Characterization of porosity in vapor-deposited amorphous solid water from methane adsorption

Abstract

We have characterized the porosity of vapor-deposited amorphous solid water (ice) films deposited at 30-40 K using several complementary techniques such as quartz crystal microgravimetry, UV-visible interferometry, and infrared reflectance spectrometry in tandem with methane adsorption. The results, inferred from the gas adsorption isotherms, reveal the existence of microporosity in all vapor-deposited films condensed from both diffuse and collimated water vapor sources. Films deposited from a diffuse source show a step in the isotherms and much less adsorption at low pressures than films deposited from a collimated source with the difference increasing with film thickness. Ice films deposited from a collimated vapor source at 77 degrees 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 deposited from a collimated source peaks at approximately 0.083 eV for deposition at normal incidence and at approximately 0.077 eV for deposition at >45 degrees 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 the 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 deposited at 77 degrees incidence, the increase in intensity of the dangling bond absorptions modified by methane adsorption departs from linearity at large uptakes.

The vapor pressure over nano-crystalline ice

“…We interpret this behavior as the thermally activated crystallization of ASW. From our data we estimate the crystallization constant τ to be about 25 min at 140 K, which is in agreement with previously reported temperature-dependent crystallization constants and times (Dowell and Rinfret, 1960;Mitchell et al, 2017;Sack and Baragiola, 1993;Smith et al, 1996;Smith et al, 2011).…”

“…Measuring water vapor desorption rates at the temperatures under investigation is a challenging task and previous experiments were influenced by contamination issues, showed a very large degree of scattering in the data or yielded unphysically low vapor pressures below that of ice I h (Bryson et al, 1974;Fraser et al, 2001;La Spisa et al, 2001). Sack and Baragiola (1993) carefully avoided contributions of water molecules from external sources by shielding the ice sample with cold surfaces held at 12 K and measured the desorption rate at a constant temperature. We converted their data representing ice crystallized from ASW (Fig.…”

“…At temperatures below 160 K, however, only a limited number of desorption rate measurements of ice crystallized from ASW using quadrupole mass spectrometers and quartz crystal microbalances is available that may be used to calculate the saturation vapor pressure over the ice phase (Brown et al, 1996;Bryson et al, 1974;Fraser et al, 2001;La Spisa et al, 2001;Sack and Baragiola, 1993;Smith et al, 2011;Speedy et al, 1996).…”

SPECTROSCOPIC EFFECTS IN CH4/H2O ICES

“…The redshift of the DB double peak observed in CH 4 /H 2 O ice mixtures with respect to pure water ice has been observed before by Raut et al (2007) in water ices generated at 40 K and subsequentially exposed to methane. It has been interpreted as a result of an interaction of the free water molecules in the porous structure with methane.…”

“…These authors stress the importance of laboratory investigations to fully understand the effects on observed spectra of concentration and temperature of mixed samples. Horimoto et al (2002) and Raut et al (2007) have also studied CH 4 /H 2 O ice mixtures generated by CH 4 adsorption on water ice, aiming to obtain information about the morphology of some water ices.…”