Molecular Dynamics Analysis of Anti-Agglomerant Surface Adsorption in Natural Gas Hydrates

We have used molecular dynamics simulations to examine the surface adsorption of a model anti-agglomerant inhibitor (quaternary ammonium salt) binding to a hydrate surface in both aqueous and liquid hydrocarbon phases. From our molecular dynamics simulation data, we were able to identify the-preferred binding sites on the (111) crystal face of a methane-propane sII hydrate as well as characterize the equilibrium binding configurations of the inhibitor and their associated binding energies. In the aqueous phase, we observed that the inhibitor proceeds through a two-step surface adsorption mechanism, whereas in the liquid hydrocarbon phase surface adsorption occurs through a single-step mechanism. To characterize the extent of surface adsorption in each liquid phase, we calculated the standard binding free energy using the free energy perturbation method following a double decoupling thermodynamic cycle. We found that the surface adsorption in the liquid hydrocarbon phase is an exergonic process, whereas the surface adsorption in the aqueous phase is an endergonie process. Our results demonstrate that the extent of surface adsorption is much larger in the liquid hydrocarbon phase relative to the aqueous phase and suggest that the inhibitor is less effective in the aqueous phase because the surface adsorption is less favorable. Finally, we examine the effect of the inhibitor on the water structure in the liquid phase and in the hydrate phase, with the results highlighting the difference between the nature of anti-agglomerant/hydrate interactions as compared to kinetic inhibitor/hydrate interactions.