A small angle neutron scattering study of the thicknesses of vesicle bilayers formed from mixtures of alkyl sulfates and cationic bolaform surfactants

Past studies have established that the thickness of a vesicle bilayer formed from a mixture of conventional anionic and cationic surfactants is determined by a delicate balance of factors, including electrostatic interactions, van der Waals forces, and chain packing constraints. This complex balance of factors means that the bilayer thickness is not easily predicted from knowledge of the structure of conventional surfactants. In this paper, we report the synthesis of a family of rigid, bolaform surfactants with the structure bis(trimethylammoniumalkyloxy)azobenzene dibromide, where the alkyl group was butyl, hexyl, or octyl. We used small angle neutron scattering and quasi-elastic light scattering to characterize the microstructures formed when the cationic bolaform surfactants were mixed with either sodium dodecylsulfate or sodium tetradecylsulfate in aqueous solutions. Small angle neutron scattering spectra interpreted using form factor fits indicate that the three rigid, bolaform surfactants span the bilayers of vesicles formed by these surfactant mixtures, thus constraining the bilayer thicknesses to the end-to-end lengths of the bolaform surfactants. We conclude that use of rigid, bolaform surfactants provides a simple means to design vesicles from anionic and cationic surfactants that form spontaneously and have predictable bilayer thicknesses. These results suggest some simple rules that enable the rational design of surfactant-based nanostructures.