Micellization of water-soluble complex salts of an ionic surfactant with hairy polymeric counterions

Abstract

For ionic surfactants in general, a change from simple to polymeric counterions leads to increasing attraction between micelles, condensing them in a concentrated phase. In the present study, two novel “complex salts” were prepared in which the cationic surfactant hexadecyltrimethylammonium was neutralized by two different copolyions, both having poly(methacrylate) main chains randomly decorated with oligo(ethylene oxide) side chains. The presence of hydrophilic side chains in the polyion backbone is proposed as a strategy to stabilize the complex salt aggregates in aqueous solutions and prevent them from separating out in a concentrated phase. Surface tension experiments reveal that the complex salts form soluble nano-aggregates by surfactant ion self-assembly at a distinct critical micellization concentration (cmc), similar to the micellization of a conventional ionic surfactant. This is the first time that cmc values have been determined for complex salts in the absence of all other ions. The physicochemical nature of the aggregates formed was investigated by dynamic light scattering, nuclear magnetic resonance self-diffusion measurements and steady-state fluorescence spectroscopy. Much larger aggregates are formed when the temperature is increased, but the small aggregates reform at room temperature, suggesting that the soluble aggregates are equilibrium structures, much like the micelles of conventional surfactants.