Molecular dynamics study of di-CF4 based reverse micelles in supercritical CO2

Abstract

The reverse micelles (RMs) in supercritical CO2 (scCO2) are promising alternatives for organic solvents, especially for both polar and non-polar components are involved. Fluorinated surfactants, particularly the double-chain fluorocarbon surfactants, are appropriate to form well-structured RMs in scCO2. The mechanisms inherent to the self-assembly of the surfactants in scCO2 are still subject to discussion. In this study, molecular dynamics simulations were performed to investigate the self-aggregation behavior of di-CF4 based RM in scCO2 and a stable and spherical RM is formed. The dynamics process and the self-assembly structure in the RM reveal a three-step mechanism to form the RM, that is, small RMs, rod-like RMs and the fusion of rod-like RMs. The Hydrogen-bonds between headgroups and water molecules, and the salt bridges linking Na+, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The result shows the di-CF4 molecule has the high surfactant coverage at the RM interface, implying the high CO2-philicity. This mainly results from the bend of the short chain (C-COO-CH2-(CF2)3-CF3) due to the flexible carboxyl group. The microscopic insight provides in this study is helpful to understand the surfactant self-assembly phenomena and design new CO2-philic surfactants.