Interface Composition of Multiple Emulsions

the interfaces.8,9 Elasticity of the interfaces and steric repulsions between ..... Evolution of the elastic modulus (G′) as a function of time at v...
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Interface Composition of Multiple Emulsions: Rheology as a Probe F. Michaut, P. Perrin, and P. He´braud* L.P.M. ESPCI, UMR 7615, 10 rue Vauquelin, 75231 Paris Cedex 05, France Received May 25, 2004. In Final Form: July 27, 2004 We have investigated the dynamic rheological properties of concentrated multiple emulsions to characterize their amphiphile composition at interfaces. Multiple emulsions (W1/O/W2) consist of water droplets (W1) dispersed into oil globules (O), which are redispersed in an external aqueous phase (W2). A small-molecule surfactant and an amphiphilic polymer were used to stabilize the inverse emulsion (W1 in oil globules) and the inverse emulsion (oil globules in W2), respectively. Rheological and interfacial tension measurements show that the polymeric surfactant adsorbed at the globule interface does not migrate to the droplet interfaces through the oil phase. This explains, at least partly, the stability improvement of multiple emulsions as polymeric surfactants are used instead of small-molecule surfactants.

1. Introduction Emulsions are dispersions of two immiscible fluids, such as water and oil. Simple emulsions are of two different kinds: direct emulsions (O/W) are dispersions of oil into water, whereas inverse emulsions (W/O) are dispersions of water into an oil continuous phase. Multiple emulsions of water in oil in water (W1/O/W2) are direct emulsions (oil in W2) where the dispersed oil phase is replaced by an inverse emulsion (W1/O).1 Whereas a general understanding of emulsion metastability is still lacking, it is well-known that the physicochemical properties of the interfaces play a crucial role. Practically, amphiphilic molecules adsorbed at oil-water interfaces are used to improve emulsion metastability. Several empirical rules (Bancroft rule,2 hydrophilic-lipophilic balance (HLB) rule,3 for instance) aim at predicting the nature (direct or inverse) and stability of an emulsion from the thermodynamic equilibrium properties of solutions of surfactants used to stabilize their interfaces. Thus, according to Bancroft’s rule, stabilization of direct and inverse emulsions requires the use of a surfactant with high and low HLB, respectively. As a consequence, multiple emulsions are highly metastable systems, as they possess two distinct kinds of interfaces with opposite properties. In general, two surfactants are used, one of low HLB and the other one of high HLB, to stabilize the internal (W1/O) and external (W2/O) interfaces, respectively.4 The presence of these two different surfactants is a major source of instability of multiple emulsions.5,6 Indeed, the lifetime of the emulsions is considerably shortened by the rapid diffusion of the more water-soluble small-molecule surfactants toward the droplet interface. Among other mechanisms, the formation of inverse micelles allows the diffusion of molecules through the oil phase.7 To increase the lifetime of multiple emulsions, (1) Garti, N. Double emulsions: Scope, limitations and new achievements. Colloids Surf., A 1997, 123-124, 233-246. (2) Bancroft, W. D. J. Phys. Chem. 1913, 17. (3) Grossiord, J. L.; Seiller, M. Multiple emulsions: Structure, properties and applications; Editions de Sante´: Paris, 1998. (4) Florence, A. T.; Whitehill, D. Some features of breakdown in waterin-oil-in-water multiple emulsions. J. Colloid Interface Sci. 1981, 79, 243-256. (5) Ficheux, M. F.; Bonakdar, L.; Leal-Calderon, F.; Bibette, J. Some stability criteria for double emulsions. Langmuir 1998, 14, 2702-2706. (6) Wen, L.; Papadopoulos, K. D. Visualization of water transport in W1/O/W2 emulsions. Colloids Surf., A 2000, 174, 159-167.

copolymers instead of surfactants can be used to stabilize the interfaces.8,9 Elasticity of the interfaces and steric repulsions between droplets and the inner surfaces of globules were often given as reasons to explain the stability improvement.7 Nevertheless, it is still not clear whether the interfaces remain asymmetric or whether amphiphilic polymers still migrate, as surfactants, from one interface to the other. Thus, the composition of the droplet interface is a priori not known and cannot be measured but by in situ experiments. In this paper, we show that rheological measurements may be used to probe the amphiphile interfacial composition of inner droplets in multiple emulsions. By combining mechanical and interfacial tension measurements, we determine the composition of the droplet interface and show that diffusion of polymeric surfactants does not occur, leading to stability enhancement of multiple emulsions. 2. Experimental Section 2.1. Materials. The amphiphilic polyelectrolyte was a hydrophobically modified poly(sodium acrylate) having the chemical structure given in Figure 1. The molecular weight of the polymer was 50 000 g mol-1. The hydrophobic side chains are randomly distributed along the negatively charged backbone. Details of the synthesis have been reported previously.10 This polymer will be referred to as 10C12 (10% mol of dodecyl chains onto the poly(sodium acrylate) backbone). The surfactant was the commercial sorbitan monooleate (Span 80, Figure 2) supplied by Aldrich. The oil phase was n-dodecane (Prolabo), and deionized water was obtained from a Milli-Q system from Millipore. 2.2. Methods. 2.2.1. Preparation of Concentrated Multiple Emulsions. W1/O/W2 multiple emulsions were prepared following a two-step emulsification process, which allows the control of the concentration of both the primary (W1/O) and the multiple emulsions. Concentrated emulsions were obtained by swelling the dispersed water phase. In the following, Φd refers (7) Pays, K.; Giermanska-Kahn, J.; Pouligny, B.; Bibette, J.; LealCalderon, F. Double emulsions: how does release occur? J. Controlled Release 2000, 79, 193-205. (8) Michaut, F.; He´braud, P.; Perrin, P. Amphiphilic polyelectrolyte for stabilization of multiple emulsions. Polym. Int. 2003, 52, 594-601. (9) Sela, Y.; Magdassi, S.; Garti, N. Polymeric surfactants based on polysiloxanes-graft-poly(oxyethylene) for stabilization of multiple emulsions. Colloids Surf., A 1994, 83, 99. (10) Wang, K. T.; Iliopoulos, I.; Audebert, R. Viscometric behavior of hydrophobically modified poly(sodium acrylate). Polym. Bull. 1988, 20, 577-582.

10.1021/la048715t CCC: $27.50 © 2004 American Chemical Society Published on Web 08/31/2004

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Figure 1. Chemical structure of the hydrophobically modified poly(sodium acrylate).

Figure 2. Chemical structure of the sorbitan monooleate, Span 80. to the volume fraction of the droplets relative to the globule phase volume, Φg is the volume fraction of the globule phase, and Φ is that of the droplet phase, relative to the total sample volume. More precisely, the volume fractions are defined as

Φd )

Vd Vo + Vd

(1)

Φg )

Vd + Vo Vtot

(2)

Vd Vtot

(3)

Φ)

where Vd is the droplet volume, Vo is the oil volume, and Vtot is the total sample volume. We thus have the following relationship: Φ ) ΦgΦd. Moreover, dispersed phase volume fractions and radii before swelling will be indexed by superscript 0. In the first step of sample preparation, a 2% (w/v) NaCl solution was dispersed in a 15% (w/v) Span 80/dodecane solution using a rotor-stator homogenizer (Heidolph DIAX 900) at 26 000 rpm for 5 min. An inverse emulsion with a dispersed phase volume fraction of Φ0d ) 68% was obtained. The second step consists of the dispersion of the inverse emulsion in an aqueous phase containing 1% (w/v) of 10C12 (no salt added) using the same homogenizer at 8000 rpm for 10 s. The volume fraction of the direct emulsion, Φ0g, was varied from 6 to 25%. Following this two-step procedure, multiple emulsions with Φ0g higher than 25% were destroyed during the emulsification process. But NaCl was purposely added in the first preparation step in order to create an osmotic stress between the dispersed and the continuous aqueous phases. Emulsions then swell to reach a pseudostate of equilibrium so that concentrated multiple emulsions with total droplet volume fractions Φ from 60% to 80% and globule volume fractions Φg from 70% to 90% were finally obtained. 2.2.2. Characterizations. Microscopic observations were performed on an inverse Leica optical microscope (DM IRE II). The average droplet radius R0d ) 0.15 µm and uniformity U ) 0.11 were measured by dynamic light scattering (Figure 3). The globule radius distribution, estimated by optical microscopy, was centered at R0g ) 10 µm. The viscoelastic experiments were performed at 25 °C on a strain-controlled rheometer (Rheometrics RFS II) equipped with a cone-plate geometry (2°, 2.5 cm diameter). Experiments were performed in the linear response regime, at a strain of 0.2% and angular frequencies ranging from 1 to 100 rad s-1. Interfacial tensions γ were measured at 25 °C using a Tracker tensiometer from IT Concept. A pendent drop of the aqueous polymer solution was held in the oil phase containing the surfactant.

3. Results 3.1. Concentration of Multiple Emulsions by Osmotic Swelling. Concentrated multiple emulsions, which are difficult to prepare directly, are often concentrated after their preparation by osmotic swelling. As described in the Experimental Section, concentrated emulsions were formulated under drastic conditions, that is, by applying a large osmotic pressure gradient between the two aqueous

Figure 3. Size distribution of inverse emulsions stabilized by Span 80, measured by dynamic light scattering. The average droplet radius is 0.15 µm, and uniformity U ) 0.11.

compartments. The inner aqueous phase is composed of a 2% NaCl solution (Cs ) 0.34 mol L-1), and the initial internal osmotic pressure is thus Π0int ) 2Cs (in kT units). The external aqueous phase is a 1% 10C12 solution, leading to the following expression for the initial value of the external osmotic pressure: Π0ext ) φpCm (in kT units), where φp is the osmotic coefficient taking into account the condensation of the counterions on the chain and Cm is the total repeat unit concentration.11 For a poly(acrylic acid), ionized at 90%, at a concentration of 0.0625 mol L-1, a value close to that used in the present study (Cm ) 0.091 mol L-1), the φp coefficient is found equal to 0.2 by both theory and experiments,12 so we have used φp ) 0.2 for our calculations. The emulsion system evolved spontaneously so as to equilibrate the osmotic pressures in both aqueous compartments. Water thus migrates from the outer phase to the inner phase. Consequently, the droplets and hence the globules are forced to swell until the osmotic pressure gradient vanishes. The important swelling of the droplets led to the formation of highly concentrated multiple emulsions (Figure 4). The initial globule volume fractions, Φ0g, vary from 6 to 25%, whereas the initial droplet volume fraction inside the globule was 68% for all the experiments. Right after sample preparation (t ) 0), rheological measurements, reported in Figure 5, were performed in order to follow the time dependence of the elastic modulus G′. For an initial globule volume fraction lower than 7%, the elastic modulus was too small to be measured. The elastic modulus first increases before it levels off with time at a certain plateau value (G′ plateau). The increase of G′ reflects the increase in the volume fractions of both dispersed phases. Once the pressure gradient vanishes, the volume fractions have reached their pseudoequilibrium values and the system does not evolve anymore. At this point, it is important to keep in mind that no decrease of G′ was detected over the investigated time scales. Such a behavior can only be observed if the multiple emulsions are stable with respect to coalescence. The time dependence of G′ varies with the globule volume fraction. Increasing Φ0g, we observe that (11) Manning, G. S. J. Chem. Phys. 1969, 51 (3), 924. (12) Lifson, S.; Katchalsky, A. The electrostatic free energy of polyelectrolyte solutions. 2. Fully stretched macromolecules. J. Polym. Sci. 1954, 68 (13), 43-55.

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Figure 6. Variation of the elastic plateau G′ (filled symbols) and loss G′′ (open symbols) moduli as a function of frequency, measured at a shear rate of 0.2% for multiple emulsions stabilized by Span 80 (15% w/v in dodecane) and 10C12 (1% w/v in the aqueous phase). The initial globule volume fraction Φ0g is 7% (b), 8% (9), 12% (2), 16% (1), and 25% ([). Experiments were conducted at 25 °C.

Figure 4. Optical photographs (×100) of a multiple emulsion before (Φ0d ) 68%, Φ0g ) 8%) and after (Φ ) 70%) swelling. The first picture was taken immediately after preparation of the multiple emulsion. The second picture was taken 1 h after preparation.

concentrated emulsions (Φ0g > 8%), the elastic modulus is higher than the viscous modulus and parallel to it. Both moduli are almost independent of the frequency. These swollen multiple emulsions behave typically as elastic solids over the whole investigated frequency range. For Φ0g e 8%, the emulsions behave as viscoelastic fluids, both moduli having the same order of magnitude. 4. Discussion 4.1. Elasticity of Concentrated Multiple Emulsions. To discuss the rheological measurements, it is convenient to review some of the fundamentals. Simple emulsions exhibit a transition from a viscous fluid to an elastic solid at dispersed phase volume fractions close to that of the random close-packing volume fraction, Φc. At droplet concentrations lower than Φc, there is almost no effect of the interfacial films on the rheological properties of the fluid, which is essentially viscous. However, at concentrations higher than Φc, emulsions become elastic. The energy required to deform the system is stored within the films separating the close-packed droplets.13 Thus, the elasticity scales as γ/R, where γ is the interfacial tension, and Rd is the droplet radius. Mason et al.14 empirically determined the following dependence of the elastic modulus, G′, on the droplet volume fraction, Φ:

γ G′ ) R Φ(Φ - Φc) Rd Figure 5. Evolution of the elastic modulus (G′) as a function of time at various globule volume fractions (Φ0g) for multiple emulsions stabilized by Span 80 (15% w/v in dodecane) and 10C12 (1% w/v in the aqueous phase). The initial droplet volume fraction is Φ0d ) 68%. The initial globule volume fraction Φ0g is 7% (b), 8% (9), 12% (2), 16% (1), 20% ([), and 25% (]). Experiments were conducted at 25 °C.

both the rate of increase and the plateau value of the elastic modulus become larger. 3.2. Pseudostate of Equilibrium. Let us focus on the pseudostate of equilibrium of the swollen emulsions, once the osmotic pressures are equilibrated, that is, at the plateau value of the elastic modulus. For these concentrated emulsions, the elastic and viscous moduli, G′ and G′′, respectively, were measured as a function of the frequency. The results are reported in Figure 6 for the various initial globule volume fractions. For the most

(4)

where Φc is close to the random close-packing volume fraction, and R is an empirical coefficient. We first studied the dynamical rheological properties of concentrated inverse emulsions (2% NaCl, 15% Span 80 in dodecane) for various φ. The experimental results are given in the form of a plot of G′Rd/φ versus φ (Figure 7, filled symbols). From the value of the interfacial tension, γ ) 3.5 mN m-1, measured by tensiometry, and using eq 4, one can calculate values for R and φc equal to 1 and 0.66, respectively. We will therefore use this measured value of R in the subsequent analysis of concentrated multiple emulsions. (13) Princen, H. M. Rheology of foams and highly concentrated emulsions. 1. Elastic properties and yield stress of cylindrical model system. J. Colloid Interface Sci. 1983, 91, 160-175. (14) Mason, T. G.; Bibette, J.; Weitz, D. A. Yielding and flow of monodisperse emulsions. J. Colloid Interface Sci. 1999, 179, 439-448.

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Figure 7. G′Rd/Φ for the inverse emulsion (2% NaCl/15% Span 80 in dodecane) (O) and for the concentrated multiple emulsion (b). The straight line is a linear fit to the data.

Let us now focus on the highly swollen multiple emulsions, once they have reached the pseudostate of equilibrium for which the elastic modulus as a function of frequency is displayed in Figure 6. In that state, both the droplet and globule volume fractions are larger than Φc (see paragraph 2.2.1). Then, the amount of work W needed to strain the multiple emulsion is the amount of work needed to increase the surface of both droplets and globules. The work needed to strain a droplet of surface tension γ with a shear amplitude  is W ∼ ΠV ∼ γR2, where Π is the Laplace pressure of the droplet, V is its volume, and R is its radius. Then, the work needed to shear a globule of radius Rg and surface tension γg containing N droplets of radius Rd and surface tension γd is

Wtot ∝ (γgRg2 + NγdRd2)

(5)

The droplets being close-packed inside the globule, the total number of droplets in a globule is of the order N = (Rg/Rd)3. This means that the ratio of the work Wd needed to deform the droplets over the work Wg needed to deform the globule is the product of two terms:

( )( )

Rg γd Wd ) Wg Rd γg

(6)

The radii ratio, Rg/Rd, is almost constant for different initial globule concentrations (Figure 8a) and is equal to 60. The value of the second term depends on the surfactant system. In our case, the surface tension of the droplets is at least larger than that of the globule. Indeed, at the Span 80 concentration used to stabilize our multiple emulsion (2%), the tension of the globule interface reached a plateau value lower than 0.1 mN m-1. This value is lower than the surface tension of the droplet interface, whatever the droplet composition (Figure 9a,b). Thus, the ratio γd/γg reaches its minimum value, 1, if both interfaces have the same composition, that is, if the polymer migrates from globule to droplet interfaces. Consequently, in our system, the amount of work needed to deform the droplets is at least 60 times larger than the work necessary to strain the globules. Thus, the work needed to strain a concentrated emulsion is stored by droplet interfaces. Rheology then becomes a tool to probe the interfacial properties of droplet interfaces. Using Mason’s eq 4, we therefore plotted G′Rd/Φ versus Φ (Figure 7, open symbols), where Φ ) ΦgΦd is the droplet volume fraction calculated over the total sample volume after swelling and Rd is the droplet radius after swelling. Both Φg and

Figure 8. (a) Equilibrium droplet (b) and globule (9) radii, as a function of the initial globule volume fraction. Insert: Ratio of globule vs droplet radii as a function of the initial globule volume fraction after swelling. (b) Equilibrium volume fraction of the globules, Φg (b), and of the droplets, Φd (9), as a function of the initial globule volume fraction. The total volume fraction of the droplets, Φ, is also reported (2). (c) Equilibrium NaCl concentration, as a function of the initial globule volume fraction.

Φd were calculated at the osmotic pressure equilibrium thanks to the following expressions:

Φg ) Φ0g +

V Vtot

V Vtot Φd ) V Φ0g + Vtot

(7)

Φ0dΦ0g +

(8)

where V is the volume of water that migrated from the external to the aqueous phase, and Vtot is the total sample volume. V/Vtot can be calculated by writing that the internal, Πint, and external, Πext, osmotic pressures are equal at the pseudostate of equilibrium:

Φ0dΦ0g Πint ) Π0int V Φ0dΦ0g + Vtot

(9)

1 - Φ0g Πext ) Π0ext V 1 - Φ0g Vtot

(10)

The radius Rd of the droplets after swelling can be

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Figure 10. Mason’s plot: G′Rd/Φ vs Φ plotted for the multiple emulsions once they have reached a pseudostate of equilibrium (b). G′, the elastic modulus, is G′plateau; Rd is the droplet radius calculated after swelling; and Φ is the droplet volume fraction over the total sample volume after the swelling. The straight line is a linear regression to the data, from which a surface tension of 3.6 mN m-1 is calculated. The O (respectively 9) symbols are predicted G′Rd/Φ from interface tensions measured between (dodecane, 2% Span 80) vs water at NaCl concentrations ranging from 0.15 and 0.5%, when 10C12 is absent from the droplet interface (respectively when 10C12 migrates toward the droplet interface).

Figure 9. (a) Gibbs isotherm of the interface (water; NaCl, 1.9%; 10C12, 1%) vs Span 80 in dodecane. The Span 80 concentration is varied from 10-5 to 0.1%. (b) Gibbs isotherm for the interface (water, NaCl, 10C12) vs (2% Span 80 in dodecane), with 10C12 concentrations varying from 0 to 2 × 10-2%. The NaCl concentration is (b) 0.15% and (2) 0.5%. The temperature is 25 °C.

calculated by making the assumption that the number of droplets does not vary with time (eq 11). In other words, we assume that there is no coalescence, as checked by optical microscopy observations and release experiments.8 We obtain

(

Φ0dΦ0g +

Rd ) R0d

V Vtot

Φ0dΦ0g

)

1/3

(11)

The calculated radii and volume fractions of the droplets are reported in Figure 8a,b. They were used to perform the Mason’s analysis presented in Figure 10. We observe a sharp linear increase of G′Rd/Φ above a volume fraction close to random close-packing concentration, in agreement with the Mason eq 4. From the linear regression, one gets the value of the interfacial tension, γd ) 3.6 mN m-1, between droplet and oil. The verification of the Mason eq 4 for the multiple emulsions thus confirms that the elasticity of concentrated multiple emulsions is solely governed by the internal droplets. Moreover, one observes that the values of G′Rd/Φ for inverse emulsions and for multiple emulsions superimpose (Figure 7). This observation has a far reaching consequence: it implies that the interface between the droplets and globules, in multiple emulsions, has the same interfacial tension as the interface between droplets and continuous phase in the corresponding inverse emulsion. We now wish to study the implications of this result for the composition of the inner droplet/globule interface. 4.2. Characterization of the Interfaces. Let us thus consider the droplet interface after swelling. The

salt concentration of the dispersed water phase after swelling depends on the initial globule fraction, Φ0g: the lower the Φ0g, the lower the salt concentration, Ceq. We recall that the salt concentration in droplets was 2% before swelling. One may thus calculate the ionic strength of the dispersed water phase and finds that salt concentration varied from 0.15 and 0.5% (Figure 8c) within the range of studied φ0g. Moreover, one a priori does not know the quantity of 10C12 that migrated from the continuous water phase into the dispersed water. Thus, we studied the (2% Span 80 in dodecane)/(10C12 in NaCl aqueous solution) interface by tensiometry for NaCl concentrations ranging from 0.15% to 0.5%. For each salt concentration, we varied the concentration of 10C12 in the aqueous compartment. When it does not contain any 10C12, measured values of the interfacial tension are γ ) 3.3 and 3.5 mN m-1 for 0.5 and 0.15% NaCl concentration, respectively. Upon addition of 10C12, the interfacial tension decreases and eventually drops to reach a plateau value of 1.6 mN m-1 at a salt concentration of 0.15% (see Figure 9b). The value of the interfacial tension (γ ) 3.6 mN m-1) deduced from rheological experiments performed on concentrated swollen emulsions is thus very close to that measured by tensiometry, at zero concentration of 10C12. More precisely, assuming a linear variation of the interfacial tension between 3.3 and 3.5 mN m-1 when the salt concentration increases from 0.15 to 0.5%, one may deduce a Mason’s plot (empty circles in Figure 10), in good agreement with experimental data (filled circles). On the contrary, if the polymer migrated toward the inner interface, then the interfacial tension would vary from 0.9 mN m-1 (0.5% NaCl) and 1.6 mN m-1 (0.15% NaCl). Under these conditions, the calculated values of the ratio G′Rd/φ (filled squares, Figure 10) are much lower than the values measured for the multiple emulsions (filled circles). As a consequence, the surface of the droplets is essentially covered with Span 80 molecules when 10C12 is used to cover the external interface. In conclusion, direct rheological measurements show that the two interfaces in multiple emulsions are different in composition: the interface between the droplets and the oil phase is mostly covered with Span 80 molecules,

Interface Composition of Multiple Emulsions

while the outer interface is a mixed 10C12/Span 80 interface. Indeed, Span 80 molecules dispersed in the oil phase can easily diffuse toward the globule interface. As already mentioned in the Introduction, the migration of the hydrophilic emulsifier from the outer interface to the inner one is an important source of destabilization of multiple emulsions. As a matter of fact, the adsorption of

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an increasing number of hydrophilic emulsifier molecules at the surface of the droplets, which increases the value of the spontaneous curvature of the (mixed) amphiphile layer, would favor both the droplet/droplet and droplet/ globule coalescence.5 LA048715T