Effect of Adsorbing Polyelectrolytes on a Balanced Nonionic

Association between hydrophobic polyelectrolytes and surfactants. Ilias Iliopoulos. Current Opinion in Colloid & Interface Science 1998 3, 493-498...
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Langmuir 1996, 12, 4378-4384

Effect of Adsorbing Polyelectrolytes on a Balanced Nonionic Surfactant-Water-Oil System Vijay Rajagopalan,*,† Ulf Olsson,† and Ilias Iliopoulos‡ Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S221 00, Lund, Sweden, and Laboratoire de Physico-Chemie Macromole´ culaire, Universite´ Pierre et Marie Curie, CNRS URA-278, ESPCI, 10 rue Vauquelin, F-75231, Paris Cedex 05, France Received April 17, 1996. In Final Form: June 19, 1996X We have studied the effects of an adsorbing polyelectrolyte (hydrophobically modified sodium polyacrylate) on the phase behavior of a balanced nonionic surfactant-water-oil system. As seen from the variation of phase volumes in a three-phase (Winsor III) equilibrium, the effect of polymer adsorption can be described as an increase of the spontaneous curvature of the film away from water. With increasing polymer concentration the middle phase microemulsion first increases its water-to-oil ratio, whereas above a certain polymer concentration the trend is reversed. Following Kabalnov1 et al., this reversal is interpreted as a saturation in the polymer adsorbtion. The saturation value corresponds to approximately 0.2 mg of polymer per m2 of the surfactant film and is similar to that in the Kabalnov experiment with a different polymer. The phase behavior at equal volumes of water and oil was also studied as a function of surfactant concentration for various aqueous polymer concentrations. While the polymer is readily soluble in the microemulsion phase with bicontinuous topology, its solubility in the lamellar phase, where the surfactant (bilayer) film is planar, was found to depend on the polymer concentration and added salt. The results demonstrate that film topology and electrostatic interactions are important factors determining the polymer compatibility with these surfactant phases.

Introduction An increasing interest is directed today toward polymer/ surfactant systems. The effects of polymers on microemulsion structure and phase equilibria are of considerable interest, in particular in relation to enhanced oil recovery problems. Although vast literature exists on the studies of polymer-surfactant interactions,2-5 only little is known about the effects upon adding polymer to the three-component surfactant/water/oil systems. The behavior of these mixed systems strongly depends on the interaction between the polymer and the surfactant film. With adsorbing polymers (i.e., polymers that adsorb onto the surfactant film) the surfactant film properties may become affected. In the case of nonadsorbing polymers, it is the excluded volume interactions with the surfactant aggregates that dictate the behavior of the system. In the case of infinite surfactant films, such as bicontinuous microemulsions, lamellar phases, or sponge phases, the polymer size is an important parameter that determines the resultant phase behavior.6-8 Polymers larger than the “pore size” will be insoluble in the microstructured surfactant phases resulting in phase separation.9 * To whom correspondence should be addressed. † Lund University. ‡ Universite ´ Pierre et Marie Curie. X Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Kabalnov, A.; Olsson, U.; Thuresson, K.; Wennerstro¨m, H. Langmuir 1994, 10, 4509. (2) Goddard, E. D. Colloids Surf. 1986, 19, 301. (3) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants Physical Chemistry; Rubingh, D., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 189. (4) Lindman, B.; Thalberg, K. In Polymer Surfactant Interactions; Goddard, E. D., Ananthapadmanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1992. (5) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (6) Iliopoulos, I.; Olsson, U. J. Phys. Chem. 1994, 98, 1500. (7) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. Langmuir 1994, 10, 2159. (8) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, New York, 1979.

S0743-7463(96)00375-7 CCC: $12.00

An interesting class of adsorbing polymers is the socalled hydrophobically modified water-soluble polymers (HM-WSP).10-18 These copolymers consist of a watersoluble polymer backbone with covalently bound hydrophobic side chains. In most cases the degree of hydrophobic substitution is quite low. The effective strength of the hydrophobic binding between the polymer and the surfactant film can be conveniently varied by adjusting the degree of modification of the polymer and the hydrophobicity (e.g., the alkyl chain length) of the side chains. A larger number of studies have been performed on the dilute aqueous mixtures of HM-WSP and surfactants, where an interesting gel formation occurs with micelles6,16,17,19-23 or with vesicles.20,24 It has also been seen that the hydrophobically modified polyelectrolyte can be soluble in a nonionic surfactant lamellar phase, where the unmodified precursor is insoluble.6 In a more recent study Bagger-Jo¨rgensen et al.25 have shown that the (9) Ficheux, M. F.; Bellocq, A. M.; Nallet, F. J. Phys. II 1995, 5, 823. (10) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577. (11) Polymers in Aqueous Media Performance through Association; Glass, J. E., Ed.; Washington, DC, 1989; Vol. 223. (12) Water Soluble Polymers. Synthesis, Solution Properties and Applications; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; American Chemical Society: Washington, DC, 1991; Vol. 467. (13) Biggs, S.; Hill, A.; Selb, J.; Candau, F. J. Phys. Chem. 1992, 96, 1505. (14) Rauscher, A.; Hoffmann, H.; Rehage, H.; Fock, J. Tenside, Surfactants, Deterg. 1992, 29, 101. (15) Maechling-Strasser, C.; Francois, J.; Clouet, F.; Tripette, C. Polymer 1992, 33, 627. (16) Tanaka, R.; Meadows, J.; Williams, P. A.; Philips, G. O. Macromolecules 1992, 25, 1304. (17) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201. (18) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1. (19) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 89, 118. (20) Loyen, K.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1995, 11, 1053. (21) Biggs, S.; Selb, J.; Candau, F. Polymer 1993, 34, 580. (22) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838. (23) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7, 617. (24) Sarrazin-Cartalas, A.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1994, 10, 1421.

© 1996 American Chemical Society

Effect of Polyelectrolyte on Phase Behavior

Figure 1. Schematic phase behavior of a nonionic surfactantwater-oil system at zero spontaneous curvature. The microemulsion phase, L, is at lower surfactant concentrations in simultaneous equilibrium with excess oil and water, where the microemulsion phase contains equal volumes of water and oil. The spontaneous curvature of nonionic surfactant films is temperature dependent and zero spontaneous curvature corresponds to a particular temperature T ) T0. At higher surfactant concentrations a lamellar phase (LR) is stable. The corresponding section through the composition-temperature phase prism is illustrated on top.

hydrophobically modified polyelectrolyte can also be solubilized in the L1 and the L3 phases; however, the unmodified precursor was found to be soluble in only the L1 phase. Similarly, Kabalnov et al. found that hydrophobically modified EHEC could be solubilized in a bicontinuous microemulsions phase where water soluble homopolymers of the same molecular weight are insoluble. For understanding the phase behavior of surfactantwater-oil systems, it is often useful to consider the socalled spontaneous, or preferred, curvature, H0, of the surfactant film. When H0 . 0 a high curvature toward oil is preferred and an aqueous micellar phase is stable where the micelles can solubilize oil. Similarly, reverse micelles are stable for H0 , 0. For H0 ≈ 0 a bicontinuous microemulsion and a lamellar liquid crystalline phase are stable. At zero spontaneous curvature one has a bicontinuous microemulsion containing equal volumes of water and oil, whereas at a lower surfactant concentration one has a bicontinuous microemulsion in simultaneous equilibrium with excess water and excess oil (finite swelling). This equilibrium occupies a three-phase triangle diagram, as illustrated schematically in Figure 1. At higher surfactant concentrations the microemulsion phase is in equilibrium with a lamellar phase. For nonionic surfactants of the ethylene oxide type (CmEn) the spontaneous curvature has a strong temperature dependence. H0 is positive at lower temperatures and shifts gradually to negative values at higher temperatures. At a given temperature, T0, H0 ) 0. Due to the strong temperature dependence of H0, a useful representation of a nonionic (25) Bagger-Jo¨rgensen, H.; Olsson, U.; Iliopoulos, I. Langmuir 1995, 11, 1934.

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Figure 2. Schematic phase behavior of a nonionic surfactantwater-oil system at equal volumes of water and oil. At lower temperatures the spontaneous curvature is toward oil and the microemulsion phase (L) is in equilibrium with excess oil (O). At higher temperatures the situation is reversed. The spontaneous curvature is toward water and the microemulsion phase is in equilibrium with excess water (W). At T0, the spontaneous curvature is zero, and in its vicinity there is a three-phase (Winsor III) equilibrium, W + L + O, at lower surfactant concentrations. The corresponding section through the composition-temperature phase prism is illustrated on top.

surfactant-water-system is obtained by considering the phase changes as a function of temperature and surfactant concentration, keeping the water-to-oil ratio constant and equal to unity. A schematic phase diagram in this representation is shown in Figure 2. In this paper we report on the effect of an adsorbing polyelectrolyte (hydrophobically modified sodium polyacrylate) on the phase equilibria of a balanced microemulsion and lamellar phases with nonionic surfactant. In a first experiment we studied the variation of phase volumes in a Winsor III equilibrium (Figure 1) as a function the polymer concentration. Here we were able to detect when the surfactant film becomes saturated with polymer at higher polymer concentration. In a second experiment the stability and phase equilibria of the microemulsion and lamellar phases were studied for different aqueous polymer concentrations. Here, the volumes of water and oil were kept equal and the phase behavior was studied as a function of temperature and surfactant concentration (Figure 2). In this experiment the polymer compatibility with different microstructures (the bicontinuous topology of the microemulsion and the planar topology of the lamellar phase) can be compared at the same composition. Two surfactant-water-oil systems were used. The Winsor III experiment was performed on the C12E5water-decane system, in order to compare with a similar study performed previously by Kabalnov1 et al. The variable surfactant concentration study was performed on the C12E4-water-dodecane system, which is convenient because T0 is near room temperature. The two systems can be considered essentially identical and that they mainly differ in T0. Experimental Section Materials. The nonionic surfactants tetraethylene glycol dodecyl ether (C12E4) and pentaethylene glycol dodecyl ether

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Figure 3. (a) Typical structure of hydrophobically modified polyacrylate, denoted as HMPA. x is the degree of modification, in monomer percent, by C18 tails. (b) Schematic illustration of HMPA. The negatively charged hydrophilic backbone has hydrophobic C18 chains randomly attached. 〈λ〉 is the average curve linear repeat distance between two adjacent hydrocarbon tails. In our case it is 250 Å for 1C18 and 80 Å for 3C18. (C12E5) were obtained from Nikkol Ltd., Tokyo, dodecane was obtained from Merck, decane (99.9%) was obtained from Sigma, and D2O (99.8% isotopic purity) was obtained from Dr. Glaser AG, Basel (Switzerland). NaCl of analytical grade and Millipore filtered water were used. All chemicals were used as received. The origin of poly(sodium acrylate), the modification reaction, and the polymer characterization were as described elsewhere.10 The modified polymers contain 1 and 3 mol % of octadecyl side groups randomly anchored to the polymer chain.26 They are denoted as 1C18 and 3C18, respectively. The hydrophobically modified polymers were obtained in their sodium salt form and have the same degree of polymerization as the precursor polymer (∼2000). The average curve linear repeat distance, 〈λ〉, between two adjacent C18 tails is 250 Å for 1C18 and 80 Å for 3C18, based on a monomer segment length of 2.5 Å. The typical structure of the modified sodium polyacrylate is illustrated in Figure 3. Methods. (a) Winsor III Equilibrium Study. As a reference microemulsion system, we used the 0.1 M NaCl/C12E5/ decane microemulsion system close to its balanced state (38 °C). At the surfactant concentration used (3% v/v) and in the absence of polymer, this system is in the Winsor III three-phase equilibrium. The upper phase is a relatively weak surfactant (1.5% (v/v)) solution in oil; the lower phase is essentially pure water, with C12E5 concentration being less than 0.1% (v/v). The middle bicontinuous phase contains equal volumes of oil and water and ca. 7% (v/v) of C12E5. The samples with polymers were prepared in the following manner. Equal volume fractions (48.5% (v/v)) of decane and 0.1 M NaCl solution with various concentrations of polymer and 3% (v/v) of C12E5 were weighed into 5-mm NMR ampules and flame-sealed. After sealing, the samples were put into a special thermostated bath where they were rotated for several hours to ensure complete mixing of the components at the given temperature (38 °C). After this procedure the samples were rapidly transferred into a thermostated double-walled glass beaker filled with water and kept at 38 °C. Equilibrium was established within several weeks. Relative volumes of the coexisting phases were measured using a ruler. (b) Varying the Surfactant Concentration. Samples for the phase diagram were prepared by weighing in appropriate amounts of C12E4, dodecane, and aqueous polymer stock solutions. The weight fraction of surfactant was varied from 5 to 15%. The ratio of the volume fraction of polymer solution to dodecane was kept fixed at unity. The samples were prepared by weighing in the desired amounts into screw-capped test tubes or ampules, which were immediately sealed after the addition of small magnetic stirrers. The samples were initially mixed using a vortex mixer in a single phase. The sample tubes were then allowed to equilibrate in a thermostated water bath. The phase boundary temperatures were determined by visual inspection of the transmitted light and between crossed polarizers for birefringence. Macroscopic phase separation was invariably found to be very slow in the heterogeneous regions. In order to detect (26) Magny, B.; Lafuma, F.; Iliopoulos, I. Polymer 1992, 33, 3151.

Rajagopalan et al. the various phases in these multiphasic regions, we have made use of the 2H NMR technique. The volume fractions for the samples were calculated using the following densities (g/cm3): 0.967 (C12E5); 0.9502 (C12E4);27 1.105 (D2O); 0.73 (decane); 0.748 (dodecane). (c) Quadrupolar Splittings. 2H NMR experiments were performed at a resonance frequency of 15.371 MHz (2.3 T) on a Bruker DMX100 pulsed superconducting spectrometer working in the Fourier transform mode. The sample temperatures ((0.5 °C) were maintained during measurement by passing air of controlled temperature through the sample holder. The quadrupolar splittings (∆2H) were measured as the peak-to-peak distance and are given in frequency units (Hz). (d) SAXS. Small angle X-ray scattering measurements were performed on a Kratky compact small angle system equipped with a position sensitive detector (OED 50M from MBraun, Graz) containing 1024 channels of width, 53 µm. Cu KR radiation of wavelength 1.542 Å was provided by a Seifert ID-3000 X-ray generator, operating at 50 kV and 40 mA. A 10 µm thick nickel filter was used to remove the Kβ radiation, and a 1.5 mm tungsten filter was used to protect the detector from the primary beam. The sample to detector distance was 277 mm. The samples for SAXS measurements were filled into a quartz capillary using a syringe. The samples were drawn into the capillary either in the lamellar phase or in the microemulsion phase, after confirming for complete homogeneity of the sample. This quartz capillary is glued to an invar steel body which is designed to make possible simultaneous small and wide angle measurements. The cuvette is stoppered using screw caps. The sample stage K-PR permits control of temperature between 0 and 70 °C, with an accuracy of (0.1 °C, by using a peltier element. This sample stage can be moved up and down or tilted through the beam to achieve better placement for measurements.

Results and Discussion (a) Three-Phase (Winsor III) Equilibrium. Following Kabalnov et al.1 we, in a first experiment, studied the effect of polymer on the relative phase volumes of water, microemulsion, and oil in a three-phase (Winsor III) equilibrium (see Figure 1). The experiment started from a balanced condition; i.e., the middle phase microemulsion contains equal volumes of water and oil, in the absence of polymer, As in the Kabalnov experiment the microemulsion was composed of C12E5, water, and decane, except that the water here was replaced by 0.1 M NaCl in order to screen the electrostatic interactions involved with the polyelectrolyte. The effect of substitution of water with 0.1 M NaCl is to only slightly lower the balance temperature (T0) from 38.2 to 38.0 °C and does not change the qualitative behavior. The polymer was 1C18. The results are shown in Figure 4, where we have reproduced the relative phase volumes of a series of samples as a function of the aqueous polymer concentration. The effect on the relative phase volumes is reversed for lower and higher polymer concentrations, respectively. For lower polymer concentrations, the middle phase microemulsion swells at the expense of the water phase. However, this swelling reaches a maximum for an aqueous polymer concentration of approximately 0.8 wt %, and when the polymer concentration is increased further, the microemulsion phase contracts expelling water to the excess water phase. These results are very similar to those observed by Kabalnov et al.1 for a nonionic hydrophobically modified polymer in the same microemulsion and can be understood as follows. Kabalnov et al. analyzed the polymer concentration in the different phases, and found that at lower polymer concentrations the polymer dissolved predominantly in the microemulsion phase resulting in the swelling of that phase with water. However, the microemulsion phase could only accom(27) Chakovskoy, N.; Martin, R. H.; Nechel, R. V. Bull. Soc. Chim. Belges. A/III 1956, 65, 423.

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Figure 4. Relative volumes of the upper, middle, and lower phases versus the aqueous polymer concentration (in wt %) for the C12E5/decane/1C18 in the 0.1 M NaCl system.

modate a finite amount of polymer, and at higher polymer concentrations additional polymer dissolved mainly in the lower water phase, which resulted in the osmotic deswelling of the microemulsion phase. While we have not analyzed the polymer partitioning in the present system, it is reasonable to assume that a similar scenario takes place here. Since the polymer in the microemulsion phase can be assumed to be predominantly adsorbed on the surfactant monolayer, we relate the reversal in the swelling behavior to a saturation of the surfactant monolayer with polymer. The size of the water pores, ξw, in the bicontinuous microemulsion is difficult to define exactly but is similar to the water layer thickness in a lamellar phase of the same composition and can be estimated by

ξw ≈ 2ls(φw + φs)/φs

(1)

Here, φw and φs are the water and volume fractions, respectively, and ls is an effective surfactant length defined by the surfactant volume divided by the area per surfactant molecule at the polar-apolar interface, which for C12E5 has a value of approximately 16 Å.28 In the microemulsion phase we have approximately φs ≈ 0.06 and φw ≈ 0.5, which gives ξw ≈ 250 Å. This is of similar magnitude as the average distance between hydrophobic side chains on the polymer. The effective coil size of the hydrophobically modified polyelectrolyte difficult to estimate but is probably of a similar magnitude, which would explain why additional polymer prefers the water phase when the monolayer has been saturated. Also, we do not expect the water pore size to be large compared to the adsorbed polymer layer thickness. Rather, the most likely picture of the water pores in the microemulsion phase is that the adsorbed polymer layer occupies essentially the full water volume, i.e., that the water pores do not contain any polymer free water pools in their center. In a recent study Bagger-Jo¨rgensen et al.25 have studied the effects of addition of hydrophobically modified polyacrylate on the phase behavior in the water rich part of the ternary nonionic surfactant-water-oil system and have found significant differences in the phase behavior if salt was added to the above system. They have seen that in the case of the system with 3C18 there was an equilibrium between a lamellar phase and excess brine. Analysis of the phases showed that the excess brine phase was essentially free of polymer. Bearing this in mind, we (28) Rajagopalan, V.; Bagger-Jo¨rgensen, H.; Fukuda, K.; Olsson, U.; Jo¨nsson, B. Langmuir 1996, 12, 2939.

Figure 5. Partial phase diagram of the ternary C12E4/water/ dodecane system at equal volumes of water and oil.

may calculate the amount of polymer per unit area of surfactant film assuming all the surfactant to be present in the microemulsion monolayer. The film area per unit volume is given by A/V ) Φs/ls. Hence, the mass of polymer per unit area of surfactant is given by Cpls/Φs, where Cp is the overall polymer concentration. Recalling that Cp is approximately half its aqueous concentration (since approximately half the sample volume is water), we find with Φs ) 0.03 that the polymer concentration where the swelling trend reverses corresponds to approximately 0.2 mg/m2 of surfactant film. The value is of the same order of magnitude but slightly smaller than the maximum adsorbed amount (≈0.55 mg/m2) found by ellipsometry29 for the similar polymer 1C12 on a hard hydrophobic surface. A similar number, 0.6 mg/m2, was also found for the adsorption of 1C12 on carbon black from an aqueous 0.1 M NaNO3 solution (J.P. Gillet, unpublished results). Besides the difference in surfaces, the microemulsion corresponds to narrow pores where adsorbed polymer layers from adjacent surfaces overlap which probably reduces the maximum adsorbed amount. (b) Increasing the Surfactant Concentration. In a second experiment we investigated the phase behavior at equal volumes of water and oil as a function of temperature and the surfactant concentration for various polymer concentrations (see Figure 2). Here, the surfactant was C12E4 and the oil was dodecane. This system has a more convenient balance temperature, T0 ≈ 23 °C. The phase behavior of the system without polymer is given in Figure 5. This section through the temperaturecomposition phase prism is often referred to as the fish where the microemulsion and lamellar phases at higher surfactant concentrations represent the fish tail while the three-phase region W + L + O at lower concentrations resembles fish body. The balanced microemulsion swells to approximately 6 wt % of surfactant. Above approximately 9 wt % surfactant a lamellar phase is stable near T0, which transforms into a microemulsion phase at lower and higher temperatures, respectively. The lower microemulsion region coexists at yet lower temperatures with excess oil, while the upper microemulsion coexists with excess water at higher temperatures. The microemulsion at both lower and higher temperature is expected to have a bicontinuous structure within the present surfactant concentration range, although in the lower channel oil droplets in water are expected to form at higher surfactant concentrations.30 The structures in the two channels differ however in the mean curvature of the (29) Nilsson, S. Diploma Thesis, Lund University 1994. (30) Bodet, J.-F.; Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Miller, W. G. J. Phys. Chem. 1988, 92, 1898.

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a

b

Figure 6. Partial phase diagram of the quaternary system C12E4/water/1C18/dodecane at equal volumes of water and oil and a constant aqueous concentration of the polymer 1C18. The aqueous polymer concentrations are (a) 0.1% and (b) 1.0%.

surfactant film. In the upper channel the mean curvature is toward water while in the lower channel it is toward oil. The structural size in the microemulsion and lamellar phases decreases with increasing surfactant concentration approximately as φs-1. Here we are mainly concerned with the width of the water domains which is given approximately by eq 1. We stress again that this value is similar in the microemulsion and lamellar phase for the same composition but less well defined for the topologically more complex microemulsion structure. Using eq 1, we find that ξw ≈ 300 Å in the dilute end of the microemuslion phase and the value decreases to approximately 100 Å for the most concentrated sample at 15 wt % surfactant. In Figure 6 we show the phase diagram when water is replaced by 0.1 and 1.0 wt % of 1C18, respectively. The two concentrations correspond roughly to well below and near the maximum adsorbed polymer in the microemulsion phase, respectively (see above). Comparing these phase diagrams with the unperturbed system, we see that the stability of the microemulsion phase is only weakly affected by the added polymer. The balance temperature, T0, increases slightly and there is a small swelling of the balanced microemulsion phase (to approximately 5 wt % at 1 wt % 1C18). In addition, the upper phase boundary toward excess water is shifted toward higher temperatures. All these effects are similar to those obtained when adding small amounts of ionic surfactant to the originally uncharged microemulsion31 and can be ascribed to electrostatic interactions. However, parts of the swelling and increase in T0 should be due to nonelectrostatic effects from the adsorbed polymer, and to isolate those effects we (31) Strey, R. Personal Communication.

Rajagopalan et al.

also examined the phase diagrams in the presence of 0.1 M NaCl (see below). But before considering the salt effects, we discuss the effects on the lamellar phase in the absence of salt. While only minor effects on the microemulsion phase stability are observed, the effects on the lamellar phase stability are more pronounced. With 0.1 wt % 1C18, no homogeneous lamellar phase is obtained below the highest surfactant concentration studied, 15 wt %. Instead a microemulsion-lamellar phase coexistence (L + LR) is observed at lower surfactant concentrations while a coexistence of two lamellar phases (LR′ + LR′′) was observed in the 15 wt % sample. With 1 wt % 1C18 a homogeneous lamellar phase is stable at 15 wt % surfactant, while a two-phase region, L + LR, was observed at lower surfactant concentrations. A very interesting observation is that the solubility of the polymer depends on the surfactant film topology. While the microemulsion phase at lower and higher temperatures readily accommodates the polymer, this is not the case for the lamellar phase of the same composition. The polymer experience less confined in the three-dimensionally continuous water labyrinths of the microemulsion compared to the locally flat topology of the lamellar phase. A very similar behavior was also observed by BaggerJo¨rgensen et al.,25 where they found that the polymer could be solubilized in the L3 phase. The two-phase regions (L + LR) and (LR′ + LR′′) are probably separated by a three-phase region (L + LR′ + LR′′) rather than a homogeneous lamellar phase due to the relatively small water domain width. Bagger-Jo¨rgensen et al.25 studied the lamellar phase stability in the presence of 1C18, varying ξw. For an aqueous concentration of 0.2 wt % 1C18 they found that the minimum ξw required to give a homogeneous lamellar phase was approximately 300 Å. If the surfactant concentration was increased further, a phase separated into two lamellar phases, one of which having a larger water spacing and containing the polymer. For a slightly higher polymer concentration 0.6 wt % the corresponding minimum in ξw was found to be approximately 160 Å. Considering the dependence on polymer concentration, we expect the minimum ξw to be larger than 300 Å for 0.1 wt % 1C18 and less than 160 Å for 1 wt % 1C18. The relative composition of surfactant, water, and oil in a liquid crystalline phase like the lamellar phase can be determined from simultaneous measurements of the structural dimension by, for example, SAXS and NMR quadrupolar splitting of one of the solvents (often water where one for the experiment uses heavy water, D2O, rather than ordinary water). In a lamellar phase the smectic repeat distance is inversely proportional to the surfactant concentration according to

d ) 2ls/φs

(2)

where ls is the surfactant volume to area ratio defined above. The quadrupolar splitting from D2O, on the other hand is (for lower surfactant to water ratios) proportional to the surfactant to water ratio32

∆ ) Rφs/φw

(3)

The proportionality constant, R, in eq 3, which depends on the interaction between water molecules and the surfactant polar headgroups (hydration), and ls can both be determined from proper dilution experiments on the (32) Wennerstro¨m, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974, 6, 97.

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lamellar phase. ls ≈ 15 Å is known for C12E4.33 D2O quadrupolar splittings were measured for a number of homogeneous lamellar samples, with and without polymer and with and without salt. The results are shown in Figure 7 where we have plotted the quadrupolar splitting as a function of Φs/Φw. As is seen all data points lie on essentially the same straight line which may serve as a calibration line. The proportionality constant, R, being approximately 640 Hz. Samples in the L + LR coexistence region phase separate macroscopically very slowly, which makes it difficult to isolate the coexisting phases for experiments. However SAXS and 2H NMR experiments were measured on heterogeneous samples. The measurements demonstrated that the lamellar phase has a higher surfactant concentration than the coexisting microemulsion phase and a higher surfactant to water ratio. Hence, while we could not measure the polymer partitioning, by for example conductivity on the separate samples, the SAXS and NMR results indicate that polymer is predominantly present in the dilute microemulsion phase, which then swells with water at the expense of the lamellar phase. A coexistence of two lamellar phases was observed at 15 wt % surfactant and 0.1 wt % 1C18. Also here the macroscopic phase separation is very slow but the equilibrium was demonstrated by the observation of two quadrupolar splittings in the 2H NMR spectrum. In the SAXS spectrum, however only one Bragg peak could be clearly resolved, in addition to a broad hump at lower wave vectors, indicating that the second lamellar phase is very dilute in surfactant. The compositions of both lamellar phases can, apart from their polymer concentrations, be calculated from the two quadrupolar splittings and the peak position of one of the two phases. We denote the two lamellar phases A and B, respectively. The 2H NMR spectrum showed two quadrupolar splitting of magnitude ∆A ) 68 Hz and ∆B ) 277 Hz, respectively. The well-resolved Bragg reflection corresponds to a spacing of 183 Å. This Bragg reflection (d ) 183 Å) and the largest quadrupolar splitting must result from the same phase, which we have denoted B. The surfactant volume fraction in that phase is φBs ) 0.16 and from the splitting we have φBs /φBw ) 0.43, giving φBw ) 0.38. From the quadrupolar splitting of phase A we obtain φAs /φAw ) 0.11. From the conservation of volumes we have

VB B φ V s φAs ) VB 1V φs -

(4)

where VB/V is the relative volume of phase B, which can be written as

VB ) V

φw φBw

φAs φAw

- φs (5)

φAs φAw

-

φBs

Inserting the appropriate values, we obtain φAs ) 0.07 and a further φAw ) 0.66. Hence, the lamellar phase originally containing 15 wt % surfactant and equal volumes of water and oil separates into two lamellar phases: phase A, richer in water but poorer in surfactant; phase B, poorer in water (33) Carvell, M.; Hall, D. G.; Lyle, I. G.; Tiddy, G. J. T. Faraday Discuss. Chem. Soc. 1986, 81, 223.

Figure 7. Plot of quadrupolar splittings versus the ratio of the volume fractions of surfactant to water (φs/φw). The solid line is a least-squares fit to the experimental points.

and richer in surfactant. Presumably the polymer is predominantly contained in phase A. With 1 wt % 1C18, a homogeneous lamellar phase is obtained at 15 wt % surfactant. Thus, the lamellar phase first phase separates for lower concentrations of the polymer to reappear as a homogeneous phase at higher polymer concentrations. A similar behavior was observed in the study of Bagger-Jo¨rgensen et al.25 This behavior we relate to the strong concentration dependence of the rigidity of polyelectrolytes. The rigidity decreases with increasing concentration due to screening of the electrostatic interactions decreasing the energetic penalty involved in confining the polymer within the narrow water layers of the lamellar phase. The influence of rigidity receives further support from the phase diagram obtained when water is replaced by 0.1 M NaCl. Phase diagrams with 0.1 and 1.0 wt % 1C18, respectively, are shown in Figure 8. Now, in the presence of salt, the homogeneous lamellar phase swells to lower surfactant concentration. With 0.1 wt % 1C18 it swells to approximately 12 wt % surfactant, while with 1.0 wt % 1C18 it swells to approximately 10 wt % surfactant. We also investigated the phase diagrams with two concentrations, 0.1 and 0.5 wt %, respectively, of the polymer 3C18 having a higher degree of modification. (with 1% 3C18 we observed indications of a saturation of the surfactant film by polymer at lower surfactant concentrations.) The phase diagrams are shown in Figure 9. Qualitatively the same behavior is observed as with 1C18, but quantitatively the effects are slightly more pronounced. At 0.1 wt % polymer there is no homogeneous lamellar phase in the studied concentration range, but already at 0.5 wt % polymer, it has come back to essentially full swelling, i.e., down to 10 wt % surfactant. With a higher degree of modification the adsorbed polymer adopts a more compact layer with shorter loops and tails, which is expected to lower the penalty for confinement in the lamellar phase. Also the upward shift of the upper phase boundary of the microemulsion phase is stronger with 3C18 compared to 1C18. The shift with 0.5 wt % 3C18 is even slightly stronger than with 1 wt % 1C18. Hence the system with 3C18 behaves as more strongly charged compared to the same concentration of 1C18. This we believe is mainly due to a higher concentration of 1C18 in the excess water phase compared to 3C18. Conclusions In this paper we have investigated the effects of an adsorbing polyelectrolyte (hydrophobically modified sodium polyacrylate) on the phase behavior of a balanced nonionic surfactant-water-oil system. Our findings can

4384 Langmuir, Vol. 12, No. 18, 1996

a

b

Figure 8. Partial phase diagram of the quinary system C12E4/ 0.1 M NaCl(aq)/1C18/dodecane at equal volumes of brine and oil and a constant aqueous concentration of the polymer 1C18. The aqueous polymer concentrations are (a) 0.1% and (b) 1.0%.

be summarized as follows. (i) For a given concentration of surfactant, water, and oil, the solubility of the hydrophobically modified polymer depends on the microstructure, i.e., on the geometry of the confinement. While being readily soluble in the microemulsion phase (below saturation) with a bicontinuous topology, the polymer solubility in the lamellar phase, with planar parallel bilayers, was found to depend on the polymer concentration and added salt. (ii) The adsorption of polymer onto the surfactant film results in an increased spontaneous curvature away from water. (iii) With adsorbed polyelectrolyte in the absence of salt, the system behaves similar to weakly charged films. (iv) The surfactant film becomes saturated

Rajagopalan et al.

a

b

Figure 9. Partial phase diagram of the system C12E4/water/ 3C18/dodecane at equal volumes of water and oil and a constant aqueous concentration of the polymer 3C18. The aqueous polymer concentrations are 0.1% and (b) 0.5%.

with polymer at higher polymer concentrations. In one experiment, saturation was estimated to occur for a polymer-to-surfactant ratio corresponding to 0.2 mg of polymer per m2 of surfactant film. This value, however, is expected to depend on salt concentration and on the topology and the pore size of the microstructure. Acknowledgment. This work was supported by The Swedish Natural Science Research Council (NFR) and The Swedish Research Council for Engineering Sciences (TFR). LA960375R