Langmuir 1994,10,2115-2124
2115
Interaction of Alkyltrimethylammonium Surfactants with Polyacrylate and Poly(styrenesulfonate) in Aqueous Solution: Phase Behavior and Surfactant Aggregation Numbers Per Hansson* and Mats Almgren Department of Physical Chemistry, Uppsala University, P.O. Box 532, S-75121Uppsala, Sweden Received September 10, 1993. In Final Form: March 2, 1994@ The interactions between polyacrylate and cationic surfactants has been studied. Aggregation numbers of dodecyltrimethylammonium (DoTA+)micelles formed in very dilute aqueous solutions of polyacrylate
have been estimated with time-resolved fluorescence quenching, using as quencher dodecylpyridinium ion, which is distributed similar to DoTA+between micelle and water subphases. The aggregation numbers (=65)were found to be the same as in 50 mM dodecyltrimethylammonium bromide (DoTAB) solutions. The distribution of the quencher between micelles and water was also investigated. It was concluded that the quencher and the surfactant mixed ideally in the micelles. The effect of salt on the phase behavior in aqueous solutions of polyacrylatetogether with DoTAB, dodecyltrimethylammonium chloride (DoTAC), cetyltrimethylammonium bromide (CTAB), or cetyltrimethylammonium chloride (CTAC) has been investigated. The concentrationof surfactant and the aggregation number in both of the coexisting phases in the two-phase region of the phase diagram were estimated. The aggregation numbers (about the same in dilute and concentrated phases) for DoTAB, DoTAC, and CTAC, were approximately 80, 70, and 150, respectively. CTAB formed rodlike micelles in all investigated phases. We found that both the choice of salt and the size of the micelles were important for the extension of the two-phase region. Sodium polyacrylate-CTAB showed a segregativephase behavior at high concentrationsof salt. The phase behavior ofthe system sodium poly(styrenesulfonate)-DoTAB-water was also investigated. The system was found to behave differently in many respects from the system with polyacrylate and the same surfactant. The criticalaggregation concentration in dilute solutions of PSS was estimated from surfactant selectiveelectrode measurements and was found to increase with the concentration of the polyelectrolyte. Phase separation started at the same PSS-DoTAB ratio ( ~ 1 in ) both dilute and concentrated solutions:
Introduction The interactions between polyelectrolytes and ionic surfactants of opposite charge have been studied extensively for manyyears.'-1° The two main areas investigated are the phase behavior and the binding of surfactant to the polyion chains. Two reviews dealing with the phase Abstract Dublished in Advance ACS Abstracts, J u n e 1. 1994. (1)Goddard, E.D. Colloids Surf. 1986,19, 301. (2)Abuin, E.;Scaiano, J. B. J . Am. Chem. SOC.1984,106,6274. (3)Chu, D.; Thomas, J. K. J . Am. Chem. SOC.1986,108,6270. (4)(a)Dubin, P.L.; Th6, S. S.; McQuigg, D. W.; Chaw, C. H.; Gan, L. M. Langmuir 1989,5,89. (b) Dubin, P. L.; Curran, M. E.; Hua, J. Langmuir ISSO,6,707. (c) Dubin, P.L.; Vea, M. E. Y.; Fallon, M. A.; Th6, S. S.; Rigsbee, D. R.; Gan, L. M. Langmuir 1990,6,1422. (5)Satake, I.; Yang, J. T. Biopolymers 1976,15,2263. (6)(a) Hayakawa, K.; Kwak, J. J . Phys. Chem. 1982,86,3866. (b) Hayakawa, K.; Kwak, J. J . Phys. Chem. 1983,87,506.(c) Hayakawa, K.; Santerre, J. P.; Kwak, J. Macromolecules 1983, 16, 1642. (d) Malovikova, A.; Hayakawa, K.; Kwak, J.J . Phys. Chem. 1984,88,1930. (e) Santerre, J.P.; Hayakawa, K.; Kwak, J. Colloids Surf. 1985,13,35. (0 Hayakawa, K.; Ohta, J.;Maeda, T.; Satake, I.; Kwak, J. Langmuir 1987,3,377.(g)Gao, Z.; Kwak, J.;Wasylishen, R. E. J. Colloid Interface Sci. 1988,126, 371. (h) Hayakawa, K.; Satake, I.; Kwak, J.; Gao, Z. Colloids Surf. 1990,50, 309. (i) Gao, Z.;Wasylishen, R. E.; Kwak, J. J . Phys. Chem. 1990,94,773. (7)Skejanc, S.;Kogej, K.; Vesnaver, G. J . Phys. Chem. 1988,92, 6382. (8)(a)Thalberg, K.; Lindman, B.; Karlstrijm, G. J . Phys. Chem. 1990, 94,4289.(b) Thalberg, K.; Lindman, B.; Karlstrom, G. J . Phys. Chem. 1991,95,3370. (c) Thalberg, K.; Lindman, B.; Karlstrom, G. Prog. Colloid Polym. Sci. 1991, 84, 8. (d) Thalberg, K.; Lindman, B.; Karlstrom, G. J . Phys. Chem. 1991,95,6004.(e)Thalberg, K.;Lindman, B. Langmuir 1991,7,277.(0 Thalberg, K.;Lindman, B.; Bergfelt, K. Langmuir 1991,7,2893. (9)Wong, T. C.; Thalberg, IC;Lindman, B.; Gracz, H. J . Phys. Chem. 1991,95,8850. (10)Thalberg, K.;van Stam, J.;Lindblad, C.; Almgren, M.; Lindman, B. J . Phys. Chem. 1991,95,8975. @
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behavior have appeared recently.11J2 One of them draws the attention to the similarities between polymersurfactant systems on the one hand and systems of two different polymers on the other. Karlstrom et al.13showed that the phase behavior in nonionic polymer-surfactant systems can be well described in the framework of the Flory-Huggins theory14 if the surfactant is treated as a polymeric component. It has been shown in a series of papers by Thalberg et a1.8 that the introduction of surfactant into a polyelectrolyte solution, or vice versa, induces phase separation already a t low concentrations, as would also be expected for systems of two different polymers or polyelectrolytes in a common s01vent.l~By assigning appropriate polymerization numbers for the surfactant component and values of the pair-interaction parameters between polyelectrolyte, solvent, and micelles, they were able to theoretically produce qualitatively correct phase diagrams with a similar Flory-Huggins type model as was used in the nonionic case. The use of surfactant selective electrodes,16-21replacing methods such as dialysis, have made the binding studies (11)Piculell, L.; Lindman, B. Adu. Colloid Interface Sci. 1992,41, 149. (12)Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993;Chapter 5. (13)Karlstrom, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94,5005. (14)Flory, P. J. J . Chem. Phys. 1942,10,51. (15)Scott, R. L. J . Chem. Phys. 1948,17,279. (16)Gavach, C.; Bertrand, C. Anal. Chim. Acta 1971,55,385. (17)Birch, B. J.;Clarke, D. E. Anal. Chim. Acta 1973,67,387. (18)Cutler, S. G.; Meares, P.;Hall, D. G. J . Electroanal. Chem. 1977, 85,145. (19)Shirahama, K.; Yuasa, H.; Sugimoto, S. Bull. Chem. SOC.Jpn. 1981,54, 375. (20)Maeda, T.; Ikeda, M.; Shibahara, M.; Haruta, T.; Satake, I. Bull. Chem. SOC.Jpn. 1981,54,94.
0743-746319412410-2115$04.50/00 1994 American Chemical Society
2116 Langmuir, Vol. 10, No. 7, 1994
easier and more reliable. Kwak and co-workers6a-ehave published a large number of binding isotherms for the binding of cationic surfactants to both synthetic and naturally occurring linear anionic polymers in dilute aqueous solutions. Their results clearly show the cooperativity in the binding process and the sensitivity of the critical aggregation concentration (cac, the total concentration of surfactant a t the onset of cooperative binding) to the addition of simple salt. Very important, however, is the specific nature of the polymer backbone. For instance, the difference between sodium polybtyrenesulfonate) (PSS)and sodium polyacrylate (PA)on binding dodecyltrimethylammonium ions is striking. In the presence of 0.01 m sodium chloride, cooperative binding starts a t about 10 times lower surfactant concentration in the PSS case, although both have the same linear charge separation. In both cases, however, a large amount of surfactant may bind to each chain. More than 50% of the polymeric charges can be neutralized before precipitation occurs, but a s shown by Thalberg et a1.8fthis is possible for PA only in very dilute systems. At higher concentrations, phase separation almost immediately takes place when the surfactant concentrations exceeds the cac. The phase behavior of PSS-surfactant mixtures has not been studied explicitly. Nevertheless, various properties have been i n v e ~ t i g a t e dat~PSS ~ ~ ~concentrations ~ ~ , ~ ~ ~ ~ ~(as monomers) between 0.5 and 300 mM together with surfactant (dodecyltrimethylammonium bromide (DoTAB) or cetyltrimethylammonium bromide (CTAB) a t concentrations corresponding to half of the PSS concentration. The system remained a one-phase system even though almost all surfactant could be considered as bound t o the polyelectrolyte. Early investigators5pointed out the similarities between the binding of surfactant ions to polypeptides and the formation of micelles in pure surfactant solutions. A sparsely investigated3J0jz2but very important property for the understanding of both the binding and the phase behavior is the size of the surfactant aggregates formed. In an earlier studyz2we reported surfactant aggregation numbers for the PSS-DoTAB system as estimated by time-resolved fluorescence quenching measurements. Here we will investigate the molecular structure and the phase behavior of PA-surfactant aggregates and compare it with the behavior of the PSS-DoTAB system.
Experimental Section Chemicals. Sodium polyacrylate (PA) from Fluka ( M , = 170000) was dried and kept in a desiccator. Sodium poly(styrenesulfate) (PSS), prepared from polystyrene (Pressure Chemical Corp., M, = 90 000) was a gift from Professor Hans Vink.24 Dodecyltrimethylammonium bromide (DoTAB) from Tokyo Casei, Inc., cetyltrimethylammonium bromide (CTAB) from Aldrich (cryst. pure), and dodecyltrimethylammoniumchloride (DoTAC) from Eastman were all used without further purification. Cetyltrimethylammonium chloride was prepared by ion-exchange from CTAB on a Dowex 1-X8resin. The product was freeze-driedand stored in a desiccator. N-Dodecylpyridinium chloride (DoPC) from Aldrich was recrystallized several times from acetone. N-Cetylpyridinium chloride (CPC) and sodium chloride (pa grade), both from Merck, and sodium bromide (analytic grade) from Bakers, were used as supplied. Pyrene and dimethylbenzophenone (DMBP) from Aldrich were recrystallized twice from ethanol. (21) Hayakawa, K.; Kwak, J. In Cationic Surfactants: Physical Chemistry. Surfactant Science Series; Rubingh, D., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; Vol. 37, Chapter 5. (22) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (23) Hansson, P. Unpublished results. (24) Vink, H. Mucromol. Chem. 1981,182, 279.
Hansson and Almgren Phase Diagrams. The concentration of surfactant in coexisting phases was determined spectroscopicallyusing DoPC and CPC as probes for the distribution of DoTABlC and CTABIC, respectively between phases. For DoTABlC and CTAC an aqueous stock solution (inthe one-phase region) of PA, surfactant, and NaCl5r was diluted in a test tube with an aqueous solution of DoPC (or CPC). Adding different amounts of solid NaCYBr resulted in a series of samples containing equimolar amounts of PA and surfactant (0.150 m monomers) and simple salt in the range from 0.15 to 0.50 m. The concentration of DoPC and CPC was kept low, about 100 times smaller than the main surfactant Concentration. In the same way a correspondingseries of samples was prepared without DoPC. All samples were slowly rotated end over end for at least 2 days, centrifuged at 3500 rpm for 10 min, and then kept at rest for a month. The steps following the mixing procedure were performed at 25 "C in an air thermostat. In the CTAB case the surfactant, PA, and NaBr were all added as solids,before the addition ofwater or the aqueous CPC solution. When analyzing a sample the amount of dilute phase was estimated by weighing. Its density was determined by weighing several portions with known volumes from a calibrated micropipet. Before the absorbance of the dilute phase was measured, it was mixed with an equal volume of 1 M NaCYBr solution to prevent further phase separation (the samples with coexisting phases were very sensitive to temperature changes) which would cause scattering of light during the measurement. For each sample the corresponding solution without DoPC (or CPC) was used as reference. Absorbances were measured at 262 nm on a Varian Cary 2400 spectrophotometer. By use of molar extinction coefficients, estimated from calibration solutions of DoPC in 25 mM DoTAB and CPC in 11mM CTAB, the concentration ofDoPC or CPC in the dilute phase ofeach sample was calculated. It was then easy to calculate the distribution of these species between the two phases. Since, as will be shown later, DoTA+and CTA+ is distributed in the same way as DoP+ and CP+, respectively, the weight percentage of the former in each phase could be calculated. The phase diagram for the PSS-DoTAB system was determined in the corresponding way. In this case the concentration ofPSS in the phases could also be estimated. In samples without DoPC the absorbance from the styrenesulfonate group was measured at 262 nm. Time-&solved Fluorescence Quenching. The fluorescence decay data were collected with the single photon counting technique. A detailed description of the experimental technique and equipment used is given elsewhere.22 All measurements were performed at 25 "C in equilibrium with air. For the phase separating systems the same preparations were used as for the absorption measurements, simply by mixing them with the fluorescenceprobe (pyrene). Thus, DoPC and CPC were used as quenchers in this case. For details about the distribution of probe and quenchers see below. Steady-StateFluorescence Measurements. Pyrene fluorescence spectra were recorded on a SPEX Fluorolog 1680 combiend with SPEX DM3000 software. Surface Tension Measurements. The cmc values for DoTAB and DoPB in 0.50 m aqueous solutions of NaBr were determined by surface tension measurements using the dropweight method. An apparatus built at the department's work shop was used. Electrode Measurements. The cac values for DoTAB in aqueous solutions of PSS were determined with a surfactant selective electrode at 25 "C. The electrode and the setup have been described elsewhere.22 In principle, the differencein activity of the surfactant between a reference solution and a test solution positioned on each side of a plastic membrane, containing a charge-carrier complex, gives rise to a measurable potential difference over the membrane.
Results and Discussion The Distribution of DoPB between Micelle and Water Subphases. The difficulties encountered when employing photochemical techniques to investigate hydrophobic domains in very dilute surfactant systems were briefly discussed by Abuin and Scaiano.z Time-resolved fluorescence quenching is a very useful technique for
Interaction between Polyacrylate and Surfactants estimating aggregation numbers of surfactant micelles. The rapid quenching of a fluorescent probe confined to the same micelle as a quencher molecule gives rise to a The result is shown in Figure 1,where the mole fraction characteristic fluorescence decay. From fittings to the ofDoPB in micelles, as calculated from eq 2 with (a) equal Infelta-Tachiya m0de1,2~-~' the average number of quenchto 86 and 90, is plotted as a function of the total surfactant ers per micelle can be estimated. The average aggregation concentration. The solid line is calculated from ideal number can then be calculated if the concentration of mixing theory using the cmc values given above and eqs quencher and surfactant in the micellar subphase is 9,12, and 13 in ref 32. The values calculated from (a)= known. The traditional way is to use probes and quenchers 86 agree well with what is expected from ideal mixing. that will distribute themselves almost exclusively in the The use of (a) = 90, on the other hand, would give mole hydrophobic domains. Nevertheless, reducing the hyfractions lower than 0.01 a t surfactant concentrations drophobic volume will favor partitioning into the water m. Since DoPB has a lower cmc higher than 10 x phase and, consequently, it becomes important to know than DoTAB, this is not realistic. However, due to the the distribution coefficient for quencher (and probe) polydispersity of micellar solutions, the aggregation between micelles and water. Methods were both micellar numbers as determined from time-resolved fluorescence and water phases are saturated with quencher and quenching is dependent on the ratio between quencher analyzed spectroscopically have been used to get the and surfactant in micelles.33 This is usually expressed as distribution coefficient.28These methods can sometimes a series expansion34 introduce artifacts to the system or be impossible to use.29 We have taken a different approach by using a quencher (a), = (a), - 1/2B~g,mic/~s,mic + 1/6K(CB,mic/c,,mic)2that is itself a surfactant. DoP+ ions are well-known quenchers of pyrene fluorescence and the critical micelle (3) concentration (cmc) values for the bromide and chloride salts are close to the cmc's for DoTAB and DoTAC, where (a), is the measured average at a given quencher respectively. Since deviations from ideal mixing are concentration and (a), is the weight average aggregation expected to be small in mixed cationic one can number as obtained by extrapolating to zero quencher expect DoPB to be distributed almost like DoTAB between concentration. C Q , and ~ ~ cs,fic ~ are the concentrations of micelles and water. By assuming ideal mixing of the two quencher and surfactant in micelles, respectively, and u surfactants in micelles, the mole fraction of DoPB in is the variance. Since aggregation numbers obtained from micelles, XD,,PB,~~~, can be calculated from the cmc values light-scattering would correspond to (a),, the value 90 for the two surfactants. To check this experimentally, we can still be in accordance with our results: Putting (a), took advantage of a study of Anacker et al.31on DoTAB = 86 and (a), = 90 into eq 3 and neglecting higher order micelles in the presence of 0.50 M NaBr. From lightterms gives u = 28 in this case. A value of u = 29 was scattering measurements they reported a n aggregation reported from quenching studies of CTAC micelles ((a), number of 90 for the DoTAB micelles, although in the = same paper they also reported a value of 86 from Due to the difference in the cmc values, the micelles experiments a t 0.51 M NaBr. Important here is that a t become enriched in DoP+ when the concentration of this high ionic strength the average aggregation number, surfactant is low. But, on the other hand, if the main part a t least close to the cmc, can be considered as independent of the surfactant is present in micelles, X D ~ P Bwill , ~ ,in~ of the surfactant concentration. From time-resolved practice be the same a s the mole fraction counted on the fluorescence quenching measurements we estimated the total amounts of surfactant. In the polyelectrolyte average number of quenchers (DoPB) per micelle, (n), for systems, the enrichment of DoPB in the micellar aga number of DoTAB concentrations in the range from 1.5 gregates and its depletion in the surrounding solution x to 10 x m, keeping the DoP+/DoTA+ratio will depend on the local concentration of surfactant close fixed a t 0.01 and the NaBr concentration a t 0.5 m. The to the polyelectrolyte coils. This local concentration was cmc for DoTAB in the presence of 0.50 m NaBr was high in our experiments, even when the total concentration of surfactant was very low, and we have assumed, estimated by surface tension measurements to 1.51 x m. The corresponding value for DoPB was 0.93 x m. therefore, that is best given by the mole fraction The concentration of DoPB in micelles, C D ~ P Bwas , ~ ~ ~ , DoPB of the total surfactant. Because of its long fluorescence lifetime, pyrene is often calculated from used as probe. The distribution constant for pyrene between micelles and water is also very favorable, making the contribution to the fluorescence intensity from pyrene in the water unimportant even a t very low micelle where C D ~ T B is the total concentration of DoTAB and (a) concentrations. Even a t such a low concentration as 0.25 is the aggregation number. The mole fraction of DoPB in mM DoTAB in micelles, about 95% of the pyrene molecules micelles can be expressed as should be in micelles according to the distribution results by Almgren et (25)Infelta, P.P.;Gratzel, M.; Thomas, J. K. J. Phys. Chem. 1974, Aggregation Numbers in Dilute Solutions of PA 78,190. and DoTAB/C. Isotherms for the binding ofDoTA+,both (26) (a) Tachiya, M. Chem. Phys. Lett. 1976,33,289. (b)Tachiya, in the presence of 0.010 m NaCl and without salt, to 0.5 M. J.Chem. Phys. 1982,76,340.( c ) Tachiya, M.J.Chem. Phys. 1973, x m (residue) PA were published by Kwak et a1.&We 78,5282. (27)Almgren, M.;Lofroth, J.-E.; van Stam, J. J.Phys. Chem. 1986, used the method described above with DoP+as a quencher 90, 4431. of pyrene fluorescence to estimate the aggregation number (28)Almgren, M.;Grieser, F.;Thomas, J. K. J.Am. Chem. SOC.1979, 101,2021. (29)Alsins, Jan, private communication. (30)(a) Scamehom, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982,85,479.(b)Holland, P.M.; Rubingh, D. N. J.Phys. Chem. 1983,87,1984. (31)Anacker, E. W.;Rush, R. M.; Johnson, J. S.J.Phys. Chem. 1964, 68,81.
(32)Clint, J. H. J. Chem. SOC.,Faraday Trans. 1 1975,71, 1327. (33)Warr, G. G.; Grieser, F. J.Chem. Soc., Faraday Trans. 1 1986, 82,1813. (34)Almgren, M.; Lofroth, J.-E. J. Chem. Phys. 1982,76, 2734. (35)Almgren, M.; Alsins, J.;van Stam, J.; Mukhtar, E. Prog. Colloid Polym. Sci. 1988,76,68.
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2118 Langmuir, Vol. 10, No. 7, 1994
75 70
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Figure 1. The mole fraction of DoPB in mixed micelles of DoTAE and DoPB, X D ~ P Bgiven , ~ ~ as ~ , a function of the total concentrationof surfactant in micelles, csurfpetant,mic.Total mole fractionof DoPB is 0.0100. Symbols: calculated from eq 2 using experimental (n)values and (a)= 86 (filled)and(a) = 90 (open). Line: calculated from ideal mixing theory (see text).
Figure 3. Results from time-resolvedfluorescence quenching measurements on the same system as in Figure 2 at different quencher concentrations. Aggregation numbers,(a),,are given as a function of molefraction of quencher (DoP+)in micelles, X D ~ P BThe , ~ ~line ~ . represents a least-squares fit to the data.
are compared with values for related systems, indicate that the aggregation numbers in the PA systems are not affected by the presence of salt. In Figure 3, the aggregation numbers in the presence of 0.010m NaCl are given for different quencher to surfactant ratios. The mean aggregation number is 65, and, according to eq 3 the results indicate a narrow distribution. In the presence of salt the concentration of free surfactant is larger than that bound in micelles. DoP+ may have been somewhat enriched in the micelles, therefore, which would imply that the aggregation numbers where somewhat overestimated. However, since the addition of salt would not be expected to decrease the aggregation numbers, the effect is probably small. In contrast to what happens in pure surfactant solutions, 0 200 400 600 800 where DoTAB forms larger micelles than DoTAC, the Time I nr replacement of NaCl by NaBr does not affect the agFigure 2. Pyrene fluorescence decays from time-resolved gregation number. We assume that the binding isotherm fluorescence quenching experiments using the single photon is the same in the presence of NaBr as with NaC1; ions counting technique, with DoP+ as a quencher, in 0.010 m like bromide or chloride are not expected to be directly m DoTAB (0.27 x solutionsofNaCl containing0.64 x involved in the surfactant binding process because of the m in micelles) together with 0.50 x m PA. Relative fluorescenceintensity,F, is given as a function of time. Decay highly negative electrostatic potential near the polyion. m a is without quencher and b is in the presence of 5.3 x The pyrene lifetimes in Table 1 clearly support this view. DoP. Since bromide ions are good quenchers of pyrene fluorescence, while chloride ions are not, the lifetimes in pure Table 1. Aggregation Numbers for DoTA+ and Pyrene surfactant solutions are much shorter for DoTAB than Fluorescence Lifetimes for DoTAC. However, for the micelles formed a t the PA system (a). ro (nsY chains there is no significant difference even though the PA-DoTABb 65 212 added salt is in excess. The lifetimes are even higher PA- DoTAC-NaClc 65 213 than in pure DoTAC systems, revealing some further PA-DoTAB-NaBrC' 69 205 protection from oxygen quenching. PSS-DOTAB~ 38 251 It is interesting to note that the aggregation numbers DoTAC, 50 mM 5 7f 184 in the PA systems do not differ from what is found for DoTAB, 50 mM 65f 122 normal DoTAB micelles. This suggests that the formation a Pyrene lifetimes, all samples in equilibrium with air. 0.50 x of micelles in the field of the polyion is under the influence 10-3 m PA, 0.25 x 10-3 m DoTAB, #? = 0.44. Concentrations as of simple packing conditions. The size of a DoTA+micelles in Figure 2. As in footnote c with NaBr replacing NaC1. e 5 x is dependent on the surface potential. Thus, from a n m to 3.6 x m DoTAB, #? = 0.25-0.53 (see m PSS, 1.3 x electrostatic point of view, our results suggest that the ref 22). f Weight-averaged aggregation number (see eq 3). From ionic atmosphere surroundingthe micelle with PA acting ref 22. a s counterion gives rise to a surface potential in the same order of magnitude as in dilute DoTAB solutions. The of the DoTA+ micelles formed in these systems, assuming comparatively small aggregation number found in the is that the mole fraction of DoPB in micelles, XD,,PB,~~~, PSS-DoTAB system may be attributed to the specific equal to its mole fraction ofthe total amount of surfactant. interaction between the styrene groups and the surfactant Representative fluorescence decays are shown in Figure mo1ecules.6asg,iAs was discussed in a n earlier paper,22the 2. All quenched curves, when fitted to a generalized Infelta gave good statistics. The concentration of sureffective head group area per surfactant monomer is large compared to unrestricted packing, possibly due to that factant was 0.25 x m in the salt-free case and 0.64 the styrene groups occupy space in the micellar surface. x m in the presence of 0.010m NaC1. According to Phase Behavior in Dilute Solutions of PSS and the binding isotherms, this corresponds to 0.22 x and 0.27 x m as bound to the polyelectrolyte, DoTAFS. The phase behavior of PA together with DoTAB respectively. The results in Table 1,where aggregation in water was studied by Thalberg et al.8fThey found that numbers and pyrene fluorescencelifetimes from this study adding DoTAB to dilute solutions of PA causes precipita-
Interaction between Polyacrylate and Surfactants 1 0 ’ e
Langmuir, Vol. 10, No. 7, 1994 2119 i
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Figure 4. cac values (open symbols) and phase separation limits (filled symbo1s)forsolutionsof DoTAB together with PA
and PSS. cac values: PSS-DoTAB (0)from surfactantselective electrode measurements from this work; PA-DoTAB from ref ref 8f (A),and ref 37 (0).Phase separating data: PSS6c (O), DoTAB ( 0 )from this work PA-DoTAE3 from ref 8f (B) and from this work (+I. 250 L
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-2
Ig ( CDoTAB 1 molal ) Figure 5. emfvaluesin millivolts, as a functionof the logarithm
ofthe DoTAB concentration,lOg(cDoTA$”, from measurements with the DoTAB-selective electrodein solutionsof (frombottom to top) 0.01 m ofNaC1,O.lO x m PSS,0.43x m PSS, and 5.0 x m PSS. Arrows indicate the onset ofcooperative binding. tion already at very low concentrations; see Figure 4. Prior to precipitation, micelles seem to be present in the solutions.36At 0.5 mM it is possible to bind a considerable amount C/3 > 0.5) of DoTA+ before precipitation occurs. Interestingly, the maximum degree of binding decreases with increasing concentration of PA, and a t 5 mM precipitation starts immediately above the cac. Note that at higher concentrations of PA, the degree of binding a t the phase boundary is about the same as for 5 mM PA. The PSS-DoTAB system behaves differently. We estimated cac values with a surfactant selective electrode. The electrode is very sensitive to changes in the activity of DoTA+in the solution. Measured emf values a t different total concentrations of DoTAB in aqueous solutions of PSS are shown in Figure 5. Included is also the response from DoTAB in 0.010 m NaCl solutions. Unfortunately, as was mentioned in a previous paper,22it is not straightforward t o use these data to construct binding isotherms due to the fact that the PSS, probably by adsorbing to the membrane, affects the response from the electrode. This is also obvious from Figure 5 since, in the presence of PSS, the measured emf can be even larger than what is (36)Chandar, P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988,21, 950.
-5.5
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Ig ( cDofAB i molal )
Figure 6. Relative intensity of the third to the first peak (IIU I) in the fluorescenceof pyrene m) in solutions of 0.50 x m PA and DoTAB.
possible to obtain in the PSS-free solutions. The different features ofthe interaction between PSS and the surfactant, such as the onset of cooperative binding and the saturation a t higher degrees of binding, are still reflected in the recorded curves, and the cac values are readily obtained. As is obvious from Figure 4,the cac values were very low, reflecting the strong interaction between PSS and DoTAB, as was also found by Haykawa and Kwak from investigations of the effect of salt.6a The cac values increase with increasing PSS concentration. This is understood as follows: Cooperative aggregation of the surfactant, facilitated by interactions with the polyion, will occur when the local concentration of surfactant at, or close to, the polyion exceeds a critical value. In other words, micelles start to form at some critical degree of binding. If the surfactant molecules are distributed over a larger number of polyions, the critical degree of binding will correspond to a larger cac. It is important to remember, however, that the cac cannot be interpreted as the concentration of free surfactant a t the onset of micelle formation since a fraction of the surfactant molecules would always bind to the polyions a s statistically distributed counterions, even if only electrostatic interactions were present. Thus, a t the cac the concentration of free surfactant is lower than the cac. However, when the cooperativity of the micellization is low, the free concentration will rapidly increase when more surfactant is added. These are important differences between free aggregation of surfactant and aggregation in the field of a polyelectrolyte, showing that the cac cannot be used as a value of the concentration of free surfactant. Binding isotherms should thus be the basis for investigations of oppositely charged polyelectrolyte-surfactant systems. The cac value for PA-DoTAB in Figure 4 is taken from the binding isotherm reported by Kwak et a1.6c To check this we measured the vibronic band intensities in pyrene fluorescence. The relative intensity of peak I11 to peak I is a monitor of the polarity of the envir~nment.~’ When the probe is positioned in a micelle the IIYI ratio is higher than that found in water. The results from adding DoTAB to a solution of 0.50 x m PA are shown in Figure 6. The cac (3 x m) is in good agreement with the value taken from the binding isotherm. Nevertheless, we would like to comment on the binding isotherm. It was constructed from experiments with a surfactant selective electrode. In such experiments the test solution and a reference solution are placed on each side of a membrane. The difference in activity will give rise to a measurable (37) Kalyanasundaram, K.; Thomas, J. K. J.Am. Chem. SOC.1977, 99,2039.
2120 Langmuir, Vol. 10, No. 7, 1994 potential difference (emf) over the membrane. When constructing the binding isotherm, the free concentration of surfactant in the PA-DoTA+ sample was taken as the concentration of surfactant in a calibration solution, containing surfactant and simple salt but no polyelectrolyte, which gave rise to the same emf.6c The concentration of bound surfactant is taken as the difference between the total and the free concentrations. Now, the free concentration thus estimated corresponds to the concentration in the field-free regions between the polyelectrolyte molecules; in the cell model of polyelectrolyte solutions it would be the concentration a t the cell boundary. It is obvious that the concentration of bound surfactant with this bookkeeping will include all surfactant molecules, in excess of the free ones, in the diffise double layer, and not only those immediately adjacent to the polyelectrolyte chain. Formation of micelles will occur where the local concentration of surfactant exceeds the normal cmc, or below that if the polyelectrolyte is intimately involved in the aggregation. The extension of the double layer, and the fraction of surfactant molecules loosely "bound" in it, are largest when no excess salt is added. The binding isotherm for the salt-free case could thus be influenced by this effect. A simple calculation of the fraction of free counterions, from a solution of the Poisson-Boltzmann equation for the case without salt, suggests that the concentration of free surfactant near the cac may be underestimated by a factor of 2. Note that the interpretation of the cac, which is the sum of the free and bound concentration a t the onset of cooperative binding, is not affected by this. For PSS-DoTAB there seems to be a simple relation between phase separation (precipitation) and cac. When the samples start to get turbid, the DoTAB-PSS ratio is about the same at all PSS concentrations (0.9,1.0, and 0.8 a t 0.1 x low3,0.5 x and 5 x m PSS, respectively). Thus, the degree ofbinding is the important parameter for the onset of both micellization and precipitation in the PSS-DoTAB system. Apparently the behavior is more complex in the PA system, as will be discussed below. I t should be noted that for both PSS and PA phase separation in the dilute region generally results in formation of precipitates. Unlike the binding of simple counterions, the binding of surfactant to the polyelectrolyte will make the complex more hydrophobic. When their net charge becomes sufficiently low, the complexes will start to attract each other, and, due to the hydrophobic nature of the interaction, the water content will be low in the concentrated phase; i.e. a precipitate will be formed. Effect of Salt on Binding and Aggregation. Kwak and co-workers have investigated the effect of simple salt on the binding isotherms for DoTA+ in 0.5 x m solutions of PSPaand PA.6c For the PA system they found that, in the presence of 0.010 m NaCl, the free concentration of DoTA+was more than 10 times higher a t the onset of cooperative binding than in a salt-free solution. The PSS system, on the other hand, was much less sensitive to salt. For instance, the free concentration of surfactant m when cooperative binding started was about 4 x in the presence of 1.1m (!) NaC1. This is approximately the free concentration at the cac in the salt-free PA system. The authors attributed the small effect in the PSS case to hydrophobic interactions between the surfactant and the backbone of the PSS chain. In the PA system no such specific interaction between the polyelectrolyte and the surfactant is present. An interaction is still very favorable due to the hydrophobic effect from the surfactant tails. At concentrations below the cac the binding of surfactant is governed by the statistical distribution of ions in the field of the polyelectrolyte. However, a t the cac the surfactant
Hansson and Almgren
ions can rule out the binding of normal counterions by forming micelles with the polyelectrolyte acting as counterion. Since the binding of a surfactant ion is accompanied by the release of a sodium ion, neither the internal energy nor the entropy of mobile ions is expected to change very much during the process. Therefore, the hydrophobic interaction between the surfactant tails must be the main driving force. To illustrate the electrostatic influence on the cac we can apply the simplest form of the counterion condensation model proposed by Manning38 to the binding isotherms reported by Kwak et a1.6c We consider here only DoTAB concentrations less than or equal to cac and assume that the sodium ions and the surfactant molecules are either condensed (bound) or free. Within the model the degree of dissociation, a, is equal to 1 - (V2.85) = 0.35for the PA charges at all salt concentrations, where the dimensionless polyion charge density parameter is equal to 2.85. Accordingly, the concentration of bound counterion is 0.33 x m a t 0.5 x m PA. If DoTA+ and Na+ are distributed in the same way, the free concentration of DoTAB, cfieee, will be related to the degree of binding, B, and the total concentration of DoTAB, ctot, according to eqs 4 and 5 (4)
where CPA and CNaCl are the concentrations of PA and NaCl, respectively. In the salt-free case, cooperative binding starts at a degree of binding corresponding to about 0.02 according to the results presented in Figure 1of ref 6c; i.e. at this point the curve starts to deviate from what would be expected from simple condensation. Taking this degree of binding as critical also in the presence of 0.01 m NaCl, the cooperative binding is predicted to start a t 0.32 x m (log cfiee= -3.49). Indeed, according to experiment, the binding starts to increase rapidly at about l o g c h = -3.5 (Figure 3 of ref 612). One obvious objection to this treatment is of course that the critical degree of binding may be different in the presence of extra salt (the PA will be more flexible, etc.) Effect of Salt on the Phase Behavior in the System of PA Together with DoTAEVC and CTABK. The starting point for this study is the phase diagram reported by Thalberg et al.,8f for PA and DoTAB in water. In a three-component representation of the phase diagram, a two-phase region, anchored in the water-rich corner, is extended alongthe PA-DoTAB equimolarline. The phase separation is associative and generally results in a dilute phase in equilibrium with a phase that is concentrated with respect to both surfactant and polyelectrolyte. As was mentioned in the Introduction, this behavior has been d e s ~ r i b e dand ~ s ~seems ~ to be quite general for systems of oppositely charged polyelectrolytes and surfactants."J2 In the present investigation we have probed the strength of the associative interaction between PA and the surfactants DoTAB, CTAB, DoTAC, and CTAC by adding a simple electrolyte. Samples with different concentrations of salt (NaBr or NaC1) together with equimolal concentrations of polyelectrolyte and surfactant were prepared. All samples that resulted in phase separation were analyzed with respect to the concentration ofsurfactant. The result is shown in Figure 7, where the weight percentages of surfactant in two phases in equilibrium are given for (38)(a) Manning, G.S . J . Chem. Phys. 1969,51,924.(b)Manning, G. S.J. Chem. Phys. 1969,51,934. (39)Thalberg,K.Ph.D. Thesis, University of Lund,Lund,Sweden, 1990.
Langmuir, Vol. 10,No. 7, 1994 2121
Interaction between Polyacrylate and Surfactants 0.7
a
b
-
0.6
;
m
t
0.3 Y
DoTA0
-. .
0.2
".
n r
0
29 wt%
10
30
40
1
1
DofAC
u
0
10
20 wt%
30
40
Figure 7. Influenceof simple salt on the phase behavior of PA together with DoTAB and CTAB (Figure 7a), and DoTAC and CTAC (Figure 7b) at 25 "C. The concentration(weight percentage)of surfactant in each phase, as estimated spectroscopically(see text), is given for differentconcentrationsof added salt. Two phases in equilibrium(filledsymbols)are connected to the mixing composition (open symbols)with a line. The segregativephase behavior of PA-CTAB at high concentrationsof NaBr is schematicallyindicated with a dotted line in Figure 7a. Table 2. Surfactant Aggregation Numbersa and Pyrene Lifetimes* from Phase Separating Polyelectrolyte-Surfactant Solutions 70 (ndb concenconcentrated dilute trated phase phase phase
(4s"
system
PA-DoTABd PA-CTABd PA-DoTACd
PA-CTACd PSS-DOTAB' PSS-DoTABf DoTAB, 5.6 mM DoTAB, 26 wt %
extra dilute salf ( m ) phase 0.150 0.200 0.227 0.284 0.301 0.599 0.303 0.405 0.301 0.404 0.450 0.301
73 81 78
83 84 82
rods
rods
rods
rods
64 66 145 147 147 63
rods 71 70 148 152 149 68
rods
65 81 82 141 132
116 114 116
136 128 124
175 179 191 186 187 111
201 195 203 205 196 143 124 109 111 178 174
CTAC, 16 mM 0.550 CTAC, 24 w t % From time-resolved fluorescence quenching measurements at 25 "C, with DoP+or CP+as quencher. The quencher to surfactant ratio was 0.013 in all samples. * Measured at 25 "C. All samples in equilibrium with air. NaBr and NaCl for PA-Do/CTAB and PA-Do/CTAC, respectively. Mixing composition: 0.15 m with respect to both PA and surfactant. The concentrationof surfactant in the separated phases as in Figure 7. e Mixing composition (wt %): 3.5% PSS, 11.5% DoTAB. Compositionof dilute phase: 0.04% PSS, 4.1% DoTAB. Concentrated phase: 11% PSS,28% DoTAB. fcomposition (wt %): 3.6% PSS, 23% DoTAB (one-phaseregion). different salt concentrations. The mixing composition of each sample is connected with a line to the compositions of the two equilibrium phases. It is obvious that the phase separation is of associative nature in all cases, except for high concentrations of salt in the CTAB system. With increasing salt concentration the composition of the phases in equilibrium becomes more and more similar, and, a t high enough salt content each system remains in one phase. It can be mentioned that the volume of the concentrated phase increased at the expense of the dilute phase, with increasing salt concentration. The concentrated phases were viscous and very sticky, but became less viscous due to the swelling at higher salt concentrations. In all CTAC samples and in DoTAC near the onephase region, the density of the viscous phase was lower
than that of the dilute phase. A similar observation has been reported when water was replaced by heavy water.8a At salt concentrations above 0.1 m the phase separation always resulted in two clear phases, but at lower concentrations this was not the case. Instead a sticky white precipitate was formed. The addition of salt to such a sample rapidly transformed the precipitate into a clear viscous phase coexisting with the dilute phase. Thalberg et a1.8freported clear isotropic phases a t all compositions, even without the presence of salt, once the initial concentration of PA exceeded 100 mM. This discrepancy can be explained, however, by the fact that they used a not fully neutralized polyacrylic acid (pH = 7). In the same paper they reported experiments with a cationic polyelectrolyte and anionic surfactants where phase separation, like in our PA-DOTAB samples without salt, resulted in the formation of precipitates. They suggested that these precipitates were formed due to the strong interaction between the polyelectrolyte and the surfactant in this case and that the precipitates may not be in equilibrium with the supernatant. Clearly, a weaker interaction between polyelectrolyte and surfactant,either as a consequence of the addition of salt or a reduced linear charge density of the polyelectrolyte, must make it easier for the system to establish equilibrium between the phases. Both with bromide and chloide as counterion, more salt is needed to transfer a system into a one-phase system for the longer surfactant. Interestingly, bromide is more effective than chloride in this sense. The size and shape of surfactant micelle^^^^^^ can be affected by both the presence of salt and polyelectrolyte,22 as well as of the concentration of the surfactant itself. In order to check the influence of such factors on the phase behavior, we made a n investigation of the micellar aggregation numbers. The results from time-resolved fluorescence quenching measurements are given in Table 2. We see that the aggregation numbers are determined by the choice of salt. The presence of PA does not affect the aggregation numbers, Interestingly, the aggregation numbers are about the same for two separated phases in equilibrium, even though in the dilute phases the concentration of surfactant is in some cases very low. (40) Tanford, C. The hydrophobic effect: Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley & Sons: New York, 1980. (41)WennerstrBm, H.; Lindman, B. Phys. Rep. 1979, 52, 1.
2122 Langmuir, Vol. 10,No. 7, 1994
The viscosity in all phases with CTAB was higher than for the other surfactants, and even samples in the onephase region were almost gel-like. Indeed, no stationary state in the quenched fluorescence decays could be observed on a 2-ms time scale for any of the phases with CTAB. This is an indication that very long rodlike micelles are This type of micelle has been reported for CTAB by other^.^^,^^ The phase separation starting a t about 0.6 m NaBr in the CTAB system must likewise be a n effect of the large micelles. No such phase separation was observed for the other surfactants, except for CTAC a t very high NaCl concentrations ( > 3 m). It can be mentioned here that when sodium chlorate was added to a system of PA and CTAC, a behavior similar to that for PA-CTAB -NaBr appeared. Again fluorescence measurements indicated rodlike micelles. At high concentrations of NaBr in solutions of sodium hyaluronate and tetradecyltrimethylammonium bromide (TTAB), a segregative phase separation was reported by Thalberg et ahsb This behavior is characterized by one phase enriched in the surfactant and the other phase enriched in polyelectrolyte. In our investigation we have only estimated the surfactant content in the phases, but it seems reasonable to assume that what we observe is a segregation in the PA-CTAB system as well. The pseudo-three-component phase diagram referred to in the beginning of this section suggests that in the associative regime, the PA is probably distributed in a way similar to the surfactant. All our samples were mixed t o lie on the equimolar line, which is parallel to the tie lines in this particular region of the phase diagram. The pyrene lifetimes are also reported in Table 2. A somewhat longer lifetime is found in the concentrated phases than in the corresponding dilute ones. Since this is observed both with bromide and chloride salt, the effect is probably due to a difference in oxygen solubility. This probably also explains the shorter lifetimes found in the PA-free samples. In the following we will try to explain our results. We emphasize that many of the ideas have been discussed by others.8 As a starting point we can mention that both experim e n t ~ ~and ' theoretical consideration^^^ suggest that phase separation (coacervation) in a system of rodlike cylindrical polyelectrolytes of reasonable charge densities does not take place if there are only univalent counterions. With di- and multivalent counterions such an effect is possible however, which suggests that the surfactant behaves like a polyionic species when inducing phase separation in a polyelectrolyte solution. In order for the system to phase separate (associatively) there must first of all be a n attraction between the surfactant and the polyelectrolyte. Since the total concentration of surfactant is well above the cmc the system will always contain micelles. An interaction will be electrostatically favorable since both the PA chains and DoTA+micelles can release their counterions. In contrast to the dilute systems discussed above, where the interaction is strong due to a hydrophobic effect, the driving force here is mainly of electrostatic origin. However, as will be (42)Almgren, M.; Alsins, J.;Mukhtar, E.; van Stam, J.J. Phys. Chem. 1988,92,4479. (43)Medhage, B.; Almgren, M. J . Fluoresc. 1992,2,7. (44)Medhage, B.; Almgren, M.; Alsins, J . J . Phys. Chem. l993,2,7. (45)Reiss-Husson, F.; Luzzati, F. J . Phys. Chem. 1964,68, 3504. (46) Ulmius, J.; Wennerstrom, H.; Johansson, L. B.-A.; Lindblom, G.; Gravsholt, S. J . Phys. Chem. 1993,2,7. (47)Bungenberg de Jong, H. G. In Kruyt, H. R. Colloid Science; Elsevier: Amsterdam, 1948;Vol. 2,Chapter 10. (48)Michaeli, I.; Overbeek, J. Th. G.; Voorn, M. J . J . Polym. Sci. 1957,23,443.
Hansson and Almgren
discussed further in the PSS-DoTAB case, a strong interaction is not sufficient for a phase separation to occur. The gain in free energy of interaction following a phase separation must be large enough to compensate for the loss in entropy of mixing. Due to the fact that the surfactant micelle and the polyelectrolyte contribute little to the entropy of mixing, a much smaller free energy gain is required for phase separation to occur, as compared to systems of simple solutes. From the theory of polyelect r o l y t e we ~ ~know ~ that, in general, the electrostatic molar free energy of a polyelectrolyte solution increases with decreasing concentration. This is largely a n entropic effect due to that the distribution of counterions deviates more from ideality in dilute solutions. In the present case however, this entropic contribution is less important due to the small number of micelles. Instead, as suggested by Thalberg et a1.,8athe reason for the phase separation may be that in a concentrated phase there are more possibilities for the components to make favorable contacts. The introduction of extra salt weakens the interaction between polyelectrolyte and surfactant, as is obvious from Figure 6 and the binding isotherms discussed above. At high enough salt concentration the large number density of salt ions will rule out the possibility of one polyion acting as a counterion for the other. This is an entropic effect, now acting to weaken the interaction between polyelectrolyte and surfactant. This explains why less salt is needed to obtain a one-phase system for PA-DoTAB than for PA-CTAB; the smaller DoTAB micelles will give the surfactant component a larger contribution to the entropy of mixing. Surprisingly, NaCl has a much weaker effect than NaBr, although the latter gives bigger micelles. Note that even for DoTAC with a n aggregation number of about 70, more salt is needed than for CTAB rods to get one phase. This suggests that other forces than pure electrostatic ones are important. The polarizability and the hydration energies of the ions would make the binding of bromide ions to the micelles stronger than the binding of chloride. The segregation of CTAB micelles and PA chains (the latter probably in the form of random coils at this high ionic strength) indicates a repulsive interaction between them a t high concentrations of NaBr. When modeling the phase behavior of hyaluronate and TTAB, Thalberg et aLMfound that if the interaction between polyelectrolyte and solvent is more favorable than that between polyelectrolyte and micelle, it is possible to obtain a segregative type of phase diagram, but only if the surfactant component is given a polymerization number. (See also Gustafsson et aL50 and ref 51.) One of the questions raised by Piculell and Lindman in their review1' concerns to what extent the aggregation numbers vary in different regions of the phase diagram. In particular, it would be interesting to see if the aggregation numbers were different in two phases in equilibrium, in analogy with the well-known fractionation of polymers.52 No such effect was observed in the present investigation, except perhaps in the DoTAB system a t the lowest salt concentrations. Although such a behavior cannot be ruled out from the present results, because of the quite high salt concentration in all samples, a simple size sorting mechanism as that in polymer fractionation should perhaps not be expected. Unlike normal polymers, micelles rapidly adjust their aggregation number and size distribution according to the conditions. (49)Lifson, S.;Katchalsky, A. J . Polym. Sci. 1954,13, 43. (50)Gustafsson, A.; Wennerstrom, H.; Tjerneld, F. Polymer 1986, 27,1768. (51)Sjoberg, A.;Karlstrom, G. Macromolecules 1989,22,1325. (52)See for instance Floly, P. J. Principles of Polymer Chemistry; Cornel1 University Press: New York, 1953;Chapter 13.
Langmuir, Vol. 10, No. 7, 1994 2123
Interaction between Polyacrylate and Surfactants DoTAB / I
-- PSS 0
10
20
30 %
PSS
40
50
--f
Figure 8. A schematicpseudo-three-component representation of the phase diagram for the systemof PSS together with DoTAB in water at 25 “C.The composition (given as weight percentage) of coexisting phases in equilibrium (dots)is connected with a line through the initial composition (squares).
The Phase Diagram for PSS Together with DoTAB. The experimental findings in dilute solutions of PSS together with DoTAB suggest that the phase behavior of the system differs from what is found in other systems of polyelectrolyte together with oppositely charged surfactant (for instance PA-DoTABBf and HyaluronateDoTABsa). To investigate this we employed DoP+to probe the distribution of DoTAB between phases. Samples containing 3.5%(w/w)of PSS and different concentrations of DoTAB were mixed. All samples resulted in clear phases. The concentrations of DoTA+ and styrene sulfate ions in the dilute phase of all phase separating samples were estimated, and the weight percentages of DoTAB and PSS in both phases were calculated. The result is shown in Figure 8. The phase boundary is schematically indicated by a dotted line, and the composition of each phase is connected with a line to the initial composition of the sample. It is important to note that this is a three-component representation of a fourcomponent system, giving no information of the distribution of the small ions. Even though the phase diagram is not complete, and have some inconsistencies, it gives important information of this system. The general feature of the phase diagram is similar to what is found in PADO TAB,^^ with a two-phase region reaching out from the water-rich corner. Furthermore, phase separation in the surfactant-rich region results in a concentrated phase enriched in both components in equilibrium with a phase consisting almost exclusively of surfactant and water, i.e. in principle, only very small amounts of PSS can be added to a solution of DoTAB before phase separation occurs. In the polyelectrolyte-rich region the two systems behave differently. In the PA-DoTAB system almost all added surfactant forms a concentrated phase together with an equimolar amount of PA, leaving the excess PA in the supernatant. The PSS-DoTAB system, on the other hand, does not phase separate as long as the polyelectrolyte is in some excess, and the phase boundary seems to be an extension of the line for the precipitation limit in Figure 4.
Most interesting is the phase separation in the sample with the smallest initial amount of DoTAB, where a concentrated phase is in equilibrium with a more dilute phase, both having a DoTABPSS molar ratio approxi-
mately equal to unity. Unfortunately, due to experimental difficulties, we were not able to investigate this sample with the fluorescence quenching technique. Another phase separating sample, containing a n excess of the surfactant, was investigated. The aggregation number (Table 2) in the more dilute phase, containing 4.1% (w/w) 3 and only tracer amounts of PSS, is of DoTAB ( ~ 0 . 1 M) approximately what is expected for the surfactant at that concentration (giving credit also to the assumption that DoP+ is distributed as DoTA+ between the phases in this system). In the concentrated phase, where the concentration of DoTAB is 28%, the aggregation number is smaller thanwhat is found in a 26% solution of only DoTAB (Table 2). It is obvious that the presence of PSS in the concentrated phase restricts the growth of the micelles. The same low aggregation number is also found for a single-phase sample containing the surfactant in large excess (3.6%PSS, 23%DoTAB, Table 2). Experiments s ~ o wthat ~ ~the, PSS-DoTAB ~ ~ complexes formed when the polyelectrolyte is in excess are stable even after additions of large amounts of salt. This means that the hydrophobic contribution to the interaction between PSS and DoTA+ is dominating. Therefore, even though the ionic strength is very high, the concentrated phases may contain micelles intimately bound to the PSS ((a)sz 40, see Table 1) together with “normal” micelles ((a)sz 8090). The aggregation numbers reported here could thus be a n average with contributions from two populations of micelles. But, since we have no information of the polydispersity, we cannot exclude other possibilities. We have seen that, although both PSS and PA are vinylbased polyelectrolytes with the same linear charge separation, they display great differences in their interactions with surfactants. Most striking is the stability of the PSS-DoTAB aggregates. As long as PSS is present in some excess there will be no phase separation. Furthermore, both binding isotherms and aggregation numbers are little affected by the addition of even large amounts of salt. On first thought it may be tempting to assume that the strong interaction would immediately lead to phase separation. However, a consideration of the microstructure ofthe PSS-DoTAB complex, as it was described by Kwak et a1.,69,’ is illuminating. According to their results the first two methylene groups next to the surfactant head group are close to the benzene ring of the styrene groups of the PSS, suggesting that units of the PSS chain are incorporated in the micelle surface. As a n effect of this inherent surface activity of the PSS, the charges on polyelectrolyte and micelle are in close contact, thus giving the complex also a low electrostatic internal energy. We suggest, with support from the discussion above, that these energetically favorable aggregates make the phase separation “unnecessary” as long as the PSS is in some excess. Another way of expressing it is that PSS and DoTA+ behave like one single species, thus leaving us with a system of one macroion-with reduced charge-in a salt solution. In systems of hydrophobically modified polyelectrolytes together with oppositely charged surfactants, a n observed increased stability against phase separation, as compared with systems of the unmodified polyelectrolyte, was reported recently.M We believe that the increased stability is governed by the same principles as in the PSS-DoTAB (53) Results from fluorescence quenching measurements in 0.1 m (2% (w/w))solutions of PSS together with 0.05 m DoTAB indicate that the aggregates formed are similar to those in dilute solutions (ref 22). The addition of large amounts of NaBr ( 22 M)lowered the fluorescence lifetime but did not by other means affect the shape of the quenched decavs. -. .. .
(
