Cosolvent effects on the stability of catanionic vesicles formed from ion-pair amphiphiles

Four ion-pair amphiphiles (IPAs), derived from the pairing of alkyltrimethylammonium chlorides and sodium alkyl sulfates, were used to form catanionic vesicles in water upon the mechanical dispersion method. For the first time in the literature, short-chained alcohols (methanol, ethanol, 1-propanol, and 1-butanol) were added as cosolvents at a variety of concentrations and systematically studied for their effects on the stability of the ensuing vesicles. Dynamic light scattering measurements indicated that vesicles formed from one of the IPAs (i.e., dodecyltrimethylammonium dodecyl sulfate) could be efficiently and successfully stabilized by the addition of appropriate amounts of 1-propanol and 1-butanol. Maximum lifetimes of more than 1 year and 132 days for stable vesicles in 5% 1-butanol and 15% 1-propanol solutions, respectively, were observed, and this demonstrates that a novel method for the stabilization of catanionic vesicles formed from IPAs becomes available by means of cosolvent addition. Furthermore, the stability of catanionic vesicles was found to be strongly dependent on cosolvent concentration. In general, the vesicle stability increased with increasing the cosolvent concentration, reached a maximum at a specific concentration, and thereafter decreased with further increasing the concentration. The vesicles finally disintegrated into constituent molecules in solutions of high cosolvent concentrations. An explanation of cosolvent effects based on the medium dielectric constant was proposed.