Structure, dynamics and vibrational spectrum of supercritical CO2/H2O mixtures from ab initio molecular dynamics as a function of water cluster formation

In this study, we investigate the effect of water-cluster formation in the supercritical (SC) systems CO2/(H2O)(n) as a function of water content using DFT-based molecular dynamics simulations. The dependence of the intermolecular and intramolecular structure and dynamic properties upon water concentration in the supercritical CO2/H2O phase at a density of 0.74 g cm(-3) and temperature of 318.15 K is investigated in detail and compared to previous studies of the pure sc-CO2 system, single D2O in sc-CO2, and Monte-Carlo simulations of a single water molecule in sc-CO2 phase. Analysis of radial and orientational distribution functions of the intermolecular interactions shows that the presence of water molecules does not disturb the previously established distorted T-shaped orientation of CO2 molecules, though there is evidence of perturbation of the second shell structure which enhances the preference for the slipped parallel orientation in this region. There is also evidence of short-lived hydrogen bonds between CO2 and water molecules. For higher water concentrations, water clustering is observed, consistent with the low solubility of water in CO2 under these conditions of temperature and pressure. Finally, the water-water and water-CO2 interactions are discussed and analyzed in terms of the water self-association and thermodynamic quantities derived from the molecular dynamics simulations.