Interactions between Water-Soluble Polymers and Surfactants: Effect of the Polymer Hydrophobicity. 1. Hydrophilic Polyelectrolytes

Abstract

The binding of DTAC (dodecyltrimethylammonium chloride) to two polyelectrolytes 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 investigated by potentiometry with a surfactant ion-specific electrode, by time-resolved fluorescence quenching, by fluorescence anisotropy, and by viscosimetry. The surfactant binding isotherms, the number of surfactants (N) making up polymer-bound aggregates, the lifetime of pyrene (τ2) in these aggregates, and their microviscosity (ηi) were thus obtained as a function of the surfactant concentration and copolymer neutralization degree α, which determines its electrical charge density. As observed in other studies, the binding is cooperative in the whole range of α for PS1 and for α ⩾ 0.5 for PS4. Aggregation numbers, pyrene lifetimes, and aggregate microviscosities were all found to be independent of the surfactant concentration but to depend strongly on α. N increased with α for PS1 but decreased for PS4. For PS1, τ2 and ηi increased when α decreased, i.e. with decreasing micelle size. The comparison of the τ2 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 where polymer chains tightly wrap around aggregates with their charged groups in contact with surfactant charged groups. The difference of behavior between PS1 and PS4 is attributed to the contribution of hydrophobic interactions between PS4 butyl side chains and surfactant alkyl chains. This contribution is small in the case of PS1, where binding is of essentially electrostatic nature. The high microviscosity of polymer-bound aggregates relative to free DTAC micelles is attributed to the electrostatic binding of surfactant ions to the polyelectrolyte 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 other reactant) in the aggregates and thus makes quenching (and other) processes much less efficient than in free micelles.