Self-assembly based on hydrotropic counterion-single-chain amphiphile ion pairs

We investigate the effect of organic hydrotropic counterions on the self-assembled structures formed by pure counterion-single-chain amphiphile ion pairs. The effect of inorganic counterions on single-chain amphiphiles has been studied for years, taking into account the Hofmeister series that directly affects the micellization. Here, hexadecyldimethylbenzylammonium salicylate (C16Sal) in aqueous solution is used as a model for the influence of organic counterions, and the results have been compared with those previously published for inorganic counterions, specifically hexadecyldimethylbenzylammonium chloride (C16Cl). The studies have been performed by using conductivity, dynamic light scattering, as well as atomic force microscopy. We demonstrate the formation of vesicles and suggest the presence of a vesicle-to-micelle transition at higher concentrations. The Gibbs free energy associated with the self-assembly process has been estimated on the basis of the well-known mass-action model. The main conclusion is that the use of hydrotropic counterions instead of classical inorganic ions dramatically changes the packing parameter of single-chain amphiphiles to higher values, resulting in bilayer structures. We propose that these systems are good and cheap alternatives to double-chain amphiphiles for forming more complex structures like vesicles.