcia-Celma, Nano-emulsions,Curr. Opin. Colloid Interface Sci. 10 (2005) 102–110.

[65] P.A. Rehbinder, V. Lichtman, Effect of surface active media on strains and rupturein solids, Proc. 2nd Int. Congr. Surf. Act. London, vol. 58, 1957, p. 549.

[66] M. Ostrovsky, R. Good, Mechanism of microemulsion formation in systems withlow interfacial tension: occurrence, properties, and behavior of microemulsions,J. Colloid Interface Sci. 102 (1984) 206–226.

[67] K.J. Ruschak, C.A. Miller, Spontaneous emulsification in ternary systems withmass transfer, Ind. Ing. Chem. Fundam. 11 (1972) 534–540.

[68] P. Winsor, Solvent Properties of Amphiphilic Compounds, Butterworth, London,1954.

[69] H. Fessi, J.P. Devissaguet, F. Puisieux, Procédés de préparation de systèmescolloïdaux dispersibles d'une substance sous forme de nanocapsules, FrenchPatent 8618444.

[70] H. Fessi, F. Puisieux, J.P. Devissaguet, N. Ammoury, S. Benita, Nanocapsuleformation by interfacial polymer deposition following solvent deplacement, Int. J.Pharm. 55 (1989) 25–28.

[71] H. Fessi, J.P. Devissaguet, F. Puisieux, Procédé de préparation de systèmescolloïdaux dispersibles d'une substance, sous forme de nanoparticules, EP 0275796 B1 (1992).

[72] K. Bouchemal, S. Briançon, E. Perrier, H. Fessi, I. Bonnet, N. Zydowicz, Synthesisand characterization of polyurethane and poly(ether urethane) nanocapsulesusing a new technique of interfacial polycondensation combined to spontaneousemulsification, Int. J. Pharm. 269 (1) (2004) 89–100.

[73] I. Montasser, H. Fessi, S. Briançon, J. Lieto, Method of preparing colloidal particlesin the form of nanocapsules, World Patent WO0168235.

[74] L. Marszall, Nonionic Surfactants, Vol. 23 of Surfactant Sciences, Marcel Dekker,New York, 1987.

[75] P. Taylor, R.H. Ottewill, The formation and ageing rates of oil-in-waterminiemulsions, Colloids Surf. A 88 (1994) 303–316.

[76] P. Taylor, R.H. Ottewill, Ostwald ripening in o/w miniemulsions formed by thedilution of o/w microemulsions, Prog. Colloid Polym. Sci. 97 (1994) 199–203.

[77] A. Forgiarini, J. Esquena, C. González, C. Solans, Formation of nano-emulsions bylow-energy emulsification methods at constant temperature, Langmuir 17 (2001)2076–2083.

[78] H. Wu, C. Ramachandran, N.D. Weiner, B.J. Roessler, Topical transport ofhydrophilic compounds using water-in-oil nanoemulsions, Int. J. Pharm. 220(2001) 63–75.

[79] M. Porras, C. Solans, C. González, A. Martínez, A. Guinart, J.M. Gutiérrez,Studies of formation of w/o nano-emulsions, Colloids Surf. A 249 (2004)115–118.

[80] N. Usón, M.J. García, C. Solans, Formation of water-in-oil (w/o) nano-emulsions ina water/mixed non-ionic surfactant/oil systems prepared by a low-energyemulsification method, Colloids Surf. A 250 (2004) 415–421.

[81] I. Solè, A. Maestro, C.M. Pey, C. González, C. Solans, J.M. Gutiérrez, Nano-emulsions preparation by low energy methods in an ionic surfactant system,Colloids Surf. A 288 (2006) 138–143.

[82] I. Solè, A. Maestro, C. González, C. Solans, J.M. Gutiérrez, Optimization of nano-emulsion preparation by low-energy methods in an ionic surfactant system,Langmuir 22 (2006) 8326–8332.

[83] M. Kahlweit, R. Strey, P. Firman, D. Haase, Phase behavior of ternary systems:water–oil-nonionic surfactant as a near-tricritical phenomenon, Langmuir 1(1985) 281–288.

[84] M. Kahlweit, Mikroemulsionen—eine qualitative beschreibung ihrer eigenschaf-ten, Tenside Surf. Det. 30 (1993) 83–89.

[85] K.V. Schubert, R. Strey, M. Kahlweit, A new purification technique for alkylpolyglycol ethers andmiscibility gaps for water-c[i]e[j], J. Colloid Interface Sci. 141(1991) 21–29.

[86] K. Shinoda, Solution behavior of surfactants: the importance of surfactant phaseand the continuous change in hlb of surfactant, Prog. Colloid Polym. Sci. 68 (1983)1–7.

[87] K. Shinoda, S. Friberg, Microemulsions colloidal aspects, Adv. Colloid Interface Sci.4 (1975) 281–300.

[88] H. Kunieda, K. Shinoda, Correlation between critical solution phenomena andultralow interfacial tensions in a surfactant/water/oil system, Bull. Chem. Soc. Jpn.55 (1982) 1777–1781.

[89] R. Aveyard, B.P. Binks, P.D.I. Fletcher, Interfacial tensions and aggregate structurein pentaethylene glycol monododecyl ether/oil/water microemulsion systems,Langmuir 5 (1989) 1210–1217.

[90] N. Anton, P. Gayet, J.P. Benoit, P. Saulnier, Nano-emulsions and nanocapsules bythe pit method: An investigation on the role of the temperature cycling on theemulsion phase inversion, Int. J. Pharm. 344 (1–2) (2007) 44–52.

[91] N. Anton, J.P. Benoit, P. Saulnier, Particular conductive behaviors of emulsionsphase inverting, J. Drug Del. Sci. Tech. 18 (2) (2008).

[92] H. Kunieda, H. Fukui, H. Uchiyama, C. Solans, Spontaneous formation of highlyconcentrated water-in-oil emulsions (gel–emulsions), Langmuir 12 (1996)2136–2140.

[93] J.L. Salager, L. Marquez, A. Graciaa, J. Lachaise, Partitioning of ethoxylatedoctylphenol surfactants in microemulsion-oil–water systems: influence oftemperature and relation between partitioning coefficient and physicochemicalformulation, Langmuir 16 (2000) 5534–5539.

[94] J.L. Salager, R. Anton, J.M. Anderez, J.M. Aubry, Formulation des microémulsionspas la méthode du hld, Techniques de l'Ingénieur Génie des Procédés J2 (2001)1–20.

[95] J.L. Salager, Handbook of Detergents—Part A: Properties, vol. 82, 1999, pp. 253–302.[96] J.L. Salager, Pharmaceutical emulsions and suspensions, Ch. Formulation Concepts

for the Emulsion Maker, Marcel Dekker, New York, 2000, pp. 19–72.[97] M. Bourrel, J.L. Salager, R.S. Schechter, W.H. Wade, Correlation for phase behavior of

nonionic surfactants, J. Colloid Interface Sci. 75 (1980) 451–461.[98] J.L. Salager, Formulation Concepts for the Emulsion Maker, Marcel Dekker, New

York, 2000, pp. 19–72, Ch. 2.[99] A. Béduneau, P. Saulnier, N. Anton, F. Hindré, C. Passirani, H. Rajerison, N. Noiret, J.P.

Benoit, Pegylated nanocapsules produced by an organic solvent-free method:evaluation of their stealth properties, Pharm. Res. 23 (2006) 2190–2199.

[100] M. Rondón-González, V. Sadtler, L. Choplin, J.L. Salager, Emulsion inversion fromabnormal to normal morphology by continuous stirring without internal phaseaddition. effect of surfactant mixture fractionation at extreme water–oil ratio,Colloids Surf. A 288 (2006) 151–157.

[101] M. Rondón-González, V. Sadtler, L. Choplin, J.L. Salager, Emulsion catastrophicinversion from abnormal to normal morphology. 5. Effect of the water-to-oil ratioand surfactant concentration on the inversion produced by continuous stirring, Ind.Eng. Chem. Res. 45 (2006) 3074–3080.

[102] E. Tyrode, J. Allouche, L. Choplin, J.L. Salager, Emulsion catastrophic inversion fromabnormal to normal morphology. 4. Following the emulsion viscosity during threeinversion protocols and extending the critical dispersed-phase concept, Ind. Eng.Chem. Res. 44 (2005) 67–74.

[103] E. Tyrode, I. Mira, N. Zambrano, L. Márquez, M. Rondón-González, J.L. Salager,Emulsion catastrophic inversion from abnormal to normal morphology. 3.Conditions for triggering the dynamic inversion and application to industrialprocesses, Ind. Eng. Chem. Res. 42 (2003) 4311–4318.

[104] I. Mira, N. Zambrano, E. Tyrode, L. Márquez, A. Pena, A. Pizzino, J.L. Salager, Emulsioncatastrophic inversion from abnormal to normal morphology. 2. Effect of thestirring intensity on the dynamic inversion frontier, Ind. Eng. Chem. Res. 42 (2003)57–61.

[105] N. Zambrano, E. Tyrode, I. Mira, L. Márquez, M.P. Rodríguez, J.L. Salager, Emulsioncatastrophic inversion from abnormal to normal morphology. 1. Effect of the water-to-oil ratio rate of change on the dynamic inversion frontier, Ind. Eng. Chem. Res. 42(2003) 50–56.


[106] B. Heurtault, P. Saulnier, B. Pech, J.E. Proust, J. Richard, J.P. Benoit, Patent W001/64328.

[107] B.Heurtault, P. Saulnier, B. Pech, J.E. Proust, J.P. Benoit, A novel phase inversion-basedprocess for the preparation of lipid nanocarriers, Pharm. Res. 19 (2002) 875–880.

[108] A. Lamprecht, Y. Bouligand, J.P. Benoit, New lipid nanocapsules exhibit sustainedrelease properties for amiodarone, J. Control. Release 84 (2002) 59–68.

[109] D. Hoarau, P. Delmas, S. David, E. Roux, J.C. Leroux, Novel long-circulating lipidnanocapsules, Pharm. Res. 21 (2004) 1783–1789.

[110] K.D. Collins, M.W.Washabaugh, The hofmeister effect and the behaviour of water atinterfaces, Q. Rev. Biophys. 18 (1985) 323–422.

[111] J.L. Kavanau,Water and Solute–Water Interactions, Holden-Day, San Fransisco,1964.[112] R.J. Kjellander, Phase separation of non-ionic surfactant solutions. a treatment of the

micellar interaction and form, Chem. Soc. Faraday Trans. 12 (1982) 2025–2042.[113] P.G. Nilsson, H. Wennerstrom, B. Lindman, Structure of micellar solutions of

nonionic surfactants. nuclear magnetic resonance self-diffusion and protonrelaxation studies of poly(ethylene oxide) alkyl ethers, J. Phys. Chem. 87 (1983)1377–1385.

[114] P.G. Nilsson, B. Lindman, Water self-diffusion in nonionic surfactant solutions.hydration and obstruction effects, J. Phys. Chem. 87 (1983) 4756–4761.

[115] B. Jonsson, P.G. Nilsson, B. Lindman, L. Guldbrand, H. Wennerstrom, Surfactants inSolution, vol. 1, Plenum, New York, 1984.

[116] S.S. Funari, M.C. Holmes, G.J. Tiddy, Intermediate lyotropic liquid crystal phases inthe c16eo6/water system, J. Phys. Chem. 98 (1994) 3015–3023.

[117] L.S. Romsted, J. Yao, Arenediazonium salts: new probes of the interfacialcompositions of association colloids. 4. estimation of the hydration numbers ofaqueous hexaethylene glycol monododecyl ether, c12e6, micelles by chemicaltrapping, Langmuir 12 (1996) 2425–2432.

[118] J. Szymanowski, M. Wisniewski, E. Pietrzak, K. Prochaska, The emulsion inversionpoint and the temperature of emulsion inversion for model emulsions stabilised byethylene and butylene oxide block copolymers of type ebe, Tenside Deterg. 20(1983) 188–191.

[119] S.K. Goel, Selecting the optimal linear alcohol ethoxylate for enhanced oily soilremoval, J. Surfactants Deterg. (AOCS) 2 (1998) 213–219.

[120] N. Anton, P. Saulnier, A. Béduneau, J.P. Benoit, Salting-out effect induced bytemperature cycling on a water/nonionic surfactant/oil system, J. Phys. Chem. B 111(2007) 3651–3657.

[121] J. Kreuter, Colloidal Drug Delivery Systems, Marcel Dekker, New York, 1994 Ch.Nanoparticles.

[122] E.F. Craparo, G. Cavallaro, M.L. Bondì, D. Mandracchia, G. Giammona, Pegylatednanoparticles based on polyaspartamide. Preparation, physico-chemical character-ization, and intracellular uptake, Biomacromolecules 7 (2006) 3083–3092.

[123] M.A. Bradley, S.W. Prescott, H.A.S. Schoonbrood, K. Landfester, F. Grieser,Miniemulsion copolymerization of methyl methacrylate and butyl acrylate byultrasonic initiation, Macromolecules 38 (2005) 6346–6351.

[124] G. Qi, C.W. Jones, F.J. Shork, Enzyme-initiated miniemulsion polymerization,Biomacromolecules 7 (11) (2006) 2927–2930.

[125] K. Landfester, N. Bechthold, F. Tiarks, M. Antonietti, Formulation and stabilitymechanisms of polymerizable miniemulsions, Macromolecules 32 (1999)5222–5228.

[126] K. Landfester, M. Willert, M. Antonietti, Preparation of polymer particles innonaqueous direct and inverse miniemulsions, Macromolecules 33 (2000)2370–2376.

[127] K. Wormuth, Superparamagnetic latex via inverse emulsion polymerization, J.Colloid Interface Sci. 241 (2001) 366–377.

[128] J. KwonOh, C. Tang, H. Gao, N.V. Tsarevsky, K.Matyjaszewski, Inverseminiemulsionatrp: a new method for synthesis and functionalization of well-defined water-soluble/cross-linked polymeric particles, J. Am. Chem. Soc. 128 (2006) 5578–5584.

[129] C.Y. Li,W.Y. Chiu, T.M. Don, Preparation of polyurethane dispersion byminiemulsionpolymerization, J. Polym. Sci. Pol. Chem. 43 (2005) 4870–4881.

[130] C. Maitre, F. Ganachaud, O. Ferreira, J.F. Lutz, Y. Paintoux, P. Hémery, Anionicpolymerization of phenyl glycidyl ether in miniemulsion, Macromolecules 33(2000) 7730–7736.

[131] A. Tomov, J.P. Broyer, R. Spitz, Emulsion polymerization of ethylene in watermedium catalysed by organotransition metal complexes, Macromol. Symp. 150(2000) 53–58.

[132] S. Magdassi, L. Spernath, A method for the preparation of nanoparticles fromnanoemulsions, Patent WO 2005/102507 A1.

[133] M.M. Gallardo, G. Couarraze, B. Denizot, L. Treupel, P. Couvreur, F. Puisieux,Preparation and purification of isohexycyanoacrylate nanocapsules, int J. Pharm.100 (1993) 55–64.

[134] G. Puglisi, M. Fresta, G. Giammona, C.A. Venture, Influence of the preparationconditions in poly(ethylcyanoacrylate) nanocapsules formation, Int. J. Pharm. 125(1995) 283–287.

[135] K. Landfester, R.Montenegro, U. Scherf, R. Güntner, U. Asawapirom, S. Patil, D. Neher,T. Kietzke, Semiconducting polymer nanospheres in aqueous dispersion preparedby a miniemulsion process, Adv. Mater. 14 (2002) 651–655.

[136] T. Kietzke, D. Neher, K. Landfester, R. Montenegro, R. Güntner, U. Scherf, Novelapproaches to polymer blends based onpolymer nanoparticles, Nat.Mater. 2 (2003)408–412.

[137] Z. Yang, W.T.S. Huck, S.M. Clarke, A.R. Tajbakhsh, E.M. Terent Jev, Shape-memorynanoparticles from inherently non-spherical polymer colloids, Nat. Mater. 4 (2005)486–490.

[138] W.K. Lee, J.Y. Park, S. Jung, C. Woo Yang, W.U. Kim, H.Y. Kim, J.H. Park, J.S. Park,Preparation and characterization of biodegradable nanoparticles entrappingimmunodominant peptide conjugated with peg for oral tolerance induction, J.Control. Release 105 (2005) 77–88.

[139] M.D. Blanco, M.J. Alonso, Development and characterization of protein-loadedpoly(lactide-co-glycolide) nanospheres, Eur. J. Pharm. Biopharm. 43 (1997)287–294.

[140] C. Perez, A. Sanchez, D. Putnam, D. Ting, R. Langer, M.J. Alonso, Poly(lactic acid)-poly(ethylene glycol) nanoparticles as new carriers for the delivery of plasmid dna, J.Control. Release 75 (2001) 211–224.

[141] R. Montenegro, M. Antonietti, Y. Mastai, K. Landfester, Crystallization in miniemul-sion droplets, J. Phys. Chem. B 107 (2003) 5088–5094.

[142] A. Taden, K. Landfester, Crystallization of poly(ethylen oxide) confined inminiemulsion droplets, Macromolecules 36 (2003) 4037–4041.

[143] F. Quaglia, L.Ostacolo,G.DeRosa,M.I. LaRotonda,M.Ammendola,G.Nese,G.Maglio,R. Palumbo, C. Vauthier, Nanoscopic core-shell drug carrier made of amphiphilictriblock and star-diblock copolymers, Int. J. Pharm. 324 (2006) 56–66.

[144] D. Quintanar-Guerrero, E. Allémann, E. Doelker, H. Fessi, A mechanistic study of theformation of polymer nanoparticles by the emulsification-diffusion technique,Colloid Polym. Sci. 275 (1997) 640–647.

[145] T. Trimaille, C. Pichot, A. Elaïssari, H. Fessi, S. Briançon, T. Delair, Poly(d,l-lactic acid)nanoparticle preparation and colloidal characterization, Colloid Polym. Sci. 281(2003) 1184–1190.

[146] S. Galindo-Rodrigez, E. Allémann, H. Fessi, E. Doelker, Physicochemical parametersassociated with nanoparticle formation in the salting-out emulsification–diffusion,and nanoprecipitation methods, Pharm. Res. 21 (8) (2004) 1428–1439.

[147] D. Moinard-Checot, Y. Chevalier, S. Briançon, H. Fessi, S. Guinebretière, Nanopar-ticles for drug delivery: review of formulation and process difficulties illustrated bythe emulsion–diffusion process, J. Nanosci. Nanotechnol. 6 (9–10) (2006)2664–2681.

[148] J.C. Leroux, E. Allemann, E. Doelker, R. Gurny, New approaoch for the preparation ofnanoparticles by an emulsification method, Eur. J. Pharm. Biopharm. 41 (1995)14–18.

[149] N. Csaba, L. González, A. Sánchez, M.J. Alonso, Design and characterisation of newnanoparticulate polymer blends for drug delivery, J. Biomater. Sci. Polym. Edn. 15(2004) 1137–1151.

[150] N. Csaba, P. Camaño, A. Sánchez, F. Domínguez, M.J. Alonso, Plga:poloxamer andplga:poloxamine blend nanoparticles: new carriers for gene delivery, Biomacro-molecules 6 (2005) 271–278.

[151] M.J. Santander-Ortega, N. Csaba, M.J. Alonso, J.L. Ortega-Vinuesa, D. Bastos-González, Stability and physicochemical characteristics of plga, plga:poloxamerand plga:poloxamine blend nanoparticles. a comparative study, Colloids Surf. A 296(2007) 132–140.

[152] N. Csaba, A. Sánchez, M.J. Alonso, Plga:poloxamer and plga:poloxamine blendnanostructures as carriers for nasal gene delivery, J. Control. Release 113 (2006)164–172.

[153] R.H.Müller, A. Dingler, Thenext generation after liposomes: solid lipid nanoparticles(sln, lipopearls) as dermal carrier in cosmetics, Eurocosmetics 7–8 (1998) 19–26.

[154] R.H. Müller, K. Mäder, S. Gohla, Solid lipid nanoparticles (sln) for controlled drugdelivery—a review of the state of the art, Eur. J. Pharm. Biopharm. 50 (2000)161–178.

[155] S.A. Wissing, R.H. Müller, Cosmetic applications for solid lipid nanoparticles (sln),Int. J. Pharm. 254 (2003) 65–68.

[156] C. Freitas, R.H. Müller, Correlation between long-term stability of solid lipidnanoparticles (sln) and crystallinity of the lipid phase, Eur. J. Pharm. Biopharm. 42(2) (1999) 125–132.

[157] R.H. Müller, M. Radtke, S.A. Wissing, Nanostructured lipid matrices for improvedmicroencapsulation of drugs, Int. J. Pharm. 242 (1–2) (2002) 121–128.

[158] C. Olbrich, A. Gessner, O. Kayser, R.H. Müller, Lipid-drug conjugate (ldc)nanoparticles as novel carrier system for the hydrophilic antitrypanosomal drugdiminazenediaceturate, J. Drug Target. 10 (5) (2002) 387–396.

[159] R.H. Müller, C. Scharwz, W. Mehnert, J.S. Lucks, Production of solid lipidnanoparticles (sln) for controlled drug delivery, Proc. Int. Symp. Control. ReleaseBioact. Mater. 20 (1993) 480–481.

[160] R.H. Müller, J.S. Lucks, Arzneistoffträger aus festen lipidteilchen, feste lipidnano-sphären (sln), European Patent No. 0605497.

[161] K. Jores, W. Mehnert, M. Drechsler, H. Bunjes, C. Johann, K. Mäder, Investigations onthe structure of solid lipid nanoparticles (sln) and oil-loaded solid lipidnanoparticles by photon correlation spectroscopy, field-flow fractionation andtransmission electron microscopy, J. Control. Release 95 (2004) 217–227.

[162] C. Schwarz, W. Mehnert, J.S. Lucks, R.H. Müller, Solid lipid nanoparticles (sln)for controlled drug delivery. i. Production, characterization and sterilization,J. Control. Release 60 (1994) 83–96.

[163] H. Weyhers, Feste lipid-nanopartikel (sln) für die gewebsspezifishe arzneistoffap-plikation, Ph.D. thesis, University of Berlin (1995).

[164] M. García-Fuentes, D. Torres, M.J. Alonso, Design of lipid nanoparticles for the oraldelivery of hydrophilic macromolecules, Colloids Surf. B 27 (2002) 159–168.

[165] H. Fukui, T. Koike, A. Saheki, S. Sonoke, J. Seki, A novel delivey system foramphotericin b with lipid nano-spehre (lns), Int. J. Pharm. 265 (2003) 37–45.

[166] J. Seki, S. Sonoke, A. Saheki, H. Fukui, H. Saski, T. Mayumi, A nanometer lipidemulsion, lipid nano-sphere (lns), as a parenteral drug carrier for passive drugtargeting, Int. J. Pharm. 273 (2004) 75–83.

[167] M. Bockisch, Nahrungsfette und-öle, Ulmer Verlag, Stuttgart, 1993.[168] M.R. Gasco, Method for producing solid lipid microspheres having a narrow size

distribution, US Patent 5 250 236.[169] L. Boltri, T. Canal, P.A. Esposito, F. Carli, Lipid nanoparticles: evaluation of some critial

formulation parameters, Proc. Int. Symp. Control. Release Bioact. Mater. 20 (1993)346–347.

[170] M.R. Gasco, Solid lipid nanospheres from warm microemulsions, Pharm. Technol.Eur. 9 (1997) 52–58.


[171] S. Morel, E. Terreno, E. Ugazio, S. Aime, M.R. Gasco, Nmr relaxometric investigationsof solid lipid nanoparticles (sln) containing gadolinium (iii) complexes, Eur. J.Pharm. Biopharm. 45 (1998) 157–163.

[172] F.Q. Hu, H. Yuan, H.H. Zhang, M. Fang, Preparation of solid lipid nanoparticles withclobetasol propionate by a novel solvent diffusion method in aqueous system andphysicochemical characterization, Int. J. Pharm. 239 (2002) 121–128.

[173] M. Trotta, F. Debernardi, O. Caputo, Preparation of solid lipid nanoparticles by asolvent emulsification?diffusion technique, Int. J. Pharm. 257 (1–2) (2003) 153–160.

[174] F.Q. Hu, Y. Hong, H. Yuan, Preparation and characterization of solid lipidnanoparticles containing peptide, Int. J. Pharm. 273 (1–2) (2004) 29–35.

[175] R. Pandey, S. Sharma, G. Khuller, Oral solid lipid nanoparticle-based antitubercularchemotherapy, Tuberculosis 85 (5–6) (2005) 415–420.

[176] F.Q. Hu, S.P. Jiang, Y.Z. Du, Preparation and characterization of stearic acidnanostructured lipid carriers by solvent diffusion method in an aqueous system,Colloids Surf. B 45 (2005) 167–173.

[177] Y. Hong, F.Q. Hu, H. Yuan, Effect of peg2000 on drug delivery characterization fromsolid lipid nanoparticles, Pharmazie 61 (4) (2006) 312–315.

[178] F.Q. Hu, S.P. Jiang, Y.Z. Du, H. Yuan, Y.Q. Ye, S. Zeng, Preparation and characteristics ofmonostearin nanostructured lipid carriers, Int. J. Pharm. 314 (2006) 83–89.

[179] G. Lambert, J. Bertrand, E. Fattal, F. Subra, H. Pinto-Alphandary, C.Malvy, C. Auclair, P.Couvreur, Ews fli-1 antisense nanocapsules inhibits ewing sarcoma-related tumorin mice, Biochem. Biophys. Res. Commun. 279 (2) (2000) 401–406.

[180] G. Lambert, E. Fattal, H. Pinto-Alphandary, A. Gulik, P. Couvreur, Polyisobutylcya-noacrylate nanocapsules containing an aqueous core as a novel colloidal carrier forthe delivery of oligonucleotides, Pharm. Res. 17 (6) (2000) 707–714.

[181] G. Lambert, E. Fattal, H. Pinto-Alphandary, A. Gulik, P. Couvreur, Polyisobutylcya-noacrylate nanocapsules containing an aqueous core for the delivery of oligonu-cleotides, Int. J. Pharm. 214 (2001) 13–16.

[182] G. Lambert, E. Fattal, P. Couvreur, Nanoparticulate systems for the delivery ofantisense oligonucleotides, Adv. Drug. Deliv. Rev. 47 (1) (2001) 99–112.

[183] N. Toub, C. Angiari, D. Eboué, E. Fattal, J.P. Tenu, T. Le Doan, P. Couvreur, Cellular fateof oligonucleotides when delivered by nanocapsules of poly(isobutylcyanocrylate),J. Control. Release 106 (2005) 209–213.

[184] N. Toub, J.R. Bertrand, A. Tamaddon, H. Elhamess, H. Hillaireau, A. Maksimenko, J.Maccario, C. Malvy, E. Fattal, P. Couvreur, Efficacy of sirna nanocapsules targetedagainst the ews-fli1 oncogene in ewing sarcoma, Pharm. Res. 23 (2005) 892–900.

[185] H. Hillaireau, T. Le Doan, M. Besnard, H. Chacun, J. Janin, P. Couvreur, Encapsulationof antiviral analogues azidothymidine-triphosphate and cidofovir in poly(iso-butylcyanoacrylate) nanocapsules, Int. J. Pharm. 324 (2006) 37–42.

[186] H. Hillaireau, T. Le Doan, H. Chacun, J. Janin, P. Couvreur, Encapsulation ofmono- andoligo-nucleotides into aqueous-core nanocapsules in the presence of water-solublepolymers, Int. J. Pharm. 331 (2007) 148–152.

[187] N. Anton, P. Saulnier, C. Gaillard, E. Porcher, S. Vrignaud, J.P. Benoit, Aqueous-corenanocapsules for encapsulating fragile hydrophilic and/or lipophilic molecules.Unpublished.

[188] A.M. Pensé, C. Vauthier, F. Puisieux, J.P. Benoit, Microencapsulation of benzalkoniumchloride, Int. J. Pharm. 81 (1992) 111–117.

[189] A.M. Pensé, C. Vauthier, J.P. Benoit, Study of the interfacial polycondensation ofisocyanate in the preparation of benzalkonium chloride loaded microcapsules,Colloid Polym. Sci. 272 (1994) 211–219.

[190] A.J.P. van Zyl, R.F.P. Bosch, J.B. McLeary, R.D. Sanderson, B. Klumperman, Synthesis ofstyrene based liquid-filled polymeric nanocapsules by the use of raft-mediatedpolymerization in miniemulsion, Polymer 46 (2005) 3607–3615.

[191] S. Torza, S.G. Mason, Three-phase interactions in shear and electrical fields, J. ColloidInterface Sci. 33 (1970) 6783.

[192] N. Al Khouri Fallouh, L. Roblot Treupel, H. Fessi, J.P. Devissaguet, F. Puisieux,Development of a new process for manufacture of polyisobutylcyanoacrylate, Int. J.Pharm. 28 (1986) 125–132.

[193] N. Al Khouri Fallouh, H. Fessi, L. Roblot Treupel, J.P. Devissaguet, F. Puisieux, Anoriginal procedure for preparing nanocapsules of polyalkyl-cyanoacrylates forinterfacial polymerization, Pharm. Acta Helv. 61 (1986) 274–281.

[194] J.M. Rollot, P. Couvreur, L. Roblot Treuquel, F. Puisieux, Physicochemical andmorphological characterization of polyisobutylate nanocapsules, J. Pharm. Sci. 75(1986) 361–364.

[195] F. Chouinard, F.W. Kan, J.C. Leroux, C. Foucher, V. Lenaerts, Preparation and purifi-cation of polyhexylcyanoacrylate nanocapsules, Int. J. Pharm. 72 (1991) 211–217.

[196] M. Gallardo, G. Couarraze, B. Denizot, L. Treupel, P. Couvreur, F. Puisieux, Studyof themechanisms of formation of nanoparticles and nanocapsules of polyisobutyl-2-cyanoacrylate, Int. J. Pharm. 38 (1993) 293–299.

[197] C. Losa, L. Marchal-Heussler, F. Orallo, J.L. Vila Jato, M.J. Alonso, Design of newformulations for topical ocular administration: polymeric nanocapsules containingmetipranolol, Pharm. Res. 10 (1993) 80–87.

[198] F. Chouinard, S. Buczkowski, V. Lenaerts, Poly(alkylcyanoacrylate) nanocapsules:physicochemical characterization and mechanism of formation, Pharm. Res. 11(1994) 869–874.

[199] F. Cournarie,M. Chéron, M. Besnard, C. Vauthier, Evidence for restrictive parametersin formulation of insulin-loaded nanocapsules, Eur. J. Pharm. Biopharm. 57 (2004)171–179.

[200] H.K. Mahabadi, T.H. Ng, H.S. Tan, Interfacial/free radical polymerization micro-encapsulation: kinetics of particle formation, J. Microencapsul. 13 (5) (1996)559–573.

[201] S.K. Yadav, N. Ron, D. Chandrasekharam, K.C. Khilar, A.K. Suresh, Polyureas byinterfacial polycondensation: preparation and properties, J. Macromol. Sci.-Phys.B35 (1996) 807–827.

[202] C. Ramarao, S.V. Ley, S.C. Smith, I.M. Shirley, N. De Almeida, Encapsulation ofpalladium in polyurea microcapsules, Chem. Commun. 10 (2002) 1132–1133.

[203] M. Takasu, H. Kawaguchi, Preparation of colored latex with polyurea shell byminiemulsion polymerization, Colloid Polym. Sci. 283 (2005) 805–811.

[204] I. Montasser, H. Fessi, S. Briançon, J. Lieto, New approach of the preparation ofnanocapsules by an interfacial polycondensation reaction. Polym. Bull. 50 (2003)169–174.

[205] I. Montasser, H. Fessi, S. Briançon, J. Lieto, Method for preparing colloidal particles inthe form of nanocapsules, US Patent Application, Number 20030059473.

[206] I. Montasser, S. Briançon, H. Fessi, The effect of monomers on the formulation ofpolymeric nanocapsules based on polyureas and polyamides, Int. J. Pharm. 335(2007) 176–179.

[207] M. Aboubakar, F. Puisieux, P. Couvreur, M. Deyme, C. Vauthier, Study of themechanism of insulin encapsulation in poly(isobutylcyanoacrylate) nanocapsulesobtained by interfacial polymerization, J. Biomed.Mater. Res. 47 (4) (1997) 568–576.

[208] D. Quintanar-Guerrero, E. Allémann, E. Doelker, H. Fessi, Preparation andcharacterization of nanocapsules from preformed polymers by a new processbased on emulsification–diffusion technique. Pharm. Res. 15 (1998) 1056–1062.

[209] V.C.F. Mosqueira, P. Legrand, H. Pinto-Alphandary, F. Puisieux, G. Barrat, Poly(d,l-lactide) nanocapsules prepared by a solvent displacement process: influence of thecomposition on physicochemical and structural properties, J. Pharm. Sci. 89 (5)(2000) 614–626.

[210] A.R. Pohlmann, V. Weiss, O. Mertins, N.P. da Silveira, S.S. Guterres, Spray-driedindomethacin-loaded polyester nanocapsules and nanospheres: development,stability evaluation and nanostructuremodels, Eur. J. Pharm. Sci.16 (2002) 305–312.

[211] M. Teixeira, M.J. Alonso, M.M.M. Pinto, C.M. Barbosa, Development and character-ization of plga nanospheres and nanocapsules containing xanthone and 3-methoxyxanthone, Eur. J. Pharm. Biopharm. 50 (3) (2005) 491–500.

[212] C. Prego, M. García, D. Torres, M.J. Alonso, Transmucosal macromolecular drugdelivery, J. Control. Release 101 (2005) 151–162.

[213] C. Prego, D. Torres, E. Fernandez-Megia, R. Novoa-Carballal, E. Quiñoá, M.J. Alonso,Chitosan-peg nanocapsules as new carriers for oral peptide delivery. effect ofchitosan pegylation degree, J. Control. Release 111 (2006) 299–308.

[214] C. Prego, M. Fabre, D. Torres, M.J. Alonso, Efficacy and mechanism of action ofchitosan nanocapsules for oral paptide delivery, Pharm. Res. 23 (3) (2006) 549–556.

[215] P. Saulnier, J.P. Benoit, Nanoparticulates as drug carriers, Ch. Lipidic CoreNanocapsules as New Drug Delivery Systems, Imperial college press, London,2006, pp. 213–224.

[216] I. Minkov, T. Ivanova, I. Panaiotov, J.E. Proust, P. Saulnier, Reorganization of lipidnanocapsules at air/water interface: I. Kinetics of surface film formation, ColloidsSurf. B 45 (2005) 14–23.

[217] I. Minkov, T. Ivanova, I. Panaiotov, J. Proust, P. Saulnier, Reorganization of lipidnanocapsules at air–water interface. part 2. Properties of the formed surface film,Colloids Surf. B 44 (4) (2005) 197–203.

[218] I. Minkov, T. Ivanova, I. Panaiotov, J. Proust, R. Verger, Reorganization of lipidnanocapsules at air–water interface: 3. Action of hydrolytic enzymes hll andpancreatic pla2, Colloids Surf. B 45 (1) (2005) 24–34.

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