Cosolvent Effects on the Stability of Catanionic Vesicles Formed from Ion-Pair Amphiphiles

Abstract

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.

Recent Strategies in the Development of Catanionic Vesicles

“…Thus, the presence of nonaqueous polar solvents effectively induced catanionic vesicle formation from catanionic precipitates. This finding was extended to the formation of catanionic vesicles from IPAs as well 14,27 . In addition, Huang s group revealed that the catanionic vesicles fabricated from bolaamphiphiles or gemini surfactants and oppositely charged surfactants possessed superior thermo-stability in aqueous solutions.…”

“…Catanionic vesicles prepared from pure aqueous-phase IPAs usually exhibit short-term physical stability 8,12,14 . To overcome this shortcoming, various approaches have been employed to adjust the inter-vesicle and/or intra-vesicle interactions and thereby enhance the catanionic vesicles physical stability.…”

“…Since Huang s group proposed fabricating catanionic vesicles from several mixed cationic-anionic surfactant systems by means of co-solvent addition in water 24 26 , using a co-solvent to decrease the aqueous-phase dielectric constant has been employed to create catanionic vesicles from IPAs with either a spontaneous or mechanical disruption approach 14,69,70 . Thus, catanionic vesicles have been prepared by dissolving an IPA in alcohol and then adding water with mixing by a homogenizer.…”

“…Moreover, because of their thermodynamically unstable characteristics, IPA vesicle dispersions usually exhibit short-term physical stability 8,12,14 .…”

“…1 14, 27 , inducing strong attraction between the polar groups. As a result, the average molecule-occupied area decreased and fewer co-solvent molecules existed in the polar group layer of the vesicles, affecting the amphiphiles critical packing parameters and changing the solubility and vesicle-forming ability of the IPAs 14,24 . Thus, the presence of nonaqueous polar solvents effectively induced catanionic vesicle formation from catanionic precipitates.…”