Monomer Transfer and Solubilization of Bilayer Vesicles from Double-Chained Ammonium Amphiphiles Carrying Aromatic Units

Abstract

Double-chained ammonium amphiphiles with terminal (4‘-cyanobiphenyl-4-yl)oxy, (4‘-cyanoazobenzen-4-yl)oxy, or (4‘-nitroazobenzen-4-yl)oxy moieties form vesicles upon sonication in water. The UV absorption maxima of the vesicle dispersions are blue shifted due to formation of H aggregates of the aromatic units in the bilayer. When these vesicles are mixed with dialkyldimethylammonium bromide vesicles the H aggregates disappear, as can be monitored by UV spectroscopy. By changing the concentration of both vesicle dispersions, it was concluded that the spectral changes are due to a monomer exchange process instead of a vesicle fusion process. The same phenomenon was used to monitor the solubilization of these vesicles by hexadecyltrimethylammonium bromide micelles. For vesicles from amphiphiles with (cyanobiphenylyl)oxy units at the termini of both chains, a unidirectional diffusion of monomers to didodecyldimethylammonium bromide vesicles is found. The reverse process is hampered by the rigidity of the bilayer of the former compound due to stacking of the aromatic units. For vesicles from amphiphiles with only one azobenzene unit, the exchange with didodecyldimethylammonium bromide vesicles is bidirectional, due to the reduced rigidity of these membranes. The monomer exchange with dodecyloctadecyldimethylammonium bromide and dioctadecyldimethylammonium bromide vesicles is again a unidirectional diffusion of the azobenzene-containing amphiphiles. Solubilization of vesicles from amphiphiles with (cyanobiphenylyl)oxy units at the termini of both chains by hexadecyltrimethylammonium bromide micelles is an unidirectional diffusion process of monomers from the vesicles to the micelles. Solubilization of vesicles from amphiphiles with only one azobenzene unit is accompanied by penetration of hexadecyltrimethylammonium bromide monomers into the bilayers.