Dissociation and Reformation of CO2 Clathrate Hydrate Cages in Amorphous Ice Thin Film under Ultrahigh Vacuum

The formation of clathrate hydrates (CHs) within amorphous ice holds significant astrochemical importance. CO₂ typically forms structure I (sI) CH under vacuum, upon nucleation from an amorphous water-CO₂ ice mixture. This study presents the first report of structure II (sII) CO₂ CH, where CO₂ occupies large cages in the presence of dimethyl ether (DME) or tetrahydrofuran (THF) under ultrahigh vacuum (UHV) conditions. Mixed CHs of DME-CO₂ were prepared by sequential vapor phase deposition of CO₂:water mixture over DME at 10 K and thermally annealing this mixed ice film to 130 K. During the formation of large cages of sII CH of DME, partial dissociation of the preformed small cage of sI of CO₂ CH occurs along with the formation of new large cages of sII CO₂ CH. Similar results were observed for THF-CO₂ mixed CHs. Additionally, mixed CHs of THF-DME-CO₂ formed sII hydrates when annealed at 130 K. Prolonged annealing (37 h) at 130 K led to the dissociation of mixed CHs, releasing CO₂ and DME and increasing the THF CH fraction. These observations highlight the greater stability of THF CH compared with those of CO₂ and DME under identical conditions. These findings enhance our understanding of the structural dynamics and formation mechanisms of mixed CHs under simulated interstellar conditions.