The self-assembly structure and the CO2-philicity of a hybrid surfactant in supercritical CO2: effects of hydrocarbon chain length

Abstract

Hybrid surfactants containing both fluorocarbon (FC) and hydrocarbon (HC) chains, as effective CO₂-philic surfactants, could improve the solubility of polar substances in supercritical CO₂. Varying the length of the HC of hybrid surfactants is an effective way to improve the CO₂-philicity. In this paper, we have investigated the effects of the HC length on the self-assembly process and the CO₂-philicity of hybrid surfactants (F7Hn, n = 1, 4, 7 and 10) in water/CO₂ 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 CO₂ phase shows that F7H7 has the strongest CO₂-philicity because it has the best ability to separate water and CO₂. 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 CO₂. Our simulation results demonstrate that the increased volume and the curved conformation of the hybrid surfactant tail could enhance the CO₂-philicity in F7Hn surfactants. It is expected that this work will provide valuable information for the design of CO₂-philic surfactants.