Stimuli-responsive emulsions stabilized bypolymeric surfactants†

Patrick Perrin,* Iolanda Porcar and Christophe TribetLaboratoire de Physico-Chimie Macromoleculaire, ESPCI-UMPC-CNRS, UMR 7615, 10 rue Vauquelin, 75231 Paris Cedex 05, France

Abstract: We present physicochemical routes to the development of soap-free simple emulsions (n-

dodecane–water) stabilized by ion-containing polymers as primary emulsifiers. First, we show how to

control the type of emulsion (oil-in-water or water-in-oil) using a series of hydrophobically modified

poly(sodium acrylate)-based polymers with a broad range of hydrophilic–lipophilic (HL) properties.

The relevant parameters used to monitor the emulsion type are the degree of hydrophobic

modification of the emulsifier, the type of the hydrophobic moiety, the ionic strength and pH of the

system. We also present original results demonstrating how light can be used as a new trigger to set the

emulsion type. Light-induced control is obtained using an appropriate combination of a photo-

responsive (PR) polymer and an amphiphilic polyelectrolyte possessing well-balanced HL properties.

The chromophore groups along the PR polymer allow the adjustment of its hydrophobicity upon

irradiation and, hence, the overall hydrophobicity of the system. In turn, the macroscopic consequence

of the effect of irradiation results in the control of the emulsion type.

# 2003 Society of Chemical Industry

Keywords: emulsion; emulsion type; emulsion stability; polymeric surfactant; responsive system; light responsivesystem; polyelectrolyte; ion-containing polymer

INTRODUCTIONUnderstanding the mechanisms governing the inver-

sion and stabilization/breaking processes of simple

emulsions (O/W for oil-in-water, and W/O for water-

in-oil) is a central problem of emulsion science.1

Traditionally, oil and water are emulsified using

(small-molecule) surfactants. However, possible toxi-

city of these surfactants, which arises mainly from their

small size, has recently stimulated great interest in

replacing them by polymers.2 These initial remarks

indicate the importance of creating emulsion systems

formulated with polymeric emulsifiers which present

the required emulsion type and stability for given

industrial applications. More specifically, we describe

in this paper how to control the type of emulsion (O/W

or W/O) stabilized by hydrophobically modified

poly(sodium acrylate) (HMPAANa) and poly(acrylic

acid) (HMPAAH) polymers containing various

amounts of n-dodecylacrylamide or di-n-dodecylacry-

lamide hydrophobic groups. As reported below, this

can be achieved by suitably changing the appropriate

physicochemical parameters, such as polymeric emul-

sifier structure and composition, ionic strength and

pH. Furthermore, we also describe an original emul-

sion system formulated using a combination of two

polymers. The first one is a HMPAANa possessing

well-balanced hydrophilic–lipophilic (HL) properties

and the other is a photoresponsive (PR) polymer. With

this system, we demonstrate that a new trigger, UV–

visible light, represents an original method to fine tune

the type of emulsion. To the best of our knowledge, we

are reporting a new application of photoresponsive

macromolecules.3

EXPERIMENTALMaterialsAqueous solutions were prepared with deionized water

(Milli-Q system from Millipore). n-Dodecane (Recta-

pur) and sodium nitrate (NaNO3, analytical reagent)

were purchased from Prolabo. The synthesis4,5 and

characterization5–8 of the HMPAA polymers were

detailed previously. Therefore, we detail here only the

chemical structures of the various amphiphilic copoly-

mers used in this study. To synthesize the HMPAA

polymers, we used a poly(acrylic acid) precursor

polymer with an average molecular weight of

50000g mol�1. Single-tailed HMPAANa copolymers

with a wide range of degrees of hydrophobic modifi-

cation (304t%480 where t is the molar fraction of

hydrophobes) outlined in Fig 1(a) were synthesized.

(Received 7 February 2002; revised version received 25 March 2002; accepted 26 April 2002)

* Correspondence to: Patrick Perrin, ESPCI-LPM, 10 rue Vauquelin, 75005 Paris, FranceE-mail: patrick.perrin@espci.fr† Oral presentation – Paper presented at the Formula III Conference: New Concepts and Strategies in Formularies, from Laboratory toIndustry, 13–16 October, 2001, Herault, France

# 2003 Society of Chemical Industry. Polym Int 0959–8103/2003/$30.00 465

Polymer International Polym Int 52:465–470 (2003)DOI: 10.1002/pi.1024

The chemical structure of the twin-tailed series of

HMPAANa copolymers is given in Fig 1(b).

The single-tailed (S) and twin-tailed (T) HMPAA-

Na copolymers are denoted t%SNa and t%TNa

respectively. t%SH is the name given to the acidic

form of the t%SNa. By analogy with the correspond-

ing low hydrophobically modified polymers (typically

t420%),9,10 both n-alkyl and di-n-alkyl side-chains

of the HMPAA polymers are likely to be ‘randomly’

distributed along the polymer backbone. The PR

polymer is a poly(sodium acrylate)

(M=150000g mol�1) randomly grafted with 7mol%

of azobenzene chromophores.11

As shown in Fig 2, the trans-azobenzene group

(apolar isomer) can be converted into the cis-isomer

(polar isomer with a dipole moment of 3.1 D) upon

irradiation at 365nm.11,12 The cis to trans reverse

isomerization occurs either in darkness or upon visible

irradiation at 436nm.11,12 Only the cis to transrelaxation in darkness is considered in this paper.

Method of preparation of emulsion samplesWe first describe the preparation of emulsion samples

formulated using a single amphiphilic copolymer.

Polymer concentration in emulsions is 0.5% (weight

of polymer/volume of emulsion) and oil volume

fraction, f, was kept constant at 50%. 0.05g of

emulsifier were first dissolved in 5ml of sodium nitrate

aqueous solution by gently stirring the solution for

24h. Then, 5ml (3.75g) of n-dodecane were added to

the aqueous polymer solution and the two phases were

left in contact at rest for 24h. Oil and aqueous phases

were finally mixed for 5min at 24000rpm (ambient

temperature) using a rotor-stator type of disperser

(Heidolph DIAX 600). To investigate the effect on the

emulsion type of changing the COONa groups of the

polymeric emulsifiers into COOH groups, the method

of sample preparation was the same as that described

above, except that the emulsifier was first swollen in n-

dodecane instead of water.

The method for the preparation of the light-

responsive system, which requires the presence of

two copolymers, is now presented. Two per cent

(weight of polymer/weight of solvent) of concentrated

60SNa polymer solutions were prepared by swelling

the polymer for 18h in 4ml of NaNO3 aqueous

solution. An appropriate volume (varying from 60 to

200ml) of a non-irradiated (ie dark-adapted for 24h)

or irradiated 0.5wt% aqueous solution of the PR

polymer (polymer solutions with a thickness of 3mm

were vertically irradiated at 365nm for 30min) was

then added to the 60SNa polymer solution. (The PR

polymer concentrations, CPR, are given in weight of

polymer per volume of aqueous phase.) The mixture

of the two polymers was left in the dark under

magnetic stirring for 90min. Four milliliters of n-

dodecane were then added to the aqueous mixture.

The oil and aqueous phases were then kept at rest for

another 90min. Emulsions were prepared by mixing

the two phases for 3min at 24000rpm using a

Heidolph Diax 900 homogenizer. Finally, the emul-

sion type was determined by observing the dilution of

the emulsion in both oil and water. A drop of a direct

emulsion is immediately dispersed in water but not in

oil. In contrast, a drop of an inverse emulsion is

dispersed in oil but not in water.

RESULTS AND DISCUSSIONStimuli-responsive emulsion systems containingonly one polymeric emulsifierThe type of emulsion was first investigated as a

function of the copolymer composition (t), copolymer

structure (S-tailed versus T-tailed) and sodium nitrate

concentration (Fig 3). The emulsion-type diagram

Figure 1. Single-tailed (a) and twin-tailed (b) HMPAANa copolymers.

Figure 2. Cis–trans isomerization ofthe PR polymer.

466 Polym Int 52:465–470 (2003)

P Perrin, I Porcar, C Tribet

shows that the S-copolymers with an amount of

hydrophobe t, up to 50% lead exclusively to the

formation of direct (O/W) emulsions, irrespectively of

the salt concentration (from 10�3 M to 2 M). A

completely different situation is observed with 80SNa

since only inverse (W/O) emulsions could be obtained

within the investigated sodium nitrate concentrations.

The 60SNa copolymer exhibits a remarkable inter-

mediate behaviour. With this batch of 60SNa, the type

of emulsion switches from one type to the other at a

salt concentration of the order of 1 M (0.86 M), O/W

and W/O emulsions forming at low and high sodium

nitrate concentrations, respectively. Table 1 shows the

effect of changing the sodium carboxylate groups of

the emulsifier into carboxylic acid groups on the

emulsion type. In these experiments, the sodium

nitrate concentration was 10�3 M. In contrast to the

30SNa and 40SNa poly(sodium acrylate)-based poly-

mers, which give direct emulsions, the corresponding

30SH and 40SH poly(acrylic acid)-based emulsifiers

yield inverse emulsions, hence suggesting pH as a

parameter to controlling emulsion type. Let us now

compare the type of emulsions prepared with S-

copolymers with that of emulsions stabilized by

T-copolymers. With T-tailed 50TNa copolymer,

emulsions are, respectively, of the water and oil

continuous type, at salt concentrations of 10�3M and

0.1 M. Consequently, adding a salt provides a tool to

control the emulsion type with 50TNa contrasting

with the corresponding S-tailed copolymer of same

degree of grafting (50SNa), in agreement with

previous studies on surfactants.13 Furthermore, com-

paring the types of emulsion formulated with 40SNa

and 20TNa, 60SNa and 35TNa and, 80SNa and

50TNa, we come to the conclusion that S-tailed

(t1SNa) and T-tailed (t2TNa) modified ion-contain-

ing polymers give emulsions of the same type provid-

ing that their grafting degrees are roughly in a ratio of 2

(t1/t2=2).

Several approaches can be used to determine

whether an emulsion will be oil or water continuous:

the empirical Bancroft rule,14 the conditions of validity

of which were recently discussed in a paper by

Ruckenstein,15 the HLB (hydrophile-lipophile bal-

ance) concepts,16,17 the Shinoda’s phase inversion

temperature18 and the oriented wedge theory19,20

further revisited by Kabalnov and Wennerstrom.21

These strongly interrelated concepts are most often

used to explain experimental observations on liquid–

liquid dispersions stabilized by surfactants since there

still constitute by far the most important class of

emulsifiers. In contrast, the type of emulsions contain-

ing polymers have not been investigated in detail

although it has long been understood that it was

dependent on the structure of the surfactant macro-

molecules.22–25 However, the revisited oriented wedge

theory can certainly explain, at least from a qualitative

viewpoint, our experimental data regarding the de-

pendence of the emulsion type on the chemical

structure and composition of the emulsifier, the pH

and the NaNO3 electrolyte concentration. According

to this theory, the emulsion type is completely

determined by the value and sign of the emulsifier

spontaneous curvature, H0. The effects of any par-

ameters that are able to modify the overall hydro-

philic–lipophilic properties of the emulsion systems are

all reflected in the change of H0, which in turn

determines the emulsion type. Unfortunately, the

meaning of H0 in the case of randomly grafted

copolymers is not as clear as for surfactants and we

are not aware of any measurements or calculations of

H0 for polymers with structures similar to those used in

this study. Consequently, a quantitative interpretation

of our experimental findings is clearly far outside the

scope of this report. Nevertheless, one can qualita-

tively understand that increasing the amount of

hydrophobes or salt concentration, decreasing the

pH and replacing linear hydrophobic side-chains by

bulkier hydrophobic moieties tend to decrease the

value of H0, and hence favour the formation of inverse

emulsions.

We now consider the stimuli-responsive emulsion

system stabilized by the 60SNa copolymer. Like with

35TNa, the use of 60SNa allows the formation of both

Figure 3. Emulsion type of n-dodecane/NaNO3 aqueous phase emulsionsystems stabilized by various HMPAANa copolymers.

Table 1. Effect of changing the COONa groupsof the polymeric emulsifier into COOH groupson the emulsion type.

f=0.5; NaNO3: 10�3 M Emulsion type

30SNa O/W

30SH W/O

40SNa O/W

40SH W/O

Polym Int 52:465–470 (2003) 467

Stimuli-responsive emulsions stabilized by surfactants

types of emulsions, depending on the salt concentra-

tion. Figure 4 presents the stability behaviour of

emulsions at electrolyte concentrations close to the

inversion point. The stability of the dispersion samples

was assessed by measuring volumes of emulsion

(within an error of 5%) remaining at different times

of observation. Due to breakdown mechanisms, the

emulsified volume is only a fraction of the total volume

of the emulsion. As shown in Fig 4, increasing the salt

concentration from 0.6 to 0.85M causes the destabi-

lization of direct emulsion samples (Fig 4(A)). At

concentrations of 0.8 and 0.85M, total coalescence is

achieved within one week. Two weeks after sample

preparation, the complete phase separation of the

emulsions into two phases is observed at a concentra-

tion of 0.8M, while at the lower salt concentration,

0.6M, the emulsified volume is still around 60%.

Further increase in NaNO3 concentration (to 0.87M)

leads to the formation of inverse emulsions which

phase separate within one week (Fig 4(B)). At 0.9M,

W/O emulsions become more stable and an emulsified

volume close to 60% is again measured two weeks

after sample emulsification. The ability of the 60SNa

copolymer to stabilize both types of emulsion is due to

its balanced HL properties. In other words, within the

formalism of Kabalnov and Wennerstrom,21 the H0

value of the 60SNa emulsifier (n-dodecane–�1M

NaNO3 aqueous phase) is probably close to zero.

Farther from the inversion point, ie within salt

concentration domains where jH0j values are larger,

both types of concentrated (f larger than 0.9)

emulsions could be stabilized, therefore proving the

remarkable potential of 60SNa as an emulsifier.26

Light-responsive emulsion systems containing60SNa stabilizer and PR polymerIn this study, our experiments aiming to control the

emulsion type were carried out in the vicinity of the

inversion point of the emulsion system stabilized by

the balanced 60SNa polymer. The idea was to add

small amounts of a PR polymer to the emulsions to

create systems for which the emulsion type could be

triggered by light. Before going further, we note that

the PR polymer in the absence of the 60SNa polymer

was not able to provide the emulsions with long term

stability, at least within the range of CPR investigated.

Typically, only a few seconds were required to observe

the complete breakdown of the emulsion samples.

Nevertheless, it was possible to determine that the

rather unstable emulsions were water continuous. We

also checked that the stability and emulsion type

behaviour of the emulsions solely stabilized by the PR

polymer were not dependent on irradiation. Hence,

the 60SNa polymer is unambiguously the primary

emulsifier of the n-dodecane–water system, and the

PR polymer can be regarded as an additive. This is not

too surprising since, as shown above, 60SNa can

adequately be used to prepare both types of stable

emulsions. The emulsion type of a large number of

samples was investigated as a function of both sodium

nitrate and PR polymer concentrations (Fig 5). With-

out PR polymer, CPR=0%, direct and inverse emul-

sions were obtained at salt concentrations smaller and

larger than 1.1M, respectively. Note that the inversion

point (1.1M) is not the same as the one (0.86M)

reported in the preceding paragraph. As already

discussed elsewhere,5 the difference arises from the

fact that we have used two different 60SNa synthesis

batches. The direct and inverse emulsion domains

were still observed in the presence of the non-

irradiated PR polymer. At PR polymer concentra-

tions40.01%, the emulsion type diagram does not

change. However, when the PR polymer concentra-

tion increases, the inversion point moves to higher salt

concentrations. For instance, for polymer concentra-

tions of 0.018 and 0.025%, the inversion points are

located at salt concentrations of 1.6 and 2.8M,

respectively.

Let us now discuss the effect of irradiation on the

emulsion type. For PR polymer concentra-

tions50.012%, the UV light (365nm) irradiation is

responsible for the shift of the inversion point towards

higher NaNO3 concentrations compared to the corre-

sponding non-irradiated emulsion sample. Note that

the difference in salt concentrations between the two

Figure 4. Stability of n-dodecane/NaNO3 aqueous phase/60SNa direct (A)and inverse (B) emulsions at various salt concentrations, in the vicinity ofthe inversion point, which is about 0.86M. The emulsified volume is thevolume of emulsion remaining at different times of observation. The erroron the volume is�5%.

468 Polym Int 52:465–470 (2003)

P Perrin, I Porcar, C Tribet

inversion points is about 0.5M (see for instance at

CPR=0.018%, the shift of the boundary separating the

direct from the inverse emulsion domain is

2.1�1.6=0.5M). The effect of irradiation on the

displacement of the inversion point is thus important.

In consequence the emulsion type can actually be

controlled by irradiation. Again, a qualitative explana-

tion for this can be given using the revisited wedge

theory. According to the theory, direct and inverse

emulsions will break if H0 decreases and increases,

respectively. In experimental terms, the parameters

favouring the formation of direct (or inverse) emul-

sions must be activated to break inverse (or direct)

emulsions. This would explain why the presence of

non-irradiated hydrophilic PR polymer, which dis-

places H0 towards larger values, favours the formation

of direct emulsions. The presence of the chromophore

(azobenzene) groups along the poly(sodium acrylate)

backbone of the PR polymer enables the adjustment of

its HL properties upon irradiation by near-UV light.

Upon irradiation at 365nm, the PR polymer becomes

more hydrophilic because the cis-isomer is polar. In

other words, its H0 value increases, which would

explain why the O/W domain grows upon irradiation

at the expense of the W/O emulsion domain.

The stability behaviour of our emulsion samples

near inversion point is now briefly presented. In

general, for some unexplained reasons, direct emul-

sions were found to coalesce much more rapidly than

inverse emulsions. To illustrate this particular aspect,

the stability of emulsions with CPR=0.012% was

studied at a constant salt concentration of 1.2M (Fig

5: sample A is a non-irradiated sample, and sample B is

an irradiated sample). The non-irradiated sample

(circles, Fig 6) is an oil continuous stable emulsion

whereas the irradiated one is a direct emulsion which

breaks rapidly with time (squares, Fig 6). The

emulsified volume (V%) of the inverse emulsion

reaches a plateau value at V=65–70%, which corre-

sponds to dispersed phase volume fractions of 0.76–

0.71 in the sedimented layer (Fig 6). The top inset

sketch in Fig 6 gives a schematic idea of the long-term

stability of a typical inverse emulsion. Due to the fast

coalescence process, a stable creamy layer with an oil

volume fraction close to 0.65–0.75 could not be

observed in the case of direct emulsions as depicted

by the inset bottom drawings in Fig 6. Since the size of

the droplets on both sides of the inversion point is of

the order of several micrometers (r=4�2mm), the

creaming/sedimentation destabilization phenomenon

was obviously expected.

CONCLUSIONWe developed stimuli-responsive surfactant-free

emulsion systems. Hydrophobically modified poly

(sodium acrylate)-based polymers were used to con-

trol emulsion type. In the first part of the paper, we

showed that increasing the degree of hydrophobic

modification of the copolymers, replacing S-tailed

hydrophobes by the corresponding T-tailed hydro-

phobic groups, increasing electrolyte concentration

and decreasing pH, favour the formation of inverse

emulsions rather than direct emulsions. All these

physicochemical parameters, which can in principle

be continuously adjusted, are suitable variables to

change the overall HL properties of the system and

hence, to control the emulsion type. In the second part

of this report, we showed that, by adding to the

Figure 5. Light-induced control of emulsion type: Effect of both thephotoresponsive polymer and irradiation of the aqueous phase (365nm) onthe type of emulsion.

Figure 6. Stability behaviour of photoresponsive emulsions formulatedusing both the 60SNa (2%) and PR (0.012%) copolymers: (circles), nonirradiated inverse emulsion corresponding to sample A in Fig 5; (squares),irradiated direct emulsion corresponding to sample B in Fig 5. In bothemulsions, the sodium nitrate concentration is 1.2M. The volume V wasmeasured as explained in Fig 4.

Polym Int 52:465–470 (2003) 469

Stimuli-responsive emulsions stabilized by surfactants

formulation a PR polymer, the emulsion type could

also be controlled by irradiation. The experimental

conditions under which the light-induced sweep of the

O/W-breaking-W/O sequence was achieved, are

described in this paper. We demonstrate, for the first

time, that light could be used as an original trigger to

monitor the emulsion type. Irrespective of the stimuli,

the observed change in emulsion type is the macro-

scopic consequence of perturbations occurring at the

molecular level. Consequently, it would certainly be

interesting to perform systematic studies to character-

ize the molecular arrangement of the macromolecules

in the bulk and at the oil–water interface as a function

of the various parameters used to select the emulsion

type. At the end of our conclusion, we would like to

point out that our experimental observations, although

lacking a clear interpretation at the moment, are of

great practical importance for the development of

future applications in stimuli-responsive emulsion

systems. They certainly demonstrate the potential of

polymeric surfactants, which can advantageously re-

place normal surfactants in some industrial formula-

tions in order to meet environmental requirements.

These results show that very slight modifications, on a

molecular level, provide enough change to switch the

emulsion type. The small amount of PR–polymer

additives required to adjust the emulsion type with

light (0.012 wt%) provides interesting opportunities in

terms of formulation. Finally, we believe that the

creation of emulsion systems that could be reversibly

flipped from one type to the other by a change of the

external light source wavelength would probably help

to find new practical applications for such oil–water

systems.

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