Quasi-Liquid Layer Reconstruction and Entropic Stabilization during Hydrate Particle Agglomeration and Particle-Droplet Adhesion

Abstract

Natural gas hydrate particle agglomeration and adhesion to unconverted water droplets are key microscopic processes underlying cluster growth and pipeline plugging in oil-continuous multiphase flows. However, a molecular-level understanding of whether particle–particle agglomeration and particle-droplet adhesion follow analogous interfacial evolution pathways, and how their free energy stabilization mechanisms differ, remains lacking. Here, molecular dynamics simulations, umbrella sampling, and temperature-dependent free energy decomposition were combined to compare the two processes. Hydrate particles formed stable agglomerates through diffusive approach, collision, interfacial rearrangement, and cluster relaxation. Droplet deformation, wetting, and spreading facilitated earlier particle-droplet contact formation and expansion of the solid–liquid contact region. The hydrate cores remained largely ordered, whereas oil-film displacement induced localized quasi-liquid layer reconstruction and interfacial hydrogen-bond network formation. Under the present models and reaction-coordinate definitions, the total potential of mean force well depths relative to the respective separated states were approximately 384 and 600 kJ/mol for the particle–particle and particle-droplet systems, respectively. Both greater kinetic accessibility and stronger free energy stabilization were observed for particle-droplet contact. Enthalpy–entropy decomposition showed that both contact processes were entropy-dominated, with favorable entropic contributions overcoming positive enthalpic contributions. Particle-droplet adhesion exhibited particularly strong enthalpy–entropy compensation accompanying oil-film displacement, droplet deformation and spreading, and interfacial-water rearrangement. These results identify oil-film displacement, quasi-liquid layer reconstruction, droplet deformation, and hydrogen-bond bridging as coupled controls on hydrate adhesion and provide a molecular basis for interfacial-film stabilization and suppression of water-mediated contact.