Clathrate structure-type recognition: Application to hydrate nucleation and crystallisation.

Area of Science:

• Computational Chemistry
• Materials Science
• Physical Chemistry

Background:

• Clathrate hydrates exhibit polymorphism, with structure I (sI) and structure II (sII) being common.
• Understanding the nucleation and growth of these structures is crucial for various applications, including natural gas storage and carbon capture.

Purpose of the Study:

• To develop and validate geometric recognition criteria for distinguishing between sI and sII clathrate hydrate structures.
• To investigate the interplay and development of sI and sII motifs during hydrate nucleation events using molecular dynamics simulations.

Main Methods:

• Development and validation of geometric criteria for clathrate hydrate structure recognition.
• Direct and enhanced-sampling molecular dynamics (MD) simulations of methane and hydrogen sulfide (H2S) hydrate nucleation.
• Analysis of enclathrated molecule distribution and crystallite structure at transition states.

Main Results:

• Geometric criteria successfully differentiate between sI and sII clathrate hydrate structures.
• For methane hydrate nucleation, enhanced-sampling MD simulations show approximately 80% of methane molecules form sII-like crystallites at the transition state.
• Direct MD simulations of H2S hydrate nucleation reveal polymorphic diversity (sI/sII) and highlight the importance of realistic energy dissipation for determining structural propensity.

Conclusions:

• The study provides robust geometric criteria for clathrate hydrate structure identification.
• Methane hydrate nucleation demonstrates a strong preference for the sII structure early in the process.
• The formation of sI versus sII motifs in H2S hydrate nucleation is sensitive to the simulation's energy dissipation pathway.