Large scale simulations of a detailed molecular model of seawater: ionic conductivity and diffusion coefficients of CO2

The physical chemistry of oceans is simplified by the fact that marine waters are essentially an electrolyte solution. However, the experimental study of several important physical and chemical processes occurring in oceans remains challenging. In such cases, molecular simulations may be of great help. We have recently demonstrated that molecular dynamics calculations using a state-of-the-art force field can accurately describe the thermophysical properties of seawater by employing a detailed chemical model of the solution. In this study, we extend our previous work by investigating additional properties that require simulations on larger sample and time length scales. The aim of our work is twofold. First, the extended time and size scales of our simulations allow for a relatively precise determination of the electrical conductivity. This is a fundamental property of seawater for which accurate experimental data are available, serving as a further test of the employed force field. Second, the incorporation of CO2 to the sample enables us the evaluation of its diffusion coefficient. To the best of our knowledge, there is not a single estimate (neither experimental nor computational) of the carbon dioxide diffusivity at realistic oceanic conditions. To validate our results, we have also determined DCO2 in pure water. Our simulation results show excellent agreement with experimental data in pure water, which reinforces our confidence in the predicted CO2 diffusivity in seawater. This study, along with previous works, provides a rigorous test of the reliability of the Madrid-2019 force field (together with TraPPE for CO2 ) in saline environments. From this perspective, relevant challenges can be addressed, such as the sink of atmospheric carbon dioxide into the deeper ocean, CO2 sequestration in deep saline aquifers, and seawater freezing (for desalination purposes).