J. Phys. Chem. 1995,99, 9201-9209
9201
Interactions in Aqueous Mixtures of Hydrophobically Modified Polyelectrolyte and Oppositely Charged Surfactant. Mixed Micelle Formation and Associative Phase Separation Franqois Guillemet*l? and Lennart Piculell Physical Chemistry 1, Lund University, Chemical Center, Box 124, S-221 00 Lund, Sweden Received: January 5, 1995; In Final Form: March 27, I995@
Aqueous mixtures of sodium dodecyl sulfate (SDS) with QUATRISOFT LM200, a cellulose derivative substituted with cationic hydrophobic side chains, have been investigated by various techniques, in the absence and in the presence of added salt. Steady state fluorescence measurements show that hydrophobic microdomains are formed in aqueous solutions of LM200 already at low concentrations ( 4 % ) . On adding SDS to a solution of LM200 in the range 0.02-1%, liquid-liquid phase separation occurs near charge neutralization (for the same amount of polymer and surfactant charges) for the salt-free mixture arid earlier in the presence of salt. In both cases, redissolution occurs upon further SDS addition. The total SDS concentration at redissolution increases linearly with polymer concentration, from a limiting value, at vanishing polymer content, close to the cmc of the polymer-free solution. Viscosity measurements show that SDS associates to LM200 already at very low SDS concentrations M) and that the binding continues even after redissolution up to the highest investigated ratios of SDS to LM200. In a 1% solution, a very high viscosity is found on both sides of the two-phase area, as previously shown by Goddard and Leung (Colloids Surf. 1992, 65, 211). The results are interpreted in terms of a binding isotherm of surfactant to polymer, analogous to isotherms observed for surfactants binding to proteins or to micelles of other surfactants. The first stages of the isotherm involve binding of individual surfactant molecules to the mixed micelles, and the last stage, occurring when the free surfactant concentration approaches cmc, is a strong and cooperative binding related to the self-association of the surfactant. High SDSLM200 binding ratios seem required for redissolution and even higher for breaking the micellar cross-links responsible for the enhanced viscosity. Such high binding ratios are only obtained near or within the cooperative binding region, i.e. when the free surfactant concentration is close to the cmc.
Introduction Polymer-surfactant systems receive a steadily growing interest due to their wide range of technical applications and also, from a scientific standpoint, to their fascinating propensity to form complex systems.'-3 By now, it is well established that surfactants can interact with polymers at a critical aggregation concentration, cac, in forming micelle-like aggregates with the p01ymer.~ The association has been shown to depend on both the relative charge and the hydrophobicity of the polymersurfactant pair.5 A class of polymers that interact particularly strongly with surfactants are the so-called hydrophobically modified water soluble polymers (HMWSP). The HMWSP are water soluble polymers containing a small number of strongly hydrophobic substituents. Since the contact between the hydrophobic groups and water is unfavorable, these polymers have a strong tendency to self-associate andor to associate with Progressive addition of surfactant typically gives rise to an increase in the viscosity of the solution, followed by a decrease at higher surfactant concentrations. The maximum is generally explained as follows:6.s,'2The enhancement in viscosity is ascribed to the formation of mixed micelles between the polymer alkyl chains and the surfactant molecules, reinforcing polymer intermolecular cross-links. Upon further surfactant addition, the number of polymer alkyl chains in the mixed micelles decreases, reducing in this way the number of polymer bridges. Thereby, with
' Present address: Physique Thermique, CNRS URA 857, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France. Abstract published in Advance ACS Abstracts, May 1, 1995. @
0022-365419512099-9201$09.00/0
excess surfactant, the connectivity of the network is lost and the solution recovers a lower viscosity. Another type of system, where strong polymer-surfactant interactions are found, is a mixture of a polyelectrolyte with an oppositely charged surfactant. Here, the association generally results in an associative phase ~ e p a r a t i o n . ' ~ In - ~ mixtures ~ of a cationic cellulose derivative and an anionic surfactant, Goddard et al.15-17 observed a maximum of precipitation at charge neutralization, i.e. for equal amounts of polymer and surfactant charges. Redissolution of the complex by an excess of surfactant has been ascribed to a charge reversal of the c0mp1ex.I~ In studies of mixtures of sodium polyacrylate (NaPA) or hyaluronate (NaHy) and cationic surfactants, Thalberg et a1.'8-20pointed out that the association, in this case, was essentially purely electrostatic in nature. Thus, the associative phase separation could be suppressed by the addition of salt. Some important variation in the phase behavior has been noticed, depending on the hydrophobicity of the polyelectrolyte. Thus, with NaPA or NaHy, phase separation on surfactant addition occurs essentially at cac, and prior to this point, no surfactant binding can be d e t e ~ t e d . ' ~ .Mixtures '~ of sodium polystyrene sulfonate and cationic surfactant, on the other hand, do not phase separate until charge neutralization.21.22 Systematic studies have been performed on the binding of surfactant to oppositely charged polyelectrolytes where the hydrophobicity of the polymer was ~ a r i e d . ~ However, ~-~~ comparatively little attention has been paid to the phase behavior of oppositely charged HMWSP and s ~ r f a c t a n t s . ~ , ' ~ The present work concems aqueous solutions of a hydrophobically modified cationic cellulose derivative, QUATRISOFT LM200 (henceforth referred to as LM200), mixed 0 1995 American Chemical Society
9202 J. Phys. Chem., Vol. 99, No. 22, 1995 OH
Guillemet and Piculell CH3
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1-7 Figure 1. Chemical structure of Quatrisoft LM200. t = 5.4 mol %.
with an anionic surfactant, sodium dodecyl sulfate (SDS). The polymer charges are located on the hydrophobic side chains, since the hydrophobic modification is made by grafting a cationic surfactant onto the hydrophilic polymer backbone. Thereby, a very strong polymer-surfactant interaction is expected, since both electrostatic and hydrophobic interactions promote the formation of mixed micelles between the polymer alkyl chains and the surfactant molecules. This expectation is bome out by the results of previous studies by Goddard and Leung on the same system.I0 In particular, these authors showed that mixtures with an anionic surfactant displayed two monophasic gel regions at sufficiently high polymer concentrations. One region was situated in the one-phase area at low surfactant concentrations, before phase separation, whereas the other gel region appeared at higher surfactant concentration, after redissolution had occurred. Recently, the adsorption of LM200 to hydrophilic surfaces in the presence of various surfactants has also been i n ~ e s t i g a t e d . ~ ~ The present paper provides detailed data on molecular interactions and phase behavior of LM200/SDS mixtures in the bulk and attempts to give a unified molecular interpretation of the findings. A preliminary account of this study has been given el~ewhere.~'
Experimental Section Materials. QUATRISOFT LM200 was supplied by Union Carbide Chemicals and Plastics Company, Inc. It is the chloride salt of an N,N-dimethyl-N-dodecyl derivative of hydroxyethyl cellulose (Figure 1) with a molar mass of about 100 OOO g/mol.'O The polymer was dialyzed against pure water and freeze-dried. The degree of side chain substitution was obtained by nitrogen analysis of the dry polymer and found to be 2.0 x moles of hydrophobic chains per gram of polymer. This roughly corresponds to 5.4 side chains per 100 sugar residues. Therefore, LM200 is a weakly charged hydrophobically modified cationic polyelectrolyte. SDS was obtained from BDH. Pyrene, from Jansen Chimica, was purified by recrystallization from ethanol. Methods. Sample preparation and all experiments were performed at room temperature (25 "C). Concentrations are given either in weight percent (%) or in moles of charges per liter of solution (M). Steady state fluorescence measurements of the emission spectra were performed using an AMINCO 500 SPF spectrofluorimeter at an excitation wavelength of 334 nm. Excitation and emission slit widths were open at 5 and 0.5 nm, respectively. The polymer solutions were prepared from a filtered stock solution of water saturated with pyrene. Also, at this low pyrene concentration ( ~ x5 lo-' M), no excimer emission was observed. The pyrene emission spectrum exhibited five distinct vibronic peaks, the first and the third ones being located at 373
and 383 nm, respectively. The ratio of their intensity, 11/13, yields an estimate of the polarity sensed by the pyrene molecules in their solubilization sites.32 Viscosities in dilute polymer solutions were measured using an Ostwald capillary viscometer. The specific viscosity, vs,was obtained as vs = ( t - to)/to, where t and to are the flow times of polymer solutions and polymer-free reference solutions (containing the same concentration of SDS), respectively. The viscosity of 1% polymer solutions was measured with a cone and plate geometry on a Carrimed Controlled Stress rheometer. Since the polymer-surfactant mixtures were shear thinning, stress sweeps were applied in order to locate the Newtonian plateau, which gave the zero-shear viscosity. In the gel region, where the Newtonian viscosity could not be measured, the viscosity at 0.01 s-' is reported. In the whole range of surfactant concentrations investigated, the viscosity of the SDS solutions was always negligible compared to the polymer-surfactant solutions. For the determination of the phase diagrams, samples were prepared from polymer and surfactant stock solutions, the vials were sealed off with Teflon-lined screw caps, and the samples were homogenized by smooth heating at 50 "C accompanied by gentle shaking in order to avoid foam formation. They were then left to equilibrate at room temperature for at least 1 week. In order to enhance the rate of sedimentation, centrifugation was carried out at 3000 rpm during 12 h. After this treatment, the phase-separated samples displayed two neatly separated isotropic phases. The top phases were transparent and had a viscosity close to that of pure water, while the bottom phases were translucent and gel-like. Transparent samples showing no macroscopic phase separation after centrifugation were considered to be monophasic. In some of the phase-separated samples, the composition of the two phases was determined. Both phases were carefully separated and weighed. The water content in the bottom phase was measured by weighing before and after freeze-drying. SDS concentrations were measured by sulfur analysis, using filamentpulse-pyrolysis gas chromatography with a sulfur photometric detector. A detailed description of the analysis procedure, which allowed the determination of the SDS concentration in mixed samples down to a concentration of M, is given elsewhere.33 Polymer concentrations were measured by a colorimetric method using a W-visible Perkin-Elmer spectrophotometer.34
Results Fluorescence Measurements. Figure 2 displays the effect of polymer concentration on Il/Z3, the ratio of the intensities of the first and third vibronic peaks of the pyrene fluorescence emission spectrum. For comparison, similar measurements are presented on dodecyltrimethylammonium chloride (DoTAC), a surfactant which closely resembles the isolated hydrophobic substituent of LM200. For both systems, the ratio 11/13 decreases monotonically with the concentration. This shows the development of strongly hydrophobic microdomains in solution, owing to the association of the hydrophobic chains. For DoTAC, 11/13 drops sharply to a value of 1.45 just before cmc, as it has previously been demonstrated for a number of surfactants (including DoTAC*'). For LM200, the decrease in 11/13 occurs at lower equivalent concentrations and is less sharp. The results at high concentrations indicate a leveling off at a value similar to that obtained for DoTAC micelles. Owing to the limited solubility of LM200, it was not possible to measure on polymer samples beyond 2%. The onset of formation of hydrophobic regions at lower concentrations for the HMWSP is an expected consequence of
J. Phys. Chem., Vol. 99, No. 22, 1995 9203
Hydrophobically Modified Polymer and Surfactant 0.01
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Figure 2. Change of 11/13 as a function of polymer or surfactant concentration given in molarity of charges. The filled and open circles correspond to LM200 and C 12TAC. respectively. The concentration of LM200 is also given in percent (top axis). the fact that, in this case, many surfactant molecules are already tied together on a macromolecule, possibly also in a rather blocky structure (depending on the details of the grafting procedure). Because of this, the decrease in the entropy of mixing of the surfactant chains, which accompanies the formation of a micelle, is considerably smaller than for free surfactant molecules. Thus, for certain nonionic HMWSP (depending on the detail of the hydrophobe modification and on the length and stiffness of the polymer backbone), intramolecular micelle formation should be possible also at infinite dilution. For ionic HMSWP, however, we also have to consider that the electrostatic contribution to the free energy of forming a micelle depends on the total concentration. Therefore, a finite cmc, even for intramolecular micelles, may be expected for ionic HMWSP. The more gradual decrease of 11/13 for the HMWSP may be due to one or more of a number of factors. An obvious source of the “smearing out” of the transition is the polydispersity that undoubtedly exists in the polymer sample. Thus, it is likely that different fractions of the HMWSP have different cmc’s, depending on the details of their pattem of substitution. Furthermore, it is possible that the micelle formation on the polymer is intrinsically less cooperative. Lastly, it may be questioned whether the concentration of hydrophobic regions at very low polymer concentrations is really sufficiently large to solubilize all of the dissolved fluorescence probe molecules. For our present purposes, the important conclusion of Figure 2 is the evidence that hydrophobic microdomains are formed already at very low polymer concentrations ( 4 % ) . Phase Diagrams. Figure 3 illustrates the phase diagram of the pseudoternq LM200-SDS-water system. The two-phase region is located in the water corner, with two isotropic phases in equilibrium. As expected, the phase separation is associative (cf. the direction of the tie-line in Figure 3), with a bottom phase, rich in both polymer and surfactant, in equilibrium with a dilute top phase. The two-phase region exhibits a strong asymmetry with respect to the bisector of the water comer: Addition of small amounts of SDS to a polymer solution causes almost immediately coacervation (liquid-liquid phase separation), while some polymer can still be soluble in a more concentrated micellar solution. As will become clear below, this asymmetry is due to the weak charge of the polyelectrolyte. The two-phase region is limited by two boundaries, one at low surfactant concentrations, referred to as the coacervation line, and the other at high surfactant concentrations, referred to as the redissolution line. Both these lines are very nearly straight in the investigated range of compositions.
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dilute phase. Charge Neutralization 4.i
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Figure 4 gives an alternative log-log representation of the phase diagram, which shows the dilute part of the diagram more clearly. The figure also includes data for mixtures in the presence of 10 mM NaCl. It is seen that the coacervation line for salt-free mixtures is near and parallel to the charge neutralization line, which indicates systems containing equal amounts of polymer and surfactant charges. This representation reveals another type of asymmetry of the phase diagram, with respect to the charge neutralization line: The two-phase region is stable with a large equivalent excess of surfactant, but is unstable at a small excess of polymer. With salt addition, the coacervation line is shifted to lower surfactant concentrations, while the redissolution line is moved to higher surfactant concentrations at high polymer concentrations, but to lower surfactant concentrations at low polymer concentrations. (The curvatures of the redissolution lines are due to the log-log scale; both lines are essentially straight in a linear-linear plot.) A strikinge feature of these phase diagrams is that the redissolution line, at very low polymer concentrations, nearly coincides with the cmc of the surfactant (the cmc of SDS is 8 mh4 in water and 5 mh4 in 10 mM NaC135).
9204 J. Phys. Chem., Vol. 99, No. 22, 1995
Guillemet and Piculell
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Phase Compositions. Phase compositions of samples belonging to the two-phase region are described in Figure 5. The samples studied had a concentration of 1% LM200, without added salt or in the presence of 10 mM NaC1, and the total surfactant concentration was varied. Figure 5a illustrates the changes in the swelling of the bottom phase as surfactant is added. First, the bottom phase rapidly shrinks down to a minimum value of 15% of the total sample mass. Within the accuracy of the determination, this minimum swelling occurs at charge neutralization (2 mM SDS). On further surfactant addition, the bottom phase swells gradually. The water content of the bottom phase goes through a similar variation, with a minimum at the charge neutralization of the system. It is to be noted that the water content is always quite high, even at the minimum in swelling. The variation in swelling and water content of the bottom phase may be ascribed to the osmotic pressure exerted by the counterions of the polymer-surfactant complex.36 Anticipating our conclusions below, we may liken the bottom phase to a charged chemical gel, to which an oppositely charged surfactant is added. (This case has been treated by Khoklov et ~ 1 . Surfactant ~ ~ ) molecules are incorporated into the gel network both before and after charge neutralization. When the gel network carries a net charge, it also has to contain an excess of counterions, owing to the condition of macroscopic electroneutrality. These counterions give an important contribution to the osmotic pressure of the gel and thus to its swelling. The net charge of the bottom phase will be 0 at charge neutralization, since all added surfactants bind to the polymer up to this point (cf. below), hence, the minimum in swelling. The variation in swelling of the bottom
phase before and after this point is an indication of a changing stoichiometry of the polymer-surfactant complex over the entire two-phase region. Figure 5b presents the SDS and LM200 concentration profiles in the dilute top phase. The surfactant content in the top phase is first very low and continues diminishing down to 7 x low6 M at charge neutralization, after which it increases up to concentrations close to the cmc. This means that, before charge neutralization, essentially all of the surfactant binds to the polymer. In the whole range of concentrations, the polymer content in the top phase remains quite low. Data are only given for top phases where the surfactant concentration was low, since it was found that SDS disturbed the analytical method for determining the polysaccharide concentration. However, at higher surfactant concentrations, the viscosity of the top phase was still very low, showing that most of the polymer was located in the bottom phase. The leveling off of the polymer concentration near charge neutralization parallels the initial decrease of the surfactant concentration in the top phase. Our interpretation of these initial trends is that they reflect the solubility of the polymer-surfactant complex, which is expected to decrease as the net charge of the complex approaches 0. The increased surfactant concentration after charge neutralization, however, is believed to mainly be due to free surfactant molecules. This interpretation is justified below. The fact that a significant fraction of the polymer remains in the top phase at charge neutralization, although virtually all of the surfactant is in the bottom phase, deserves comment. We believe that this is due to the uneven substitution of the polymer, so that the fraction that remains in the top phase has a significantly lower degree of hydrophobe modification. Nitrogen analyses of the dry content of separating phases confirm that such a fractionation occurs. Unfortunately, due to lack of material, no quantitative information on the fractionation of the polymer batch used in Figure 5b was obtained. The samples containing 10 mM NaCl exhibit a behavior similar to the salt-free samples, although the two-phase area in this case is extended both to higher and to lower surfactant concentrations. It may be noted that, at a given total surfactant concentration above charge neutralization, the degree of surfactant binding to the bottom phase is higher (cf. the free SDS concentration in the top phase in Figure 5b) and the swelling is lower (Figure 5a) in the presence of salt. Both these effects are expected consequences of the enhanced electrostatic screening at higher ionic strength. Viscosity. Figure 6a shows the specific viscosity of a 0.1% LM200 solution as a function of NaCl or SDS concentration. When NaCl is added, the viscosity starts to decrease around 0.1 mM NaC1, where the concentration of added salt becomes comparable to the concentration of polymer charges, and continues to decrease up to 16 mM NaCl, where phase separation occurs. Further salt addition does not lead to a redissolution of the polymer. The loss of solubility of LM200 at high ionic strength demonstrates that the polymer derives its solubility from the presence of charges which counteract the attraction of the hydrophobic side chains. LM200 is thus intrinsically (Le. in the absence of charge effects) water insoluble. The decrease in specific viscosity on salt addition indicates that, at 0.1% polymer, the screening of the polymer charges predominantly leads to a promotion of intramolec~lar,~~ rather than intermolecular, hydrophobic interactions. This results in a more compact polymer conformation and thus in a lower viscosity. In contrast to the results with NaC1, the viscosity increases when SDS is added to the 0.1% LM200, at a concentration as
J. Phys. Chem., Vol. 99, No. 22, 1995 9205
Hydrophobically Modified Polymer and Surfactant
cross-links are disrupted, since the increased fraction of SDS in the mixed micelles leads to a decreased functionality. Viscosity experiments performed at 1% LM200 are shown in Figure 6b. Here, the difference between adding salt and surfactant is much more striking, although the onset of surfactant binding is more difficult to extract. In accordance with the findings of Goddard and Leung,Io added surfactant at 1% polymer leads to the formation of very viscous systems, both just before coacervation and just after redissolution. This rise in viscosity clearly attests that mixed micelles are formed in this range of compositions. At very high surfactant concentrations ([SDS] > 0.1 M), the final increase in viscosity of the solution is again larger than the contribution from free SDS micelles.
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low as M SDS. The augmentation was even more pronounced for 0.2% LM200.31 The reason for the increase may be complicated; both the possible change in the numbers and sizes of the micellar cross-links and the competition between inter- and intramolecular interactions have to be considered.7-38,39However, the important conclusion in the present context is that the difference between the results obtained for surfactant and for simple salt, at the same polymer concentration, provides strong evidence that the surfactant binds to the polymer at these very low concentrations. Upon further surfactant addition, the viscosity drops down just before the two-phase region. We attribute this drop to the fact that intramolecular polymer-surfactant complexation should be favored, at this low concentration, as the charge density of the complex decreases. At 8 mM SDS, the system is again monophasic and the specific viscosity increases with added surfactant. This augmentation is not due to a trivial increase in the concentration of free surfactant micelles, since the specific viscosity in Figure 6a is given relative to the polymer-free surfactant solution (cf. the Experimental Section). Rather, the rise in viscosity after redissolution could indicate that some of the added surfactant molecules enter into the mixed polymer-surfactant aggregates, possibly leading to a more expanded polymer conformation. An important result in Figure 6a is that the viscosities of the surfactant-rich solutions after redissolution are lower than that of the surfactant-free solution. We attribute the higher viscosity of the surfactant-free solution to the existence of intermolecular cross-links, formed by the hydrophobic side chains, even in the absence of surfactant (cf. the fluorescence results above). At sufficiently high surfactant concentrations, the intermolecular
Surfactant Binding and Mixed Micelle Formation. In order to interpret the phase behavior and the gel formation in LM200/SDS mixtures, it is essential to have a good understanding of the binding of SDS to LM200. We should also appreciate how it differs from the binding of an ionic surfactant to a hydrophilic oppositely charged polyelectrolyte. A specific case could be the binding of alkyltrimethylammonium surfactants to sodium In the latter case, a cooperative binding of the surfactant to the polyelectrolyte is found at some concentration, cac, which is much lower than the cmc. The binding here is of a purely electrostatic nature, and the effect of the polyelectrolyte is essentially to act as a multivalent counterion to the surfactant, thereby dramatically lowering the cmc. To speak about a mixed micelle formation in this case is, in our opinion, misleading, although this terminology can be found in the literature. The complex formed here is no more a mixed micelle than is any other micelle with mixed, possibly multivalent, counterions. In conformity with the usage of the term in the context of surfactant solutions, we will henceforth reserve the term “mixed micelle” for systems where the micellar aggregate itself consists of two or more species in molecular contact. In the present study, we are dealing with the binding of a surfactant to a hydrophilic polymer which is substituted with surfactant-like side chains (unsubstituted hydroxyethyl cellulose does not associate with surfactant^^^^^). The binding of surfactant here is quite different from the purely electrostatic binding just described. First, we expect that truly mixed micelles will indeed be formed here, involving surfactant molecules and the hydrophobic side chains. Second, as has been evidenced above, the side chains of the polymer associate into hydrophobic microdomains, already at very low concentrations. Thus, in the concentration range relevant here and in most other studies of similar systems, HMWSP “micelles” are present even before surfactant addition. (This conclusion does not always hold for highly charged HMWSP, where electrostatic repulsion may prevent the formation of HMWSP micelles at low ionic These micelles have the capacity to solubilize individual surfactant molecules, a feature that distinguishes the HMWSP from a water soluble homopolymer interacting with a surfactant. Indeed, we have in the present case evidence of polymer-surfactant association at SDS concentrations as low M, i.e. almost 3 decades lower than the cmc: An as associative phase separation occurs at these concentrations when SDS is added to 0.02% LM200, and the viscosity rises at these concentrations for 0.1% polymer. Thus, the binding of a surfactant to a “micellar” solution of HMWSP should be closely analogous to the binding of a surfactant to a micellar solution of another surfactant, and we
9206 J. Phys. Chem., Vol. 99, No. 22, 1995
Guillemet and Piculell
may expect to gain important insight from the results of such experiments. Particularly illuminating for our present purposes are studies reporting on the binding of ionic surfactants to nonionic surfactant micelles4’ or phospholipid v e s i ~ l e s . 4In~ ~ ~ ~ the latter studies, ionic surfactants were added to systems having a constant concentration of nonionic surfactants or phospholipids, and the results were analyzed as binding isotherms: that is, the concentration of bound ionic surfactant, cb, was plotted against the concentration of free (monomeric) ionic surfactant molecules, cf. Naturally, cf may never exceed the cmc of the 0 1 ionic surfactant alone. The binding isotherms obtained in these 0 2 4 6 8 l o 3 1 lo-* studies had a quite general shape, which may be divided into C,/ M three more or less distinct regions: (1) At very low cf, there is a noncooperative high-affinity Figure 7. Binding isotherm of SDS to LM200. The binding ratio, p binding of the ionic surfactant. This is a Langmuir-type binding (see text), is plotted as a function of the free surfactant concentration, cf. Open and filled symbols correspond to samples without added salt of individual surfactant molecules into the nonionic micelles and in the presence of 10 mM NaCl, respectively. The solid and dotted or vesicles. The bound surfactant molecules are still sufficiently lines represent the expected complete isotherms. The open and filled far apart, so that their mutual electrostatic repulsion is negligible, triangles correspond to the redissolution coordinates (Table 1) as and the aggregates are essentially unchanged by the added determined by eq 1. surfactant. (2) Already at low degrees of binding (for mixed micelles, polymer. The binding isotherm determined in this way shows roughly when the number of bound ionic surfactants is on the the expected behavior. The binding is very strong up to charge order of one per micelle), an extended region of anticooperative neutralization, after which the increase is much slower. The binding is found. This happens when the electrostatic repulsion larger binding obtained in the presence of 10 mM NaCl is due between the increasingly charged mixed micelle and the ionic to the less negative electrostatic potential on the micellar surface surfactant molecule starts to become important. In this region, at enhanced ionic strength. The highest slope of the isotherm cb increases very slowly, and the addition of ionic surfactant in the presence of salt is also an expected consequence of the mainly results in an increase in cf. same effect. (3) As cf approaches the cmc of the ionic surfactant, a The cooperative part of the isotherm was not seen by this cooperative binding sets in. This process is most closely related method, but is nevertheless sketched in Figure 7 for later use. to the self-associationof the added ionic surfactant. The activity The qualitative behavior of this part of the isotherm is based (cf)of the latter is now so high that it is on the verge of forming on the following arguments: (i) The free surfactant concentramicelles on its own. In this region, almost all of the added tion can never exceed the cmc of the polymer-free surfactant ionic surfactant is incorporated into the mixed micelles, and solution under the same ionic conditions. (ii) All micellar these may now be regarded essentially as ionic micelles, into aggregates are mixed micelles, and the fraction of pure SDS which some nonionic surfactant is solubilized. (Pure ionic micelles may be neglected. The latter assumption is supported micelles should not appear to any important extent until the by the presence of the “gel” region and by the decrease in number of micelles exceeds the number of nonionic surfactant viscosity just after redissolution, as was pointed out above. molecules.) The effect of the nonionic surfactant here is to Binding isotherms of ionic surfactants to oppositely charged lower the cmc of the ionic surfactant by binding to the ionic protein^‘"'^^^ or hydrophobic polyelectrolyte^^^^^^ show similar surfactant micelles. features. In particular, they exhibit a strong binding up to charge neutralization, followed by a flat part and then by a strong The isotherm describing the binding of SDS to “micellar” cooperative binding at a free surfactant concentration close to solutions of LM200 is expected to be similar to the one the cmc of the surfactant for such an ionic strength. described above, with some modifications due to the fact that the initial LM200 micelles are oppositely charged to the SDS We may summarize the conclusions of this section as follows. surfactant molecules. As a consequence of this, the binding is Experimental evidence strongly suggests that SDS-LM200 expected to be of a high affinity type (essentially all SDS is aqueous mixtures form mixed micelles essentially over the entire bound) all the way until charge neutralization has been reached. composition range investigated here. Indeed, both the entropy Only when the mixed micelles have the same charge as the of mixing of the alkyl chains and electrostatic interactions should added surfactant molecules should we see a decreased affinity favor the formation of mixed micelles, rather than pure micelles in the binding and a buildup of the free surfactant concentration. of either kind. There is a possibility that repulsion between micelles on the same polymer could lead to saturation at high In Figure 7, we have used data on the phase compositions of binding ratios, but there is no evidence that free SDS micelles phase-separated mixtures (Figure 5b) to construct the binding are ever formed in the composition range investigated here. The isotherms of SDS to LM200, in the presence and in the absence affinity of the micelles to added surfactant is found to vary of added NaC1. The free surfactant concentration has been taken greatly over the isotherm: Before charge neutralization, and as the SDS concentration in the top phase, assuming negligible when the cf is close to cmc, essentially all added surfactant binds the amount of polymer (and, thus, the amount of bound to the complex. In the intervening region, however, the added surfactant) in the top phase. This approximation should not be surfactant molecules mainly stay as free monomers in solution. valid before charge neutralization (cf. above), and data from This binding behavior has interesting consequences for the phase this region are therefore omitted. For the same reason, the behavior and rheology of the mixtures, as will be discussed highest values of cf might be overestimated. The concentration below. of bound surfactant is then obtained as the total SDS concentration less the SDS concentration in the top phase. The binding Phase Behavior: Coacervation. When a hydrophilic polyratio, B, is the concentration of bound surfactant molecules electrolyte is mixed with an oppositely charged surfactant, divided by the concentration of cationic alkyl chains on the coacervation occurs already at the cac of the surfactant.’* At
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J. Phys. Chem., Vol. 99, No. 22, 1995 9207
TABLE 1: Redissolution Coordinates As Extracted from this point, the surfactant forms a micelle, and the system the Fit of the Redissolution Lines with Eq 1, without Added becomes analogous to a mixture of oppositely charged hydroSalt and in the Presence of 10 mM NaCP philic polyelectrolytes. In such a mixture, phase separation [NaCl] (mM) Bredlss Ctred1ss cmc (mM) occurs over a wide range of compositions, whereby a concentrated phase separates from a more dilute phase mostly contain0 5 .O 6.8 8 10 9.5 5.1 5 ing the excess polyelectrolyte. The tendency toward association is due to the release of the counterion atmosphere of the two a The free surfactant concentration at redissolution is compared to polyions which accompanies the formation of the concentrated the cmc of the surfactant found in the l i t e r a t ~ r e . ~ ~ ~oacervate.'~This driving force for association disappears in the presence of extemal salt, and, consequently, the tendency area, where the total concentration mainly affects the volumes, for oppositely charged hydrophilic polyelectrolytes to coacervate and not the concentrations, of the separating phases. However, decreases on addition of salt.20 we should consider the fact that the concentration of excess salt (i.e. all ions in excess of the counterions required to balance The situation is very different for the present mixtures of a the net charge of the complex) does depend on total concentrahydrophobically modified polyelectrolyte with an oppositely tion, in systems with the same concentration of free surfactant. charged surfactant. Here, molecular complexes (Le. the mixed At redissolution, the excess salt concentration is given by the micelles) are formed, which carry a net charge almost until free surfactant concentration (which is on the order of the cmc), charge neutralization is reached. Since the polymer is intrinsiplus the concentration of added salt, plus the contribution from cally water insoluble (cf. the loss of solubility upon salt the counterions released on forming the polymer-surfactant addition), coacervation occurs as soon as the fraction of complex. The latter contribution is equal to the total concentraneutralized charges exceeds some critical value. In salt-free tion of polymer charges. In the investigated range, this is on solutions, this value is apparently quite high (0.73), and the the order of 2 mM or less and is therefore always smaller than coacervation line is close to the charge neutralization line (Figure the other contributions. Thus, we will assume that the binding 4). However, when salt is added, a smaller fraction of isotherms drawn in Figure 7 will apply independently of the neutralized charges (0.3) is required for coacervation, owing to total polymer concentration. the lower solubility of the polymer-surfactant complex at (2) Redissolution occurs at some well-defined value of the enhanced ionic strength. Hence, the coacervation line shifts to binding ratio, Prediss, which is also independent of polymer lower surfactant concentrations in the presence of 10 mM NaCl. concentration over the investigated range. This assumption is In fact, in the systems where the polyelectrolyte contains some less obvious, but is nevertheless warranted by the success of hydrophobic groups, the presence of polymer-surfactant mixed micelles will delay the phase s e p a r a t i ~ nsometimes ~ ~ ~ ~ ~ all ~ ~ , ~ our ~ analysis (cf. below) and by the fact that the coacervation line (at least in the salt-free case) corresponds to a fixed charge the way up to charge neutralization, as compared to mixtures density, independent of polymer concentration. In analogy with of a hydrophilic polyelectrolyte and an oppositely charged the salt dependence of the coacervation line, Prediss should be surfactant. Likewise, such enhancement of solubility due to larger in the presence of salt, since a larger net charge of the the presence of hydrophobic groups has been observed in complex should then be needed for redissolution. mixtures of oppositely charged polymers: mixing sodium A consequence of the two assumptions above is that redispolyacrylate with LM200 induces a large two-phase region, solution should also occur at a well-defined concentration of whereas substituting 3 mol % of dodecyl chains on the free surfactant, Cf,rediss, which is independent of polymer polyacrylate backbone causes a drastic diminution of the twoconcentration, but dependent on the concentration of added salt. phase region.46 This leads to the following simple equation for the total Phase Behavior: Redissolution. Beyond charge neutralizaconcentration of surfactant required for redissolution. tion, the charge reversal of the mixed micelles should eventually lead to a redissolution of the polymer-surfactant complex. Ctot,rediss - 'f,rediss + Prediss'pol However, the phase diagram suggests that a quite large charge density is required here. Indeed, the redissolution is induced Here cpolis the total concentration of polymer alkyl chains. at a binding ratio of 5 without added salt and 9.5 in the presence Equation 1 gives excellent fits to the phase diagrams in Figure of 10 mM NaCl. Most likely, this is due to the presence of 4,with the values of the redissolution coordinates given in Table free surfactant in solution which delays the redissolution in 1. The redissolution coordinates (triangles in Figure 7) are seen contributing to the ionic strength of the solution. The coacervate to connect smoothly to the data points obtained independently is probably best viewed as a gel, swollen to some equilibrium from the SDS concentrations in the top phases of the biphasic water content (which is by no means low; cf. Figure 5a) regions. In principle, the assumptions above allow these determined by the balance of chain entropy, chain-chain coordinates to fall on any point on the binding isotherm above interactions, and counterion e n t r ~ p y . ~ Thereby, ~ . ~ ~ the swelling charge neutralization. The exact position should depend on the of the bottom phase is a function of the amount of bound balance between the intrinsic (in)solubility of the complex, the surfactant and of the ionic strength of the solution (NaCl and solubilizing contribution from the dissociated counterions, and free surfactant concentration). As expected, the bottom phase the free salt concentration. The plot in Figure 7 indicates that swells smoothly over the low-affinity part of the surfactant redissolution of LM200 with SDS occurs in the vicinity of the binding isotherm. In the presence of 10 mM NaCl, the higher cooperative binding. In general, we may expect that, for a binding ratio of the surfactant to the polymer is counterbalanced sufficiently insoluble complex, Prediss should be so large as to by the higher ionic strength of the solution, so that the swelling fall within the cooperative binding region. Thus, for such is slightly affected by the addition of salt. complexes Cf,rediss should be close to the cmc. Indeed, a Now, to account for the redissolution line, we make the correlation between the cmc and redissolution has also been following assumptions: observed in systems of nonionic hydrophobic polymers and (1) The binding isotherm is nearly independent of the total surfactants.40,47-49 polymer concentration, but depends on the concentration of Equation 1 gives a natural explanation to the crossing of the added salt. This should be a good assumption in the two-phase redissolution lines in the presence and absence of NaCl (cf.
9208 J. Phys. Chem., Vol. 99, No. 22, 1995
Figure 4). At enhanced ionic strength, the charge density required for redissolution of the complex is larger, while the cmc of the surfactant is lower (cf. Table 1). At low polymer concentration, the constant term of eq 1 is predominant, and redissolution occurs at a lower surfactant concentration. At high polymer concentrations, on the other hand, the constant term is negligible, and the redissolution is determined by the higher surfactant binding required to compensate the screening of the charges. We have quantified the redissolution only in terms of charge density of the polymer-surfactant complex and of ionic strength of the solution. Nevertheless, the solubilization of the polymer hydrophobic side chains into surfactant micelles should also favor the redissolution of the complex. Indeed, such type of redissolution has been reported in mixtures of HMWSP and nonionic surfactant?8 Rheology. The binding isotherm has consequences not only for the phase separation but also for the rheology of HMWSPsurfactant complexes. A quite general finding of HMWSPsurfactant mixtures is the maximum in the viscosity (or the shear storage modulus, G') with increasing surfactant concentration. The ultimate drop in viscosity is commonly explained in terms of the breaking of the mixed micellar cross-links, owing to the increasingly lower probability of finding side chains from two different macromolecules in the same mixed micelle. Thus, this loss in connectivity is expected to happen at large surfactant binding ratios (of the order of 10-100, depending on the aggregation number of the mixed micelle and on the number of side chains from the same macromolecule that belongs to the same micelle7), which, as we argued above, are only found in the cooperative region of the binding isotherm. This explains the sharp drop in the viscosity of the 1% LM200 system shortly after redissolution. This decrease occurs when the binding ratio (assuming high affinity binding) changes from 11 to 46, entirely in agreement with our expectations. In fact, this experimental result provides additional evidence that the surfactant molecules keep adding to the mixed micelles, also after redissolution. Relation between the Phase Separation and the Viscosity Maximum. Although both these features are common to aqueous mixtures of oppositely charged HMWSP and surfactant, it is important to realize that they depend on different surfactant/ polymer stoichiometries. The decrease in viscosity depends on the stoichiometry of polymer side chains in the mixed micelles and on the polymer concentration (through the competition between inter- and intramolecular association). Phase separation, on the other hand, depends on the charge stoichiometry of the entire polymer-surfactant complex. It should thus be possible to move the viscosity maximum and the phase separation relative to each other by changing the ratio between charged and hydrophobic groups on the polymer. For LM200/ SDS this ratio is unity, and, at polymer concentrations on the order of 1%, the breakup of bonds evidently requires a higher binding ratio than does the redissolution. This is why redissolution here occurs before the drop in viscosity. Hydrophobically modified polyacrylate, on the other hand, is an example of a polyelectrolyte where the number of charged groups greatly exceeds the number of hydrophobic groups. In this case, the binding ratio is expected to be so large at phase separation that the intermicellar cross-links should be broken already before this point. Indeed, this is what is found e~perimentally.~
Conclusions In the range of compositions investigated, mixed micelles of SDS molecules and alkyl side chains of LM200 dominate. The alkyl side chains of LM200 self-associate into hydrophobic
Guillemet and Piculell microdomains at comparatively low concentrations. SDS molecules bind very early to LM200 and continue to bind to the mixed micelles even at very high binding ratios. Thus, at any given mixing ratio, an aqueous SDSLM200 mixture may be approximated as containing a single type of aggregate of a fixed stoichiometry. The phase behavior of the mixture is essentially determined by the stoichiometry of this quasicomponent and by the amount of extemal salt (including monomeric surfactant) present in solution. The dependence of the phase behavior on aggregate concentration is only weak. In contrast, the viscosity of the solution depends on the concentration of aggregates not only trivially but also due to the competition between inter- and intramolecular mixed micellar cross-links. It is gratifying that, although LM200 is presumably a quite heterogeneous polymer, many important aspects of its behavior in mixtures with oppositely charged surfactants are possible to rationalize by quite simple arguments. The interaction of other hydrophobically modified water soluble polymers with surfactants should be possible to understand in similar terms, emphasizing both the tendency to form mixed micelles and the qualitative appearance of the surfactant binding isotherm.
Acknowledgment. We are grateful to Ilias Iliopoulos, Bjorn Lindman, Svante Nilsson, and Krister Thuresson for inspiring and helpful discussions. This study was supported by grants from the Swedish Natural Science Research Council and from Bo Rydin's Foundation for Scientific Research. F.G.'s stay in Lund was supported by a grant from Elf-Atochem. References and Notes (1) Lindman, B.; Thalberg, K. In Interaction of Surfactanfs with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 203. (2) Goddard, E. D. Colloids Surf. 1986, 19, 255. (3) Goddard, E. D. Colloids Surf. 1986, 19, 301. (4) Cabane, B.; Duplessix, R. J . Phys. (Paris) 1982, 43, 1529. ( 5 ) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactanfs; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 189. (6) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 89, 188. (7) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180. (8) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838. (9) Tanaka, R.; Meadows, J.; Williams, P. A,; Phillips, G. 0. Macromolecules 1992, 25, 1304. (IO) Goddard, E. D.; Leung, P. S. Colloids Surf. 1992, 65, 211. (1 1) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991, 7,905. (12) Gelman, R. A. In International Dissolving Pulps Conference; TAPPI: Atlanta, GA, 1987; p 159. (13) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992,41, 149. (14) Dubin, P. L.; Oteri, R. J. Colloid Inrerfuce Sci. 1983, 95, 453. (15) Goddard, E. D.; Hannan, R. H. J . Am. Oil Chem. SOC. 1977, 54, 561. (16) Leung, P. S.; Goddard, E. D. Colloids Surf. 1985, 13, 47. (17) Ananthapadmanabhan, K. P.; Leung, P. S.; Goddard, E. D. Colloids Surf. 1985, 13, 63. (18) Thalberg, K.; Lindman, B.; Bergfeld, K. Langmuir 1991, 7, 2893. (19) Thalberg, K.; Lindman, B.; Karlstrom, G.J . Phys. Chem. 1990, 94, 4289. (20) Thalberg, K.; Lindman, B.; Karlstrom, G.J . Phys. Chem. 1991, 95, 6004. (21) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (22) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (23) Hayakawa, K.; Kwak, J. C. T. J . Phys. Chem. 1982, 86, 3866. (24) Shimizu, T.; Kwak, J. C. T. Colloids Surf. A 1994, 82, 163. (25) Binana-Limbele, W.; Zana, R. Macromolecules 1987, 20, 1331. (26) Binana-LimbelC, W.: Zana, R. Macromolecules 1990, 23, 273 1. (27) Benrraou, M.; Zana, R.; Varoqui, R.; Pefferkorn, E. J. Phys. Chem. 1992, 96, 1468. (28) Shirahama. K.: Tashiro, M. Bull. Chem. SOC.Jun. 1984, 57, 377. (29) Ohbu, K.; Hiraishi, 0.;Kashiwa, I. J. Am. Oil' Chem. SOC. 1982, 59, 108. (30) Shubin, V. Langmuir 1994, 10, 1093.
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