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.

Molecular-Scale Design of Hydrocarbon Surfactant Self-Assembly in Supercritical CO2

Forming wormlike reverse micelles (RMs) by hydrocarbon surfactant self-assembly is an economic and environmental strategy to improve the physicochemical properties of supercritical carbon dioxide (scCO₂), but it remains challenging. Introducing cosurfactant in hydrocarbon surfactant self-assembly system is a potential method to generate wormlike RMs. Here, adopting molecular dynamics simulations, we performed hydrocarbon surfactant (TC14) self-assembly with introducing cosurfactants (C₈Benz). It is found that adding the C₈Benz molecules will induce the spherical RMs to a short rodlike form. In this case, the microstructure of the short rodlike RMs shows a dumbbell-like form that is composed by three parts including a middle part of C₈Benz and two parts of TC14 aggregation at both ends of rodlike RMs, which is regarded as the origin of RMs shape transition. Further, the analysis of free energy for RMs fusion indicates that the high fusion ability of C₈Benz aggregation drives the formation of the dumbbell-like RMs. Accordingly, enhancing the affinity of the C₈Benz is found to be effective strategy to further fusion of rodlike RMs in end-to-end manner, yielding a wormlike RMs with a beads-on-a-string structure. It is expected that this work will provide a valuable information for design the hydrocarbon wormlike RMs and facilitate the potential application of scCO₂.

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

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.