Formation of Methanol Clathrate Hydrate in Simulated Interstellar Ices

Area of Science:

• Astrochemistry
• Materials Science
• Physical Chemistry

Background:

• Clathrate hydrates (CH) are ice structures with cage-like lattices.
• Methanol (CH3OH) is generally considered an inhibitor of clathrate hydrate formation.
• Understanding ice formation in extraterrestrial environments is crucial for astrochemistry.

Purpose of the Study:

• To investigate the potential for methanol incorporation into structure-I clathrate hydrates.
• To explore the conditions under which methanol can form clathrates, contrary to its known inhibitory role.
• To understand the relevance of methanol clathrate formation to interstellar and cometary ice composition.

Main Methods:

• Deposition of layered dimethyl ether (DME)-methanol-water ices at 10 K.
• Thermal annealing of ices to 125 K under ultrahigh vacuum (10^-10 mbar).
• Analysis using Reflection-Absorption Infrared Spectroscopy (RAIRS) and Density Functional Theory (DFT) calculations.

Main Results:

• A distinct IR band at 1021 cm^-1 indicates methanol incorporation into structure-I clathrate cages.
• Methanol occupies the 5^12 6^2 cage, requiring a clathrate-promoting guest like DME or ethylene oxide.
• Methanol enclathration involves CH3OH···H2O hydrogen-bonded environments.

Conclusions:

• Methanol can be incorporated into structure-I clathrate hydrates under specific non-equilibrium cryogenic conditions.
• This process is morphology-dependent and requires a promoter molecule.
• Findings offer insights into volatile trapping mechanisms in interstellar and cometary ices.