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

Abstract

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.