The differences in sterol chemical structures influenced the membrane characteristics of liposomes, especially the physicochemical changes during storage. Therefore, the effects of sterols (cholesterol, β-sitosterol, and stigmasterol) on membrane characteristics were investigated and compared with liposomes without sterols. The incorporation of sterols increased the particle size of liposomes. Transmission electron microscopy images showed that sterols improved the rigidity and decreased the deformability of membranes. Cholesterol produced a more condensing effect on membranes, whereas stigmasterol reduced the effectiveness of lipid packing as shown by Raman spectroscopy. The addition of sterols provided good vesicle size stability during storage. Fluorescence probe analysis demonstrated decreased membrane hydrophobicity and micropolarity with prolonged storage. Phospholipid hydrolysis and oxidation played a major role in regulating membrane characteristics. Among the three sterols, β-sitosterol was the most effective for the inhibition of hydrolysis and oxidation.
Efficient synthesis of four star-like triethanolamine based cationic trimeric surfactants T n (where n is the tail chain length, n = 10, 12, 14, and 16) with ester groups in spacers has been successfully achieved, where mild reaction conditions were employed by substituting triethylamine (TEA) with potassium carbonate as the acid binding agent. The physicochemical properties of these surfactants were investigated by means of surface tension, electrical conductivity, and steady-state fluorescence measurements. The trimeric surfactants have been found to be capable of adsorbing to the air/water interface and orienting themselves through the interactions among the hydrocarbon chains. The conductivity measurement revealed the formation of premicellar aggregations in the T 16 solution. It is noteworthy that the micropolarity of the surfactant aggregations decreased with the elongation of the hydrophobic chain length. Typically, dynamic light scattering and transmission electron microscopy studies showed that the aggregations of the four surfactants changed from larger ellipsoidal vesicles to smaller globule micelles with the increase of the surfactant concentration. It is assumed that this unusual aggregation behavior is related to the transformation of molecular conformation from stretched configuration to a pyramid-like shape.
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