Steric effects stabilize reverse micelle domains in supercritical CO2 by determined conformation: restrictions of water and cations

Abstract

Previous research into designing CO₂-philic surfactants has certain limitations, necessitating the exploration of effective design concepts for hydrocarbon surfactants, which are far less expensive and less toxic than fluorocarbon surfactants. In this study, molecular simulations were employed to extensively investigate preassembled models of four representative surfactants and elucidate the structure of water-in-carbon dioxide (W/C) microemulsions. Innovative strategies for evaluating the stability of surfactant microemulsions and designing CO₂-philic surfactants were developed. Thorough investigations into the microemulsion structure revealed that fluorocarbon surfactants formed stable micelles due to the steric effect arising from the determined conformation, which is restricted by hydrogen bonds. Fluorination at the terminal end of the surfactant tail will lead to a more restrictive structure, acting as a steric hindrance. Investigation into a hydrocarbon surfactant also determined that the group at the surfactant tail terminal formed hydrogen bonds with water molecules, restricting the conformation of the surfactant. This is a feasible method via fluorine substitution. This work reveals that steric effects arising from water molecules and cation restrictions can guide surfactant synthesis strategies or stabilize reverse micelles in scCO₂ systems. With the development of these basic design principles, the synthesis of efficient hydrocarbon surfactants will be achieved in the foreseeable future.