INTERACTIONS BETWEEN WATER-SOLUBLE POLYMERS AND SURFACTANTS - EFFECT OF THE POLYMER HYDROPHOBICITY .1. HYDROPHILIC POLYELECTROLYTES

The binding of DTAC (dodecyltlimethylammonium chloride) to two polyele ctrolytes of differing hydrophobicity, the alternated copolymers poly( maleic acid-co-methyl vinyl ether) and poly(maleic acid-co-butyl vinyl ether) referred to as PS1 and PS4, respectively, has been investigate d by potentiometry with a surfactant ion-specific electrode, by time-r esolved fluorescence quenching, by fluorescence anisotropy, and by vis cosimetry. The surfactant binding isotherms, the number of surfactants (N) making up polymer-bound aggregates, the lifetime of pyrene (tau(2 )) in these aggregates, and their microviscosity (eta(i)) were thus ob tained as a function of the surfactant concentration and copolymer neu tralization degree alpha, which determines its electrical charge densi ty. As observed in other studies, the binding is cooperative in the wh ole range of alpha for PS1 and for alpha greater than or equal to 0.5 for PS4. Aggregation numbers, pyrene lifetimes, and aggregate microvis cosities were all found to be independent of the surfactant concentrat ion but to depend strongly on alpha. N increased with alpha for PS1 bu t decreased for PS4. For PS1, tau(2) and eta(i) increased when a decre ased, i.e. with decreasing micelle size. The comparison of the tau(2) values obtained with DTAC and dodecyltrimethylammonium bromide in the presence of PS1 revealed that the surfactant counterions are expelled from the surface of polymer-bound aggregates. This leads to a model wh ere polymer chains tightly wrap around aggregates with their charged g roups in contact with surfactant charged groups. The difference of beh avior between PS1 and PS4 is attributed to the contribution of hydroph obic interactions between PS4 butyl side chains and surfactant alkyl c hains. This contribution is small in the case of PS1, where binding is of essentially electrostatic nature. The high microviscosity of polym er-bound aggregates relative to free DTAC micelles is attributed to th e electrostatic binding of surfactant surfactant ions to the polyelect rolyte and to the presence of the polymer main chain at the aggregate surface. The large values of the pyrene fluorescence lifetime are the result of the high microviscosity of the bound aggregates, which slows down the diffusive motion of reactants (probe and quencher, or any ot her reactant) in the aggregates and thus makes quenching (and other) p rocesses much less efficient than in free micelles.