Molecular dynamics simulations of interfacial structure, dynamics, and interfacial tension of tetrabutylammonium bromide aqueous solution in the presence of methane and carbon dioxide.

The interfacial behavior of tetrabutylammonium bromide (TBAB) aqueous solutions in the absence of gas and the presence of methane and carbon dioxide gases is studied by molecular dynamics simulations. The aqueous TBAB phase, at concentrations similar to the solid semiclathrate hydrate (1:38 mol ratio), has a smaller interfacial tension and an increase in the gas molecules adsorbed at the interface compared to that in pure water. Both these factors may contribute to facilitating the uptake of the gases into the solid phase during the process of semiclathrate hydrate formation. At similar gas pressures, CO is adsorbed preferentially compared to CH, giving it a higher surface density, due to the stronger intermolecular interactions of CO molecules of the solution at the interface. The increase in relative adsorption of CH at the solution surface compared to that in pure water surface is due to the hydrophobic interactions between the n-alkyl chains of the TBA cation and methane gas.