Surfactant and Gelation Properties of Acetylsalicylate Based Room Temperature Ionic Liquid in Aqueous Media

An amphiphilic room temperature ionic liquid (RTIL) containing acetylsalicylate anion of type 1-dodecyl-1-methylpiperidinium acetylsalicylate, [C₁₂mpip]­[AcSa], is synthesized from the precursor [C₁₂mpip]­[Cl] by an ion exchange process. The sample is characterized, and its surface active and aggregation behavior in water has been studied and explained. The critical aggregation concentrations (CACs) are determined by a variety of methods, namely, electrical conductivity, surface tension, steady state florescence, and isothermal titration calorimetry (ITC) at different temperatures. As compared to its precursor, [C₁₂mpip]­[AcSa] has low CAC values, indicating enhanced favorable interactions between the [alkylmpip]⁺ cation···bulky [AcSa]⁻ anion and also hydrogen bonding of both of the ions with water. The free energy of aggregation ΔG⁰_a is always negative, and both enthalpy and entropy of aggregation drive the aggregation process. The micelle-like aggregates are ellipsoidal in shape. The aggregation numbers are determined from translational diffusion coefficients and florescence quenching measurements. Aggregates of [C₁₂mpip]­[AcSa] are larger than those of its precursor IL with chloride anion. Therefore, it is evident that the close interactions between the ion pairs of [C₁₂mpip]⁺···[AcSa]⁻ facilitate packing of more molecules in an aggregate. The steady state and oscillatory rheology measurements in aqueous solutions consisting of mixtures of [C₁₂mpip]­[AcSa] and sodium salicylate (SS), an hydrotope additive, were carried out. The analysis of zero shear viscosity and moduli properties as a function of concentration and temperature reveals that the addition of SS promotes the growth of small ellipsoid aggregates into large worm-like structures with a typical viscoelastic gel behavior. The moduli properties vs temperature profiles are complex and no hysteresis was produced in heating and cooling modes, suggesting the thermoirreversibile and complex nature of the network structures. The release of the acetylsalicylate anion from the gels could be triggered by simple dilution, and the release occurs due to surface erosion and demicellization.