Molecular dynamics study on microstructure of supercritical CO2 microemulsions containing ionic liquids

Abstract

Supercritical CO2 microemulsions containing ionic liquid have been widely used in materials preparation, wastewater treatment, biological medicine, and other fields due to its polar microenvironment successfully constructed in the non-polar environment. The focus of this research is to study the microstructure and thermodynamic properties of the microemulsion system through molecular dynamics simulation, with the aim of further expanding its application. Therefore, it is essential to study microstructure at first. In this paper, molecular dynamics simulations were performed to investigate the thermodynamic properties of supercritical CO2 microemulsions containing three different ionic liquids ([bmim][PF6], [bmim][BF4], and [bmim][Ac]) and surfactant Ls-mn (dodecyl polyoxyethylene (m) polyoxypropylene (n) ether) in the presence or absence of water. The simulation results identified the necessary condition for preparing scCO2 microemulsion containing ILs as presence of water. For the quaternary system, the fixed ILs content and surfactant concentration had the optimal water content value. Moreover, in the core area, except for the ionic liquid domain, part of the water was also dissolved. Simultaneously, the stretching angle of the surfactant tail chain and the radius of each system were significantly different. Ultimately, the comparative analysis of scCO2/H2O/ILs microemulsion systems based on Ls-mn hydrocarbon surfactants was conducted, and the system aggregation change law of the m/n ratio was proposed. When the m/n ratio of surfactants is smaller, that is, the hydrophilic EO groups are the less, the faster the aggregation rate of surfactants is, the higher the aggregation efficiency is, but the less the amount of water captured by clusters will be. The Ls-36 system used only 12 ns to complete the aggregation process, and the system could achieve 100 % aggregation efficiency within 150 ns. However, this system had the least amount of captured water, only 93.75 %.