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Langmuir 1998, 14, 3730-3739
Articles Phase Equilibria of Polymer-Containing Microemulsions A. M. Bellocq Centre de Recherche Paul Pascal, CNRS, avenue A. Schweitzer, 33600 Pessac, France Received July 23, 1997. In Final Form: March 23, 1998 This paper reports experimental results of the effect of poly(ethylene glycol) (PEG) on the phase behavior of the system made of sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT)/water/isooctane. Although the microemulsions and the lamellar phases still exist when pure water is replaced with aqueous solutions of PEG, their stability is significantly affected. It has been found that (1) large amounts of polymer can be solubilized in the lamellar and microemulsions phases, up to 17% in weight of water can be replaced by polymer, (2) the solubility of PEG in the water-in-oil droplets depends on the relative sizes of the water core (RW) and the polymer (RG) and on temperature; (3) at high RW/RG ratios, PEG enhances the solubilization of water in oil, and (4) PEG changes the sign of the spontaneous curvature of the surfactant film and induces an inversion of microemulsion type from oil-in-water to water-in-oil. This effect is similar to that produced by addition of NaCl. Interestingly enough, there exists a PEG concentration range, where the hydrophile-lipophile property of AOT is just balanced at T ) 25 °C. Finally electrical conductivity and light scattering data indicate that the presence of PEG in the water core of inverse micelles leads to a decrease of the attractions between the micelles.
I. Introduction Surfactant molecules forming flexible layers in solution often give rise to a wide variety of phases corresponding to different topologies of the interfaces between the hydrophobic and hydrophilic regions. Thus droplet or bicontinuous microemulsions, sponge phases, and oil or water swollen lamellar phases can be formed in surfactant-water-oil mixtures.1 Several theoretical2-7 and experimental8-10 studies have shown that the structuresdroplet or spongelikesof a surfactant dispersion is determined by the surfactant film bending elasticity, which can be described by three parameters: the spontaneous curvature Co and two bending moduli κ and κj, associated with the mean curvature H and the Gaussian curvature G, respectively. Co describes the tendency of the surfactant film to bend toward either the water or the oil. It arises from the competition in the packing of the polar heads and hydrocarbon tails of the surfactant molecule.11 Roughly, one can say that if Co is sufficiently large, a droplet structure is formed while when Co vanishes a lamellar phase and/or a random bicontinuous structure can be obtained. For these balanced systems with Co ) 0 the type of structuresliquid or orderedshas been (1) For a review see for instance: Micelles, membranes, microe´ mulsions and monolayers; Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer-Verlag: Berlin, 1994. (2) De Gennes, P. G.; Taupin, C. J. Phys. Chem. 1982, 86, 2294. (3) Safran, S. A. In ref 1, p 427. (4) Huse, D.; Liebler, S. J. Phys. (Paris) 1988, 49, 605. (5) Andelman, D.; Cates, M. E.; Roux, D.; Safran, S. A. J. Chem. Phys. 1987, 87, 7229. (6) Golubovic, L.; Lubensky, T. C. Phys. Rev. A 1990, 41. (7) Wennestrom, H.; Olsson, U. Langmuir 1993, 9, 365. (8) Auvray, L. In ref 1, p 347. (9) Porte, G.; Appell, J.; Bassereau, P.; Marignan, J. J. Phys. (Paris) 1989, 50, 1335. (10) Herve´, P.; Roux, D.; Bellocq, A. M.; Nallet, F.; Gulik-Krzywicki, T. J. Phys. II 1993, 3, 1255. (11) Israelachvili, J. M.; Mitchell, D. J.; Ninham, B. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
discussed in terms of flexibility of the surfactant.5 De Gennes and Taupin have discussed the role of the film undulations:2 stiff systems with large values of the elastic constant κ form a lamellar phase, while very flexible ones form bicontinuous microemulsions. The role of κj is important in the phase transitions where the topology of the interface is changed: positive κj favors connected structures such as bicontinuous microemulsions or sponge phases, while negative κj favors lamellar phases or spherical droplets. In microemulsions, the type of microstructure (oil-in-water (O/W) or water-in-oil (W/O)) is closely related to the sign of Co.1 In addition, in droplet microemulsion the solubilization is mainly determined by the amplitude of Co and is further influenced by interdroplet interactions.3,12 An increasing interest is directed today toward polymer-surfactant systems.13 Recently there has been some speculation concerning the effect the addition of a neutral flexible adsorbing polymer would have on the elasticity parameters of flexible surfactant layers.14-16 While some studies predict an increase in the elastic bending modulus κ,14 some other studies predict κ to decrease and the Gaussian rigidity modulus κj to increase.15,16 Thus polymers interacting with flexible surfactant layers should deeply affect the structure and phase behavior of these systems. These effects begin to be investigated experimentally in lamellar phases17-22 and also in micro(12) Leung, R.; Shah, D. O. J. Colloid Interface Sci. 1987, 120, 320. (13) For a review see: (a) Goddard, E. D. In Interactions with polymers and proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Ed.; CRC Press Inc.: Boca Raton, FL, 1993. (b) Brackmann, J. C.; Engbert, B. F. N. Chem. Soc. Rev. 1993, 85. (14) De Gennes, P. G. J. Phys. Chem. 1990, 94, 8407. (15) Brookis, J. T.; Marques, C. M.; Cates, M. E. J. Phys. II 1991, 1, 673. (16) Brooks, J. T.; Cates, M. E. J. Chem. Phys. 1993, 99, 5467. (17) Kekicheff, P.; Cabane, B.; Rawiso, M. J. Colloid Interface Sci. 1984, 102, 51. (18) Ligoure, C.; Bouglet, G.; Porte, G. Phys. Rev. Lett. 1993, 71, 3600.
S0743-7463(97)00821-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/05/1998
Phase Equilibria of Microemulsions
emulsions.23-32 Several studies indicate that the solubilization of an adsorbing water-soluble polymer in the water core of a inverse microemulsion changes the flexibility of the interface,28,31 the size of the droplets,24-26 and also the mutual interactions between the droplets.27,30 It has been shown that the adsorption of poly(ethylene glycol) (PEG) at the interface of water-in-oil microemulsions made of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT, AOT)/water/isooctane leads mainly to an increase of the elastic bending modulus κ and to a less extent to a modification of the spontaneous curvature.31 This last finding suggests that the solvent properties of AOT solutions are unaffected as the polymer concentration is varied. The present investigation has been undertaken to examine the effects of PEG concentration and molecular weight on the solvent properties of AOT and the phase behavior of the ternary system made of AOT/water/ isooctane. This study should allow the variation of the spontaneous curvature Co to be followed. In a more chemical language, the spontaneous curvature and the solubilizing power are related to the hydrophile-lipophile balance (HLB) of the surfactant. Since a more balanced surfactant exhibits larger solubilizing power, various ways were examined to obtain such balanced states.33 In ionic surfactants this can be obtained by partially replacing the ionic surfactant with lipophilic cosurfactant or by increasing the concentration of counterions. Salt and alcohol have the same effect on the spontaneous curvature of an ionic surfactant:1 increasing their concentration induces a monotonic decrease of Co. Salt makes the ionic surfactant less hydrophilic because the salt ions compete with the counterions and the headgroups for water of hydration. The added cosurfactant acts somewhat differently. By interposing between the charged surfactant headgroups, alcohol reduces the surface charge density and relieves part of the electrostatic strain due to headgroup repulsions. Both additives in making the ionic surfactant less hydrophilic render the surfactant aggregation process more favorable as manifested by a drop in the critical micelle concentration (cmc). The binding of hydrophilic polymers such as PEG to the micellar surface of ionic surfactants also promotes micellization at a lowered cmc.13,34 This is thought to result in a screening of the electrical charges and a diminution of the exposed hydrocarbon areas of the micelle to water.13 Then one might expect that adsorption of the neutral polymer in changing the solvent structure and the polar head repulsive interactions induces qualitatively the same effect on Co and the HLB of the surfactant with variations in alcohol or salt concentration. Therefore, in the following (19) Singh, M.; Ober, R.; Kleman, M. J. Phys. Chem. 1993, 97, 11108. (20) Ficheux, M. F.; Bellocq, A. M.; Nallet, F. J. Phys. II 1995, 5, 823. (21) Radlinska, E. Z.; Gulik-Krzywicki, T.; Lafuma, F.; Langevin, D.; Urbach, W.; Williams, C. E.; Ober, R. Phys. Rev. Lett. 1995, 74, 4237. (22) Zhang, K.; Linse, P. J. Phys. Chem. 1995, 99, 9130. (23) Radiman, S.; Fountain, L. E.; Troprakcioglu, C.; de Vallera, A.; Chieux, P. Prog. Colloid Polym. Sci. 1990, 81, 54. (24) Lianos, P.; Modes, S.; Staikos, G.; Brown, W. Langmuir 1992, 8, 1054. (25) Papoutsi, D.; Lianos, P.; Brown, W. Langmuir 1993, 9, 663. (26) Papoutsi, D.; Lianos, P.; Brown, W. Langmuir 1994, 10, 3402. (27) Suarez, M. J.; Levy, H.; Lang, J. J. Phys. Chem. 1993, 97, 9808. (28) Lal, J.; Auvray, L. J. Phys. II 1994, 4, 2119. (29) Kabalnov, A.; Olsson, U.; Thuresson, K.; Wennerstro¨m, H. Langmuir 1994, 10, 4509. (30) Suarez, M. J.; Lang, J. J. Phys. Chem. 1995, 99, 4626. (31) Meier, W. Langmuir 1996, 12, 1188. (32) Schu¨bel, D.; Ilgenfritz, G. Langmuir 1997, 13, 4246. (33) Shinoda, K. Pure Appl. Chem. 1980, 52, 1195. (34) Ficheux, M. F.; Bellocq, A. M.; Nallet, F. Colloids Surf., in press.
Langmuir, Vol. 14, No. 14, 1998 3731 Table 1. Characteristics of PEG Samples product PEG 1450 PEG 3350 PEG 8000 (8K) PEG 20 000 (20K) PEG 35 000 (35K) PEG 100 000 (100K)
Sigma Sigma Fluka Fluka Fluka Polysciences
RG (Å)
C* (g/L)
9 16 30 ∼45 ∼70
180 85 35 25 5
I will compare the effects of PEG and NaCl on the phase behavior of the AOT/water/isooctane system. In the absence of additive, the ternary system forms in certain conditions of temperature and composition a lamellar phase LR and two microemulsions L1 and L2. L1 is of type oil-in-water while L2 is of type water-in-oil. In this paper, I examine to what extent polymer chains can be dissolved inside microemulsion droplets and lamellar structures and I describe the structural changes induced by the polymer. Since the solubilization also depends on the interdroplet interactions, I determine by means of light scattering and conductivity the perturbations of the droplet structure and intermicellar interactions when the polymer chains are solubilized in the water core of the droplets. These questions have already been studied by several authors in quaternary microemulsions, composed of alcohol/ surfactant/water and oil,24-27,30 but they led to contradictory conclusions. II. Experimental Section Sample Preparation. The surfactant AOT (Fluka) was purified following the procedure described by Rogers and Winsor.35 Five poly(ethylene glycol)s with molecular weights ranging from 103 to 105 g mol-1 were obtained from Sigma, Fluka, and Polysciences. They were purified by repeated precipitation from methanol and diethyl ether. Table 1 lists for each product the radius of gyration RG of the isolated macromolecule and the overlap concentration C*. These values were determined by neutron and light scattering measurements for PEG 20 K and PEG 8 K20 and were deduced from relations RG vs MW or C* vs MW for the other products.13 The preparation of spatially homogeneous samples at thermodynamic equilibrium is crucial in the study of polymercontaining phases. To achieve this result, the samples were prepared by mixing a solution of AOT in oil and stock solutions of the polymer in water. The sample tubes were then equilibrated at 25 °C for several days. All the diagrams are expressed in weight. Several hundred of samples were prepared to construct the phase diagrams shown in this paper. The lamellar phase was identified by examination of the texture with a polarizing optical microscope. In the following Cp represents the polymer concentration in water and Xwp the weight fraction of the aqueous polymer solution in the sample. I also use ω and ωp expressed as the molar concentration ratios to define the composition
ω)
[H2O] [AOT]
,
ωp )
[H2O] + [Pm] [AOT]
(1)
where Pm represents the monomer molar concentration of the polymer. Analysis of light scattering data in terms of size and interaction requires an extrapolation of the results to zero concentration; therefore it is necessary to use a dilution procedure that keeps constant both size and composition of micelles. AOT is completely miscible with oil where it does not aggregate in the absence of water. Thus in the ternary L2 microemulsions, the oil domains contain a small amount of molecularly dispersed surfactant. Measurements of the scattered intensity I by a given microemulsion L2 versus the AOT volume fraction φs shows that I goes to zero for a positive value φS* ) 0.007. Thus we considered that φS* is the volume fraction of AOT solubilized in the oil continuous phase. Within the experimental accuracy, this value does not (35) Rogers, J.; Winsor, P. A. J. Colloid Interface Sci. 1969, 30, 247.
3732 Langmuir, Vol. 14, No. 14, 1998
Bellocq
seem to be affected by the solubilization of PEG in the micellar water core. As a consequence, the micelle volume fraction is defined as
φ 1 ) (1 + Bφ) I KV
φ ) φS - φS* + φwp
In W/O microemulsions the droplets essentially behave as hard spheres. The reference value of the virial coefficient is in this case B ) +8. However, in many systems, an attractive contribution to the interactions exists, leading to negative values of B.8 Moreover the autocorrelation function g(2)(τ) of the scattered light is given by38
φS and φwp are the volume fractions of surfactant and aqueous polymer solutions, respectively. To calculate these volume fractions, we use the following densities dAOT ) 1.067 and d of the aqueous polymer solution ranging from 1 to 1.0042 as CP increases from 0 to 45 g/L. Measurements. Refractive indexes were measured using a Pulfrich refractometer. Static and dynamic light scattering measurements were performed with a 72-channel, 4-bit digital correlator (Brookhaven Instruments) processing the signal of a photon counting PM (Hamamatsu). I used the 6471 Å red line of a krypton laser source (Coherent) irradiating a sample immersed in a thermostat tank filled with CCl4 as index matching liquid. The usual sin θ correction was made to account for the angular variation of the size of the scattering volume. Measurements of conductivity were performed by using a Tacusel conductometer operating at a frequency of 1 kHz.
The excess scattering of the particles of assumed constant size over that of the continuous phase is expressed as36
I(q) ) KV φ S(q) P(q)
(2)
where q ) 4πn/λ0 sin θ/2 is the scattering wave vector, θ is the scattering angle, V is the volume of the micelle, φ the micellar volume fraction, and
(λ (dn dφ) 2
4 -1 0
)
(3)
with n the refractive index and λ0 the light wavelength in vacuo. P(q) is the intraparticle form factor; as the droplet size is small compared to λ0, we set P(q) ∼ 1. S(q) is the structure factor. In the limit q f 0, S(q) is related to the osmotic pressure Π by the compressibility relation37
S(O) )
kBT ∂Π V ∂φ
-1
( )
(4)
where kB is the Boltzmann constant and T the absolute temperature. One of the simplest ways to have an idea of the interactions is to develop the osmotic pressure according to the virial formula. Π can be written as a function of different powers of φ:
kBT C B 1 + φ + φ2 ... V 2 3
Π)φ
(
)
(5)
where B and C are virial coefficients. B is directly related to the interaction potential by38
B)
-4π [e-U(r)/kBT - 1]r2 dr V
∫
g(2)(τ) ) 1 + 2-2Dq τ 2
(8)
where D is the translational diffusion coefficient. D is related to osmotic pressure and to the friction coefficient f between micelles and continuous phase by
D)
V ∂Π f ∂f
(9)
In the low concentration range, D can be written as39
III. Methods
K ) 2π2n2
(7)
(6)
Far from a critical point, the intensity scattered by microemulsions is independent of the scattering angle. Therefore in the limit of very small volume fractions one can deduce (36) Riley, D. P.; Oster, G. Discuss. Faraday Soc. 1951, 11, 107. (37) Ornstein, L. S.; Zernicke, F. Proc. Acad. Sci. (Amsterdam) 1914, 17, 793. (38) Berne, B. J.; Pecora, R. Dynamic Light/scattering; Wiley: New York, 1976.
D ∼ Do(1 + Rφ) with Do )
kBT 6πηRH
(10)
η is the viscosity of the continuous phase, RH the hydrodynamic radius of the micelle. The virial coefficient R is related to that of the osmotic pressure by the equation
R)B-β
(11)
β represents the dynamic part that takes into account the volume fraction dependence of the friction coefficient f. R is close to 2 for repulsive interactions between droplets and increasingly negative for attractive ones. IV. Phase Behavior Results IV.1. PEG 20K: Effect of Concentration. A part of the phase diagram of the ternary system AOT/water/ isooctane at T ) 25 °C is shown in Figure 1.40 For AOT weight fractions below 50%, the diagram consists of three one-phase regions: L1 and L2 are two isotropic microemulsions and LR is a lamellar phase (Figure 1). The water-in-oil microemulsion L2 phase is bounded by a straight line along which the molar ratio ω between water and AOT contents is constant and equal to 55. This limit represents the maximum of water solubilization in the inverse droplets at 25 °C. Let us recall that in L2 microemulsions, the ratio ω is directly related to the droplet size.41 The lamellar phase LR and the L1 phase extend from the water/AOT side into the triangle and solubilize large amounts of oil. Depending upon the AOT concentration, the change from L1 to L2 occurs via the LR phase or a three-phase region (t1). The addition of water to a solution with 20% AOT in oil leads to the sequence of phases L2 f L2/LR f LR f LR/L1 f L1 f 0/L1 (equilibrium 2) (0 designates oil) whereas for a dilute AOT solution, the sequence is L2 f L2/LR f 0/LR/L1 (three-phase equilibria t1) f 0/L1. Although the AOT rich region of the phase diagram has not been investigated in detail in this paper, one finds that the extent of the reverse hexagonal phase in the triangle is very restricted. Isooctane is more
(39) Felderof, B. U. J. Phys. A 1978, 11, 929. (40) Bellocq, A. M. In Physics of complex and supermolecular fluids; Safran, S. A., Clark, N. A., Eds.; John Wiley: New York, 1987; p 41. (41) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480.
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Langmuir, Vol. 14, No. 14, 1998 3733
Figure 1. Phase diagram at T ) 25 °C of the ternary system AOT/water/isooctane. L1 and L2 are isotropic microemulsion phases; LR is a lamellar phase. The dashed line with 20% AOT in isooctane is line I. t1 is a three-phase region where pure oil, L1, and LR coexist, HR and I are respectively an hexagonal phase and a cubic phase, t1′ is a three-phase region L2/LR/HR.
Figure 2. Cut in the composition tetrahedron at constant AOT/ isooctane ratio giving a two-dimensional phase diagram.
soluble in LR (∼58%) than in HR (∼10%) while the opposite was found for aromatic oils.42 At constant temperature the phase diagram of the quaternary system water/PEG/AOT/isooctane has to be discussed in a three-dimensional space, a tetrahedron for instance. To determine the effect of PEG concentration in water on the stability of the L1, L2, and LR phases, I investigate a cut through the tetrahedron at a constant AOT/isooctane weight ratio of 20/80, as illustrated in Figure 2. This ratio has been selected because the line (line I) going from this mixture (designated M in the following) to pure water crosses the three phases L2, LR, and L1 (dashed line in Figure 1). In the cut investigated, Cp and the aqueous polymer weight fraction Xwp are the variables. The data points in Figure 3 were obtained by determining the occurring phases keeping Cp constant and varying Xwp. The polymer is soluble in the three phases L1, LR, and L2. As Cp goes from 0 to 160 g/L, the solubilization of the aqueous polymer solution in the L2 phase increases from ω ) 55 to ωp ) 150 (ω ) 135). Both the microemulsions L2 with Cp above 150 g/L and the microemulsions L1 with Cp above 50 g/L are flow birefringent and strongly scatter light. For Cp above 130 g/L, LR does not exist, and around Cp ) 160 g/L, the L2 and L1 channels merge in a single phase denoted L*. The most drastic changes produced by PEG occur in region L2. The solubility of PEG in L2 depends on the (42) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1970, 30, 215.
Figure 3. (a) Partial phase diagram of the system AOT/ isooctane/water/PEG. Effect of Cp for PEG 20K on the stability of the phases existing along the line with 20% AOT in isooctane (see dashed line in Figure 1). The variable along the path is expressed as the weight fraction of the aqueous polymer solution Xwp. L2, L*, and L1 are microemulsions; LR is a lamellar phase. In region 2 h a microemulsion (upper phase) coexists with an aqueous PEG solution. In region 2, a microemulsion (lower phase) coexists with an organic phase; a1 is a two-phase region where solid PEG is in equilibrium with a microemulsion. t2 and t3 are three-phase equilibria. In t2, solid PEG coexists with an aqueous PEG solution and a microemulsion; in t3, the microemulsions L1 and L2 coexist with the liquid crystalline phase (LR). (b) Schematic representation of the effect of PEG on the stability of the L2 phase.
water content. It is extremely low (Cp < 1 g/L) in the water range below 15% and it starts to increase significantly for Xwp > 17% (Figure 3b). Above the solubility boundary as a minute amount of aqueous polymer solution is added to the mixture M (AOT + oil in the ratio AOT/oil ) 0.25) PEG precipitates, then when Xwp increases, it separates in an aqueous phase and finally at higher Xwp content it dissolves in the L2 microemulsion. In the Xwp range 1-3% (region a1) PEG solid coexists with an essentially free polymer microemulsion L2, then in the range 3-5% (region t2), most of the polymer is partitioned into a solid phase and an aqueous solution, and finally at higher water content, the solid-phase disappears and PEG partitions into a microemulsion L2 and a lower aqueous phase (region 2 h ). From 1H NMR it was found that this lower phase consists essentially of water and PEG. Region t2 is hard to detect and was previously regarded as a monophasic microemulsion.43 At least two reasons might be invoked to explain why the three-phase equilibria existing along line I are difficult to observe. First, the volume fractions of the solid and aqueous phases are extremely small, typically less than 0.25% of the sample volume. Second, the aqueous phase is concentrated in polymer; consequently it is very viscous and does not flow as the sample tube is tilted. (43) Bellocq, A. M. Prog. Colloid Polym. Sci. 1997, 105, 290.
3734 Langmuir, Vol. 14, No. 14, 1998
Figure 4. Effect of NaCl concentration, Cs, on the phases found along the dashed line in Figure 1; the variable along the path is expressed as the weight fraction of brine XWS.
The microemulsions in the equilibria 2h are likely to be oil-continuous; indeed the microemulsion is the upper phase and coexists with an aqueous polymer solution. Inversely, the microemulsions in the equilibria 2 are likely to be water-continuous; in these equilibria, the surfactantrich microemulsion phase separates with an upper oilrich phase. These assumptions are confirmed by electrical conductivity data. The phase behavior observed between L2 and L1 along lines with constant Cp depends on the Cp value. For Cp below 130 g/L the sequence is identical to that found without polymer L2 f LR f L1. For Cp ranging between 130 and 160 g/L, the change from L2 to L1 occurs via three-phase equilibria t3 where L1, L2, and LR phases are in coexistence. Equilibria t3 are different from the three-phase equilibria t1 found in the ternary system AOT/ water/isooctane. A preliminary study of cuts corresponding to an AOT/isooctane ratio less than 0.25 shows that the three-phase region t3 originates from a four-phase region involving the phases L1, L2, LR, and oil. Equilibria t1 and t3 are two of the four three-phase regions surrounding the four-phase region. Finally for Cp above 160 g/L, it is possible to proceed continuously from the L2 phase to the L1 phase through the L* one-phase channel. Accordingly a bicontinuous structure may be anticipated for the L* mixtures. Let us note that these mixtures contain comparable volumes of oil and water. h involves an The boundary between regions L2 and 2 incomplete solubilization of the internal solvent; above this limit the microemulsion expels a separate water phase. This boundary has been termed the emulsification failure.3 Along this boundary, as Cp increases the spontaneous curvature decreases and the solubilization of water in oil enhances. Figure 3 shows that one of the main effects of polymer concentration on mixtures with Xwp above 0.50 is to induce a structural inversion from the L1 phase, an oil internal, water-continuous structure at low Cp, to the L2 phase, a water internal, oil-continuous structure at high Cp. This sequence of phases with increasing polymer concentration corresponds to a change in the sign of the spontaneous curvature. With an increase of the PEG concentration in water, AOT becomes more hydrophobic. Figure 4 presents the effect of salt concentration on the phases found along the line investigated above with 20% AOT in isooctane. In this case the variables are the brine concentration Xws and the salt concentration in brine Cs. The patterns observed with increasing PEG or NaCl concentrations in water are very similar. Each additive enhances the solubilization of water in oil and induces a change in the sign of curvature. At 25 °C, in the presence of PEG, the aqueous solutions of surfactant (L1) change to oil continuous phase (L2) via either a bicontinuous phase
Bellocq
Figure 5. Effect of temperature on the phases existing along the line with 20% AOT in isooctane and aqueous PEG 20K solutions for two Cp: a, Cp ) 50 g/L; b, Cp ) 100 g/L.
L* or a lamellar phase while in the presence of salt this inversion occurs via the lamellar phase only. Previous results obtained by Shinoda et al. indicate that in the quaternary system AOT/isooctane/water/NaCl there exists a temperature (HLB temperature) at which a bicontinuous balanced microemulsion is found.44 This HLB temperature, which is a function of AOT and NaCl concentrations, is around 50 °C for Cs ) 5 g/L and XAOT ) 5%. NMR investigations confirm that at the HLB temperature oil and water form continuous layers.45 IV.2. Effect of Temperature. The solubility of the polymer in the AOT system studied is sensitively affected by the temperature. The effect of temperature on the stability of phases found along the lines corresponding to various Cp for PEG 20K is shown in Figure 5. The same trends are observed whatever Cp is. Increasing temperature shifts the three regions L2, LR, and L1 to lower Xwp; as a consequence the extent of region 2 h is reduced. Increasing temperature leads also to the disappearance of the LR phase and to the merging of the L2 and L1 channels. A gradual lowering of these phase transitions on the temperature scale is found as Cp increases. In particular the L1 L2 merging temperature decreases from 60 to 25 °C as Cp increases from 50 to 160 g/L. Thus a rise in temperature produces two main effects: first it favors the solubilization of the polymer in inverse microemulsions with intermediate values of ωp and second it causes a phase inversion from a W/O microemulsion L2 phase at low temperature to an O/W phase at high temperature. A similar transfer is also observed in the brine/AOT/isooctane system at constant salt concentration44 (Figure 6). Thus in the presence of either PEG or salt, AOT changes from lipophilic to hydrophilic with raising temperature because of effective fractional dissociation of the counterion increase and also because of changes in the conformation of the surfactant molecules as evidenced by NMR.46 It is worthwhile to note that the sequence of phases obtained by raising temperature at fixed Cp (L2 f LR f L1) is reversed with respect to that found at fixed temperature with increasing Cp (L1 f LR f L2). IV.3. Effect of Polymer Molecular Weight. Figure 7 shows the effect of polymer molecular weight MW on the (44) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. Shinoda, K.; Kunieda, H. J. Colloid Interface Sci. 1987, 83, 480. (45) Carnali, J. O.; Lindman, B.; Soderman, O.; Walderhang, H. Langmuir 1986, 2, 51. Carnali, J. O.; Ceglie, A.; Lindman, B.; Shinoda, K. Langmuir 1986, 2, 417. (46) Maitra, A.; Vasta, G.; Eicke, H. F. J. Colloid Interface Sci. 1983, 93, 383.
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Langmuir, Vol. 14, No. 14, 1998 3735
diagrams at lower Cp (5, 2.5 g/L) show that in the dilute polymer regime, the solubility of the polymer in the microemulsion phase L2 starts to increase significantly when the water core radius Rw of the droplets is comparable to the polymer radius of gyration. This is achieved around ω ) 16 (Rw ∼ 25 Å) for PEG 8 K, ω ) 25 (Rw ∼ 38 Å) for PEG 20 K, ω ) 30 (Rw ∼ 45 Å) for PEG 35 K, and ω ) 40 (Rw ∼ 65 Å) for PEG 100 K. Thus in the dilute polymer regime, the stability of the L2 phase is affected by both the polymer size and polymer concentration, while in the semidilute polymer regime it is mainly dependent on the monomer concentration. V. Electrical Conductivity and Light Scattering Results Figure 6. Temperature dependence of the stability of the phases existing along the line going from the mixture with 20% AOT in isooctane to brine (Cs ) 2.5 g/L) for the system AOT/ isooctane/water/NaCl.
Figure 7. PEG molecular weight dependence of the stability of the phases existing along the line at 20% AOT in isooctane for two Cp: a, Cp ) 5 g/L; b, Cp ) 120 g/L.
stability of phases found along line I, for two polymer concentrations. Depending upon the value of the polymer concentration Cp compared to the polymer overlap concentration C*, two types of behavior are seen. For Cp larger than C* (Figure 7b), all the phase boundaries are almost vertical, and they shift to higher Xwp as Cp increases. For Cp below C* (Figure 7a), the phase diagram exhibits vertical boundaries (phases L1 and LR) and also limits which change with the polymer size. That is the case for the L2 phase boundary. At low water content, the h continuously moves to higher boundary between L2 and 2 Xwp values with increasing Mw. Investigations of phase
To ascertain the structures of L1 and L2 mixtures, I carried out electrical conductivity measurements in each region as a function of ωp and Cp concentrations (Table 2). The extremely low values of the conductivity σ measured at T ) 25 °C in the L2 phases are consistent with a waterin-oil structure. The data show that this structure exists over a very large range of ωp values and volume fractions of the dispersed phase. For instance, at Cp ) 150 g/L, inverse droplets with ωp ) 135 can be concentrated up to φ ) 0.50. Conversely the high values of σ obtained for the L1 samples clearly show that water forms a continuous domain in this phase. Along the L1 channel, as Xwp increases the surfactant concentration decreases, which partly accounts for the decrease of σ. One sees also that conductivity takes high values in the region L*, which connects the L1 and L2 channels. These data suggest that L*, which is in the continuity of the lamellar phase where the spontaneous curvature of the interface is zero, has a bicontinuous structure.45 Results obtained at T ) 25 °C for L2 phases show that the conductivity strongly drops by addition of PEG (Table 2). This finding is confirmed by the comparison of the electrical conductivity data for samples located along the dilution lines defined by ω ) 55 at Cp ) 0 and Cp ) 20 g/L. The results are plotted in log σ vs φ in Figure 8. The sharp increase of σ observed around φ ) 0.22 in the microemulsions without polymer is eliminated by addition of polymer (Cp ) 20 g/L). The disappearance of the percolation behavior as PEG is added suggests that the polymer reduces the rate of exchange of material between two colliding droplets as well as the attractions between droplets.48-50 To check the above finding, I have measured the light scattered intensity I at T ) 25 °C, as a function of φ for two L2 microemulsions with Cp ) 0 and Cp ) 45 g/L (PEG 20K) and defined by ω ) 54 and ωp ) 56.5, respectively. By extrapolation of φ/I at infinite dilution, the radius R of the droplets and the second virial coefficient B are derived (eq 7). The comparison of the results presented in Table 3 reveals that the addition of PEG induces a slight growing in size of the droplets, a correlative decrease of the area per polar head, and a change in the sign of B, which goes from a slightly negative value (B ) -1) to a positive one (B ) +7) in the presence of PEG. This effect is illustrated in Figure 9 where the reduced osmotic compressibility KVφ/I is plotted against φ. Dynamic light scattering measurements have also been done on the microemulsion with Cp ) 45 g/L and ωp ) 56.5 as a function (47) Lagues, M.; Ober, R.; Taupin, C. J. Phys. Lett. 1978, 39, L-487. (48) Cazabat, A. M.; Chatenay, D.; Langevin, D.; Meunier, J. Faraday Discuss. Chem. Soc. 1982, 76, 291. (49) Safran, S.; Webman, I.; Grest, G. S. Phys. Rev. A 1985, 32, 506. (50) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10.
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Table 2. Values of Electrical Conductivity σ (S cm-1) at T ) 25 °C of Polymer Containing Microemulsions vs the Weight Fractions of PEG 20K Aqueous Solutions XWP for Various Cp (g/L)h Cp ) 0
Cp ) 45
Cp ) 100
Cp ) 150
Cp ) 170
Xwp
σ
Xwp
σ
Xwp
σ
Xwp
σ
phase L2
0.20 0.30
1.8 × 10-7 9.7 × 10-5 5.25 × 10-3 3.83 × 10-3
0.40 0.42 0.43 0.46 0.58d 0.79e 0.89f
1.2 × 10-7 2.2 × 10-7 1.9 × 10-7 4.5 × 10-7 2.92 × 10-3 2.55 × 10-3 2.23 × 10-3
9.8 × 10-7
0.48a 0.90
1 × 10-7 1.5 × 10-7 1.9 × 10-7 3.8 × 10-7 4.05 × 10-3
0.52
phase L1
0.30 0.32 0.38 0.40 0.54c
0.62g
2.12 × 10-3
phase L*
a AOT weight fraction in the L samples: a, 0.103; b, 0.065; c, 0.091; d, 0.084; e ) 0.091; f ) 0.057; g ) 0.076. 1 the ratio AOT/isooctane ) 0.25. The AOT concentration in L1 samples is given in the table.
h
Xwp
σ
0.60 0.61 0.62
1.35 × 10-3 1.57 × 10-3 1.82 × 10-3
In the L2 and L* samples
Figure 9. Effect of PEG 20K concentration in water on the reduced osmotic compressibility KVφ/I versus volume fraction φ of micelles for the ternary microemulsions AOT/water/ isooctane, T ) 25 °C: (O) microemulsion without polymer, 14% AOT, 31% H2O, 55% isooctane; (b) microemulsion with Cp ) 45 g/L, 13.72% AOT, Xwp ) 32.31%, 53.97% isooctane. Figure 8. Variations of electrical conductivity with the micelle volume fraction φ for the water/AOT/isooctane L2 microemulsions without PEG (b) and with PEG 20K (Cp ) 20 g/L) (O). In both microemulsions ω ) 55, T ) 25 °C. Table 3. Values Obtained at T ) 25 °C from Static and Dynamic Light Scattering Measurements for AOT/ Isooctane/Water/PEG 20K Microemulsions Cp
102(dn/dφ)
R (Å)
σ (Å)2
B
D0 × 1011, m2 S-1
0a 45b
1.64 1.58
126 ( 10 154 ( 15
58 51
-1 +7
3.4
RH (Å)
R
130 ( 10 3
Microemulsion with Cp ) 0; AOT 14%, isooctane 55%, water 31%. b Microemulsion with PEG 20K Cp ) 45 g/L: AOT ) 13.72%, WWP ) 32.31%, isooctane ) 53.97%. a
of the micelle volume fraction. Figure 10 shows the φ dependence of the translational diffusion coefficient D, and Table 3 gives the values of the hydrodynamic radius and the virial coefficient R obtained from these data. The high values of both B and R indicate that the microemulsion with Cp ) 45 and ωp ) 56.5 behaves as a hard-sphere liquid. In summary, both conductivity data and the values of the second virial coefficients B and R provide clear evidence that the interactions between AOT W/O droplets are less
Figure 10. Variation of the diffusion coefficient D with the micelle volume fraction for the microemulsion AOT/water/ PEG/isooctane: Cp ) 45 g/L, AOT ) 13.72%, Xwp ) 32.31%, isooctane ) 53.97%. T ) 25 °C.
attractive in the presence of PEG. They even show that, at 25 °C, a small addition of PEG changes the intermicellar interactions from attractive (B ) -1) to repulsive (B ) +7). The same trends have been previously observed by Suarez et al.27 in microemulsions made with AOT/alcohol/ water/decane and several water soluble polymers. How-
Phase Equilibria of Microemulsions
Langmuir, Vol. 14, No. 14, 1998 3737
Figure 11. Effect of PEG 20K concentration in water on the temperature dependence of the electrical conductivity σ for two AOT/water/isooctane microemulsions with the following compositions: (a) Xwp ) 32%, AOT ) 13.6%, isooctane ) 54.5%; (b) Xwp ) 35%, AOT ) 13%, isooctane ) 52%.
Figure 12. Temperature percolation threshold vs weight fraction of PEG 20K aqueous solution Xwp for different Cp. For all the samples AOT/isooctane ) 0.25.
ever the effect of PEG on the alcohol-containing microemulsions was found to be less strong than that in the microemulsions without alcohol. This might be explained by the fact that PEG and short chain alcohols produce opposite effects on intermicellar interactions. This finding has been confirmed recently by Suarez et al. who found that an increase in the cosurfactant concentration decreases the effect of addition of water-soluble polymers.30 The effects of adding small amounts of PEG 20K on the percolation behavior of AOT inverse micelles have been previously studied by Suarez et al.27,30 and Meier.31 In the present paper, I extent the investigation to include concentrated polymer solutions. Figure 11 presents the effect of Cp on the temperature dependence of σ for two microemulsions with Xwp ) 0.32 (Figure 11a) and Xwp ) 0.35 (Figure 11b). All the curves exhibit an electrical percolation phenomenon; the temperature Tp* corresponding to the threshold of electrical percolation depends on Cp and Xwp. Tp* is found to increase at fixed Xwp with Cp and at fixed Cp as Xwp decreases (Figure 12). The shift of Tp* to higher temperatures observed as Cp increases confirms that the presence of PEG in the water core of the inverse micelles reduces the attractions between micelles. As already mentioned in the preceding section, salt and
Figure 13. Variations at T ) 25 °C of electrical conductivity with the brine weight fraction XWS for microemulsions located along line I: Cs ) 0, 9; Cs ) 2.5 g/L, b.
PEG induce similar changes in the phase behavior of the AOT/water/isooctane system. Therefore it was interesting to compare the influence of these additives on the micellar size and intermicellar interactions. Previous small angle neutron scattering studies have shown that NaCl leads to a slight growth of the water core51 while a time-resolved fluorescence method gives an opposite trend.52 Figure 13 shows the effect of NaCl at T ) 25 °C upon the variation of electrical conductivity σ with Xws along line going from the mixture M (20% AOT + 80% isooctane) to the brine corner. σ rapidly increases for Xw ) 0.3 in pure water and for Xws ) 0.6 in salted water. Thus, at T ) 25 °C the (51) Cabos, C.; Delord, P. J. Phys. Lett. 1980, 41, L 455. (52) Lang, L.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946.
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nonmiscibility gap region 2 h increases with Cp. Indeed the boundary between region 2h and L2, which can be identified as the emulsification failure boundary, shifts to higher Xwp with increasing Cp. Along this line, the radius R of the droplets determined by the water and surfactant volume fractions φw and φs, respectively, is equal to the spontaneous radius of curvature C ˜ 0-1 fixed by the elastic bending energy.3 Along this limit
φs/φw ) 3δC ˜0 where δ is the surfactant size3 and
C ˜ 0 ) C0[1 + κj/2κ]-1 Figure 14. Temperature dependence of electrical conductivity for the AOT/isooctane/water microemulsion with XWS ) 30%, AOT/isooctane ) 0.25 at different salinities: Cs ) 0, b; Cs ) 1 g/L, O; Cs ) 2 g/L, 2; Cs ) 2.5 g/L, 0.
addition of salt suppresses the percolative behavior of inverse droplets suggesting that salt reduces the intermicellar attractive interactions. This effect has been previously observed in microemulsions.53,54 However, the percolation phenomenon is recovered by raising the temperature. Results presented in Figure 14 show that the percolation threshold temperature Tp* increases with salinity. Consequently the comparison of Figures 8 and 13 on one hand and 11 and 14 on another hand reveals that PEG and NaCl induce the same effect on the electrical conductivity of AOT inverse micelles. These results suggest that both additives reduce the attractions between AOT inverse droplets. VI. Discussion The addition of the water soluble polymer PEG induces important changes in the phase behavior of the AOT/water/ isooctane system. The droplet and lamellar phases found in absence of polymer still exist when pure water is replaced with aqueous solutions of PEG, but their extent is strongly influenced by the polymer size, the water/ surfactant ratio, and the temperature. The solubilization of PEG in W/O droplets depends on the ratio between the size of the water droplets Rw and the size of the polymer RG: for Rw/RG < 1, the solubility of PEG in L2 is extremely low and for Rw/RG > 1, the solubility increases with Rw. Above the solubility boundary PEG is partly expelled in an aqueous solution. A similar behavior is also found in the ternary system water/AOT/PEG.34 Indeed the polymer is soluble in dilute lamellar phases with a thickness of the water layer dW larger than the diameter of gyration of PEG. Here again, for the intermediate water layer sizes, the polymer separates in an aqueous solution. These results demonstrate the effect of confinement of the polymer on the stability of the mixed system. One of the most predominant effects of PEG is to increase the efficiency of the amphiphile. The polymer enhances the maximum solubilization of water in inverse microemulsions in stabilizing very large droplets with ω values ranging from 55 to 150. This effect is similar to that produced by the addition of salt at low concentration. In these droplets, PEG is present either as isolated chains or in semidilute regime. The largest droplets may contain up to 200 PEG 20 K chains in semidilute regime. Simultaneously, PEG produces an opposite effect on the solubilization of water in oil since the extent of the (53) Garcia-Rio, L.; Leis, J. R.; Mejuto, J. C.; Pen˜a, E.; Iglesias, E. Langmuir 1994, 10, 1676. (54) Lague¨s, M.; Sauterey, C. J. Phys. Chem. 3508.
The experimental phase diagrams presented in Figure 3 show that the amplitude of the spontaneous curvature of the AOT monolayer continuously decreases along emulsification failure boundary as Cp increases. At high C p, C ˜ 0 tends to zero as the L2 mixtures approach the L* phase. Another important finding is that PEG modifies the effective hydrophilicity of AOT. At constant composition, PEG changes the sign of the spontaneous curvature of the surfactant film. The addition of PEG to a given AOT/ water/isooctane mixture induces an inversion of microemulsion type from oil-in-water (phase L1) to water-in-oil (phase L2). This effect of PEG on the curvature is fully analogous to that produced by the addition of salt.55 However in the presence of PEG, depending upon Cp, the inversion takes place through either a lamellar phase or a bicontinuous phase while in the presence of salt, only the first mechanism is operative at ambient temperature. The elastic constants Co, κ, and κj of the AOT monolayer at the interface brine/alkane have been measured in the vicinity of the Winsor II/III transition.55,56 Close to the optimal salinity at which the spontaneous curvature of the AOT monolayer is zero, the values of κ and κj reflect the differences found in the phase diagrams of the AOT/ brine/alkane mixtures. Negative values of κj along with high values of κ are obtained in the systems forming a lamellar phase while positive values of κj and low values of κ are found in those forming a bicontinuous phase. The observation of a bicontinuous phase at high Cp (phase L*) in the AOT/isooctane/water/PEG system strongly suggests that PEG reduces the rigidity of the AOT monolayer and increases the saddle-splay modulus κj. This is in agreement with the most recent theoretical predictions15,16,57 and with the occurrence of very dilute lamellar phases swollen with oil as PEG is added. At Cp ) 0, lamellar phases with surfactant concentrations around 10% can incorporate up to 58% of isooctane. At Cp ) 100 g/L the lamellar phase is now stable at lower surfactant concentration (about 3%) and the oil concentration reaches 76%.58 This large oil swelling exhibited by the mixed system is attributed to a significant increase in the flexibility of the inversed bilayer with the appearance of undulation repulsions.1 However an opposite conclusion has been found upon addition of dilute PEG solutions (Cp e 12 g/L) to inverse AOT microemulsions. In a recent Kerr effect study, Meier finds that κ is going from 0.7kBT in pure water up to 1kBT for the polymer concentration Cp ) 12 (55) Kellay, H.; Meunier, J.; Binks, B. P. Phys. Rev. Lett. 1993, 70, 1485. (56) Binks, B. P.; Kellay, H.; Meunier, J. Europhys. Lett. 1991, 16, 53. (57) Clement, F.; Joanny, J. F. J. Phys. II 1997, 7, 973. (58) Bellocq, A. M. To be published.
Phase Equilibria of Microemulsions
g/L.59 But as pointed out by Borkovec et al.60 these measurements are very sensitive to the polydispersity of the droplets, which is still unknown in the mixed system. The solubility of PEG in inverse microemulsion droplets is dependent on temperature. While at 25 °C a small amount of PEG enters in droplets having a water radius comparable to RG, an increase in temperature facilitates its solubilization. Electrical conductivity data show that as T increases, a percolation transition occurs that implies aggregation of droplets to allow charge transfer. In W/O microemulsions exhibiting a percolation behavior, an appreciable number of dimers and clusters exist, and their amount increases when attractive interactions increase.61,62 The existence of dimers and more extended structures at high temperatures explains the higher solubilization of PEG in microemulsions with RW comparable to RG; in these structures the polymer chains are less confined than those in a single droplet.23,31 Finally the solubilization of PEG in the water core of the L2 microemulsion strongly affects the interactions between the droplets. At 25 °C the second virial coefficient B increases from a negative value (B ) -1) at Cp ) 0 to a positive one (B ) +7) at Cp ) 45 g/L. These data show that PEG produces a decrease of the interdroplet attractive interactions in the AOT system. This finding is consistent with the increased water solubilization as PEG is added.12 Two mechanisms have been invoked to explain the origin of the attractive interactions in inverse microemulsions.
Langmuir, Vol. 14, No. 14, 1998 3739
First, Lemaire et al. assume the W/O droplets prefer to interpenetrate each other rather than to be in contact with oil.63 In this model, the van der Waals attraction increases with the volume of overlapping of the surfactant films. The penetrated volume depends on the size R of the droplets and on the area σ per polar head of the surfactant molecule.62,63 Light and neutron scattering data32 show that the solubilization of PEG within the inverse AOT droplets leads only to a slight change of R and σ. Therefore this mechanism cannot explain the reduction of attractions between inverse micelles generated by PEG. It has been shown that in some cases, the interpenetration effects could be in concurrence with curvature effects.65 In dilute W/O microemulsions where the droplet curvature R-1 is slightly larger than the spontaneous curvature C0, a possible fluctuation at thermodynamic equilibrium is the fusion of two droplets in a dimer of low curvature energy. This process gives an attractive contribution Ba to the droplet second virial coefficient B. The value of Ba depends on several parameters -κ, C0, and the shape of the dimerstherefore it is difficult to clarify the origin of the effects induced by PEG on the intermicellar interactions. Acknowledgment. I express my thanks to Maryse Maugey for her essential technical assistance and to V. Roux for his help in preparing and measuring salted samples. LA970821Q
(59) Meier, W. J. Phys. Chem. B 1997, 101, 919. (60) Borkovec, M.; Eicke, H. F. Chem. Phys. Lett. 1989, 157, 457; erratum 1991, 187, 554. (61) Chatenay, D.; Urbach, W.; Cazabat, A. M.; Langevin, D. Phys. Rev. Lett. 1985, 54, 2253. (62) Ober, R.; Taupin, C. J. Phys. Chem. 1980, 84, 2418.
(63) Lemaire, B.; Bothorel, P.; Roux, D. J. Phys. Chem. 1983, 87, 1023. (64) Dichristina, T.; Roux, D.; Bellocq, A. M.; Bothorel, P. J. Phys. Chem. 1985, 89, 1433. (65) Auvray, J. J. Phys. Lett. 1985, 46, L-163.
