Morphology and structure transition of surfactant/scCO2 system: A dissipative particle dynamics simulation

Dissipative particle dynamics (DPD) simulations have been carried out to study the morphology and structure of a surfactant/supercritical CO2 (scCO2) system. A copolymer-type surfactant model was used, which consists of two building blocks representing the CO2-philic unit and the CO2-phobic unit, respectively. Unlike traditional DPD simulation, which used equal repulsive interactions between like beads, the DPD simulation scheme in this work relates the conservative interactions directly to the solubility parameters. It can reasonably represent the density dependence of scCO2 repulsive parameters in DPD simulation. The DPD simulation is able to describe the effect of supercritical fluid density (pressure) on the morphology transition of surfactant self-assembly in scCO2 fluid. The obtained phase diagram in the relation of surfactant concentration with the scCO2 density is qualitatively consistent with the experimental result. We also simulated the density-induced structure transition for the surfacetant/scCO2 system in confined pores. An increase in supercritical fluid density can not only destroy the surface micelle structure in confined pores, but also modify the surface adsorption behavior of surfactant molecules.