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

Phase equilibria of water/CO 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 CO (PGC) model to represent the water/CO mixture. The HBP water model describes hydrogen bonds between water and CO explicitly. All models underestimate CO solubility in water if standard combining rules are used for the dispersion interactions between water and CO. With the dispersion parameters optimized to phase compositions, the BK3 and GCP models were able to represent the CO solubility in water, however, the water composition in CO-rich phase is systematically underestimated. Accurate representation of compositions for both water- and CO-rich phases cannot be achieved even after optimizing the cross interaction parameters. By contrast, accurate compositions for both water- and CO-rich phases were obtained with hydrogen bonding parameters determined from the second virial coefficient for water/CO. 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.