Molecular Modeling of Diffusion Coefficient and Ionic Conductivity of CO2 in Aqueous Ionic Solutions

Mass diffusion coefficients of CO2/brine mixtures under thermodynamic conditions of deep saline aquifers have been investigated by molecular simulation. The objective of this work is to provide estimates of the diffusion coefficient of CO2 in salty water to compensate the lack of experimental data on this property. We analyzed the influence of temperature, CO2 concentration,and salinity on the diffusion coefficient, the rotational diffusion, as well as the electrical conductivity. We observe an increase of the mass diffusion coefficient with the temperature, but no clear dependence is identified with the salinity or with the CO2 mole fraction, if the system is overall dilute. In this case, we notice an important dispersion on the values of the diffusion coefficient which impairs any conclusive statement about the effect of the gas concentration on the mobility of CO2 molecules. Rotational relaxation times for water and CO2 increase by decreasing temperature or increasing the salt concentration. We propose a correlation for the self-diffusion coefficient of CO2 in terms of the rotational relaxation time which can ultimately be used to estimate the mutual diffusion coefficient of CO2 in brine. The electrical conductivity of the CO2 brine mixtures was also calculated under different thermodynamic conditions. Electrical conductivity tends to increase with the temperature and salt concentration. However, we do not observe any influence of this property with the CO2 concentration at the studied regimes. Our results give a first evaluation of the variation of the CO2 brine mass diffusion coefficient, rotational relaxation times, and electrical conductivity under the thermodynamic conditions typically encountered in deep saline aquifers.