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 / 2 HCl) 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 ()[NaNphS]/[C14DMAO‚ 1 / 2 HCl]) at 25°C: elongated micelles ( < 0.25) f positively charged unilamellar vesicle dispersion (0.3 < < 0.5) f sediments consisting of aggregated multilamellar vesicles (0.55 < < 0.75) f negatively charged unilamellar vesicle dispersion (0.75 < < 5). The -dependent aggregation behavior of the vesicles correlated well with the change in the potential of the vesicles, and it was described by the normal DLVO theory. This indicates that the repulsive doublelayer 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 / 2 HCl-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.

Temperature-induced sedimentation to dispersion of ionic vesicles

“…This implies that the potential barrier against the aggregation of vesicles is not much larger than kT in the case of the vesicles with a ζ potential (∼16 mV). This is consistent with the previous report that the potential barrier was on the order of kT (∼5 kT ) for the vesicle with the surface potential φ 0 = 15 mV [25].…”

“…A similar temperatureinduced vesicle-to-micelle transition has been also reported for cetyltrimethylammonium 5-ethylsalicylate (C16TA5ES) vesicles [32]. In the system of C14DMAO· 1 2 HCl-NaNphS, on the other hand, the surfactant ion pairs forming the vesicle are likely to be stabilized by the hydrogen bonding [25]. The superior thermal stability of vesicles in the C14DMAO· 1 2 HCl-NaNphS system may be attributed to the pseudo double-tailed surfactants stabilized by hydrogen bonding.…”

“…This in turn suggests that the temperature T Dis is different from the chain melting temperature of the vesicle. For the vesicles in the C14DMAO· 1 2 HCl-NaNphS system, the melting temperature of the hydrocarbon chains in the complex was 21 • C [25], which is much lower than T Dis (∼50 • C).…”

“…In a previous paper, we reported that the zeta potential of the aggregated vesicles in the C14DMAO· 1 2 HCl-NaNphS system at 25 • C is small because of the complete binding of counterions NphS [25]. It was suggested that the borderline of the zeta potential between the rapid coagulation and the dispersion is about 10 mV on the basis of the ELS measurement and the DLVO theory.…”

“…Recently, we reported on vesicle formation by mixing of two solutions: a micellar solution of tetradecyldimethylamine oxide hemihydrochloride (C14DMAO· 1 2 HCl) and a sodium 2-naphthalenesulfonate (NaNphS) solution [25]. The stability of the vesicle dispersion and the sign of the vesicle charge depended on the mixing mole ratio β (= [NaNphS]/[C14DMAO· 1 2 HCl]) 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).…”