Insights into the self-assembly of water reverse micelles and solubilization process in liquid and supercritical solvents: A molecular dynamics study

Abstract

This article reports on a full-atomic classical molecular dynamics simulation investigating the self-assembly process of reverse micelles in carbon tetrachloride and supercritical carbon dioxide (scCO2). The study explores the impact of solvent type, surfactant structure (sodium dodecyl sulfate and sodium bis(3,5,5-trimethyl-1-hexyl) sulfosuccinate), water/surfactant ratio, and surfactant concentration on the self-assembly, size, and shape of reverse micelles. The results indicate that at low water/surfactant ratios in scCO2, irregularly shaped reverse micelles are formed, whereas spherical micelles form as the water/surfactant ratio increases. In scCO2, there is always a small fraction of free water molecules present, whereas in CCl4, all water molecules are surrounded by surfactants. The distribution of head groups of di-chain surfactants over the surface of micelles is observed to occur in groups of several surfactants, forming salt bridges through sodium cations. The solubilization of methyl orange by reverse micelles in pure scCO2 and scCO2 modified with ethanol has also been investigated. The study reveals that the hydrophilic part of the solute is located in the water core of the micelle, while the tail is positioned in the hydrophobic crown. Moreover, the presence of ethanol increases the number of reverse micelles compared to the system without ethanol. Ethanol molecules primarily reside on the surface of the water core, forming stable hydrogen bonds with surfactants, as well as shorter-lived hydrogen bonds with water molecules.