Equilibrium and Transport Properties of Methane at the Methane-Water Interface with the Presence of SDS

Molecular dynamics simulations were conducted to study the equilibrium and transport properties of methane/water two-phase systems with sodium dodecyl sulfate (SDS) at the interface. In particular, the properties were determined at different SDS packing fractions, 0%, 25%, 50%, 75%, and 100%. The calculated interfacial tension, determined from the difference of normal and transverse components of the pressure tensor, was found to decrease with increasing SDS concentration at the methane/water interface, whereas the equilibrium partition of methane in the gas and water phases remained nearly unchanged. Both of these results were in quantitative agreement with experimental observations. The presence of SDS at the interface increased the free energy barrier of methane transport through the interface. Therefore, it was found that the rate of methane transport across the interface slowed down with increasing SDS concentrations. In other words, the thermodynamic driving force was unchanged, but the kinetic barrier was enhanced from 9.07 kJ/mol at 0% SDS to 14.17 kJ/mol at 100% SDS coverage at 260 K for the transport of methane to the aqueous phase. Our simulation results have a significant implication on the promoting effect of methane hydrate formation with the presence of SDS.