Phase Equilibria of Water/CO₂ and Water/n-Alkane Mixtures from Polarizable Models

Phase equilibria of water/CO₂ and water/n-alkane mixtures over a range of temperatures and pressures were obtained from Monte Carlo simulations in the Gibbs ensemble. Three sets of Drude-type polarizable models for water, namely the BK3, GCP, and HBP models, were combined with a polarizable Gaussian charge CO₂ (PGC) model to represent the water/CO₂ mixture. The HBP water model describes hydrogen bonds between water and CO₂ explicitly. All models underestimate CO₂ solubility in water if standard combining rules are used for the dispersion interactions between water and CO₂. With the dispersion parameters optimized to phase compositions, the BK3 and GCP models were able to represent the CO₂ solubility in water, however, the water composition in CO₂-rich phase is systematically underestimated. Accurate representation of compositions for both water- and CO₂-rich phases cannot be achieved even after optimizing the cross interaction parameters. By contrast, accurate compositions for both water- and CO₂-rich phases were obtained with hydrogen bonding parameters determined from the second virial coefficient for water/CO₂. Phase equilibria of water/n-alkane mixtures were also studied using the HBP water and an exponenial-6 united-atom n-alkanes model. The dispersion interactions between water and n-alkanes were optimized to Henry's constants of methane and ethane in water. The HBP water and united-atom n-alkane models underestimate water content in the n-alkane-rich phase; this underestimation is likely due to the neglect of electrostatic and induction energies in the united-atom model.