Effects of the Na⁺/Ca²⁺ Ratio and Cation Type in the Montmorillonite Interlayer on the Intercalated Methane Hydrate Formation: Insights from Molecular Dynamics Simulations

Ubiquitous natural gas hydrates in marine sediments that are rich in clay minerals exhibit heterogeneous phase behavior in gas hydrate formation, particularly in the presence of swelling montmorillonite. Na⁺ and Ca²⁺ naturally coexist in the interlayer space of montmorillonite as compensation cations. In this study, the intercalated methane hydrate formation in the interlayer space of montmorillonite with various ratios (Na⁺/Ca²⁺) and types (Na⁺/Ca²⁺/Li⁺/K⁺/Cs⁺) of interlayer cations was investigated. The results showed that a small number of methane molecules were distributed near the montmorillonite surface, where siloxane–oxygen hexagonal rings provide a basic site for intercalated methane hydrate nucleates. The outer-sphere hydration structure of Na⁺/Ca²⁺/Li⁺ and the inner-sphere hydration structure of K⁺/Cs⁺ were formed. Most of the methane molecules were distributed in the middle region of the interlayer space due to the adsorbed surface cations. The diffusion capacity of water molecules gradually decreased with the increase of Na⁺ content in the montmorillonite interlayer. Of particular interest is that the tetrahedral arrangement of interlayer water molecules increased with the decrease of Na⁺ content in the montmorillonite interlayer. Furthermore, the interlayer water molecules in Na-Mt and Ca-Mt have a less tendency to form a good tetrahedral structure than those in K-Mt and Cs-Mt. The present findings provide a basic knowledge of the effect of interlayer cations in swelled clay minerals on the formation behavior of methane hydrate in clay-mineral-rich sediments.