Molecular Insights into the Inhibition Mechanisms of Modified PVCap Kinetic Inhibitors in Hydrogen Sulfide-Containing Gas Hydrate Systems

Abstract Gas hydrate blockage threatens pipeline safety in sour gas reservoirs, where hydrogen sulfide (H2S) accelerates hydrate formation and the conventional kinetic inhibitor poly N-vinylcaprolactam (PVCap) severely degrades. Microsecond-level molecular dynamics (MD) simulations and quantum chemical calculations were performed to investigate hydrate nucleation and growth in a 30% H2S environment using unmodified PVCap and three functionalized copolymers: PVCap-co-Apy, PVCap-co-Pip, and PVCap-co-n-butyl. Based on cage type distributions, molecular diffusion, and interaction energies, the inhibitory efficacy follows PVCap-co-Apy > PVCap-co-n-butyl > PVCap-co-Pip > PVCap. The inhibitors barely delay initial nucleation but significantly suppress postnucleation rapid growth. PVCap-co-Apy shows the best performance and a pronounced selective inhibition of 51262 cages, driven by its highly hydrophilic N-acryloylpyrrolidine group that preferentially hydrogen-bonds with water and disrupts local hydrogen-bonded networks. PVCap-co-n-butyl regulates kinetics via a hydrophobic mechanism: the n-butyl group forms local hydrophobic domains that preferentially accumulate methane rather than H2S, modifying the local guest composition and weakening the driving force for cage formation, though steric hindrance partially shields adjacent polar groups. PVCap-co-Pip shows limited efficacy because of weaker hydrogen bonding with polar molecules, and unmodified PVCap performs the poorest. The novelty of this work is 3-fold. It provides the first microsecond-scale molecular comparison of unmodified and functionally modified PVCap under identical high-H2S conditions; it resolves inhibitor action into a nucleation stage that is barely affected and a postnucleation growth stage that is strongly suppressed, a distinction that macroscopic induction-time measurements cannot make; and it identifies two distinct and independently tunable mechanisms, hydrophilic hydrogen-bond disruption and hydrophobic modification of the local guest composition, that map directly onto the choice of pendant functional group. The work indicates structure–activity relationships governed by functional group polarity and hydrophobic–hydrophilic balance, providing guidance for designing high-performance kinetic inhibitors for sour gas fields.