A Fluorescence Study of Divalent and Monovalent Cationic Surfactants Interacting with Anionic Polyelectrolytes † Per Hansson* Department of Physical Chemistry, Uppsala University, PO Box 532, S-75121 Uppsala, Sweden Received April 10, 2000. In Final Form: April 5, 2001 Self-assemblies of the divalent surfactant dodecyl-1,3-propylenepentamethylbis(ammonium chloride) (DoPPDAC) and the monovalent dodecyltrimethylammonium bromide (DoTAB) were investigated in dilute solutions of anionic polyelectrolytes. Pyrene probing showed that the critical aggregation concentration (cac) was about the same for the two surfactants in solutions of a given polyion. The polyions could be arranged in order of increasing cac: dextran sulfate < polyvinyl sulfate < polyacrylate < poly- (styrenesulfonate) < carboxymethyl cellulose. The surfactant aggregation number (N) for DoPPDAC, obtained from time-resolved fluorescence quenching, was 2-3 times smaller than that for DoTAB in the presence of all polyions. With poly(styrenesulfonate) taken out of consideration, it was found that a large N correlated with a low cac, and vice versa. For both surfactants, pyrene lifetime measurements indicated an insignificant binding of negatively charged quenchers to the micelles, showing that the micelle charges were neutralized mainly by the polyions. The presence of polyion reduced the quenching by oxygen dissolved in the water. The effect was larger for DoPPDAC than for DoTAB, suggesting that the micelles of the former are surrounded by a more dense layer of polyion. Introduction Aqueous mixtures of ionic surfactants and oppositely charged polyions have been studied in great detail recently. 1-4 One reason for this interest is the desire to control the stability, rheology, and water content of technical dispersions containing both components. It is generally believed that flexible polyions can neutralize charged surfactant micelles by folding around them. 5 From the point of view of surfactant self-assembly this is very favorable as the entropic penalty of binding simple counterions is removed. 6 Thus, the critical aggregation concentration 7 (cac) in a polyelectrolyte solution is much lower than the critical micelle concentration (cmc) of the pure surfactant. 8 The neutralization of the micelles by a “layer” of polyion has important consequences also for the phase behavior. 9 Noticably, the absence of long-range repulsive forces between the micelles strongly diminishes the stability range of the micellar solution phase found in related binary ionic surfactant/water systems. Instead a phase separation takes place, 6,10-13 resembling the “com- plex coacervation” of oppositely charged polyions. Fur- thermore, the possibility of the micelles to interact with either polyion chains or simple ions (or both), with different effects on curvature and micelle-micelle interactions, gives rise to a rich phase behavior. 14-16 In our lab we have investigated 17-22 the influence of polyion on self-assemblies of alkyltrimethylammonium ions in dilute dispersions close to the cac. Under these conditions the presence of the polyion was found to completely rule out the binding of simple counterions to the micelles. Thus it was possible to directly relate variations of surfactant aggregation number (N) and cac to certain properties of the polyions. For instance, both N and cac were found to depend on the polyion linear charge density, backbone stiffness, and nature of the charged group. N was obtained from time-resolved fluorescence quenching (TRFQ), and the cac from surfactant binding studies, the latter complemented with earlier data from Kwak and co-workers. 8,23 Interestingly, a correlation between cac and N was found 17-21 for dodecyltrimethy- lammonium bromide (DoTAB), resembling the effect of different monovalent counterions on cmc and N in polymer- * E-mail: per.hansson@fki.uu.se. † This work was initiated when the author was working at the Division for Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University. (1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Hayakawa, K.; Kwak, J. C. T. Interactions Between Polymers and Cationic Surfactants. In Cationic Surfactants: Physical Chemistry; Rubingh, D., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; Vol. 37. (3) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (4) Polymer-surfactant systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; Vol. 77. 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