The interface between water and a hydrophobic gas

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Methane hydrate formation is crucial for energy resources and climate.
• Understanding the water-methane interface is key to controlling hydrate formation.
• Previous studies lack detailed molecular insights into interfacial water structure.

Purpose of the Study:

• To investigate the molecular structure and dynamics of water at the liquid water-methane interface.
• To compare interfacial water structure with bulk water, ice, and methane hydrate phases.
• To elucidate the role of interfacial water structure in methane hydrate formation.

Main Methods:

• Classical molecular dynamics simulations were employed.
• Order parameters were used to distinguish between water, ice, and methane hydrate phases.
• Surface tensions were calculated and compared with experimental data, including long-range corrections.

Main Results:

• The water surface exhibits a tetrahedral, clathrate-like structure, distinct from bulk water and ice.
• Interfacial methane concentration is sufficient for clathrate formation at higher pressures.
• Water molecule orientation around methane is weakly pressure-dependent, with tangential availability being a key factor for hydrate cage formation.

Conclusions:

• The unique interfacial water structure promotes methane hydrate formation.
• Disordered water at the interface, with clathrate-like orientations, is essential for hydrate stability.
• Simulation results align with experimental surface tension data when long-range corrections are applied.