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

Abstract

Oceans are essentially an electrolyte solution. The experimental study of physical and chemical processes occurring in oceans remains challenging, so molecular dynamics simulations may be of great help. We have recently demonstrated that simulations 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. Here, we extend our previous work by investigating additional properties that require simulations on larger samples and time length scales. First, the extended time and size scales of our simulations allow for a relatively precise determination of the electrical conductivity, 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 into the sample enables us to evaluate its diffusion coefficient DCO2. No experimental measurements or computational simulations have yet provided estimates of carbon dioxide diffusivity at salinity levels and compositions representative of actual oceanic environments. 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 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).