Reverse micelles (RMs) in supercritical CO (scCO) are promising alternatives for organic solvents, especially when both polar and non-polar components are involved. Fluorinated surfactants, particularly double-chain fluorocarbon surfactants, are able to form well-structured RMs in scCO. The inherent self-assembly mechanisms of surfactants in scCO are still subject to discussion. In this study, molecular dynamics simulations are performed to investigate the self-aggregation behavior of di-CF4 based RMs in scCO, and stable and spherical RMs are formed. The dynamics process and the self-assembly structure in the RMs reveal a three-step mechanism to form the RMs, that is, small RMs, rod-like RMs and fusion of the rod-like RMs. Hydrogen-bonds between headgroups and water molecules, and salt bridges linking Na ions, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The results show that di-CF4 molecules have a high surfactant coverage at the RM interface, implying a high CO-philicity. This mainly results from bending of the short chain (C-COO-CH-(CF2)-CF3) due to the flexible carboxyl group. The microscopic insight provided in this study is helpful in understanding surfactant self-assembly phenomena and designing new CO-philic surfactants.

Hybrid surfactants containing both fluorocarbon (FC) and hydrocarbon (HC) chains, as effective CO-philic surfactants, could improve the solubility of polar substances in supercritical CO. Varying the length of the HC of hybrid surfactants is an effective way to improve the CO-philicity. In this paper, we have investigated the effects of the HC length on the self-assembly process and the CO-philicity of hybrid surfactants (F7Hn, n = 1, 4, 7 and 10) in water/CO mixtures using molecular dynamics simulations. It is found that the self-assembly time of F7Hn exhibits a maximum when the length of the HC is equal to that of the FC (F7H7). In this case, the investigation of H-bonds between the water core and CO phase shows that F7H7 has the strongest CO-philicity because it has the best ability to separate water and CO. To explain the origin of the differences in separation ability, the analysis of the structures of the reverse micelles shows that there are two competing mechanisms with a shortening HC. Firstly, the volume of F7Hn is reduced, which thus decreases the separation ability. Moreover, this also leads to the curved conformation of the FC. As a result, the separation ability is enhanced. These two mechanisms are balanced in F7H7, which has the best ability to separate water and CO. Our simulation results demonstrate that the increased volume and the curved conformation of the hybrid surfactant tail could enhance the CO-philicity in F7Hn surfactants. It is expected that this work will provide valuable information for the design of CO-philic surfactants.