Unexpectedly stable nitrogen, oxygen, carbon monoxide and argon clathrate hydrates from vapour-deposited amorphous solid water: an X-ray and two-step differential scanning calorimetry study
Clathrate hydrate (CH) formation from vapour-deposited amorphous solid water (ASW), with N2, O2, CO and Ar as guest species at ca. 1 bar ‘external’ pressure, was investigated by X-ray diffraction and two-step differential scanning calorimetry. For a heating rate of 10 K min–1, peak temperatures for formation of CH by reaction of trapped gas with ASW and/or cubic ice in the closed pores are 201 K (O2), 206 K (Ar) and 208 K (N2 and CO). The four CHs decompose only close to the m.p. of ice, apparently because they are stabilized by enclosing layers of ice. The flow characteristics of water vapour during preparation of ASW influence the subsequent yield of CH formation, supersonic flow giving ca. 15%(O2–CH), ca. 7%(Ar–CH), ca. 4%(CO–CH) and ca. 5%(N2–CH)‘yield’, whereas baffled flow gives yields which are lower by a factor of ca. 4. Decomposition of CHs is also influenced, supersonic flow giving decomposition of O2–CH in two steps, at ca. 222 and ca. 246 K, whereas samples made by baffled flow decompose in one step, i.e. at the higher temperature. Astrophysical implications are discussed briefly.