Molecular simulation of swelling and interlayer structure for organoclay in supercritical CO2

In this work, Monte Carlo simulations have been carried out to investigate the swelling stability and interlayer structures of alkylammonium-modified montmorillonite both in vacuum and in supercritical CO2 (scCO2) fluid. In the vacuum (dry) condition, the stable spacing for this kind of organoclay was determined based on the energy minimum. In the stable spacing, the corresponding interlayer structure of dry organoclay is the monolayer arrangement with the intercalated surfactant chains lying parallel to the silicate surface. In scCO2 fluid medium, the normal pressures within the organoclay gallery and the swelling free energy have been obtained from Gibbs ensemble Monte Carlo simulation. The mechanically and thermodynamically stable spacings of the organoclay have been determined. As compared with the case in vacuum, the simulation shows that the swelling of the organoclay is thermodynamically favorable in the environment of scCO2 fluid. The interlayer structure and conformation have been used to analyze the mechanism of swelling. The headgroups of surfactant cations are distributed close to the clay surfaces. The presence of CO2 molecules within the clay gallery can cause a specific steric arrangement of the long-chain alkylammonium cations.