Vesicle−Micelle Transition and the Stability of the Vesicle Dispersion in Mixtures of Tetradecyldimethylamine Oxide Hemihydrochloride and Sodium Naphthalenesulfonate

Abstract

Spontaneous vesicle formation was found to occur on simple mixing of two solutions: a micellar solution of tetradecyldimethylamine oxide hemihydrochloride (C14DMAO·1/2HCl) and a sodium 2-naphthalenesulfonate (NaNphS) salt solution. The stability of the vesicle dispersion and the sign of the vesicle charge depended on the mixing mole ratio β (=[NaNphS]/[C14DMAO·1/2HCl]) at 25 °C: elongated micelles (β < 0.25) → positively charged unilamellar vesicle dispersion (0.3 < β < 0.5) → sediments consisting of aggregated multilamellar vesicles (0.55 < β < 0.75) → negatively charged unilamellar vesicle dispersion (0.75 < β < 5). The β-dependent aggregation behavior of the vesicles correlated well with the change in the ζ potential of the vesicles, and it was described by the normal DLVO theory. This indicates that the repulsive double-layer force is a major factor in stabilizing the vesicle dispersion, while the main driving force of the aggregation is an attractive van der Waals force between the vesicle bilayers. CryoTEM pictures demonstrated that the vesicles showed a drastic change in microstructure upon aggregation. In the aggregated multilamellar vesicles, the electrostatic repulsion between the bilayers is suppressed by the complete binding of counterions NphS-, leading to the change from unilamellar vesicles to aggregated multilamellar vesicles. It is suggested that vesicle formation in the C14DMAO·1/2HCl−NaNphS system can be attributed to the combined effect of the hydrogen bonding between the cationic and the nonionic headgroups (−N+−OH···O−N−) and the strong counterion NphS- binding.