Insights into CO2-hexane phase behavior under inorganic nanopore confinement: A Wang-Landau transition-matrix Monte Carlo study

The confined phase behavior of CO2-alkane mixtures in nanopores is of great relevance to industrial applications such as CO2 capture, utilization and storage (CCUS). In this study, the Wang-Landau transition-matrix Monte Carlo (WL-TMMC) method was employed to investigate the phase behavior of CO2-hexane mixtures confined within three representative mineral nanopores: quartz, calcite, and muscovite mica. Due to the stronger adsorption of CO2, the pressure (P)-composition (x) phase diagrams of all three pores exhibit a highly asymmetric shape, characterized by a sharp increase in CO2 mole fraction at relatively low pressures. The dominance of the CO2 adsorption layer also leads to nearly overlapping vapor binodal curves for calcite and mica in the P-x diagrams, and liquid densities that exceed the corresponding bulk values in the P-density (ρ) diagrams. However, the differences in solid-fluid interactions result in notable variations in both liquid compositions and vapor spinodal points among the pores, emphasizing the importance of distinguishing different mineral pores. Nevertheless, the simulations also indicate that, when the same effective pore size is considered under identical bulk conditions, the pressure of binodal and spinodal points across the three pores are highly similar. This finding suggests that, for studies focusing solely on phase transition pressures, different mineral pores can be approximated as a single representative pore type. This work provides key validation data for theoretical models such as modified equations of state and offers a simplified basis for upscaling confined phase behavior to larger scale.