Mechanism for Transition of Reverse Cylindrical Micelles to Spherical Micelles Induced by Diverse Alcohols

Herein, the effects of various alcohols on lecithin/CaCl₂ organogels are investigated. Mixtures of lecithin and CaCl₂ form reverse cylindrical micelles, resulting in optically transparent organogels. The addition of various alcohols to a mixture of lecithin and CaCl₂ induces a decrease in viscosity through which reverse cylindrical micelles are transformed into spherical micelles (or short cylindrical micelles). Long-hydrocarbon-chain alcohols decrease the viscosity of lecithin/CaCl₂ mixtures more efficiently. Hydrogen bonding and hydrocarbon chain interactions between lecithin and alcohol play important roles in the morphological transition. More importantly, isothermal titration calorimetry was conducted to obtain thermodynamic variables such as the enthalpy, equilibrium constant, Gibbs free energy, entropy, and stoichiometry of the associated molecules observed in the transition. It was found that the transition is an entropically driven process, in which the endothermic and exothermic behaviors were observed depending on the hydrocarbon chain length in the alcohol. In addition, the enthalpy for the association of the alcohol with lecithin showed a linear relationship depending on the hydrocarbon chain length, in which the magnitude of hydrogen bonding and hydrocarbon chain interactions was obtained quantitatively. To the best of our knowledge, this is the first study reporting the thermodynamic properties of the morphological transition observed in a reverse self-assembly process.