Micelle—polymer complexes: Aggregation numbers, micellar rate effects and factors determining the complexation process

Abstract

This paper describes a study of the interaction of anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB) micelles with two structurally related, neutral polymers, poly (ethylene oxide) (PEO) and poly (propylene oxide) (PPO) of different hydrophobicity. Mean aggregation numbers ( n) were measured for PEO- and PPO-complexed SDS micelles both in water and in aqueous solutions containing 0.4 M NaBr. Interaction of the SDS micelles with the polymers results in decreased n values, the effect being more pronounced for PPO than for PEO. The n values were employed in a detailed kinetic analysis of the effects of PEO and PPO on the SDS-induced inhibition of the neutral hydrolysis of 1-benzoyl-3-phenyl-1,2,4-triazole (1) in water and aqueous 0.4 M NaBr solutions. Particular emphasis is placed on the binding of 1 to the micellar aggregates. It was found that 1 binds less effectively to the polymer-complexed micelles than to unperturbed SDS micelles, most likely as a result of decreased surfactant headgroup packing in the SDS-polymer complexes. The effect of PPO on the reaction of alkyl- and arylsulfonylmethyl sulfonates with hydroxide ions catalyzed by CTAB micelles was also studied. PPO greatly decreases the micellar catalysis, most likely by reducing the effective substrate binding to the surfactant assemblies. The present and previous results suggest that the hydrophobicity of the polymer is a major factor determining micelle-polymer interaction. The general finding, that anionic micelles bind more strongly to uncharged water-soluble polymers than cationic micelles, is rationalized in terms of hydration shell overlap effects of polymer and surfactant headgroup in the micelle-polymer complex.