Weak interactions between water and clathrate-forming gases at low pressures

Using scanning probe microscopy and temperature programed desorption we examined the interaction between water and two common clathrate-forming gases, methane and isobutane, at low temperature and low pressure. Water co-deposited with up to 10(-1) mbar methane or 10(-5) mbar isobutane at 140 K onto a Pt(111) substrate yielded pure crystalline ice, i.e., the exposure to up to similar to 10(7) gas molecules for each deposited water molecule did not have any detectable effect on the growing films. Exposing metastable, less than 2 molecular layers thick, water films to 10-5 mbar methane does not alter their morphology, suggesting that the presence of the Pt(111) surface is not a strong driver for hydrate formation. This weak water-gas interaction at low pressures is supported by our thermal desorption measurements from amorphous solid water and crystalline ice where 1 ML of methane desorbs near similar to 43 K and isobutane desorbs near similar to 100 K. Similar desorption temperatures were observed for desorption from amorphous solid water. (C) 2015 Elsevier B.V. All rights reserved.