Langmuir zyxwvu 1992,8, zyxwvu 2405-2412 2406 zyxwvutsrqp Aggregation of Alkyltrimethylammonium Surfactants in Aqueous Poly(styrenesu1fonate) Solutions Mats Almgren,*Per Hansson, Emad Mukhtar, and Jan van Stam Department zyxwvuts of Physical Chemistry, Uppsala University, P.O. Box zyxw 532, S-751 21 Uppsala, Sweden Received February 14, 1992. In Final Form: July 13,1992 The aggregation of alkyltrimethylammoniumsurfactants &TA+ and ClsTA+in dilute water solutions of sodium poly(styrenesu1fonate)has been investigated. Aggregation numbers were estimated with the time-resolved fluorescence quenching technique. In the calculations,results from binding isotherms and solubility measurements were used. Binding isotherms for dodecyltrimethylammonium bromide to the polyelectrolyte were determined using a surfactant-selective electrode. The aggregation numbers were found to be independent of the concentration of surfactant and type of counterion, but to increase with increasingsurfactant tail length. From the kinetics of the quenching of pyrenefluorescence with hydrophobic and hydrophilic quenchers,it was concludedthat compact aggregateswith net negative charge were formed, in which the polyelectrolyte is intimately associated with the surfactant. The aggregates are joined by surfactant-free parta of the polyelectrolyte chain, the lengths of which depend on the amount of bound surfactant. The quencher dimethylbenzophenone was found to migrate between the aggregates at the highest concentration of the long-tailed surfactant. Introduction The strong interaction between polyelectrolytes and ionic surfactants of opposite charge type has dramatic effects on solution properties and phase behavior.'-3 At low additions of surfactant to a dilute polyelectrolyte solution a cooperative binding of the surfactant sets in above a critical aggregation concentration, cac, which is lower than the ordinary critical micellization concentra- tion, cmc, of the pure surfactant. The binding has been characterized in a number of systems through measure- ments with surfactant-sensitive electrodes.41~ Most studies of binding and solution properties have been carried out in very dilute systems. Even in these, a precipitate is formed at surfactant concentrations close to charge neutralization. At higher polyelectrolyte concentrations a second, gel-like, liquid phase starts to form and can be separated already at surfactant concentrations much lower than those corresponding to charge neutralization. The phase behavior has been studied in detail recently, both experimentally and theoretically, for some surfactant- polyelectrolyte systems.6 The interaction is dominated by strong electrostatic effects, and may therefore be tuned down by addition of salt. The cooperativity, however, which emanates from the hydrophobicity of the surfactant alkyl chains, and results in formation of micellar-like aggregates, persists also when the electrostatic effects have been weakened by salt,5 or by using mixtures of ionic and nonionic surfactants.' As is well known, formation of micelles occurs also a t concentrations well below the cmc (1) Goddard, E. D. zyxwvutsrqpon Colloids Surf. 1986,19, 301. (2) Hayakawa, K.; Kwak, J. In Cationic Surfactants: Physical Chemistry; Rubingh, D.; Holland, P. M., Eds.; Surfactant Science Series 37; Marcel Dekker: New York, 1991; Chapter 5. (3)Robb, I. D. In Anionic surfactants in physical chemistry of surfactant action; Lucassen-Reynders, E., Ed.; Surfactant Science Series 11; Marcel Dekker: New York, 1981; Chapter 3. (4) Hayakawa, K.; Kwak, J. J. Phys. Chem. 1982,86,3866. (5) Skerjanc, zyxwvutsrqpo 5.; Kogej, K.; Vesnaver, G. J. Phys. Chem. 1988,92,6382. (6) (a) Thalberg, K.; Lindman, B.; Karlstrom, G. J. Phys. Chem. 1990, 94,4289. (b) Thalberg, K.; Lindman, B.; Karlstrdm, G. J. Phys. Chem. 1991,95,3370. (c)Thalberg,K.;Lindman,B.;Karlstrdm,G.Progr. 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. by anionic surfactants interacting with nonionic polymers like poly(ethy1ene oxide) (PEO) or poly(viny1 alcohol) (PVA) The aggregation behavior in nonionic polymers has been rather thoroughly studied, e.g., by small-angle neutron scattering (SANS)* and fluorescence quenchinglOJ1 meth- ods. Small aggregates (aggregation number ( a ) = 20) start to form at the cac, which is about 5 mM in the SDS-PEO system. Further addition of surfactant results in agrowth of the bound aggregates, and a t a second critical concen- tration the polymer becomes saturated with surfactant; formation of free micelles starts just after this point. The aggregates formed by SDS at the point of saturation are somewhat smaller than the free ones. Depending on its length, each polymer chain can be associated with many aggregates, and it appears as if the polymer wraps around the aggregates and shields the unfavorable contact between the hydrophobic core of the aggregates and the surrounding aqueous solution.10 Much less is known about the aggregates formed in contact with polyelectrolytes. Abuin and Scaiano12 studied the effect of such aggregates on the photochemical behavior of several probes in aqueous poly(styrenesulfonat) (PSS)- dodecyltrimethylammonium bromide (DoTAB) solutions and found that several small ((a) = 7-10) clusters were formed with each polyelectrolyte chain, and that these had a negative surface charge. In contrast, Chu and Thomas13reported an aggregation number of 105 (which would be reduced to about 60 if corrected for the concentration of unaggregated surfactant) for decyltri- methylammonium bromide (DeTAB) in poly(methy1 acrylate) at pH 8. These polyelectrolytes have high charge densities, and interact stronger with the surfactants. (7) (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.Longmuir 1990 6, 707. (c) Dubin, P. L.; Vea, M. E. Y.; Fallon, M. A.; Th6, S. S.; Rigsbee, D. R.; Gan L. M. zyxwv Langmuir 1990,6,1422. (8) Goddard, E. D. Colloids Surf. 1986, 19, 255. (9) (a) Cabane, B.; Duplessix, R. J. Phys. 1982,43,1529. (b) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19. (c) Cabane, B.; Dupleeeix, R. J. Phys. 1987,48,651. (10) van Stam, J.; Almgren, M.; Lindblad, C. Progr. Colloid Polym. Sci. 1991, 84, 13. (11) Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem. 1985,89, 41. (12) Abuin, E.; Scaiano, J. B. J. Am. Chem. SOC. 1984, 106, 6274. (13) Chu, D.; Thomas, J. K. J. Am. Chem. SOC. 1986,108, 6270. 0743-7463/92124Q8-2405$03.0010 0 1992 American Chemical Society