Adsorption and self-assembly of surfactant/supercritical CO2 systems in confined pores: a molecular dynamics simulation

A coarse-grained molecular dynamics simulation has been carried out to study the adsorption and self-organization for a model surfactant/supercritical CO2 system confined in the slit-shape nanopores with amorphous silica-like surfaces. The solid surfaces were designed to be CO2-philic and CO2-phobic, respectively. For the CO2-philic surface, obviously surface adsorption is observed for the surfactant molecules. The various energy profiles were used to monitor the lengthy dynamics process of the adsorption and self-assembly for surfactant micelles or monomers in the confined spaces. The equilibrium properties, including the morphologies and micelle-size distributions of absorbed surfactants, were evaluated based on the equilibrium trajectory data. The interaction between the surfactant and the surface produces an obvious effect on the dynamics rate of surfactant adsorption and aggregation, as well as the final self-assembly equilibrium structures of the adsorbed surfactants. However, for the CO2-phobic surfaces, there are scarcely adsorption layers of surfactant molecules, meaning that the CO2-phobic surface repels the surfactant molecules. It seems to conclude that the CO2 solvent depletion near the interfaces determines the surface repellence to the surfactant molecules. The effect of the CO2-phobic surface confinement on the surfactant micelle structure in the supercritical CO2 has also been discussed. In summary, this study on the microscopic behaviors of surfactant/Sc-CO2 in confined pores will help to shed light on the surfactant self-assembly from the Sc-CO2 fluid phase onto solid surfaces and nanoporous media.