Formation of Methanol Clathrate Hydrate in Simulated Interstellar Ices

We report that methanol, widely regarded as an inhibitor of clathrate hydrate (CH) formation, can be incorporated into a structure-I CH relevant to interstellar ices, under ultrahigh vacuum (10^-10 mbar) and cryogenic (125 K) conditions. In the experiment, layered dimethyl ether (DME)-methanol-water ices were deposited at 10 K and thermally annealed to 125 K. Reflection-absorption infrared spectroscopy (RAIRS) reveals the emergence of a distinct band at 1021 cm^-1 that is absent in pure CH₃OH and binary CH₃OH-H₂O ices. Density functional theory (DFT) calculations assign this band to the C-O stretching mode of methanol encapsulated in the 5¹²6² cage of structure-I CH. This clathrate occurs only in sequentially layered ices and requires a clathrate-promoting guest, such as DME. Similar behavior was observed with ethylene oxide, and methanol consistently occupies the 5¹²6² cage independent of the promoter molecule. RAIRS and DFT analyses further indicate that CH₃OH···H₂O hydrogen-bonded environments are involved in the methanol enclathration. These results identify a specific, morphology-dependent pathway through which methanol, traditionally considered a clathrate inhibitor, can be incorporated into the clathrate cages under nonequilibrium cryogenic conditions, with possible relevance to volatile trapping in thermally processed interstellar and cometary ices.