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J. Phys. Chem. 1994,98, 1500-1505
Polyelectrolyte Association to Micelles and Bilayers? I. Iliopoulos'~~~~ and U. Olssod Laboratoire de Physico- Chimie Macromolbculaire, Universitb Pierre et Marie Curie, CNRS URA 278, ESPCI, 10 rue Vauquelin, F-75231 Paris Cedex 05 France, and Physical Chemistry 1 , Chemical Center, Lund University, P.O.B. 124, S-221 00 Lund, Sweden Received: July 7, 1993; In Final Form: October 13, 1993' W e have investigated the association of hydrophobically modified poly(sodium acrylate) (HMPA) to micelles and bilayers of nonionic surfactant. The H M P A bears octadecyl chains randomly anchored on 1 or 3 mol 5% of the monomer units, and the surfactant is pentaethylene glycol dodecycl ether ( C I ~ E Sor ) octaethylene glycol dodecyl ether (C12Eg). The H M P A was introduced in small amounts to the micellar and lamellar phases of the binary nonionic surfactant/water system. The association of H M P A with the nonionic surfactant aggregate is due to the hydrophobic alkyl chains of the polymer that dissolve in the surfactant film. This association behavior has a large impact on the rheological properties of the aqueous HMPA-nonionic micelles mixtures. Moreover, the bonding of the surfactant on the polymer induces a pronounced decrease of the micellar mobility as is evident from molecular self-diffusion measurements. Small amounts of HMPA dramatically increase the stability of the micellar phase. The change in phase behavior is similar to that obtained by adding small amounts of ionic surfactant to the surfactant film. Adding H M P A to the water/C12Es lamellar phase results in a strong decrease in turbidity, indicating a crossover from a dominating undulation to a dominating electrostatic repulsion between bilayers.
Introduction Polyelectrolytes associate strongly with oppositely charged s~rfactants.~-3 Electrostatic attractions between the polyion and the polar group of the surfactant as well as hydrophobic interactions between the tails of the bound surfactant molecules are the main forces stabilizing the polyelectrolyte/surfactant complex.2 When the polyelectrolyte and the surfactant are of the same charge, electrostatic repulsions dominate and the association between the polymer and the surfactant is absent or very feeble.1s4J Only polyelectrolytes having a very strong hydrophobic character exhibit association with surfactants of the same charge."" Similarly, nonionic surfactants present a very low affinity toward polyelectrolytes, and association occurs only in cases where hydrogen bonding12 or hydrophobic intera c t i o n ~between *~ polymer and surfactant are operative. A new class of water-soluble polymers has been developed during recent years termed hydrophobically modified (or associating) water-soluble polymers (HMWSP). They arecopolymers of a water-soluble monomer (in large excess) and a very hydrophobicone (a small fraction, typically lower than 5 mol %). In aqueous solution the hydrophobic moieties self-associate forming intra- and interchain (micellar type) aggregates. This association is reversible and endows intriguing rheological properties to the aqueous solutions of these polymers.I"l9 In one of our laboratories (Paris) we have recently synthesized and studied the solution properties of hydrophobically modified polyelectrolytes based on poly(sodium acrylate) (HMPA).l8,20 Their rheological properties in aqueous solution are related to the formation of hydrophobic aggregates which in turn is governed by a subtle balance of electrostatic repulsions and hydrophobic attractive interactions. As a result, the viscosity of semidilute solutions increases upon addition of salt.l*920 These polymers are good candidates to study the hydrophobic effect on the polymer/ surfactant association. For instance, HMPA interacts with cationic surfactants more strongly than the precursor unmodified poly(sodium acrylate) (PA),l3 and it can associate with anionic surfactant micelles despite the unfavorable electrostatic repulsion.7 t ApartofthisworkwaspresentedasapostertotheXIIEuropeanChemistry at Interfaces Conference held in Lund, Sweden in June 28-July 2, 1992. t Universite Pierre et Marie Curie. 8 Lund University. a Abstract published in Advance ACS Abstracts, December 15, 1993.
0022-3654/94/2098- 1500$04.50/0
In this study we focus on the interactions between HMPA and nonionic surfactants of the oligoethylene glycol monoalkyl ether type, C,E,. The solution properties of these surfactants depend very much on the relative length of the alkyl tail ( m ) and of the polar head ( n ) as well ason the t e m p e r a t ~ r e . ~ *We - ~havechosen ~ to work with two surfactants, Cl2Es and C12E5. The first one, C&8, forms small spherical micelles over a wide range of concentration and temperature.22 The other, C12E5, forms giant micelles at room temperature and bilayers (lamellar and sponge (L3) phases) at higher temperature^.^^ The HMPAs used bear 1 or 3 mol % of octadecyl side chains. The association of these modified polymers with the nonionic surfactants is studied by rheology and N M R techniques (micellar self-diffusion coefficient). Partial phasediagrams aredetermined todepict theeffects of association on the stability of various surfactant aggregate structures. Particular emphasis was given to the polymer/bilayer association. The effects of HMPA on the phase behavior of the water-ClzE5 systems are compared with the effects of adding small amounts of the anionic surfactant sodium dodecyl sulfate (SDS).
Experimental Section Materials. The origin of poly(sodium acrylate), the modification reaction, and the polymer characterization were described elsewhere.20 The modified polymers contain 1 and 3 mol % of octadecyl side groups randomly anchored onto the polymer chain.24 They are denoted 1C18 and 3C18, respectively (Figure 1). The unmodified poly(sodium acrylate) will in the text below be refered to as PA. The degree of polymerization (- 2000) is the same in the modified and nonmodified polyacrylates. The molecular weight of the mean monomer unit is 100.9 for 3C18, 96.3 for 1C18, and 94 for PA. C I ~ and E ~ of high quality were obtained from NIKKO Chemical Co., Japan. SDS was obtained from BDH. DzO was obtained from Ciba-Geigy and had >99.7% isotopic purity. NaCl was of analytical grade and water was Millipore filtered. Rheology. Most of the viscosity measurements were performed with a Contraves LS-30 viscometer a t low shear rates SI.28 s-I corresponding to the Newtonian viscosity. For very viscous mixtures (t) 2 10000cP) a Carri-Med Controlled Stress rheometer equipped with a cone and plate geometry was used, and the viscosity at a shear rate of 0.1 s-1 was reported. The temperature 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 1501
Polyelectrolyte Association
HN
0- Na+
IC'
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80A (3C18) 250A (1C18)
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Figure 2. Schematic illustration of a mixtureof hydrophobically modified polymer and surfactant. At low surfactant concentrations there are mainly free surfactant molecules and cross-linking mixed micelles. At high surfactant concentrationsfree micelles are in equilibriumwith non-crosslinking mixed micelles. Finally,at intermediate surfactantconcentrations cross-linking mixed micelles dominate and the viscosity is very high. I
Figure 1. Molecular structure and schematic illustration of hydrophobically modified poly(sodium acrylate); X is the degree of modifcation in mol %. The alkyl (C18) tails are randomly anchored on the polymer backbone. ( I ) is the average distance between two adjacent alkyl tails assuming a fully extended polymer backbone.
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was 25.0 f 0.1 "C. H20 was used as solvent. Mixtures of the desired composition were prepared by mixing stock solutions of the polymer and the surfactant. They were shaken vigorously many times and equilibrated for one or two days at room temperature. The polymer concentration was kept constant at 1% by weight. Self-DiffusionExperiments. Self-diffusion experiments were performed on a modified JEOL FX-60 NMR spectrometer operating at a lH resonance frequency of 60 Hz, using the Fourier transform pulsed gradient spin-echo (FTPGSE) technique.25In these experiments, D2O was used as solvent and the self-diffusion coefficient of the surfactant was obtained by monitoring the echo resonance of the surfactant ethylene glycol head group. Phase Diagram. The solvent was D20 or D20 containing 0.1 mol L-l NaC1. At first, stock solutions containing 2% and 10% Cl2E5 in both solvents were prepared. The final mixtures were obtained after dissolving the appropriate amount of polymer or SDS in the stock surfactant solutions. The phase transitions were determined in a thermostated water bath coupled with a magnetic stirrer and the samples were contained in screw-capped test tubes containing magnetic stirring bars. Phase transition temperatures were determined by visual inspection in transmitted light, scattered light, and between crossed polarizers. The temperature was controlled within 0.1 OC. In some cases composition analysis of the phases in equilibrium were performed by recording a lH NMR spectrum of each phase. For this purpose a Bruker WP 250 spectrometer (250 MHz) was used.
Results and Discussion Rheology. The association of surfactants with polymers may induce pronounced synergetic effects on the rheology of these systems. Such rheological effects were reported for a variety of polymer/surfactant systems, such as mixtures of ionic surfactants with neutral polymers26J7 or with polyamphoterics28or with oppositely charged polyelectrolyte^.^^ However, the most spectacular viscosity enhancements are observed in mixtures containing HMWSP.7J1J3JO-32 The hydrophobic moieties of these polymers have a strong tendency to associate with surfactant micelles. Under suitable concentration conditions, the micelles solubilize alkyl groups belonging to more than one polymer chain, and the system becomes c r o ~ s - l i n k e das~ ~ illustrated ~~~ schematically in Figure 2. This figure illustrates the general situation of a surfactant/HM WSP mixture in which free surfactant molecules,
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Figure 3. Variation of the viscosity of 1% aqueous solution of precursor and modified polymers with ClzEs concentration.
free micelles, cross-linking mixed micelles, and non-cross-linking mixed micelles are present in equilibrium. The relative concentration of the various species depend on the conditions, such as total concentrations, chain lengths, temperature, interactions, etc. At low surfactant concentrations there are not enough micelles to cross-link the polymer chains, and at high surfactant concentrations there are mainly free micelles and non-cross-linking mixed micelles (see Figure 4 in ref 13). It must be noted that the above picture describes a system in which small spherical micelles are formed. A somewhat different behavior is expected when the surfactant forms larger aggregates. The general behavior, described above, is observed in the present systems with nonionic surfactant. In Figures 3 and 4 we present the variations of the viscosity of a 1 wt % solution of polymer (PA, 1C18, and 3C18) with the concentration of added C&8 and C12E5, respectively. A pronounced increase in viscosity and even gel formation was observed over a large range of surfactant concentration. The higher the alkyl content of the polymer, the stronger the viscosity enhancement. Viscoelastic solutions were obtained with the most modified polymer (3C18) and for surfactant concentrations in the range 2 X 10-4 < [surfactant] < 2 X 1C2 mol L-l. For such viscoelastic solutions the shear viscosity cannot describe correctly the rheological behavior of the system and nonlinear phenomena become important. However, the shear viscosity measured at a low shear rate (0.1 s-1) is still reported (dashed part of the curves in Figures 3 and 4) to
1502 The Journal of Physical Chemistry, Vol. 98, No. 5, 199‘4
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SURFACTANT CONCENTRATION (mol r’) Figure 4. Variation of the viscosity of 1% aqueous solution of precursor
and modified polymers with C12E5 concentration. 80
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SURFACTANT CONCENTRATION (mol r’) Figure 5. Elastic modulus, G’, of aqueous solutions containing 1% 3C18 as a function of nonionic surfactant concentration. Frequency = 0.1 Hz.
allow for a qualitative comparison between all samples investigated. On the viscoelastic solutions, oscillatory experiments in the linear viscoelasticity region were performed. The elastic modulus, G’, measured at a frequency of 0.1 Hz is plotted as a function of surfactant concentration in Figure 5. From the results reported in Figures 3-5 it is seen that ClzE5 forms more viscous solutions (compare 1C18 systems) and more elastic gels (compare 3C18 systems) than ClzEs when mixed with the modified polymers. This difference between C12E5 and Cl2Es can be attributed to the influence of the micellar aggregation number and/or to the length of the surfactant polar head group. Concerning the aggregation numbers, it is well known that at room temperature the Cl2Es micelles grow with increasing surfactant con~entration,2~,~~ while C&8 form spherical micelles, of constant size, up to very high concentrations.22 With Cl2E8 a rather symmetric peak in the viscosity and G’, as a function of the surfactant concentration, is observed, in the presence of modified polymers. In the case of 3C18, the maximum mol in G’occurs at a surfactant concentration of about 3 X
Iliopoulos and Olsson
L-1. This is approximately the same as the concentration of C18 chains. Hence, if all the C 18 chains are solubilized in micelles, the mixed micelles contain on the average equal numbers of C 18 chains and Cl2Es molecules at the G‘maximum (the concentration of nonaggregated surfactant monomers can be neglected). Due to configurational entropy, we do not expect all C18 chains to be incorporated into mixed micelles. Nevertheless, if a majority of the C18 chains are involved in the mixed micelles, it is questionable if each of them belong to a different polymer chain. Consequently, each polymer chain is expected to contribute with many alkyl groups in a given mixed micelle. With 1C18, an elastic system is not obtained. The maximum in viscosity occurs at a surfactant concentration of about 2 X 10-3 mol L-I. Since the concentration of C18 chains is about 1 X mol L-1, this indicates a lower fraction of C 18 chains in the mixed micelles, compared to the 3C18 system. In the CI2ESsystem we also have to take into account the concentration dependence of the surfactant micellar size. Kat0 et al. have measured the collective and self-diffusion coefficients in micellar solutions of C12E5.3~At the lowest concentration of mol L-I, the value of the self-diffussion their study -7 X coefficient indicates an axial ratio of the order of 10, assuming prolate micelles. Most likely, the Cl2E5 micelles begin to grow at a concentration very close to the cmc (critical micelle concentration), and we expect the C12Es and Cl2Es to have different aggregation numbers in almost the whole concentration range studied, although in particular at higher concentrations. The growth of the micelles can explain the higher viscosities in the mixtures with PA at higher surfactant concentrations. Note that there are no attractive interactions between PA and the nonionic surfactant. Besides giving a higher viscosity and higher values of G’in the concentration range 10-3-10-2 mol L-I, the C I ~ Esystems S with modified polymer also show a significantly higher viscosity a t higher surfactant concentrations, compared to the corresponding C12E8 systems. The latter effect can be understood in terms of the formation of larger micelles in the systems. Due to larger micelles, the ratio [C18]/[micelle], where [C18] and [micelle] are the molar concentrations of octadecyl polymer side chains and micelles, respectively, is higher in the Cl2Es system as compared to the C12Es one, and therefore the systems with Cl2Es remain cross-linked at much higher surfactant concentrations. Concerning the magnitudes of the viscosity and G’, the nonionic surfactants behave here as intermediates between the cationic (higher viscosity and G? and the anionic surfactants (lower viscosity and G9.13 The alkyl groups of the modified polymers prefer to associate with the surfactant micelle in order to reduce the hydrocarbon/water contacts. For a given polymer, the magnitude of the viscosity or G’ is likely to depend on the free energy cost of removing the polymer hydrocarbon side chain from a micelle, AGremov,,which determines the lifetime of a side chain in a mixed micelle. This free energy cost is expected to depend on the length of the alkyl group of the side chain and also on the interaction between the surfactant head group and the polymer backbone. If the latter interaction is strongly repulsive AG,,,,, is low (anionic surfactants), and if attractive then AG,em,,, is very high (cationic surfactants). We note that PA and polyethylene glycol show segregation phase separation in aqueous mixtures indicating repulsive interaction^.^^ Due to the longer EO chains in C12Eg it is then likely that G,,,, is higher compared to Cl2E5, resulting in lower values of viscosity and G’. We end this section by noting that we, at present, have a reasonable understanding of the rheological behavior in these hydrophobically modified polymer/micellar mixtures on a qualitative level, It is clear however, that quantitative studies concerning the stoichiometry of the mixed micelles (fraction of
The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 1503
Polyelectrolyte Association lO0c
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Figure 6. Relative surfactant self-diffusioncoefficient,DIDO, as a function of C12E8 concentration in systems containing 1 wt % of PA and 3C18, respectively. D is the surfactant self-diffusion coefficient in the presence of polymer, and DO(taken from ref 22) is the surfactant self-diffusion coefficient in the absence of polymer.
C 18 chains involved in the micelles) of these systems are needed in order to obtain a more complete picture and to understand the rheology on a more quantitative level. Surfactant Self-Diffusion. The self-diffusion coefficient, D, of C&8 was measured at 25 "C as a function of the surfactant concentration in solutions containing 1 wt % of 3C18 and PA, respectively. The measured D values can be compared to the diffusion coefficient in the binary water/C12E8system22(absence of polymer), DO.In Figure 6 we have plotted the ratio D/Do as a function of the C&8 concentration for the two different polymercontaining systems. In the case of PA, DIDOis approximately 0.65 and independent of the surfactant concentrationin the studied concentration range. The decrease of D compared to the polymer-free system is due to obstruction by the polymer network and is in good agreement with the predictions of Johansson et al.35 A similar concentration independence of D/Do was also observed for Cl2E8 micelles in a polysaccharide gel.36 In the case of 3C18, significantly lower values of D/Do are obtained, demonstrating the associationbetween the polymer and surfactant micelles. At lower surfactant concentrations, DIDO decreases with increasing concentration. This is mainly due to a decrease of the relative concentration of surfactant monomers (the cmc of C12E8 is -7 X 10-5 mol L-l 37). The monomer contribution to the self-diffusion constant can be estimated from Dmon( Cmon/Cs) whereD,, is thediffusion coefficient of surfactant monomers, Cmon is the monomer concentration and C,is the total surfactant concentration, with Cmon/C,being the fraction of monomers. When Dmon is much larger than the micellar diffusion coefficient the monomer contribution to the self-diffusion coefficient (which is a weighted average) may still be significant, although Cmon/Cs is low. In the limit when the monomer diffusion is the dominating mechanism for the diffusion, we expect D to vary as l/Cs, assuming Dmon and Cmon to be concentration independent. If we take Dmon = 4 X 10-l0 m2 s-1,22938 the D value measured at the lowest concentration, C,= 2 X mol L-l, is consistent with Cmon = 3 X l e 5mol L-l. This value is lower than the cmc in the pure binary system as expected in a solution containing a hydrophobically modified polymer. For concentrations >3 X l e 2mol L-l, D/Do increases strongly with increasing concentration indicating the formation of free, mol L-l, nonassociated micelles. At the concentration 3 X which marks the onset of forming free micelles, the ratio [ C I ~ E/ ~ ] [C18] is about 10. Assuming a surfactant micellar aggregation number of 60, this corresponds, on the average, to 6 octadecyl side chains per micelle, assuming no free micelles and no free side mol L-l correlates chains. Note that the concentration 3 X well with the end of the viscosity maximum measured by the rheology data (Figure 3). The same ratio of surfactant to
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+
+
hydrophobic polymer side chains was found at the end of the viscosity maximum transition when these polymers were mixed with cationic surfactant^.^^ Phase Behavior. A detailed phase diagram for the binary C12E~/watersystem is given in ref 23. At lower surfactant concentrations, the binary system shows a sequence of phase transitions with increasing the temperature: it forms a liquid micellar phase (Ll) at lower temperatures ( I 3 0 "C), and it separates into two liquid phases in equilibrium (Ll' + LI") at 30 IT I 50 "C. This transition is commonly referred to as "clouding phenomenon" and the corresponding temperature as "clouding temperature". At T 1 50 "C bilayers are formed, and a lamellar liquid crystalline phase (La)appears which is transformed to a liquid bilayer continuous phase (L3) upon further heating. At higher temperatures, the L3 phase is in equilibrium with excess water. The effect of modified polymers (mainly 3C18) and other additives (PA, SDS) on the phase behavior of 10% and 2% solutions of C12E5 is shown in Figures 7 and 8, respectively. The additives to C12E5 ratio were chosen as follows: [3C18]/ [C12E5] = [PA]/[C12E5] = 1/20. Theconcentrationoftheother additives, SDS and 1C18, was chosen in such a way to have the same R = [alkyl group]/[Cl2E5] molar ratio as in the 3C18/ClzE5 mixture, Le. R = 0.006. Let us comment at first the effect of the ionic surfactant SDS. The addition of a small fraction of SDS drastically reduces the extent of the region of liquid-liquid equilibrium (Figures 7a and 8a). A similar result was reported by Douglas and Kaler40 for the effect of sodium decanesulfonate(SDeS) on the phase diagram of C12E5 and by other authors for several nonionic-ionic surfactant systems.4145 The liquid-liquid phase separation in the C12ES/ water system is similar to that in PEG/water and is associated
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The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 (a)
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Iliopoulos and Olsson
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3C18 PA SDS Figure 8. Effects of various additives on the phase transitions of 2% Cl2E5 solution: (a) in D2O and (b) in 0.1 M NaCl (in D2O). The phases are denoted as in Figure 7. L is a liquid phase exhibiting streaming birefringence. A, B, and C refers to mixtures that the composition of the two phases in equilibrium was qualitatively estimated (see Figure 9).
additive
with the changing of the solvent quality of the water toward PEG from a good to a bad solvent with increasing t e m p e r a t ~ r eWhen .~~ SDS is added to the nonionic surfactant film, electrostatic effects become important and the extent of the miscibility gap decreases. The importance of such electrostatic effects on the miscibility of aqueous two-phase systems is well established in polymer/polymer and polymer/surfactant aqueous systems.33347-49 In the 10%solution of C12E5 the presence of SDS contributes to the stability of the lamellar phase. It is extended to higher temperatures as compared with the binary ClzES/water system (Figure 7a). However,at 2% C12E5, the lamellar phase is replaced by a streaming birefringent solution, denoted L in the phase diagram (Figure 8a). This solution presumably corresponds to a Ll-La equilibrium with stable vesicle formation, which has been found to occur in the related C12E4/dodecane/water system.50 We note also that similar streaming birefringent mixtures were reported by Douglas and Kaler for the C12E~/SDeS/water system .40 In the binary water C12E5 system the lamellar phase swells under the influence of the so-called undulation force,51and there is an associated turbidity in the dilute regime due to concentration fluctuation^.^^ Addition of SDS dramatically lowers the turbidity of the lamellar phase, indicating a transition to dominating electrostatic interbilayer repulsion. Similar effects have been observed in other s y s t e m ~ . ~ ~ J ~ Very similar effects to that of SDS are found when addind 3C18 and 1C18 to the water/ClzE~system, as is seen in Figures 7a and 8a. This is a strong indication that HMPA associates with the surfactant aggregates, resulting in electrostatic effects similar to that with SDS. Of particular interest is the fact that the modified polymers can be solubilized in the lamellar phase. Also with HMPA, the turbidity of the lamellar phase is strongly decreased, indicating that the modified polyelectrolyte adsorbs
onto the surfactant bilayers resulting in a electrostatic repulsion between the polymer-coated bilayers. Addition of PA has no effect on the clouding temperature, which is consistent with no, or only negligible, interactionsbetween C12E5 and the unmodified polyelectrolyte. Furthermore, PA cannot be incorporated into the lamellar phase. This behavior is expected for polymers with dimensions larger than, or comparable to, the repeat distance in the lamellar phase. In the C12E~/watersystem the repeat distances are about 1500 and 300 A at 2% and 10% C12E5 re~pectively.~~ The radius of gyration R, of PA is about 800 A in 0.14 M NaC139 and is expected to be much larger in water. PA of low molecular weight may however be incorporated in a dilute lamellar phase. With a molecular weight of 4000 a sample containing 0.25 wt % PA and 2 wt % C12E5 showed a homogeneous lamellar phase. As in the case of high molecular weight PA, there was no effect on the clouding temperature. Since the ionic additives discussed above give effects which, at least partly, are of electrostatic origin, we have studied the effects of additives when the solvent was a 0.1 M aqueous NaCl solution. The phase behavior for 10%and 2% C12E5 with additives are presented in Figures 7b and 8b, respectively. Nonionic surfactant systems are relatively insensitive to the addition of salt.45*55In the systems without polymer or SDS, the same phase sequence and approximately the same transition temperatures are observed in 0.1 M NaCl as in the case of pure water as solvent. In the SDS case, the properties of the pure water/ClzE~(or brine/ C12E5) system are recovered when adding NaC1. Also the turbidily of the lamellar phase is recovered, indicating that when screening the long-range electrostatic repulsion with added salt the undulation force is recovered as the dominating interbilayer repulsion. The C12E5/PAsystem is practically unaffected by the addition of NaC1. PA continues to be insoluble in the lamellar phase despite the reduction of the R,. On the other hand, most spectacular changes were induced by the salt in the C12E5/3C18 system. The domain of the two liquid phases is extended to much lower temperatures than in the case without salt (Figure 8b). Moreover, the lamellar phase contracts significantly, expelling almost pure solvent (Figures 7b, 8b, and 9). Obviously, the similarities in the behavior of C12Es/SDS and C12E5/3C18 found in pure water are no more valid when 0.1 M NaCl is used as a solvent. In contrast to PA, the modified polymers associate strongly with C12E5 in 0.1 M NaCl solution and at phase separation they dissolve in the surfactant-rich phase. A qualitative analysis of the polymer partitioning is illustrated schematically in Figure 9. The analysis was performed for three mixtures containing 2% C12E5 and 0.1% PA in water (mixture A), 0.1% PA in 0.1 M
Polyelectrolyte Association NaCl (mixture B), 0.1% 3C18 in 0.1 M NaCl (mixture C). The mixtures were equilibrated for 48 hours at 55 O C . The two phases in equilibrium were then separated, their volumes were assessed, and their appearance inspected. Their lH N M R spectra were run. The top phase for all these systems is extremely concentrated in surfactant, and the polymer peaks, if present, cannot be seen on the spectrum. The spectra of the bottom phase are more informative. They show that this phaseisvery dilutein surfactant but it contains PA (mixtures A and B). The bottom phase of the mixture C contains no polymer. These observations are reported in Figure 9. In summarizing we can say that in 0.1 M NaCl the P A / C ~ Z E S system exhibits segregative phase separation while the 3C18/ ClzEs system exhibits associating phase separatior49 In the 3C18/C12Essystem theundulation forceis not recovered in the presence of 0.1 M NaCl as is the case in the SDS/ClzE5 system. Instead, the polymer-coated lamellar show a very limited swelling in brine. This can in principle be due to an increased rigidity of the bilayers with adsorbed polymer. However as indicated by the decrease of the clouding temperature, the nonswelling of the lamellar phase can also be due to a limited solubility of the polymer-coated bilayers in brine. More experiments are needed in order to reveal this point. Finally, the presence of polymer, modified or unmodified, also influences the stability of the L3 phase. In this work we did not focus on this phenomenon which will be studied in more detail in a future work. Note, however, that from data of Figure 8b it becomes obvious that the unmodified polyelectrolyte PA does not dissolve in L3 phase. In fact, similar constrains as in the L, phase concern the polymer solubility in the L3 phase. The L3 phase has a multiply connected bilayer structure56J7 with a characteristic “pore size” of the three-dimensional “sponge” structure given by 5 = a6/0where 6 is the bilayer thickness, 0 is the bilayer volume fraction, and a i= 1 . P is a numerical coefficient. In the La phase, [ can be considered as the smectic repeat distance, with a = 1. Polymers with sizes (where the size can be defined as 2 R,) larger than cannot be incorporated into the bilayer structure without constrains on the polymer configurations or, similarly, on the configurations of the bilayer structures.
Conclusions Hydrophobically modified poly(sodium acry1ate)s adsorb on the surface of nonionic surfactant films, inducing important changes in the rheology and phase behavior of these systems. Depending on the surfactant to polymer ratio, mixed micelles are formed contributing to the cross-linking of the polymer chains. The micelles associated with the polymer have a very low mobility. The HMPAs can be solubilized in the lamellar phase. They adsorb on the bilayers resulting in electrostatic repulsions between polyelectrolyte-coated bilayers. The unmodified precursor polymer, in common to most polymers, is insoluble in the lamellar phase. A very similar behavior was found when these hydrophobically modified polyelectrolytes were dissolved in nonionic microemulsions.59 Acknowledgment. We thank L. Piculell, B. Lindman, S. Nilsson, and R. Audebert for helpful discussions and encouragement. 1.1.’~stay in Lund was supported by a grant from the Swedish Board of Technical Development. U.O. acknowledges the Swedish Natural Science Research Council (NFR) for financial support. References and Notes (1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Hayakawa, K.; Kwak, J. C. T. Cationic Surfactants Physical Chemistry; Rubingh, D., Holland, P. M., Eds.; Marcell Dekker: New York, 1991; p 189. (3) Lindman, B.;Thalberg, K. Polymer-Surfactant Interactions;Goddard, E. D., Ananthapadmanabham, K. P., Eds.; CRC Press, 1992; p 203.
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