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

• The microstructure of 4FG(EO) 2 based scCO 2 microemulsion was examined using MD method. • Three hydrotropes was confirmed to have pressure reducing effect. • The origin of the synergistic effect between the hydrotropes and the surfactant was revealed. 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 scCO 2 microemulsion technology and further spurring the development of CO 2 -based green chemistry or chemical process. In this paper, we provide the direct evidence for the formation of 4FG(EO) 2 based water in scCO 2 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-CO 2 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 scCO 2 microemulsion. This work is expected to direct the better design of the surfactant and rational choice of additives to successfully construct scCO 2 microemulsion under lower pressure or even surfactant-free scCO 2 microemulsion, which facilitates the application of scCO 2 meeting industrial requirements. [ABSTRACT FROM AUTHOR]