(346b) The Effect of Pressure and Salinity on Gas/Brine Interfacial Tension for Equimolar Mixtures of H2-CH4 and H2-CO2

The effect of salinity, pressure, and different cushion gases like carbon dioxide and methane on the interfacial tension (IFT) between H2 and brine is essential in many applications like geological hydrogen storage. We have used molecular dynamics simulation to calculate the surface tension of H2-CH4/brine and H2-CO2/brine systems under varying pressures and salinities. In preparing the simulation box, the number of molecular units for the gas phase was calculated using the Peng-Robinson equation of state with simple van der Waals mixing rules. The simulation box (Lx = 4 nm, Ly = 4 nm, Lz = 12 nm) was built; energy minimization was carried out using the steepest decent algorithm until the maximum force was converged to 100 kJ/mol. Subsequently, 0.1 ns NVT equilibration and 50 ns production runs were performed. The cut-off distance for van der Waals interactions was set to 1.2 nm while electrostatic interactions were treated using the particle mesh Ewald summation.

The simulation trajectories were analyzed to construct density profiles and use the principal components of the pressure tensor in order to calculate the IFT. It was found that higher ionic strength resulted in greater surface tension which is a consequence of the perturbation of water structure brought about by the ions. Furthermore, the H2-CO2 mixture had a lower IFT compared to H2-CH4. The surface tension manifests due to the difference in net forces between molecules in the bulk liquid solution and those near the interface. Because the CO2 mixture interacts with the liquid more strongly, the surface tension will be less than that of the CH4 mixture. This was verified by calculating the interaction energies between the phases, and the resulting trend matched the IFT one.