Association of imidazolium surfactants with poly(N-isopropylacrylamide)

Abstract

Most investigations about polymer and surfactant interactions are related to standard surfactants, while the scope of literature using imidazolium surfactants is limited. In this work, conductometric and calorimetric techniques have been combined to determine the interaction of poly(N-isopropylacrylamide) with two surfactants: 1-tetradecyl-3-methylimidazolium chloride and 1-hexadecyl-3-methylimidazolium chloride. The critical aggregation concentration and the polymer saturation point have been determined as a function of the polymer concentration. To explain the conductivity results, a new model has been proposed that considers the possibility of formation of free micelles before the polymer saturation. This model allows estimating the fraction of surfactant molecules aggregated to the polymer and the ionization degree of them, as well as the fraction of surfactant in the form of free micelles. Isothermal titration calorimetry results reveal that the interaction between the polymer and both surfactants is driven by entropic processes and that the aggregation process is non-cooperative at the initial stages.