Effect of Polyelectrolyte Counterion Specificity on Dextran Sulfate

The binding isotherms for amitriptyline to DxS with 30 mM LiCl, NaCl, KCl, RbCl, and CsCl, respectively, followed the counterion sequence Li+ > Na+ > ...
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Effect of Polyelectrolyte Counterion Specificity on Dextran Sulfate-Amphiphile Interaction in Water and Aqueous/ Organic Solvent Mixtures Andreas Hugerth* and Lars-Olof Sundelo¨f Physical Pharmaceutical Chemistry, Uppsala University, Uppsala Biomedical Center, Box 574, S-751 23 Uppsala, Sweden Received December 31, 1999. In Final Form: February 24, 2000 The effect of polyelectrolyte counterion specificity on the interaction between dextran sulfate (DxS) with a charge density corresponding to 0.43, 0.7, 1.3, and 1.6 sulfate groups per monosaccharide unit, respectively, and the cationic amphiphile amitriptyline (3-(10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-ylidene)-N,Ndimethyl-1-propanamine) was investigated by means of a dialysis equilibrium technique in water and aqueous/organic solvent mixtures. The binding isotherms for amitriptyline to DxS with 30 mM LiCl, NaCl, KCl, RbCl, and CsCl, respectively, followed the counterion sequence Li+ > Na+ > K+ > Rb+ ≈ Cs+; i.e., at the same free amphiphile concentration, the degree of bound amphiphile decreased according to the order given. This sequence was retained in aqueous methanol as well as aqueous 1-propanol mixtures. Furthermore, on addition of organic solvent, the critical aggregation concentration increased and the cooperativity of the amphiphile aggregation decreased.

Introduction The interaction between polyelectrolytes and oppositely charged amphiphiles, in aqueous solutions, have been extensively studied the past few decades due to both a fundamental and a commercial interest in understanding and regulating a number of industrial and biological processes. The association between a polyelectrolyte and an oppositely charged amphiphile is primarily of electrostatic origin. In the presence of an oppositely charged polyelectrolyte, amphiphiles start to form aggregates with the polymer as the amphiphile concentration exceeds a critical value referred to as the critical aggregation concentration (cac). This is a cooperative process which can be considered analogous to the spontaneous association of amphiphiles in aqueous solution in the absence of polymer,1 which takes place within a narrow concentration interval when the amphiphile concentration exceeds the critical micelle concentration (cmc). For ionic amphiphiles, the cac may be several orders of magnitude smaller than the critical micelle concentration. This can be explained by the well-known fact that in the vicinity of a polyelectrolyte the concentration of both inorganic or organic counterions will be enhanced compared to the bulk phase.2 Thus, the amphiphile concentration will locally be considerable, thereby facilitating hydrophobic interaction between the amphiphiles and subsequent aggregate formation. As aggregates of amphiphiles are formed, the polyelectrolyte will act as a polycounterion to the micelles, thus facilitating the release of polyelectrolyte and micellar counterions. The resulting loss in configurational entropy of the polymer is less than the gain in entropy by the release of the simple polyelectrolyte and micellar counterions. As an increasing number of systems and variables have been investigated, an almost complete picture of the regulating factors have emerged. These factors can roughly * Tel.:+46-18-4714368. Fax: +46-18-4714377. E-mail: Andreas. [email protected]. (1) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 38663870. (2) Manning, G. S. J. Chem. Phys. 1969, 51, 924-933.

be divided according to the characteristics of the (i) surfactant/micelle, (ii) polyelectrolyte, and (iii) solvent. (i) The surfactant influences the polyelectrolyte-amphiphile interaction mainly by its hydrophobicity,3-6 the position and type of charge of the polar part, and the type of surfactant counterions. With respect to micelles, the surface charge density has been found to be a critical parameter.7,8 (ii) Polyelectrolyte characteristics that affect the polyelectrolyte-amphiphile interaction are the polyelectrolyte charge density,9-12 presence and distribution of hydrophobic moieties,13 chain flexibility,3,14 conformation,15 type of charges,16 and valency of the counterion.17 (iii) With respect to the solvent factors such as ionic strength,7 dielectric properties of the medium and selective adsorption of one of the solvent components by the polyelectrolyte in a solvent mixture may exert a significant effect on the interaction. Excellent reviews on the subject are given by Hayakawa and Kwak,18 Lindman and Thalberg,19 and Goddard.20 One fundamental factor of the polyelectrolyte-amphiphile interaction which has gained increased attention (3) Caram-Lelham, N.; Hed, F.; Sundelo¨f, L.-O. Biopolymers 1997, 41, 765-772. (4) Caram-Lelham, N.; Sundelo¨f, L.-O. Pharm. Res. 1996, 13, 920925. (5) Okuzaki, H.; Osada, Y. Macromolecules 1994, 27, 502-506. (6) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1930-1933. (7) Yoshida, K.; Dubin, P. L. Colloids Surf., A 1999, 147, 161-167. (8) McQuigg, D. W.; Kaplan, J. I.; Dubin, P. L. J. Phys. Chem. 1992, 96, 1973-1978. (9) Persson, B.; Hugerth, A.; Caram-Lelham, N.; Sundelo¨f, L.-O. Langmuir 1999, in press. (10) Hansson, P.; Almgren, M. J. Phys. Chem. 1996, 100, 90389046. (11) Wallin, T.; Linse, P. J. Phys. Chem. 1996, 100, 17873-17880. (12) Malovikova, A.; Hayakawa, K.; Kwak, J. C. ACS Symp. Ser. 1984, 253, 225-239. (13) Shimizu, T.; Kwak, J. C. T. Colloids Surf., A. 1994, 82, 163-171. (14) Wallin, T.; Linse, P. Langmuir 1996, 12, 305-314. (15) Caram-Lelham, N.; Sundelo¨f, L.-O. Biopolymers 1996, 39, 387393. (16) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 1669416703. (17) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983, 87, 506509.

10.1021/la9916880 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/09/2000

Effect of Polyelectrolyte Counterion Specificity

Figure 1. Primary structure formulas of amitriptyline.

is the part played by the polyelectrolyte counterions.21,22 Some findings have indicated that, in addition to the valency, the physicochemical characteristics of the monovalent7 or divalent17 polyelectrolyte counterion might also influence the polyelectrolyte-amphiphile interaction. Moreover, most polyelectrolytes, including dextran sulfate (DxS),23,24 display a selectivity in the binding of counterions. The aim of this study was thus primarily to investigate the effect of polyelectrolyte counterion specificity on the polyelectrolyte-amphiphile interaction. The effect of aqueous/organic solvent mixtures on the polyelectrolyte amphiphile interaction was also studied. To this end, a set of sodium dextran sulfate samples was employed that differed mainly with respect to charge density, the alkali ions Li+, Na+, K+, Rb+, and Cs+, and the amphiphilic drug molecule amitriptyline (See Figure 1). Amitriptyline, (3(10,11-dihydro-5H-dibenz[a,d]-cyclohepten-5-ylidene)N,N-dimethyl-1-propanamine), forms micelles with low aggregation number25,26 and is interesting as a model substance for amphiphilic drug molecules which share many of the features exhibited by traditional surfactants. Experimental Section Materials. Dextran sulfate (DxS) samples, sodium form, with a charge density corresponding to 0.43, 0.7, 1.3, and 1.6 sulfate groups per monosaccharide unit, respectively, were kindly provided by Pharmacia & Upjohn Inc. (Uppsala, Sweden), except for the sample with cd 0.43 which was a gift from TdB Consultancy AB (Uppsala, Sweden). The molecular weights of the dextran used for the synthesis of DxS received from Pharmacia & Upjohn Inc. and TdB Consultancy AB were approximately 70 000 and 150 000, respectively. Poly(sodium 4-styrenesulfonate), (PSS), of a Mw of aprox. 70 000 was obtained from Aldrich (lot no. PS03926KQ). Amitriptyline-HCl were purchased from Sigma (St. Louis, MO). All other chemicals were commercially available products of analytical grade. A stock solution of PSS was obtained by dissolving PSS in 0.1 M NaCl followed by filtration (0.8 µm Millex AA) and extensive dialysis against 0.1 M NaCl and double-distilled water. The molecular weight cut off of the dialysis bags used was 12 00014 000. DxS and amphiphile samples were used as received. Since the charge group on the polymers used is sulfate and the pKa of (18) Hayakawa, K.; Kwak, J. C. T. Interactions between Polymers and Cationic Surfactants; Rubingh, D. N., Holland, P. M., Eds; Marcel Dekker: New York, 1991; p 189. (19) Lindman, B.; Thalberg, K. Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ed.; CRC Press: Boca Raton, FL, 1993; Chapter 5, pp 203-268. (20) Goddard, E. D. Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ed.; CRC Press: Boca Raton, FL, 1993; Chapter 4, pp 171-201. (21) Konop, A. J.; Colby, R. H. Langmuir 1999, 15, 58-65. (22) Shimizu, T. Colloids Surf., A. 1994, 84, 239-248. (23) Bare, W.; Nordmeier, E. Polym. J. 1996, 28, 712-726. (24) Beyer, P.; Nordmeier, E. Eur. Polym. J. 1995, 11, 1031-1036. (25) Attwood, D.; Mosquera, V.; Garcia, M.; Suarez, M. J.; Sarmiento, F. J. Colloid Interface Sci. 1995, 175, 201-206. (26) Attwood, D.; Gibson, J. J. Pharm. Pharmacol. 1978, 30, 176180.

Langmuir, Vol. 16, No. 11, 2000 4941 amphiphiles is much higher than the pH of the solutions, which is close to neutrality, both the polyion and the amphiphile are fully charged during the experiments. All solutions were prepared as moles per kilogram solvent. Electrolyte concentrations given in the text are not corrected for contributions from the polyelectrolyte counterions. Methods. Dialysis Equilibrium. The degree of binding of the amphiphile to the polymer was determined by a previously described27 dialysis equilibrium method utilizing specially designed Lucite dialysis cells. All measurements where carried out at 23.0 ( 0.5 °C and lasted some 24 h, which was sufficient to attain dialysis equilibrium. The dialysis membrane was of a regenerated cellulose, purchased from Spectrum (CA), with a molecular cut off 12 000-14 000. In a previous study, it was shown that the adsorption of drug to these membranes was negligible.27 The concentration of amphiphile was determined spectrophotometrically. DxS did not affect the spectral properties of the drug molecules at the measuring wavelengths. The polymer concentration in all sample solutions was 0.5 mg/g.

Results and Discussion I. Effect of Counterion Specificity in Aqueous Solutions. The effect of the polyelectrolyte counterion selectivity on the adsorption isotherms of the amphiphile to DxS with a charge density corresponding to 0.43, 0.7, 1.3, and 1.6 sulfate groups per monosaccharide (cd), respectively, is shown in Figure 2a-c. All adsorption isotherms presented show the degree of binding (β) versus the free amphiphile concentration [Ami+]I. β is defined by the relation,

β ) ([Ami+]II tot - [Ami+]I)/Cp*

(1)

where [Ami+]II tot is the total concentration of amphiphile in the compartment containing polymer and [Ami+]I is the concentration of free amphiphile in the compartment without polymer. Cp* is the number of moles of polymer charges as calculated from the polymer mass concentration, the Mw of the monosaccharide unit, and the charge density. Since the concentration of polymer charge is low compared to the concentration of added salt, the Donnan effect across the dialysis membrane is considered to be practically negligible. By the same argument, the contribution of the counterions of the dissolved polyelectrolyte to the overall effect observed can also be considered as marginal. It is thus assumed that it can be omitted in the following discussion. Figure 2a-d shows the usual characteristics of polyelectrolyte-amphiphile binding isotherms. In the concentration region preceding the cac where the interaction is mainly of electrostatic nature if the polyelectrolyte is hydrophilic, the degree of binding is low. As the amphiphile concentration exceeds the cac, the degree of binding increases rapidly due to the cooperativity of the process. The decrease of the cac and the increase of the degree of cooperativity as the charge density of DxS increases is apparent for all the different types of ions. More interesting to note is that Figure 2a-d clearly shows that the type of polyelectrolyte counterion has a significant influence on the polyelectrolyte-amphiphile interaction. The order of the DxS-amphiphile adsorption isotherms follows the counterion sequence Li+ > Na+ > K+ > Rb+ ≈ Cs+; i.e., at the same free amphiphile concentration, the degree of bound amphiphile decreases in the order given. Alternatively stated, the binding isotherm is shifted to a higher concentration of free amphiphile as the type of polyelectrolyte counterion is changed according to the lyotropic sequence indicated. The sequence found for the (27) Caram-Lelham, N.; Sundelo¨f, L.-O. Int. J. Pharm. 1995, 115, 103-111.

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or Cs+ as counterions. The similarities of these adsorption isotherms can be ascribed to the “structure-breaking” properties of these ions which, in contrast to the “structuremaking” properties of Li+ and Na+, have rather featureless solvation shells. In addition, as the cd goes from 0.7 to 1.3 (Figure 2b,c), the order with respect to the cac is reversed between Rb+ and Cs+. The reversed order also seems to hold for DxS with cd 1.6. A similar reversal is also observed for the mean residence time of water in the solvation shell of alkali ions and the alkali ion mobility at infinite dilution, for Rb+ and Cs+.29 The cooperativity of the aggregation process, as indicated by the steepness of the adsorption isotherms, seems not to be influenced by the type of polyelectrolyte counterion. Intuitively there should be some effect of the type of counterion on the cooperativity since they are released as amphiphilic aggregates are formed. It may be that a more sensitive method could clarify this question. The results presented above infer that the affinity order of DxS for the different counterions and the charge density of the polyelectrolyte will determine the concentration of inorganic counterions and amphiphilic counterions in the vicinity of the polyelectrolyte, especially in the concentration interval preceding the cac. Thus, the cac and the adsorption isotherms reflect the competition between the different alkali ions relative to the amphiphilic counterions for binding to the polyelectrolyte. In analogy to the relation between the concentration of a divalent and a monovalent ion30 at a charged surface, the present case can in a simplified way be illustrated by the relation between, e.g., sodium and amitriptyline

CNa(0) CNa(bulk)

Figure 2. Binding isotherms of amitriptyline with DxS with cd (a) 0.43, (b) 0.70, (c) 1.3, and (d) 1.6, respectively, in 30 mM LiCl (3), NaCl (b) (data from Persson et al.),9 KCl (+), RbCl (0), or CsCl (9).

amphiphile adsorption isotherms correlates well with the mean activity coefficients of alkali ions in an aqueous DxS solutions which increase in the order γCs+ < γK+ < γNa+ < γLi+.23,28 Differences in the hydration (the hydration of the alkali ions decrease in the order Li+ > Na+ > K+ > Rb+ > Cs+) of the alkali ions at higher concentrations has been suggested23 to account for the observed variation in activity coefficients in the DxS case. Hence, the sequence of the adsorption isotherms can to some extent be explained by considering the nature of the hydration of the alkali ions. Further support of the influence of the hydration characteristics of the alkali ions is given by the similar features of the binding isotherms with K+, Rb+, (28) Joshi, Y. M.; Kwak, C. T. Biophys. Chem. 1978, 8, 191-201.

)

CAmi(0) CAmi(bulk)

(

exp

)

-∆(zie)effΦ0 kT

(2)

where Ci(0) is the surface concentration of an ion i, ∆(zie) is the difference in effective operative charge between the two (in this case monovalent) species, Φ0 the electrostatic surface potential at the surface, k is Boltzman’s constant, and T is the temperature. ∆(zie) expresses, in a simplified form, the degree of difference between different ions as to their charge distribution. The concentration ratio is thus critically dependent on the polyelectrolyte surface potential and subsequently on the polyelectrolyte charge density (Figure 2a-c). Hence, there are similarities between the present case and the competitive binding of inorganic/ organic counterions to, e.g., a charged surface30 or proteins.31 The change in Gibbs free energy of binding per mole amphiphile (∆Gbc°) in changing the polyelectrolyte counterion from Cs+ to Li+ can be estimated, to a first approximation, by the relation ∆Gbc° ) RT ln(cacLi+/cacCs+). The gains in free energy in the case of DxS with cd 0.70, 1.3, and 1.6 are 1.7, 2.0, and 2.1 kJ/mol amphiphile, respectively. This is equivalent to 0.68, 0.82, and 0.86kT per amphiphilic monomer. Thus, there seems to be a slight increase in the free energy gain as the polyelectrolyte charge density increases. Furthermore, the gain in free energy in changing the type of polyelectrolyte counterion can be compared with the free-energy change of 1.3kT as the hydrocarbon chain length is changed by one -CH2 group in the aggregation of pyridinium cations with DxS (ds 2.8).6 (29) Koneshan, S.; Rasaiah, J. C.; Lynden-Bell, R. M.; Lee, S. H. J. Phys. Chem., B 1998, 102, 4193-4204. (30) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain: Where Physics, Chemistry, Biology and Technology Meet; VCH Publichers: New York, 1994. (31) Tanford, C. Physical Chemistry of Macromolecules; John Wiley & Sons: New York, 1967.

Effect of Polyelectrolyte Counterion Specificity

Figure 3. (a) Binding isotherms of amitriptyline with PSS in 30 mM LiCl (3), NaCl (b), KCl (+), RbCl (0), and CsCl (9), respectively. (b) Binding isotherms of amitriptyline with PSS with 30 mM NaCl in water (b) and in 10% w/w methanol (0).

The generality of the effect of type of polyelectrolyte counterion on the polyion-amphiphile aggregation process is supported by the binding isotherms for the sodium polysterenesulfonate (PSS)-amitriptyline system presented in Figure 3. The onset of cooperativity starts at a lower concentration then that in the DxS case. This is probably due to an increased hydrophobic interaction effect originating in the polyelectrolyte and the hydrophobic part of the amphiphile.1,32,33 Despite this difference in hydrophobicity between the PSS and DxS systems, Figure 3 shows that the amphiphile adsorption isotherms for the PSS case follow the same order, although less clear-cut, as the DxS-amitriptyline adsorption isotherms. This is not surprising considering that the PSS-counterion system shows the same order of mean activity coefficients as that found for DxS.34 Furthermore, the same order of affinity, with respect to the counterion, has been found in buffered (pH 4-6) micellar solutions of lauryl sulfate and decyl phosphate.35 It should be noted though, that the reversed ion sequence has been found, for example, for dextran phosphate23 in an unbuffered solution. Hence, the type, valency, and structure of the polyion charge exert decisive influences on the interaction between the polyelectrolyte and the counterion. In addition, possible interaction between the alkali ions and the noncharged parts of the polymer as well as the amphiphile should be considered since they may have significant influence on the polyion-counterion interaction and subsequently on the solution behavior of the polyion, as may be the case for κ-carrageenan.36 II. Effect of Counterion Specificity and Organic Solvent Content in Aqueous/Organic Solvent Mixtures. The general effect on the adsorption isotherms of (32) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115-2124. (33) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405-2412. (34) Kwak, J. C. T.; Hayes, R. C. J. Phys. Chem. 1975, 79, 265-269. (35) He, Z.-M.; O’Connor, P. J.; Romsted, L. S. J. Phys. Chem. 1989, 93, 4219-4226. (36) Nilsson, S.; Piculell, L. Macromolecules 1991, 24, 3804-3811.

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Figure 4. Effect of methanol 0 (×), 10 (0), 20 (O), and 25% w/w (4) and 1-propanol 0 (×), 6.6 (9), 13.5 (b), and 15.7% w/w (+), respectively, on the binding isotherms of amitriptyline with DxS with cd (a) 0.7 and (b) 1.3 in 30 mM of NaCl. Filled and unfilled squares (r(25 °C) ) 74.1) and ditto circles (r(25 °C) ) 69.2) indicate equivalent macroscopic dielectric constant,45 respectively, and (+) designates the equivalent to 23% w/w methanol (r(25 °C) ) 67.6).

adding an organic solvent is shown for DxS with cd 0.7 and 1.3 in 30 mM NaCl in parts a and b, respectively, of Figure 4. The addition of methanol or 1-propanol resulted in an increase of the cac and a decrease in the cooperativity of the aggregation process. The same trend was also observed in the PSS case (Figure 3b). The effects of adding an organic solvent to a system such as the present one are complex and by no means straightforward to elucidate. However, the major aspects to be considered are the effect of the organic solvent component on the micellization process of the amphiphile, the solvation of the inorganic counterions, and the solution characteristics of the polyelectrolyte. A change in the cmc is, as a rule, reflected in the cac. Therefore, the cmc of amitriptyline in 30 mM NaCl with 10% w/w methanol or 6.6% w/w 1-propanol, which provides equivalent macroscopic dielectric constants, was determined by means of a dye solubilization method.4 In the methanol case, the cmc of amitriptyline was 39 mM as compared to 31 mM with no added methanol. An alcohol such as methanol is predominantly distributed to the aqueous phase and probably affects the cmc mainly by decreasing the dielectric constant of the medium.37 The addition of methanol to the DxS-amitriptyline system could thus be anticipated to increase the cac. 1-Propanol, on the other hand, produced a slight decrease of the cmc (27 mM), it is most likely due to the fact that 1-propanol participates to some extent37 in the micellization process. Addition of 1-propanol should therefore effect the cac in the opposite direction to methanol. However, the addition of methanol/1-propanol to a solution containing a weakly basic or acidic amphiphile inherently will affect the solution properties of the amphiphile by changing its pKa. In the present case, the effect of the organic solvent content (37) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1-64.

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Figure 5. Effect of methanol on the cac for amitriptyline with DxS with cd 1.3 in 30 mM LiCl (2), NaCl (b) and KCl (9), respectively. (The slope values are given in graph).

on the pKa of the amphiphile38,39 should not significantly affect the degree of dissociation of the amphiphile and hence have but minor influence on the micellization process. Thus, since the addition of methanol as well as 1-propanol both result in an increase of the cac, the results cannot be ascribed simply to the effect of methanol/1propanol on the micellization of amitriptyline. However, it should be kept in mind that addition of alcohol to a micellar solution is likely to produce a decrease of the apparent surface charge of the micelles.37 The effect on the solvation of the inorganic counterions of adding an organic solvent to the present system was studied especially for the methanol case. Figure 5 shows the effect of added methanol on the polyelectrolyteamphiphile interaction in the DxS cd 1.3 case with 30 mM LiCl, NaCl, or KCl. Increasing the fraction of methanol affected the aggregation process in increasing order for Li+ < Na+ < K+; i.e., the effect was largest in the case of K+ as the polyelectrolyte counterion. The gain in Gibbs free energy of binding per mole amphiphile obtained by replacing K+ with Li+ as counterion increased from 1.9 kJ mol-1 in water to 2.8 kJ mol-1 in 20% w/w methanol. These data correlate well with the fact that the relative change of the radius of solvation of the alkali ions is affected in the same order and that ion association usually increases with increasing size of the alkali ion.40,41 Furthermore, it is well-known that decreasing the solvation of an alkali ion increases the ion association by decreasing the contact distance between the ions or by facilitating extrusion of ion-separating solvent molecules.40 Thus, bearing in mind that the cac reflects the competition between different counterions for binding to the polyelectrolyte, the results presented in Figure 5 can be interpreted in terms of differences in the ion association between the alkali ions and -OSO3-. With respect to the general effect, the significance of the solvation of the alkali ions and the -OSO3- resides in that it is the interaction between the alkali ions and the -OSO3- that primarily determines the effective charge density of the polyelectrolyte and thus the DxS-amphiphile interaction. A decrease of the dielectric constant of the medium can be expected to increase the ion association42 and thus decrease the effective charge density of the polyelectrolyte43 as experienced by the amphiphile. (38) Thoma, v. K.; Albert, K. Pharm. Acta Helv. 1980, 55, 8-12. (39) Gelsema, W. J.; De Ligny, C. L.; Remijnse, A. G.; Blijleven, H. A. Rec. Trav. Chim. Pays-Bas 1966, 85, 647-660. (40) Gordon, J. E. The Organic Chemistry of Electrolyte Solutions; John Wiley & Sons: New York, 1975. (41) Covington, A. K.; Dickinson, T. Physical Chemistry of Organic Solvent Systems; Plenum Press: London, 1973. (42) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Butterworth Scientific Publications: London, 1959. (43) Fuoss, R. M.; Strauss, U. P. J. Polym. Sci. 1948, 3, 246-263.

Hugerth and Sundelo¨ f

On addition of organic solvent, the amphiphile adsorption isotherms (Figure 4a,b) show features which are very similar to the effect of a decrease of the charge density of the polyelectrolyte with water as solvent. Moreover, findings of Bare and Nordmeier23 concerning the binding of inorganic divalent ions to DxS in aqueous/organic solvent mixtures exhibit the same trend as in the present system. Hence, the decrease of the binding isotherms most likely reflects a general phenomenon rather than features characteristic of a specific system. Although the general characteristics of the effect of added organic solvent on the amphiphile adsorption isotherms can be interpreted by representing the solvent medium by a single uniform dielectric continuum, this is an oversimplified description of the physical reality. The polyelectrolyte characteristics determine not only the “macroscopic” overall distribution of solvent molecules (in a mixed solvent) between the bulk solution and the volume segment containing the polymer but also the “microscopic” distribution close to the polymer chain. Thus, the value of the dielectric constant will only have a meaning on the local scale. To the best of our knowledge, current polyelectrolyte theory does not account properly for either a macro- or microscopic distribution of dielectric properties. In addition, the distribution of solvent molecules will also influence the distribution of counterions and thus further affect the solvation characteristics of the alkali and -OSO3- ions. The difference between adsorption isotherms in methanol and 1-propanol, when added in proportions providing equivalent macroscopic dielectric constants of the media (Figure 4a,b), may well illustrate the influence of the individual characteristics of each chemical species on the polymer-solvent interaction. This interpretation is in agreement with theoretical calculations performed by Nyquist et al.44 which predict an increase in counterion condensation and a decrease of the effective charge density of the polyelectrolyte as the quality of the solvent is decreased, keeping the dielectric constant fixed. Conclusion The results show that the type of polyelectrolyte counterion present influences the polyelectrolyte-amphiphile interaction in water as well as in aqueous/organic solvent mixtures. The order of the DxS-amphiphile adsorption isotherms followed the counterion sequence Li+ > Na+ > K+ > Rb+ ≈ Cs+; i.e., at the same free amphiphile concentration, the degree of bound amphiphile decreases in the order given. The amphiphile adsorption isotherms indicated a close correlation between the ion specificity of the polyelectrolyte as indicated by mean activity coefficients, the solvation characteristics of the alkali ions, and the effect of the different counterions on the polyelectrolyte amphiphile interaction. Addition of organic solvent resulted in an increase of the cac and a decrease of the cooperativity of the aggregation process. The amphiphile adsorption isotherms in aqueous/organic solvent mixtures reflect the decrease of the effective polyelectrolyte charge density as the dielectric constant decreases. The gain in Gibbs free energy of binding (∆Gbc°) when exchanging the polyelectrolyte counterion Cs+ with Li+, estimated by ∆Gbc° ) RT ln(cacLi+/cacCs+), was for DxS with cd 0.70, 1.3, and 1.6 with water as solvent 1.7, 2.0, and 2.1 kJ per mole amphiphile, respectively. (44) Nyquist, R. M.; Ha, B.-Y.; Liu, A. J. Macromolecules 1999, 32, 3481-3487. (45) Åkerlo¨f, G. J. Phys. Chem. 1932, 54, 4125-4137.

Effect of Polyelectrolyte Counterion Specificity

Furthermore, the order of the polyelectrolyte counterion specificity for DxS/PSS-amphiphile adsorption isotherms correlates well with that found, for example, for lauryl sulfate micelles and alkali ions.35 Both cases simply reflect the interaction between a sulfate group and an alkali ion, which is critically dependent on ion solvation, ionic strength, and dielectric constant of the medium.

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Acknowledgment. Financial support from the Swedish Natural Science Research Council (NFR) and the Swedish Council for Engineering Sciences (TFR) is gratefully acknowledged. We also thank Dr. Per Beronius for valuable discussions. LA9916880