Molecular dynamics simulation of carbon dioxide in aqueous electrolyte solution

The structural properties and diffusion coefficients of the H2O+NaCl+CO2 ternary system at various NaCl concentrations, temperatures and pressures are investigated using molecular simulation. A CO2 potential model is selected for the simulation of CO2 diffusion coefficient in aqueous solution. As the most appropriate model, it produces simulation results which are in closest agreement with experimental data. The properties of the H2O+NaCl system are examined prior to the H2O+NaCl+CO2 system, including the radial distribution functions, coordination numbers and diffusion coefficients at various temperatures and pressures. Three aspects of the ternary system are studied. First, the diffusion coefficients of the ternary system at different NaCl concentrations are observed. The NaCl concentration is found to have a large impact on both the diffusion coefficients of the ternary system and also the cluster pattern of ions. Second, the diffusion coefficients of the system at different temperatures are studied. Raising the temperature increases the diffusion coefficients and facilitates formation of ions pairs. Finally, the diffusion coefficients of the ternary system at different pressures are investigated. Pressure also has impact, but to a much lesser degree. At 278 K, the higher the pressure, the greater value of the diffusion coefficient. In contrast at a temperature of 298 K, a pressure increase leads to lower diffusion coefficient. Hydrogen bonds at low temperatures may be the reason for the unusual phenomenon. The diffusion data are compared to predictions of two models proposed by Ratcliff and Holdcroft (1963). The first approach is based on activation theory and results in both a linear and exponential relationship. They preferred a linear model over an exponential model due to the limiting experimental data. However, we demonstrate that the exponential model is more suitable for predicting diffusion coefficient with the help of simulation data. The other approach is based on the relation between diffusion coefficient and viscosity, whereby the diffusion coefficient of the gas in electrolyte solution is derived from a given viscosity. The diffusion coefficients obtained from molecular simulation agree with the results from the two equations, demonstrating the accuracy of the two prediction equations.