Structural transition of reverse cylindrical micelles to reverse vesicles by mixtures of lecithin and inorganic salts

Hypothesis: The transformation from reverse micelles to reverse vesicles is influenced by electrostatic interactions between lecithin headgroups and inorganic salts. The electrostatic interactions are expected to influence molecular geometry of lecithin, resulting in a reduction in critical packing parameter (p). Hence, it should be possible to drive structural transitions of reverse self-assembled structures by addition of inorganic salts to lecithin solutions. Experiments: Structural transitions of reverse micelles and reverse vesicles were formulated including lecithin and inorganic salts as a function of concentration in cyclohexane. A systematic study was performed using inorganic salts with the different valences of the cations such as Li+, Ca2+, and La3+. To probe the nanodomain structures from the lecithin/salt mixtures, small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used. Findings: Adding salts to lecithin solutions induced the systematic transformation of reverse self-assembled structures from reverse spherical micelles to reverse cylindrical micelles and finally to reverse vesicles. The transformation was also correlated with interactions between lecithin headgroups and salts, that is, Li+ < Ca2+ < La3+. In addition, a water-soluble dye such as rhodamine B (RB) can be readily encapsulated into reverse micelles and vesicles, indicating that they are potentially useful for controlled solute delivery. (c) 2022 Elsevier Inc. All rights reserved.