Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 103–108

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Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

Morphological transitions in dilute solutions of sugar-basedzwitterionic dimer betaine surfactants

Peter Fischera,c,∗, Hua Wub,c

a

ransiestiga

tainetimed by don simon mzwit

Institute of Food Science and Nutrition, ETH Zurich, 8092 Zurich, Switzerlandb Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerlandc Material Research Center, ETH Zurich, 8093 Zurich, Switzerland

a r t i c l e i n f o

Article history:Received 17 March 2008Received in revised form 16 May 2008Accepted 18 May 2008Available online 27 May 2008

Keywords:Gemini surfactantDimer surfactantCatanionicZwitterionicMicellesVesicles

a b s t r a c t

In the present study the tbetaine solutions was inveshown that dimer acid bestable vesicles within thestructures can be destroyea true equilibrium situatibetaines exhibit aggregatias it has both dimeric and

1. Introduction

The equilibrium structures and properties of surfactant systemsare significantly influenced by the morphological transitions of sur-factants under non-equilibrium conditions during aggregation [1].In this contribution, we focus on the transient morphology develop-ment of dilute surfactants extracted from beetroot, which exhibitboth dimeric and zwitterionic (betaine) properties. The observedcomplex aggregation behavior suggests spontaneous formation ofvesicular structures that can be used for encapsulation purposes.In particular, sugar-based surfactants offer the advantage of beingextracted from natural sources and, therefore, are considered asinteresting candidate for enhancing the functionality of consumeror food products. The use as drug delivery systems or as replace-ments for biological lipids is reported, for example, in DNA transferstudies [2,3].

Dimer or gemini surfactants are characterized by two conven-tional single tail surfactant molecules whose head or tail groupsare covalently connected by a spacer-chain of varying chemicalnature. The structure of such a molecule is generally denoted by

∗ Corresponding author at: Institute of Food Science and Nutrition, ETH Zurich,8092 Zurich, Switzerland. Fax: +41 44 6321155.

E-mail address: peter.fischer@ilw.agrl.ethz.ch (P. Fischer).

0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfa.2008.05.031

nt morphological evolution of surfactant aggregates of dilute dimer acidted by light scattering techniques and surface tension measurements. It issolutions undergo a spontaneous formation of larger thermodynamicallyperiod of several days via intermediate aggregation states. The vesicularilution but will reversibly rebuild with the same time constants indicatingilar. In contrast to other vesicle-forming surfactant systems, dimer acid

echanism that are similar to those of two-component surfactant mixtures,terionic properties promoting vesicle formation.

© 2008 Elsevier B.V. All rights reserved.

m–s–m where m represents the tail group (CmHm+1) and s thespacer length. In general, both head groups as well as tail groupsare of the same chemical structure and length [4–6]. Heterogeminisurfactants, which are less common, are molecules with non-

identical head groups of anionic and nonionic surfactants and/ordifferent tail groups, i.e. an n–s–m structure [7–11]. Most investi-gated gemini surfactants are ionic systems, particularly based ontrimethylammoniumbromide and chloride surfactants while onlyfew are nonionic [12,13]. Dimers display some properties supe-rior to those of conventional monomeric surfactants. For example,their critical micelle concentration (cmc), can be significantly lowerthan for a single chain surfactant with the same headgroup archi-tecture. Moreover, aqueous dimer solutions have better wettingproperties, higher surface and interfacial activity, and a compara-bly high viscosity and viscoelasticity [5,14,15]. Depending on thechemical nature of the head group, spacer and tail group, a vari-ety of aggregate forms like micelles, bilayers and vesicles can beexpected [4,16–23]. For sugar-based Gemini surfactants it is knownthat they aggregate in vesicles, cylindrical micelles, and spher-ical micelles as a function of pH. In particular, as long as thedimer solution is dilute and sufficient chain length is provided,vesicle formation is spontaneous and the formed aggregates arefound to be thermodynamically stable [1,11,24–27]. Such stablevesicles do not depend on preparation, are reversible, and are inthermodynamical equilibrium with the surround bulk and other

Physic

104 P. Fischer, H. Wu / Colloids and Surfaces A:

forms of aggregation (bilayer disks, micelles). This is in contrastto concentrated systems, e.g. catanionic mixtures of single anddouble-chain surfactants in which different methods of prepara-tion (shear, sonication, and other forms of agitation) may result inmetastable, kinetically trapped vesicles [1]. As stated widely in theliterature, the debate on spontaneous formation (true equilibrium)versus non-equilibrium (kinetically trapped, metastable) states ofvesicles remains unresolved [1,28–32]. Vesicles formed by geminisurfactant can be considered as intermediates between aggregatesformed by a single-chain surfactant associated with a cosurfactantand those formed from phospholipids.

On the other hand, zwitterionic (amphoteric) surfactants con-tain both positive and negative charges in the head group and aretherefore soluble in aqueous environments over a wide pH range.Because zwitterionic surfactants do not irritate the skin and eyes,they are commonly used in detergents, disinfectants, and personalcare products like shampoos and hair conditioners due to theirproperty to stabilize foams against the antifoaming action of oildroplets [33]. Moreover, there is a growing interest in using naturalproducts to synthesize new surfactants, improved functional sur-factants, and finding a substitute for crude oil-based surfactants.With this in mind, betaines (i.e. sugar-based zwitterionic surfac-tants extracted from beetroot) seem to be ideal candidate becauseof their biodegradable and non-toxic properties [34].

The investigated betaine surfactant combines two featuresthat promote the formation of thermodynamically stable vesicles,namely a zwitterionic appearance and double-tailed geometry,which mimic a catanionic surfactant and a lipid, respectively. In thepresent work, the transient aggregation behavior of dilute dimeracid betaine solutions was investigated using light scattering, sur-face tension measurements, and electron microscopy. It has beenpreviously observed (but not reported) that the flow properties ofsuch solutions change over period of time (days, weeks, or evenmonths). This occurrence is only possible if the solution is aging andslowly changing the structure of the surfactant aggregates. Sincebetaines are used in a wide application range, the material stabilityand long term properties become an important aspect. The focus ofthis paper is therefore to investigate such transient morphologicalchanges in more detail.

2. Materials and methods

2.1. Dimer acid betaine

Dimer acid betaine was provided by Goldschmidt AG (Essen,Germany). Detailed information on the preparation and chemicalstructure is given elsewhere [5,34]. The dimer acid betaine con-tains predominantly cycloaliphatic, aromatic, and linear aliphaticcompounds (Mw ≈ 1200 g/M). Both positive and negative chargesare present in the headgroup and increasing the pH of the systemleads to a change from anionic to cationic surfactants. As a conse-quence, it is soluble in water in the entire pH range, except for theisoelectric point at pH 9 where a minimum solubility is observeddue to tail-biting between ionic groups. For electron microscopyand light scattering experiments, stock solutions of 5 mM were pre-pared with double distilled water. The solution was kept at roomtemperature for at least 1 month prior to performing experimentsto ensure that aggregation processes had reached a steady state.The aggregation dynamics were then initiated by dilution of thestock solution with double distilled water to a desired concentra-tion of 0.5 and 0.125 mM that are within and below the cmc regime.The diluted solution was immediately filtered through a 100 nmfilter (Acrodisc Syringe Filter) in order to break down the originalstructure and mimic a freshly made solution. This procedure there-

ochem. Eng. Aspects 326 (2008) 103–108

fore consists of three distinct sequences: (i) allowing the materialto equilibrate at high concentrations (i.e. 5 mM), (ii) diluting anddestroying the aggregates by filtering into a non-equilibrium state,and (iii) observing the recovery of the long-term equilibrium prop-erties in the dilute state. It also allows us to reproduce data fromdifferent solutions, i.e. different ages and concentrations. All exper-iments were performed at temperature of T = 20 ◦C and at pH 7.5.

2.2. Light scattering

The light scattering instrument used for this study was aBI-200SM (Brookhaven), with an argon-ion laser (M95-2, Lexel)operating at a wavelength of 514.5 nm and a goniometer in theangle range from 15 to 150◦. Both static and dynamic light scatter-ing experiments were carried out. The static light scattering (SLS)experiments measures the light intensity scattered by aggregates(e.g. micelles, vesicles) in the system as a function of the scatteringangle. For a monodispersed system, the scatter intensity, I(q), maybe expressed as [35,36]

I(q) = Cm2P(q) (1)

where q is the wave vector defined as

q = 4�n

�0sin

�

2(2)

where �0 is the wavelength of the incident light, n the refractiveindex of the surrounding medium, and � is the scattering angle.The other parameters and variables in Eq. (1) namely C, m, andP(q) are the concentration, mass, and form factor of the micelleor vesicle, respectively. The form factor, P(q), is a function of bothq and aggregate size. In the present case, based on the electronmicroscopy pictures of our micellar system, we assume that theaggregates are spherical. Then the form factor, P(q), in Eq. (1) can bewell approximated by the Rayleigh–Gans–Debye (RGD) expression

P(q) = 9

(sin(Rq) − Rq sin(Rq)

(Rq)3

)2

(3)

where R is radius of the micelle. Thus, in this case the q-dependenceof I(q) is only due to the form factor of the aggregates. The dynamiclight scattering (DLS) experiments measure the intensity autocor-relation function, and its decay rate determines the translationaldiffusion coefficient, D, of the micelles in the system. The hydro-dynamic radius of the micelles, Rh, can then be estimated from the

translational diffusion coefficient using the Stokes–Einstein expres-sion

D = kT

6��Rh(4)

where k is Boltzmann constant, T the temperature, and � the kine-matic viscosity.

2.2.1. Surface tension measurementsThe surface tension (liquid/air interface) was measured with a

custom made drop volume device [37]. The drop volume methodis based on the measurement of the drop volume, Vf, of a liquiddetaching (driven by gravity) from a capillary with radius, r, afterthe drop volume exceeds the maximum volume that can be held atcapillary by the surface tension. The surface tension is calculatedusing the following equation

� = ��gVf

2�rf(

r/V1/3f

) (5)

where �� is the density difference between both phases andf (r/V1/3

f ) is the empirical Harkins–Brown correction factor. The

P. Fischer, H. Wu / Colloids and Surfaces A: Physic

with the RGD expression (Eq. (3)) for the form factor, P(q). Thefitting parameter in this case is only the aggregate radius, R1,for which the fitting results are shown by the solid curves inFig. 2a. The corresponding values for the radius, R1, are reportedin Fig. 2b as a function of time. As expected, since the dimersolution has been filtered after dilution, the R1 value from thefirst set of data at t = 0.2 h is smaller than 100 nm, and subse-quently, it decreases with time and approaches a constant valuefor times larger than 6 h. However, in the case of t = 0.2 h, onlythe experimental intensity data in the small q-range can be fittedproperly. This indicates that initially the aggregates are polydis-perse and fitting with monodisperse model of Eqs. (1)–(3) is notsatisfactory. With increasing time, however, the fitting resultsare substantially improved. In the case of t = 8.6 h, good agree-ment between the experimental and calculated intensity curvesis obtained over almost the entire q-range, indicating that theaggregate size distribution is quite narrow. The steady state aver-age radius of the aggregates is R1 = 46.5 nm. Also shown in Fig. 2bis the time evolution of the hydrodynamic radius, Rh, measuredby dynamic light scattering. Similar to the R-value, the Rh valuealso decreases with time and approaches an equilibrium value.

Fig. 1. Surface tension, �, as a function of dimer concentration, cDimer. The criti-cal micelle concentration (cmc), determined by the drop detachment method, isbetween 0.6 and 0.8 mM (T = 20 ◦C).

density for the surface tension measurements was measuredwith an Anton Paar DMA38 density meter. To detect the criti-cal micelle concentration of the solution, the surface tension wasmeasured for 15 solutions covering the concentration regime of0.05 ≤ cDimer ≤ 12.5 mM. All solutions were prepared by directly dis-solving dimer acid betaine powder in double distilled water. Thetransient adsorption of surfactant at the surface was monitoredby different drop formation times up to 3 h. Precaution was takenthat the equilibrium values of the surface tension were taken atsufficient high values of the drop formation time, in our case twohours after production to avoid long-term structural buildup thatcould additionally influence the interfacial tension. In addition, theaging effect was studied for two solutions (0.5 and 5 mM) as afunction of time by performing drop detachment experiments atsolution ages of 8, 24, 32, 72, and 94 h. For all experiments theobtained surface tension data only correlate well within the statis-tical error of the first measurement at 2 h indicating that the surfacetension is not changed by the formation of vesicles in the agedsolution.

3. Results and discussion

Fig. 1 displays the surface tension as a function of dimer con-

centration. At low surfactant concentrations, the surface tension ofthe solution is only marginally influenced. Above a concentrationof 0.3 mM a pronounced drop of the surface tension is observedwhich levels off at roughly 20 mM. The inflection point of the twoslopes, which is a measure of the cmc of the solution, however, indi-cates no sharp value but a transition region. Such transition regionis either correlated to impurities or to the simultaneous formationof aggregates other than micelles. As discussed in the next para-graph, the formation of the vesicular structure contributes to theblurred transition regime. In this later case, the interfacial proper-ties are given by a mixture of both aggregates present. The criticalmicelle concentration, obtained from Fig. 1 is found to be between0.6 and 0.8 mM.

To detect the vesicle formation and their size in more detail,light scattering experiments were employed. Based on the experi-mental procedure described in the previous section, the dynamicsof the aggregation process of the dimer solution was initiated at aconcentration of c = 0.5 mM. Fig. 2a shows the evolution of scatteredlight intensity as a function of the wave vector q for differently agedsolutions. Note that all the intensity curves in Fig. 2a have been nor-malized based on the intensity value at the largest q value in the

ochem. Eng. Aspects 326 (2008) 103–108 105

case of t = 0.2 h. It is seen that with an increase in aging time, theI(q) versus q scattering curves flattens. Such a graduate flattening ofintensity indicates a decrease in mean aggregation size [36,38,39].For the dimer solution the change in aggregation size was initiatedby dilution, i.e. the size of the initial structures at a higher dimerconcentration (c = 5 mM) is larger than the structures at a lowerdimer concentration (c = 0.5 mM). After the solution is fast diluteit must establish a new equilibrium, which leads to a progressivedecrease in aggregate size over time.

Fig. 2a depicts the fitting of all intensity curves using Eq. (1)

Fig. 2. (a) Time evolution of I(q) until a sample age of 8.6 h and (b) Correspondingaggregate diameter R1 and Rh (cDimer = 0.5 mM, dotted lines are guides to the eye,T = 20 ◦C).

106 P. Fischer, H. Wu / Colloids and Surfaces A: Physic

Fig. 3. (a) Time evolution of I(q) from 8.6 h until a sample age of 93.6 h and (b)aggregate diameter for aged samples fitted (solid line) to a bimodal distributionaccording to Eq. (7) (cDimer = 0.5 mM, T = 20 ◦C).

However, in all cases, Rh is always smaller than R1. This indi-cates that although the size distribution at the equilibrium isnarrow, it cannot be considered as monodisperse [40,41]. It iswell known that if there is a distribution of particle sizes the R1

value can be substantially larger than Rh, because the two quan-tities represent different moments of the particle size distribution[42–44].

After the aggregation process had reached a steady state valueat about 8 h, measurements of the scattered intensity were con-tinued over much longer time intervals. These measurements areshown in Fig. 3a, which also includes the last set of data in Fig. 2aat t = 8.6 h. The scattered light intensity at small q-range starts toincrease progressively with time and reaches a second steady statevalue after about three days. For convenience, the earlier steadystate after about 8 h is referred to as the first equilibrium, while thelater steady state (i.e. after about 3 days) is referred to as the secondequilibrium. The upturn of the intensity curve in the small q-rangeindicates that the system now contains some secondary aggregatesof substantially larger size as compared to the size reached at thefirst equilibrium. Since the cuvette was always kept closed duringthe experiments, these particles of large size cannot be due to exter-nal contaminations and must therefore be aggregates of larger sizegenerated in situ.

Due to the limited range of the scattering angle of the instru-ment, there are insufficient data points in the small q-range to

ochem. Eng. Aspects 326 (2008) 103–108

estimate the radius of the gyration of the newly aggregated sec-ondary structures. An alternative way to estimate the size andthe amount of these larger aggregates is to fit the intensity curve,assuming that the system now contains two aggregate classes ofdifferent sizes: Primary aggregates with the radius R1 = 46.5 nm asstabilized at the first equilibrium and secondary aggregates withthe radius, R2, stabilized after 60 h. The total scattered intensity inthis case may be expressed as follows

I(q) = N1m21P1(q) + N2m2

2P2(q) (6)

where N1 and N2 are numbers of the primary and secondary aggre-gates, respectively. The I(q) can be further expressed, based on theradii, R1 and R2, of the two aggregate classes and the volume fractionx = R1/R2 of the secondary structures in total amount of aggregates,as:

I(q) = (1 − x2)R31P1(q) + x2R3

2P2(q) (7)

Using this expression, the last two sets of data at second equi-librium (i.e. at t = 69.7 and 93.6 h) in Fig. 3a are fitted consideringR2 and x as fitting parameters while utilizing the form factors, P1(q)and P2(q), given by the RGD expression (Eq. (3)). As shown in Fig. 3b(solid curve) good agreement between the calculated and experi-mental intensity curves has been obtained.

The average radius of the secondary aggregates obtained fromthe fittings is R2 = 380 ± 10 nm, which is 8 times as large as R1.The corresponding volume fraction of the secondary structure isonly x = 0.2 percent, however due to the substantially larger sizeof the aggregate the scattered intensity in the region of small qvalues is dominated by the secondary aggregates. We therefore con-clude that the dimer system contains a bimodal size distributionof aggregates and for both classes, the size distributions are rela-tively narrow. Successive light scattering experiments for solutionsaged over several weeks show that there is no further increase inthe aggregate size and, as a consequence, the secondary aggregateswith R2 = 380 ± 10 nm are regarded as thermodynamically stablevesicles.

It was postulated that the aggregation dynamics of the dimersolution might not only be a function of time but also a function ofthe concentration. Using the same procedure discussed above for adimer concentration of c = 0.5 mM two additional concentration ofc = 0.125 and 5 mM were investigated. It was found that the systemsat the second equilibrium, i.e. at times larger than 60 h, are alsocharacterized by the bimodal aggregate size distribution, which isreported in Fig. 4. The solid curves represent fittings using Eq. (7).

The fitting procedure is the same as that for the data in Fig. 3b,i.e., R1 is obtained from the first equilibrium and only R2 and x areused as the fitting parameters. For the three dimer concentrations,the size of the secondary aggregates obtained from the fittings ispractically the same (R2 = 380 ± 10 nm), and the only difference is inthe volume fraction x, which decreases as the dimer concentrationdecreases.

Fig. 5 summarizes the results by showing the radius of theprimary structures, R1, obtained from the fittings using the RGDexpression, the hydrodynamic radius, Rh, from the DLS measure-ments, as well as the percentage of the secondary structures, x, asa function of the dimer concentration. As expected, the obtaineddependence of both R1 and Rh on c indicates that the size of thesmaller aggregates decreases as the dimer concentration decreases.On the other hand, the diameter of the vesicles, R2, does not dependon the dimer concentration.

The morphological changes in dilute dimer betaine solutionsare shown in Figs. 2 and 3a where the scattering intensity,I(q), as a function of q drops to a first equilibrium (aggregatediameter about 45 nm) and then increases again to a second equi-librium. As discussed, such changes in scattering intensities at

P. Fischer, H. Wu / Colloids and Surfaces A: Physicochem. Eng. Aspects 326 (2008) 103–108 107

Fig. 4. I(q) curves of aged dimer concentration fitted to a bimodal distributionaccording to Eq. (7) (cDimer = 0.125, 0.5, 5 mM, T = 20 ◦C).

small q-values indicate the build-up of larger structures (aggre-gate radius about 380 nm). Furthermore, it can be concluded that

the dilution steps always leads to similar self-assembled struc-tures, i.e. the dilution disrupts the existing stable structures butexactly the same morphology spontaneously evolves again atlower concentrations. Similar results were found by Marques [1]where the formation path, sonication and aging were investi-gated and no difference in the final spontaneously aggregatedvesicles were observed. In the same investigation as well as sev-eral other investigations a coexistence of different aggregates anda bimodal distribution [11,28,45–48] was found which support ourresults.

The spontaneous formation of vesicles with repeatable size dis-tribution is commonly accepted as a strong indication of a trueequilibrium. Spontaneously formed vesicles exhibit, in most cases,a final size that is given by the surfactant composition itself and themolar ratio of the surfactant mixture in the inner and outer mem-brane of the bilayer but not by the surfactant concentration. Thedimer betaine vesicles are infinitely stable if solution conditionssuch as concentration, pH, and ionic strength remain unchanged.Fig. 6 shows a representative electron microscopy image of ther-modynamically stable, i.e. aged solutions at cDimer = 0.5 mM, andillustrates a multi-lamellae vesicle structure of diameter R2. Similar

Fig. 5. Radii of micelles and vesicles R1, R2, respectively and the hydrodynamicradius of the micelles Rh as a function of dimer concentration cDimer. The ratio ofmicelle to vesicle size x = R1/R2 is shown in the inlay (T = 20 ◦C).

Fig. 6. Freeze-fracture image vesicle found in the aged solutions (cDimer = 0.5 mM,T = 20 ◦C, image size 1000 nm×1000 nm).

formation of stable monodispersed vesicles (70–120 nm) in dilutebetaine solutions was observed by Ristori et al. [49]. In line withthis and other works [1,27,45] it can be therefore stated that anaged and stable equilibrium vesicular state can be reached in dilutedimer acid betaine solutions.

Since disk-like bilayer formation has been also reported inzwitterionic surfactant systems, mixtures of double-chained lipidswith asymmetric chain length, and lecithin-surfactant system[45,50–54] it is possible that such aggregates are also present in oursystem. However, open bilayers do not typically display sphericalstructures as found in our case and, as a consequence, we have ruledout the existence of disk-like bilayers for the investigated dimer sur-factant solution. As a consequence, the formation path from disks tovesicles is considered as unlikely and a micelle to vesicle formationseems more favorable.

4. Conclusion

The investigated betaine surfactant combines two features thatpromote vesicle formation namely a zwitterionic appearance anddouble-tailed geometry mimicking a catanionic surfactant and alipid, respectively. The experimental approach indicates two kindsof agglomerates of different sizes. The formation of small-sizedaggregates is triggered by the dimer concentration, cDimer, while thelarger vesicles develop as a function of time (aggregation period ofhours and days). The influence of such transient formation is seen inscattering intensity. Finally the electron microscope images man-ifest the presence of vesicles. The conclusion that the structuralevolution of vesicles is a time-driven process and not primarilydriven by the surfactant concentration is supported by the pres-ence of both aggregate sizes at different dimer concentrations. Theaggregation mechanism of the investigated surfactants, which isclearly time-driven, offers the opportunity to construct vesicleswith a thermodynamically fixed diameter R2. The use of naturalsugar-based surfactants as encapsulation material in food and con-sumer products therefore seem appropriate. Techniques such asX-ray and neutron scattering will be used to elucidate the structureand polydispersity of the aggregates formed.

Physic

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