Molecular Insights into the Growth/Dissociation of CH₄ Hydrate under the Calcite Slit-Pore Confinement: Effect of Pore Size Variation

The mechanisms of hydrate cage distortion near the hydrate–calcite surface interface and the formation of the bound interfacial water layer have been elucidated for the small slit-pore system, along with hydrogen bond analysis to explain the microscopic origins of our simulation observations. In contrast to the hydrophilic silica slit-pores studied previously (<i>Fluid Phase Equilibria</i> <b>2025</b>, <i>588</i>, 114218), the CH₄ cluster confined in the calcite slit-pore tends to remain away from the calcite surface.

This behavior arises from the strong electrostatic interaction between the calcite surface and water molecules, which creates a layer of ordered bound water. The potential of the mean force calculation reveals that this bound water layer imposes a higher energy barrier for the CH₄ molecule near the calcite surface. The relative location of the CH₄ hydrate with respect to the CH₄ cluster and/or the calcite substrate influences the hydrate growth and dissociation mechanism.