Characterization Of Porosity In Vapor-deposited Amorphous Solid Water By Methane Adsorption

Details Characterization Of Porosity In Vapor-deposited Amorphous Solid Water By Methane Adsorption Characterization Of Porosity In Vapor-deposited Amorphous Solid Water By Methane Adsorption

Oct 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dps....39.3802r&link_type=abstract

American Astronomical Society, DPS meeting #39, #38.02; Bulletin of the American Astronomical Society, Vol. 39, p.489

Astronomy and Astrophysics

Astronomy

Scientific paper

We have characterized the porosity of vapor-deposited amorphous solid water (ice) films deposited at 30-40 K using several complementary techniques: quartz-crystal microgravimetry, UV-visible interferometry, infrared reflectance spectrometry, and methane adsorption. The results, inferred from the gas adsorption isotherms, reveal the existence of microporosity in all vapor-deposited films grown from both diffuse and collimated water vapor sources. Films grown from a diffuse source show a step in the isotherms, and much less adsorption than films grown from a collimated source at low pressures, with the difference increasing with film thickness. Ice films deposited from a collimated vapor source at 77o incidence are mesoporous, in addition to having micropores. Remarkably, mesoporosity is retained upon warming to temperatures as high as 140 K where the ice crystallized. The binding energy distribution for methane adsorption in the micropores of ice films grown from a collimated source peaks at 0.083 eV for deposition at normal incidence and at 0.077 eV for deposition at > 45o incidence. For microporous ice, the intensity of the infrared bands due to methane molecules on dangling OH bonds on pore surfaces increases linearly with methane uptake, up to saturation adsorption. This shows that multilayer condensation of methane does not occur inside the micropores. Rather, filling of the core volume results from coating the pore walls with the first layer of methane, indicating pore widths below a few molecular diameters. For ice grown at 77o incidence, the increase in intensity of the dangling bond absorptions modified by methane absorption departs from linearity at large gas uptakes.
We acknowledge support from NASA Origins of the Solar System and NSF Astronomy programs.

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