Quantum Chemical Study of the Carbon Dioxide-Philicity of Surfactants: Effects of Tail Functionalization

Carbon dioxide (CO₂)-philic surfactants have broad application prospects in organic synthesis, fracture-enhanced oil recovery, polymerization, extraction, and other fields and can be used to enhance the viscosity of supercritical CO₂ (scCO₂). In this work, the relationship between the functional group of the surfactant tail and CO₂-philicity is studied from a new perspective using density functional theory. Three common functional group types (fluorinated, oxidative, and methyl groups) were investigated. The analysis of binding energy demonstrates that all three types of functional groups can improve the CO₂-philicity of the surfactant. Among these three kinds of functional groups, the strongest interaction with CO₂ molecules is observed for oxidative functional groups followed by semifluorinated, fluorinated, and methyl groups. However, the CO₂ molecules tend to be adsorbed onto the middle segment of the oxidative group, and the intrusion of the CO₂ molecules results in the low solubility of oxidative surfactants. In contrast, fluorinated and methyl groups interact with CO₂ at the end of the surfactant tail. As a result, the fluorinated surfactants show the best solubility in CO₂. Therefore, the solubility of a surfactant in CO₂ is not only related to the interaction strength between the surfactant and CO₂, it also depends on the interaction structure. The results of this study provide a new strategy for evaluating surfactant CO₂-philicity and provide guidance for the design of surfactants with high solubility in scCO₂.