Protonation of oleyldimethylamine oxide (OleylDAO) decreased the average curvature and induced a change from threadlike micelles in the nonionic state to particles (vesicles) at the half-protonated state (degree of ionization R ) 0.5), concluded from viscoelastic measurements and cryoTEM observations in water at 25 °C. The viscoelastic changes were confirmed to be reversible with respect to a pH change. The times required for the solutions to recover the behavior at R ) 0.5 after the addition of HCl to the solutions of R ) 0 were about 25 h at 0.05 mol (kg water) -1 and 14 days at 0.15 mol (kg water) -1 . The correspondence between the viscoelastic properties and the cryoTEM observations was good. It was suggested from the viscoelastic properties that the threadlike micelles grew with R at R ) 0.2, but the change from threadlike micelles to vesicles was suggested as R increased further. On increasing the concentration at R ) 0.5, the vesicles became perforated, probably due to the increased counterion concentration. The present results indicate unambiguously the important and effective action of protonation in controlling the packing parameter, or the mean curvature, of amine oxide amphiphiles.Vesicle formation by single-chain amphiphiles has been studied extensively in recent years. 1,2 Mixing of two or more single-chain amphiphiles or cosurfactants with widely different packing parameters is a popular approach. 3 Catanionic surfactants may be classified into this group. [4][5][6] For a spontaneous and reversible vesicle formation, tuning of the packing parameter by some physical factor is preferable, such as temperature, pressure or pH. Fatty acids have been known to form acid soaps on partial protonation and some of them form vesicles. [7][8][9]
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.
A preliminary study on the reversible micelle-vesicle conversion of oleyldimethylamine oxide [Kawasaki, H. et al. J. Phys. Chem. B. 2002, 106, 1524 ] is extended in the present study. In the presence of 0.01 M NaCl at a surfactant concentration of 0.05 M, a micelle-to-vesicle conversion with increasing degree of ionization alpha takes place in the following sequence: growth of fibrous micelle (alpha < 0.2), a fused network (alpha approximately 0.3), fibrous micelles + (perforated) vesicles (alpha = 0.4), and vesicles + lamellae (alpha = 0.5). Viscoelasticity correspondingly varies from the Maxwell-type behavior of the entangled network of fibrous micelles to the gel-like behavior of vesicle suspensions, via a fluid solution-like behavior of the fused network. This phase sequence is in contrast with the case of no added salt where no branching of micelles is observed, and long micelles and bilayers (vesicles + lamellae) coexist at alpha = 0.5. In water, a state of the lowest viscoelasticity occurs around alpha = 0.2 for both surfactant concentrations 0.05 and 0.15 M. Synergism between protonated and nonprotonated amine oxide headgroups is observed despite low ionic strengths. From the time course of the reversible micelle-vesicle conversion, vesicles seem to be formed from threadlike micelles within 25 h according to the shear moduli, while a longer conversion time is suggested by a flow property (viscosity). Shear thickening behavior is observed at alpha = 0.2 and 0.4 in 0.01 M NaCl but not in water.
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