Critical Microemulsion Concentration and Molar Ratio of Water-to-Surfactant of Supercritical CO2 Microemulsions with Commercial Nonionic Surfactants: Experiment and Molecular Dynamics Simulation

Abstract The critical microemulsion concentration (cμc) and the molar ratio of water-to-surfactant (W0) of supercritical CO2 (scCO2) microemulsion that uses different nonionic hydrocarbon surfactants (LS-36, LS-45, LS-54, DYNOL-604, TMN-6) were examined at temperatures from 35 to 45 °C and pressures up to 19 MPa. The results show that the cμc mainly depends on the structure of the surfactant. The surfactant with more hydrophilic structure, such as the ethylene oxide (EO) group and hydroxyl, tends to produce a higher cμc. In addition, the cμc increases with the increase of the ratio of ethylene oxide (EO) group number to the propylene oxide (PO) group number of the surfactant. The capacity of the microemulsion system to dissolve water, which is characterized by W0, is related to the concentration and structure of surfactant. It is found that a higher solubility of surfactant in CO2 favors the system to dissolve water at lower pressure. At higher pressure, the stronger hydrophilicity of surfactant and the higher surfactant concentration are beneficial for microemulsions to contain more water. The molecular dynamics (MD) simulation, which was conducted in the NPT ensemble, shows the spontaneous evolution of a surfactant cluster and microstructure of microemulsion at different conditions. It demonstrates that the microemulsion system with more water molecules can form a larger water cluster and catch more surfactants although a few surfactants dissociate in the continuous phase. The experimental data and MD simulation results provide useful infomation for the structure regulation of the scCO2 microemulsion and expand the study to the microscopic scale.