The Counter- and Coion Influence on the Interaction between Sodium

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Langmuir 1999, 15, 353-357

353

The Counter- and Coion Influence on the Interaction between Sodium Hyaluronate and Tetradecyltrimethylammonium Bromide A° sa Herslo¨f-Bjo¨rling,†,§ Mikael Bjo¨rling,*,‡ and Lars-Olof Sundelo¨f† Physical Pharmaceutical Chemistry, Uppsala University, Biomedical Center, P.O. Box 574, SE-751 23 Uppsala, Sweden, and Physical Chemistry, Department of Chemistry, The Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received June 19, 1998. In Final Form: October 28, 1998 The minimum concentration of salt (critical electrolyte concentration, CEC) required to suppress precipitation/coacervation in an aqueous solution of sodium hyaluronate (NaHy) and tetradecyltrimethylammonium bromide (TTAB) has been studied and CEC diagrams constructed for a set of inorganic salts, as a function of TTAB concentration. An initial rapid increase in the CEC is observed for low TTAB concentrations at the onset of the coacervation region. When the TTAB concentration is increased, a CEC plateau is reached at a certain TTAB concentration. Thereafter, the CEC value stays fairly constant, independent of TTAB concentration, until finally the CEC value starts to decrease at a sufficiently high TTAB concentration. The CEC plateau value and the onset of the decrease of CEC are shown to depend strongly on the valence of the ions constituting the salt. For ions of equal valency, ion-specific effects are observed.

Introduction With the increased proliferation of detergents, surfactant-polymer interaction is becoming an important aspect of everyday life.1 In pharmaceutical and cosmetic formulations, as well as in the food industry, polymers and surfactants are important ingredients. The particular choice of polymers and surfactants, their mutual interaction, and their effect on the desired formulation properties therefore becomes an essential topic of investigation. The present study focuses on the interaction between a polyelectrolyte and an oppositely charged surfactant,2,3 and how this interaction is affected by various counterand coions. The anionic polysaccharide sodium hyaluronate, NaHy, and the cationic surfactant tetradecyltrimethylammonium bromide, TTAB, are the subjects of this study. Adding TTAB to an aqueous solution of NaHy induces precipitation unless a sufficient amount of salt (or surfactant) is present.4-6 By precipitation we mean a phase separation into two aqueous phases where one is enriched in the polyelectrolyte and surfactant whereas co- and counterions are present at high concentrations in both phases. For each TTAB concentration a minimum critical electrolyte concentration, CEC, is needed to ensure a homogeneous solution. In this work, CEC diagrams have * To whom correspondence should be addressed. Tel.: +46-87908515. Fax: +46-8-7908207. E-mail: [email protected]. † Uppsala University. ‡ The Royal Institute of Technology. § Present address: Medical Products Agency, P.O. Box 26, SE751 03 Uppsala, Sweden. (1) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Goddard, E. D. Colloid Surf. 1986, 19, 301-329. (3) Wei, Y.-C.; Hudson, S. M. Rev. Macromol. Chem. Phys. 1995, C35, 15-45 and references therein. (4) Thalberg, K.; Lindman, B. J. Phys. Chem. 1989, 93, 1478-1483. (5) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1990, 94, 4289-4295. (6) Herslo¨f, A° .; Sundelo¨f, L.-O.; Edsman, K. J. Phys. Chem. 1992, 96, 2345-2348.

Figure 1. CEC diagram for the sodium salts: NaCl (- - -), NaBr (s), and Na2SO4 (- - - -). Note the broken axis.

been constructed for a number of selected inorganic salts. Interestingly, the CEC value depends not only on the valence of the ions constituting the salt but also on ionspecific effects. In addition, a mixed coion effect is observed. Experimental Section Materials. All inorganic salts used in this study were of analytical grade and obtained from Merck, Darmstadt, Germany. All salts were dried before use. NaHy samples with weightaverage molecular weights 440 000 and 740 000, as determined by low-angle laser light scattering, were kindly supplied by Pharmacia & Upjohn, Uppsala, Sweden. TTAB was purchased from Sigma Chemical Co., St. Louis, MO. Water from a Millipore ultrapure water purification unit, Milli-Q was used. The NaHy solutions were prepared by weight and once prepared they were allowed to stand with magnetic stirring overnight or longer to ensure homogeneity. The hyaluronate concentration in the stock solutions was determined by optical rotation measurements with a Perkin-Elmer 241 polarimeter at 436 nm (mercury lamp). Methods. The phase diagrams were constructed by mixing stock solutions of salt, TTAB, water, and NaHy (in the mentioned order) and the precipitation was determined by visual inspection. In uncertain cases, a scratch with a sharp metallic object on the tube walls could reveal the existence of a formed gel. The total

10.1021/la980723k CCC: $18.00 © 1999 American Chemical Society Published on Web 12/18/1998

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Figure 2. CEC diagram for the chloride salts: KCl (‚‚‚), NaCl (- - -), NH4Cl (s), MgCl2 (s s ‚) and CaCl2 (s - s). Note the broken axis.

Figure 3. CEC diagram for the bromide salts: KBr (s - - s), NH4Br (- - -), and NaBr (s). Note the broken axis. NaHy concentration in all solutions was kept constant and approximately equal to 0.1 wt % The samples were checked immediately, 1 day, and 1 week after mixing. The error in the CEC values is estimated to be less than (5 mM.

Results For all salts, the CEC diagrams as a function of the TTAB concentration contain three common traits: a rapid increase of CEC at a low concentration, a roughly constant CEC at an intermediate concentration, and a slow decrease at a high TTAB concentration. At the intermediate TTAB concentration, it is convenient to define a CECmax for which a homogeneous solution is obtained irrespective of the TTAB concentration. The CEC diagrams for selected sodium salts are presented in Figure 1. The CECmax values decrease in the following order: NaCl > NaBr > Na2SO4. For the chloride salts in Figure 2, the CECmax values decrease as KCl > NH4Cl ≈ NaCl >MgCl2 > CaCl2. The CECmax values for NaCl and NH4Cl are almost the same. The CECmax values for the bromide salts (see Figure 3) follow a similar order as the chlorides (i.e., KBr > NH4Br ≈ NaBr). For the sulfate salts in Figure 4, the order of CECmax values was (NH4)2SO4 > MgSO4 > Na2SO4. The thiocyanate salts (NaSCN, KSCN, and NH4SCN) all caused the TTAB solutions to

Figure 4. CEC diagram for the sulfate salts: (NH4)2SO4 (s), MgSO4 (- - -), and Na2SO4 (s - s). Note the broken axis.

precipitate with or without addition of NaHy. A solution of NaHy and a thiocyanate salt stayed homogeneous. The TTAB concentration where the CEC values start to decrease (i.e., at high concentrations of TTAB) differs between the CEC diagrams. In Figure 1, the suppression of the phase separation due to excess TTAB starts at approximately 50 mM of TTAB for NaCl, whereas for Na2SO4 and NaBr the suppression starts around 100 and 150 mM, respectively. It may be noted that when NaBr is used (i.e., when the system contains the minimal four ionic species (sodium and TTA cations together with hyaluronate and bromide anions)), the decay of the CEC is slower and more curved than that in the mixed systems with five ionic species where the decay is nearly linear. This leads to a crossover of the CEC lines for NaBr and NaCl at a CEC of approximately 145 mM added salt and a TTAB concentration of 230 mM (see Figure 1). At higher TTAB concentrations, NaCl suppresses the phase separation more effectively than NaBr. While this effect is much less pronounced when the cation of the added salt is varied, crossovers are observed (see Figures 2-4). Even though the reported crossover concentrations are rough estimates, their existence is less uncertain. In Figure 2, NH4Cl and KCl suppresses the phase separation more effectively than NaCl at TTAB concentrations greater than 50 and 280 mM, respectively.

Counter- and Coion Influence on NaHy and TTAB

In Figure 3, both NH4Br and KBr becomes more effective than NaBr above 210 mM of TTAB, whereas MgSO4 and (NH4)2SO4 are more effective than Na2SO4 above 160 and 220 mM of TTAB, respectively. Discussion Previous studies of homogeneous mixtures of NaHy and TTAB,6-9 slightly above CECmax for NaCl, indicate that the electrostatic attraction between the TTAB micelles and the oppositely charged hyaluronate chains leads to the formation of NaHy-TTAB complexes. Each complex is believed to consist of several hyaluronate chains with TTAB micelles acting as cross-links. The phase separation observed below CEC may be rationalized as follows: With decreasing electrolyte concentration, the electrostatic binding of micelles onto the hyaluronate chains observed in homogeneous solutions (above CEC) become stronger. Hence, more micelles bind onto the chains and the “effective” charge of the complex is decreased.8 The increased binding of micelles also appears to yield larger complexes by increasing the average number of hyaluronate chains in each complex.7-9 At sufficiently low electrolyte concentrations (i.e., below CEC) the effective charge is so low that it is more favorable to form a separate phase enriched in polymers and surfactants. In a more detailed discussion of the precipitation mechanism and the influence of added salt, we adopt either a polyelectrolyte or a surfactant perspective. It is therefore illuminating to consider each of them separately. Polyelectrolyte Perspective. From a hyaluronate perspective, TTAB micelles may be regarded as multivalent counterions. Thus, a clear analogy exists between the above phase separation and the precipitation of polyelectrolytes in the presence of multivalent counterions.10-14 In the latter case, both the nature of the backbone and the polyelectrolyte charge density influence the valency required to induce precipitation. For a given backbone, lesser valence counterions are required as the linear charge density of the polyelectrolyte increases. For a given charge density, lesser valency counterions are required as the solubility of the backbone decreases. For highly charged anionic polysaccharides, such as pectins and alginates, salts containing divalent cations are sufficient to induce precipitation (at high polysaccharide concentrations, gels are formed instead).15,16 Sodium poly(styrene sulfonate) (NaPSS) requires at least trivalent counterions to precipitate.10-13 For the moderately charged NaHy (one charge per disaccharide unit), a four-valent salt (Th4+) did not yield a precipitate. Thus, an even greater counterion valency is needed to induce precipitation in NaHy solutions. (7) Herslo¨f-Bjo¨rling, A° .; Sundelo¨f, L.-O.; Podesva, J.; Stejskal, J. Colloid Polym. Sci., submitted for publication. (8) Herslo¨f-Bjo¨rling, A° .; Sundelo¨f, L.-O.; Porsch, B.; Valtcheva, L.; Hjerte´n, S. Langmuir 1996, 12, 4628. (9) Bjo¨rling, M.; Herslo¨f-Bjo¨rling, A° .; Stilbs, P. Macromolecules 1995, 28, 6970-6975. (10) Delsanti, M.; Dalbiez, J. P.; Spalla, O.; Belloni, L.; Drifford, M. In Macro-ion characterization from dilute solutions to complex fluids; Schmitz, K. S., Ed.; ACS Symposium Series 548; American Chemical Society: Washington, DC, 1994; pp 381-392. (11) Belloni, L.; Olvera de la Cruz, M.; Delsanti, M.; Dalbiez, J. P.; Spalla, O.; Drifford, M. Nuovo Cimento 1994, 16, 727-736. (12) Olvera de la Cruz, M.; Belloni, L.; Delsanti, M.; Dalbiez, J. P.; Spalla, O.; Drifford, M. J. Chem. Phys. 1995, 103, 5781-5791. (13) Drifford, M.; Dalbiez, J. P.; Delsanti, M.; Belloni, L. Ber. BunsenGes. Phys. Chem. 1996, 100, 829-835. (14) Fang, Y.; Hoh, J. H. J. Am. Chem. Soc. 1998, 120, 8903-8909. (15) Ikeda, F.; Shuto, H.; Saito, T.; Fukui, T.; Tomita, K. Eur. J. Biochem. 1982, 123, 437-445. (16) Axelos, M. A. V.; Mestdagh, M. M.; Francois, J. Macromolecules 1994, 27, 6594-6602.

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The bridging mechanism may be qualitatively understood in terms of counterion binding onto the polyelectrolyte chain.11,12 The electrostatic interaction leads to an enhanced concentration, relative to the bulk, of counterions near the oppositely charged polyelectrolyte. The surface concentration of an ion i, ci(0), with charge zie, interacting electrostatically with a charged surface is given by17

ci(0) ) c/i 0 exp(-zieΦ0/kT)

(1)

where ci0* is the bulk concentration of the ion i, e is the elementary charge, Φ0 is the electrostatic potential at the surface, k is Boltzmann’s constant, and T is the absolute temperature. Equation 1 is a simple application of the Boltzmann distribution. For a given temperature and counterion charge, the electrostatic potential at the surface governs the degree of adsorption relative to the bulk concentration. Thus, the counterion binding is particularly strong at highly charged surfaces and at low ionic strengths. The affinity of ions to the charged surface strongly depends on the valency zi when |eΦ0/kT| e 1. Multivalent counterions therefore concentrate at the surface to a greater extent than monovalent counterions. Multivalent counterions therefore neutralize the surface charges more effectively, which may be interpreted as a lower “effective” charge of the surface. Note also that counterions with equal valency may differ in surface affinity due to nonelectrostatic contributions (i.e., ionspecific effects). Ion-specific effects are especially prominent for anionic counterions.17 A multivalent cation adsorbed onto a negatively charged polyelectrolyte chain is only partly neutralized by the local charges on the chain. The remaining effective charge may be sufficiently large so that this new counterion species (i.e., the multivalent counterion together with the nearby local charges on the polyelectrolyte) may adsorb onto another site on the same or on another chain.11,12 Consequently, the bound multivalent counterions may form intra- and interchain bridges by multisite binding.14 When the bridging tendency (“binding”) is strong, the polyelectrolytes prefer a collapsed state and expell solvent to a solvent-rich phase.11,12 A similar expulsion of solvent is observed in lamellar systems in the presence of multivalent counterions in the form of ions18 or polyelectrolytes.19 Adsorbed TTAB micelles are thought to form bridges between the hyaluronate chains and to cause phase separation by the same mechanism. Addition of excess salt decreases the degree of multivalent counterion condensation by an overall increase of the ionic strength, leading to a decrease of the effective polyelectrolyte charge, an effect referred to as electrostatic screening. In the language of eq 1, screening is due to a lowering of the surface electrostatic potential Φ0 when the bulk electrolyte concentration is increased (at constant surface charge).17 Screening increases strongly with counterion charge (the ionic strength is proporional to the square of the charge). When the original counterion of the polyelectrolyte differs from the counterion from the added salt, competitive counterion adsorption from the mixed system is observed. As evident from eq 1, counterions with higher charge tend to replace the counterions with lower charge as long as |eΦ0/kT| g 1. Nordmeier and (17) Evans, D. F.; Wennerstro¨m, H. The Colloid Domain: Where Physics, Chemistry, Biology, and Technology Meet; VCH Publishers: New York, 1994. (18) Guldbrand, L.; Jo¨nsson, B.; Wennerstro¨m, H.; Linse, P. J. Chem. Phys. 1984, 80, 2221-2228. (19) A° kesson, T.; Woodward, C.; Jo¨nsson, B. J. Chem. Phys. 1989, 91, 2461-2469.

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Dauwe studied competitive counterion condensation between mono- and divalent counterions in solutions containing a highly charged polyelectrolyte, poly(styrene sulfonate).20 Their results show that divalent ions displace the monovalent ions in good agreement with Mannings counterion condensation theory21,22 at low ionic strength. Ion-specific effects were also observed since protons appear to be easier to replace than sodium ions.20 At a greater ionic strength (|eΦ0/kT| ≈ 0), the surface shows a lesser preference for multivalent counterions. The strength of the counterion binding, and the tendency to form intraand intermolecular cross-links, decreases and at some critical electrolyte concentration, CEC, no phase separation occurs. Note that the polyelectrolyte itself contributes to the ionic strength. Increasing the polyelectrolyte concentration therefore suppresses phase separation. Theoretical calculations performed by Belloni et al.,11,12 focusing on the electrostatic interactions, show good agreement with the experimental phase diagrams of NaPSS and multivalent counterions. In addition, the experimental phase diagrams may show ion-specific effects for ions of equal charge. Axelos et al.16 found that the critical concentration of divalent salts required to cause precipitation (in solutions containing alginates or pectins) decreases in the following order: Mg2+ > Mn2+ > Ca2+ > Cu2+. With the exception of Cu2+, the critical salt concentration decreases with increasing ionic size. The degree of condensation should decrease in the same order. While the phase separation mechanism in the NaHyTTAB systems is well-understood by the analogy to the phase separation in polyelectrolyte solutions induced by multivalent counterions, a micelle is a rather complex counterion species. Its large dimensions and the intrinsic dependence of the micellar shape and size on the ionic strength call for special considerations. Surfactant Perspective. The critical micelle concentration, cmc, the micellar aggregation number, and the micellar shape roughly depend on a balance between two opposing forces: the repulsive electrostatic forces between the ionic headgroups and the net attractive forces between the hydrophobic tails. The properties of the surfactant counterion affect both of these forces. In particular, an increased counterion condensation at the micellar surface leads to a more effective screening of the electrostatic repulsion between the headgroups. An increased counterion condensation will therefore be reflected in lower cmc values. For the anionic surfactants the cmc values decrease in the following order:23 Li+ > Na+ > K+ > Cs+ > N(CH3)4+ > N(CH2CH3)4+ > Mg2+ ≈ Ca2+. Consequently, the counterion condensation increases in the same order. The effects of counterion specificity are more pronounced for cationic surfactants as compared to anionic ones.24 For the dodecyltrimethylammonium halides, the cmc value decreases in the following order: F- > Cl- > Br- > I-, a series often referred to as the Hofmeister series.17,25 (In what follows, “Hofmeister series” will refer collectively to both the anion and cation series.) The cmc values for hexadecyltrimethylammonium salts (CTAX) decrease in the same order, X being Cl- (cmc ) 1.4 mM), Br- (cmc ) 0.8 mM), and SO42- (cmc ) 0.6 mM), respectively.26 The stronger binding of Br- (as compared to Cl-) to tetrade(20) Nordemeier, E.; Dauwe, W. Polym. J. 1991, 11, 1297-1305. (21) Manning, G. S. Q. Rev. Biophys. 1978, 11, 179. (22) Manning, G. S. Acc. Chem. Res. 1979, 12, 443-449. (23) Mukerjee, P. Adv. Colloid Interface Sci. 1967, 1, 241-275. (24) Lindman, B.; Wennerstro¨m, H. In Micelles. Amphiphile Aggregation in Aqueous Solution; Boschke, F. L., Ed.; Springer: Heidelberg, 1980; pp 3-83. (25) Collins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 18, 323-422. (26) Sepu´lveda, L.; Corte´s, J. J. Phys. Chem. 1985, 24, 5322-5324.

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cyltrimethylammonium halides has been the subject of several studies. Fabre et al.27 showed that Br- ions are more tightly bound to cationic micelles than Cl- ions. Porte and Appelle28 studied the transition from spherical to rodlike micelles for cationic surfactants and showed that a closer packing between headgroups, as found in rod micelles, is promoted in the following order: Cl- , Br< NO3- < ClO3-. The observed shape transition for cationic micelles clearly adds a new aspect to the discussion of the phase separation in the NaHy-TTAB system. In addition to counterions, coions also play a role as shown by Imae and Ikeda.29 Studying the sphere-to-rod transition of TTAB micelles, they found that a higher concentration is required to induce the transition in the case of KBr as compared to NaBr; [KBr]threshold ) 0.16 M and [NaBr]threshold ) 0.12 M, respectively. This suggests that KBr screens the headgroup repulsion less efficiently and may be attributed to ion-specific effects. In the presence of polyelectrolyte counterions, experimental4,7,30-34 and theoretical35-37 studies have revealed that micellar aggregates are formed at concentrations below the normal cmc, leading to the definition of a critical aggregation concentration, cac. In particular, the Monte Carlo simulations of polyelectrolytes in the presence of charged spherical micelles by Wallin and Linse35-37 are illuminating the effects of surfactant tail length, as well as polyelectrolyte chain flexibility, and charge density, on the cac. Much smaller cac’s are found for flexible polyelectrolytes with a high linear charge density (110 times lower than cmc), as compared to rigid polyelectrolytes with a low linear charge density (10 times lower).36 The counterion binding is obviously strongest in the former case. It should be noted that good agreement with experimental cac’s was obtained, even though only electrostatic interactions were considered in the simulations.35-37 Wallin and Linse also found that the strong micellar preference for the polyelectrolyte “counterion”, as compared to monovalent, counterions are caused by (i) a decrease in the electrostatic energy due to small micellepolyelectrolyte charge separation and (ii) an increase of the entropy due to the release of monovalent counterions from the two macroions (i.e., the micelle and the polyelectrolyte). Both these contributions lead to a very tight wrapping of the polyelectrolyte around the micelle as is conjectured experimentally.34 NaHy, TTAB, and Added Inorganic Salts. Returning to the NaHy-TTAB system, under investigation in the present work, it can be concluded from the beginning that it is rather complex because of the presence of both TTAB and an inorganic electrolyte, apart from NaHy. In the worst case, six ionic components may be present. A further complication is the aggregation of the surfactant monomers into micelles that may vary in size and shape. While a surfactant monomer may be looked upon as a univalent salt, they behave as a polyelectrolyte when aggregated into a micelle. In addition, the effective charge of the NaHy (27) Fabre, H.; Kamenka, N.; Khan, A.; Lindman, B.; Tiddy, G. J. T. J. Phys. Chem. 1980, 84, 3428-3433. (28) Porte, G.; Appelle, J. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 2, p 805. (29) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216-5223. (30) Thalberg, K.; van Stam, J.; Lindblad, C.; Almgren, M.; Lindman, B. J. Phys. Chem. 1991, 95, 8975-8982. (31) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115-2124. (32) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604-613. (33) Hansson, P.; Almgren, M. J. Phys. Chem. 1996, 100, 90389046. (34) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967-1975. (35) Wallin, T.; Linse, P. Langmuir 1996, 12, 305-314. (36) Wallin, T.; Linse, P. J. Phys. Chem. 1996, 100, 17873-17880. (37) Wallin, T.; Linse, P. J. Phys. Chem. B 1997, 101, 5506-5513.

Counter- and Coion Influence on NaHy and TTAB

chains and the TTAB micelles are simultaneously affected by the addition of an inorganic salt. Using the above observation that the phase separation is caused by the electrostatic interaction between the NaHy chains and the TTAB micelles, we now attempt to rationalize the previously presented CEC diagrams. In Figure 1, the counterion species for the micelles is varied. The CECmax values decrease in the same order as cmc: NaCl (200 mM) > NaBr (160 mM) > Na2SO4 (105 mM). The greater efficiency of NaBr, as compared to that of NaCl, is primarily associated with a weaker interaction between NaHy and TTAB due to the lower effective charge of the TTAB micelles (a consequence of the more pronounced condensation of Br- onto the TTAB micelles as compared to Cl-). The greater efficiency of sodium sulfate, as compared to NaBr, is attributable to its greater impact to increase the ionic strength and thereby the electrostatic screening. Shape transitions also play a role, especially at high salt and surfactant concentrations. The micelles are expected to be more rodlike with increasing counterion condensation onto the micelles. In Figure 2 the counterion for the TTAB micelles is chloride in all cases and the counterion for the hyaluronate chain is varied. The CECmax value is found to decrease as follows: KCl (210 mM) > NH4Cl ≈ NaCl (200 mM) > MgCl2 (80 mM) > CaCl2 (75 mM). The step in CECmax values from mono- to divalent ions must be a consequence of the decreasing effective charge of the hyaluronate chains. The CECmax value for CaCl2 does not differ much from that of MgCl2, but the greater efficiency of Ca2+ could be due to its greater polarizability. Whereas the order of divalent ions can be explained by the Hofmeister series for the counterions, the order of the univalent salts cannot. The lower screening effect of K+ as a counterion for hyaluronate, as compared to NH4+ and Na+, is opposite from what is expected from the Hofmeister series. However, assuming that the coion effect on the TTAB micelles for the bromide salts (see above) is similar for the chlorides, it may explain the higher CECmax value for KCl as compared to that for NaCl. The CEC values vary less with salt composition for the bromide and sulfate salts (Figures 3 and 4). This is not unexpected, since these anionic counterions are efficient in suppressing the precipitation by themselves. The trends are roughly the same as those for the chloride salts but are less pronounced. NH4+ and Na+ have already been shown to give similar CECmax values for the chloride salts. In Figure 4, all three salts have similar CEC values. However, up to the crossover concentration of TTAB, Na2SO4 appears to be more effective than (NH4)2SO4 and MgSO4 in suppressing the phase separation. This is somewhat surprising since one divalent magnesium ion increases the ionic strength (and thereby the electrostatic screening) more than two univalent sodium ions. As suggested above, the reverse Hofmeister series observed for the cations may be due to coion effects on micellar properties such as shape. However, it may also be due to unequal distribution of the ionic species between the two prospective phases that influences the phase equilibria. It should also be noted that the micellar shape is more or less rodlike when bromide and sulfate salts are used. In fact, with a very high addition of NaBr a segregative phase separation between the polyelectrolyte and the rodlike micelles is observed.31,38 A similar phase separation may be conjectured for the other bromide and sulfate salts. (38) Thalberg, K.; Lindman, B.; Karlstro¨m, G. Prog. Colloid Polym. Sci. 1991, 84, 8. (39) Herslo¨f-Bjo¨rling, A° . Ph. D. Thesis, Uppsala University, Sweden, 1995.

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The CEC crossovers in Figures 1-4 are especially interesting since crossovers are only observed when extra ionic species are introduced. The phase diagram of NaPSS in the presence of either TTAB and NaBr or tetradecyltrimethylammonium chloride (TTAC) and NaCl, studied by Hansson and Almgren,31 shows no crossover between the two CEC lines as observed for NaHy in Figure 1 when comparing the TTAB and NABr with the mixed TTAB and NaCl system. Therefore, the crossover is probably due to the introduction of an additional ionic species (i.e., an extra degree of freedom). For the chloride salts in Figure 2, the minimum number of ionic species is five. Nevertheless, a crossover is observed when a sixth ionic species is introduced, but the differences in CEC are smaller. For MgCl2 and CaCl2, both giving two additional ionic species, no crossover is observed. In Figures 3 and 4 the CEC lines of the salts introducing an additional ionic species cross over the NaBr and Na2SO4 CEC line, respectively. The precipitating effect of SCN- on TTAB is most probably a Krafft phenomenon.17 In a solution of MSCN (M being K+, NH4+, and Na+), TTAB, and water, room temperature is too low for the cmc to be reached. Thus, the surfactant monomers prefer a crystalline state and are poorly soluble. Conclusions The CEC diagrams for aqueous solutions of NaHy as a function of TTAB concentration have been shown to depend strongly on the nature of the added inorganic salt. In general, the CEC diagrams show a rapid increase of CEC at the onset of precipitation at low TTAB concentrations; thereafter, a CEC plateau is reached and finally CEC decreases with TTAB concentration at high TTAB concentrations. The phase separation is primarily caused by the electrostatic interaction between oppositely charged surfactant micelles and the polyelectrolyte chains. The mechanism of the phase separation may be understood by using an analogy with the phase separation observed in polyelectrolyte solutions in the presence of multivalent counterions. The CEC of the added inorganic salt needed to suppress the phase separation is then rationalized in terms of electrostatic screening where multivalent counterions generally are more efficent than univalent ones. For ions of equal valency, ion-specific effects are observed. The efficiency of the sodium salts where the counterions to the micelles are varied follow a Hofmeister series where the CECmax decreases in the following order: Cl- < Br< SO42-. Varying the counterions to the hyaluronate follow an opposite Hofmeister series when the counterions have equal charge. In fact, in Figures 2 and 3 the potassium salts are less efficient in suppressing the phase separation as compared to the sodium salts. This contradiction to the Hofmeister series for the cations is perhaps due to the fact that potassium and sodium also influence the micelles as coions. This may indicate that the changes in the micelle conditions are the most important for the NaHy-TTAB interaction. However, an alternative explanation could be based on the unequal distribution of the ionic species between the two prospective phases. At high concentrations of TTAB, the salt providing additional ionic species to the system becomes more effective in suppression of the precipitation. This is manifested in a crossover of CEC lines. Acknowledgment. One referee is gratefully acknowledged for helpful comments. This work was performed as a part of A° .H.B.’s Ph. D. thesis.39 LA980723K