Thermodiffusion of CO 2 in Water by Nonequilibrium Molecular Dynamics Simulations.

The components of a fluid mixture may segregate due to the Soret effect, a coupling phenomenon in which mass flux can be induced by a thermal gradient. In this work, we evaluate systematically the thermodiffusion of the CO-HO mixture, and the influence of the geothermal gradient on CO segregation in deep saline aquifers in CO storage. The eHeX method, a nonequilibrium molecular dynamics simulation approach, is judiciously selected to simulate the phenomenon. At 350 K, 400 bar, and CO mole fraction of 0.02 (aquifer conditions), CO accumulates on the cold side, and the thermal diffusion factor is close to 1 in a number of force fields. The lower the temperature, the higher is the separation and the thermal diffusion factor. In colder regions, water self-association is stronger, whereas the CO-HO cross-association and the CO-CO interactions enhance at higher temperatures. Thermodiffusion and gravitational segregation have opposite effects on CO segregation. At typical subsurface conditions, the Soret effect is more pronounced than gravity segregation, and CO concentrates in the top (colder region). Our work sets the stage to model the effect of electrolytes on CO segregation in subsurface aquifers and other areas of interest.