Micellization of Ionic Liquid Surfactants Induced by Sodium Polystyrenesulfonate in Aqueous Solutions

The aggregation phenomena of two ionic liquid surfactants—1-decyl-3-methylimidazolium chloride (C10MeImCl) and 1-hexadecyl-3-methylimidazolium chloride (C16MeImCl) have been investigated in aqueous solutions in the presence of a polyelectrolyte, sodium polystyrenesulfonate (NaPSS), using electrical conductivity, surface tension, fluorescence and vapor pressure measurements. The counterion-condensation behavior of aqueous NaPSS has also been studied and approximately 62% of the counterions have been found to remain uncondensed in these solutions. The characteristic concentrations signifying different kinds of interactions, namely the critical aggregation concentration and the polymer saturation concentration, in the premicellar regime have been identified. Surface-active complexes with bound surfactant monomers onto the polyion backbone, non-surface active aggregates with small micellar units wrapped by polyion chains, and non-surface active micelles of ionic liquid surfactant monomers, were found to form in aqueous ionic liquid surfactant solutions in the presence of NaPSS. The surfactant concentration where polyelectrolyte–surfactant monomer complexes begin to form (cac), the concentration where the polyion chains get saturated with the small micellar units (psc), and the concentration where micelles of ionic liquid surfactant molecules begin to appear (cmc*) were identified in aqueous C16MeImCl solutions in the presence of NaPSS. Only two critical concentrations, namely the psc, and cmc* were, however, detected in aqueous C10MeImCl solutions in the presence of NaPSS. Effects of alkyl chain length of the ionic liquid surfactants, temperature, NaPSS concentration, and the charge parameter of NaPSS on the self-aggregation of the ionic liquid surfactants have been considered to elucidate the interactions in these mixed systems. The thermodynamics of micellization in these systems have also been studied and the spontaneity of the polyelectrolyte-induced micellization processes have been rationalized for the systems investigated, with the entropy terms superseding the enthalpy terms.