Molecular Dynamics Insights into the Influence of Hydrogen Sulfide on the Performance of Kinetic Hydrate Inhibitors

Molecular dynamics simulation was employed to reveal the influence mechanism of H2S on the hydrate inhibition performance of two typical kinetic hydrate inhibitors (i.e., polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap)). The results indicate that the presence of hydrogen sulfide significantly weakens the inhibitory performance of PVP and PVCap on methane hydrates, and in the growth stage, PVCap appeared to lose its inhibitory effect in our simulations. Because hydrogen sulfide itself promotes hydrate formation, it weakens the performance of the inhibitors primarily by altering the bulk hydrate-forming environment. Hydrogen sulfide and water molecules form transient "pseudo-cyclic" complexes. This structure weaken the adsorption of methane molecules on the inhibitor surface and promote the migration of methane to the hydrate clusters, which may contribute to the self-assembly and growth of hydrates. The simulations also showed a greater reduction in the inhibition performance of PVCap than of PVP. However, condensed-phase analysis indicates that this difference does not result from direct interactions between H2S and the inhibitors. The oxygen-containing functional groups remain coordinated by water molecules, and H2S is not enriched around the hydrophobic groups in either inhibitor system. Instead, the reduced inhibition performance is primarily associated with the influence of H2S on the bulk hydrate-forming environment, including enhanced methane mobility. Together, these effects lead to a significant reduction in the effectiveness of kinetic hydrate inhibitors in H2S-containing systems and provide a molecular-level explanation for the interference caused by H2S. These conclusions are based on a limited number of simulation trajectories and should therefore be regarded as representative of the dominant trends.