Surface activity, self-aggregation and antimicrobial activity of catanionic mixtures of surface active imidazolium- or pyridinium-based ionic liquids and sodium bis(2-ethylhexyl) sulfosuccionate

Abstract

The surface activity and spontaneous formation of vesicles in aqueous solution of catanionic mixtures composed of different surface active ionic liquids (SAILs), methylimidazolium- or pyridinium-based, non-functionalized or bearing an ester or amide group in the alkyl chain, and the anionic surfactant sodium bis(2-ethyl-1-hexyl) sulfosuccinate (Na-AOT) have been studied. Furthermore, the antimicrobial activity of these catanionic mixtures against bacteria and fungi has been investigated. Catanionic mixtures were prepared by mixing aqueous solutions of SAIL-Br and Na-AOT in equimolar ratio. The resulting SAIL-AOT catanionic surfactant mixtures aggregate in vesicles and display high surface activity, enhanced interfacial adsorption and low critical aggregation concentration (cac) as compared to the individual components. The critical aggregation concentration decreases with the elongation of the alkyl chain of the ionic liquid and, for the same chain length, the cac values of SAIL-AOT mixtures follow the order, non-functionalized > amide functionalized > ester functionalized. The zeta-potential of SAIL-AOT vesicles increases from negative to positive values with the alkyl chain lengthening, consistently with the composition of the aggregates determined by the Regular Solution Theory. The vesicle formation in SAIL-AOT catanionic mixtures affects the intrinsic antimicrobial activity of individual amphiphilic components. The catanionic mixtures investigated exhibit high antimicrobial activity against fungi and most Gram-positive bacteria and low toxicity to Gram-negative microorganisms.