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.

Optimal aggregation number of reverse micelles in supercritical carbon dioxide: a theoretical perspective

The aggregation number is one of the most fundamental and important structural parameters for the micelle or reverse micelle (RM) system. In this work, a simple, reliable method for the determination of the aggregation number of RMs in supercritical CO2 (scCO2) was presented through a molecular dynamics simulation. The process of pulling surfactants out of the RMs one by one was performed to calculate the aggregation number. The free energies of RMs with different numbers of surfactants were calculated through this process. We found an RM with the lowest free energy, which was considered to have the optimal number of surfactants. Therefore, the optimal aggregation number of RMs was acquired. In order to explain the existence of an optimal aggregation number, detailed analyses of surfactant accumulation were conducted by combining molecular dynamics with quantum chemistry methods. The results indicated that in the RMs with the lowest free energy, the head-group and tail-terminal of the surfactants accumulated on an equipotential surface. In this case, the surfactant film could effectively separate water and CO2; thus, the lowest free energy was expected. This method determined the aggregation number of RMs by theoretical calculations that did not depend on experimental measurements. This presented approach facilitates the evaluation of the characteristics of RMs in scCO2 and can be further applied in the RM system of organic solvents or even in the micellar system.

Dissipative Particle Dynamics Simulation on Vesicles Self-Assembly Controlled by Terminal Groups

Block copolymer vesicles have been widely used in the field of drug delivery, microreactors, and cell membrane mimetics. Introducing terminal groups to the block copolymer can control the structures of vesicles, which is important for improving the application of vesicles. In this paper, the effects of terminal groups on the structure of vesicles were studied by dissipative particle dynamics simulation. We considered different locations, hydrophobicity, and numbers of terminal groups. When the terminal group located at the end of a hydrophobic block, the increase of wall thickness and the decrease of cavity size of vesicles were observed with the hydrophobicity of the terminal group increasing. In contrast, when the terminal group located at the end of the hydrophilic block, with the hydrophobicity of terminal groups increasing, the vesicular cavity size increased but the wall thickness of vesicles remained nearly unchanged. Finally, increasing the number of terminal groups lead to a decrease of cavity size and an increase of wall thickness of vesicles. The hydrophobic changes of polymer molecules are regarded as the origin of the structural changes of vesicles. This simulation study supplies a potential approach that controls the structures of vesicles and is expected to facilitate its further applications.