Molecular dynamics simulations of interfacial properties of the CO₂-water and CO₂-CH₄-water systems

Molecular dynamics simulations were performed to study the interfacial behavior of the pure carbon dioxide-water system and a binary 40:60 mol. % gas mixture of (carbon dioxide + methane)-water at the temperatures of 275.15 K and 298.15 K and pressures near 4 MPa for CO₂ and up to 10 MPa for methane. The simulations are used to study the dynamic equilibrium of the gases at the water-gas interface, to determine the z-density profiles for the gases and water, and calculate the interfacial tension γ under the different temperature/pressure conditions close to those of the formation of clathrate hydrates of these gases. At the same hydrostatic gas phase pressure, the CO₂-water interface has a lower interfacial tension than the CH₄-water interface. A greater number of CO₂ molecules, as much as three times more than methane at the same pressure, were adsorbed at the interfacial layer, which reflects the stronger electrostatic quadrupolar and van der Waals interactions between CO₂ and water molecules at the interface. The water surfaces are covered by less than a monolayer of gas even when the pressure of the system goes near the saturation pressure of CO₂. The surface adsorbed molecules are in dynamic equilibrium with the bulk gas and with exchange between the gas and interface regions occurring repeatedly within the timescale of the simulations. The effects of the changes in the CO₂-water interfacial tension with external temperature and pressure conditions on the formation of the clathrate hydrates and other CO₂ capture and sequestration processes are discussed.