Vesicular structures of catanionic surfactants fabricated from mixed cationic and anionic surfactants in aqueous phase can be applied as drug delivery carriers. In this study, stable catanionic vesicles were fabricated from mixed cationic/anionic surfactants with different molar ratios, and then various concentrations of vitamin E acetate (VEA) as hydrophobic model drug and 30 mM arbutin as hydrophilic model drug were encapsulated. In addition, in vitro release behaviour of the encapsulated arbutin from the catanionic vesicles was investigated. Tetradecyltrimethylammonium bromide (TTMAB), a cationic surfactant, and sodium dodecylsulfate (SDS), an anionic surfactant, were used as the raw materials for the formation of the catanionic vesicles. A semi-spontaneous formation process was adopted to form the catanionic vesicles in ethanol solutions from the mixed TTMAB/SDS system with different molar ratios and various concentrations of cholesterol. The drug release measurements for arbutin were carried out for 24 h. Physical properties of the vesicles were studied by techniques of dynamic light scattering, fluorescence polarization, and transmission electron microscopy (TEM). Encapsulation efficiency and release rate of the vesicles for arbutin were analysed by high-performance liquid chromatography (HPLC). The results showed that the size of the vesicles was not significantly changed by increasing the cholesterol content. However, increasing the ethanol concentration seemed to reduce the vesicle size. In addition, increasing the cholesterol concentration would increase the vesicular bilayer rigidity and enhance the physical stability of the vesicles. The TEM images confirmed that the vesicles possessed spherical structures. The most stable vesicles were found for the TTMAB/SDS mixture with a molar ratio of 3/7 and a cholesterol content of 3 mM or 4 mM in an aqueous phase containing 10 vol.% ethanol. The lifetime of the vesicles was more than 300 days without a significant change in size.
The stable catanionic vesicles were demonstrated to be able to encapsulate model drugs, vitamin E acetate and arbutin, respectively, confirming that the TTMAB/SDS vesicles possessed bilayer structures. Encapsulation efficiency of the TTMAB/SDS (molar ratio 3/7) vesicles with 3 mM or 4 mM cholesterol was decreased by increasing the VEA concentration due to the limitation space in the hydrophobic regions of the vesicular bilayers. However, the vesicles fabricated with 3 mM cholesterol had higher encapsulation efficiency than that fabricated with 4 mM cholesterol due to the competing effect between cholesterol and VEA in the vesicular bilayers. The results showed that the vesicle size had a positive correlation with the encapsulation efficiency for arbutin. That is, the smaller the vesicle size, the less the arbutin encapsulated. At 30 vol.% ethanol, arbutin was released up to 80% after 24 h. By fitting the release curve to kinetic models, it is found that the release behaviour of the TTMAB/SDS (molar ratio 3/7) vesicles with 3 mM cholesterol followed the Higuchi model at 10 vol.% and 20 vol.% ethanol and followed the first order model at 30 vol.% ethanol. In the other hand, the TTMAB/SDS (molar ratio 3/7) vesicles with 4 mM cholesterol at 10 vol.% ethanol followed the first order model. In addition, increasing the ethanol concentration would increase the bilayer fluidity and thus the drug release rate.

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