The self-assembly and microscopic interfacial properties of a supercritical CO2 microemulsion having hydrotropes: Atom-level observation from molecular dynamics simulation

Abstract

Supercritical carbon dioxide microemulsion is a complex fluid mixture with distinct nano-scale microstructure that can be tailored to synthesize nano-particles and conduct interfacial reactions, which generally forms under high pressure. Reducing the pressure needed is significantly valuable for promoting the accessibility of the scCO2 microemulsion technology and further spurring the development of CO2-based green chemistry or chemical process. In this paper, we provide the direct evidence for the formation of 4FG(EO)2 based water in scCO2 microemulsion and study the microscopic mechanism of the positive synergistic effect of a series of sodium p-alkyl benzoate (hydrotrope) using molecular dynamics simulation method. Results show that the three additives can be effectively solubilized into the microemulsion aggregate and mainly distribute at the water-CO2 interface. Conformational analysis shows that sodium benzoate (BA0) stands at the interface with a relatively perpendicular conformation, while sodium p-ethyl benzoate (BA2) and sodium p-octyl benzoate (BA8) tend to lie on the interface by side. The packing of the surfactant molecules and interaction energy within the interfacial region are examined to reveal the specific microscopic origin of the positive synergistic effect between different hydrotropes and the surfactant in the sense of contributing to the stability of the aggregate, which is favorable for lowering the pressure needed to construct scCO2 microemulsion. This work is expected to direct the better design of the surfactant and rational choice of additives to successfully construct scCO2 microemulsion under lower pressure or even surfactant-free scCO2 microemulsion, which facilitates the application of scCO2 meeting industrial requirements.