Small-angle neutron scattering study of micropore collapse in amorphous solid water

Mitterdorfer, Christian; Bauer, Marion; Youngs, Tristan G. A.; Bowron, Daniel T.; Hill, C. R.; Fraser, H. J.; Finney, John; Loerting, Thomas

doi:10.1039/c4cp00593g

Cited by 38 publications

(57 citation statements)

References 73 publications

(95 reference statements)

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“…While the relative volume of the pores does not change drastically during the warm-up of the ices, the total surface decreases by a factor 3.5 between 10 and 120 K, influencing the increase in average pore size. This is consistent with a recent study from Mitterdorfer et al (2014) who show, by using small angle neutron scattering of ASW ice deposited at 77 K, that the specific surface area decreases from 90 to 100 K. The pores present in our simulations reach these Fig. 3.…”

Section: Pore Volume and Surface Areasupporting

confidence: 82%

Pore evolution in interstellar ice analogues

Cazaux

Bossa

Linnartz

et al. 2014

A&A

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Context. The level of porosity of interstellar ices, largely comprised of amorphous solid water (ASW), contains clues about the trapping capacity of other volatile species and determines the surface accessibility that is needed for solid-state reactions to take place. Aims. Our goal is to simulate the growth of amorphous water ice at low temperature (10 K) and to characterise the evolution of the porosity (and the specific surface area) as a function of temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are used to mimic the formation and the thermal evolution of pores in amorphous water ice. We follow the accretion of gas-phase water molecules as well as their migration on surfaces with different grid sizes, both at the top growing layer and within the bulk. Results. We show that the porosity characteristics change substantially in water ice as the temperature increases. The total surface of the pores decreases to a great extend while the total volume decreases only slightly for higher temperatures. This will decrease the overall reaction efficiency, but in parallel, small pores connect and merge, which allows trapped molecules to meet and react within the pores network and provides a pathway to increase the reaction efficiency. We introduce pore coalescence as a new solid-state process that may boost the solid-state formation of new molecules in space, and which has not been considered so far.

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Section: Pore Volume and Surface Areasupporting

confidence: 82%

Pore evolution in interstellar ice analogues

Cazaux

Bossa

Linnartz

et al. 2014

A&A

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“…It is well known that ASW might be deposited in a porous form, which depends on deposition angle, rate and temperature (Dohnalek et al, 2003;Hill et al, 2016;Kimmel et al, 2001a, b;Kouchi et al, 1994;Mayer and Pletzer, 1986;Mitterdorfer et al, 2014;Raut et al, 2007;Stevenson et al, 1999). Deposition of ASW at temperatures between 90 and 110 K revealed either small degrees of porosity (Brown et al, 1996;Chonde et al, 2006) or were nonporous (Kimmel et al, 2001b;Stevenson et al, 1999).…”

Section: The Effect Of Nano-crystalline Ice On the Vapor Pressurementioning

confidence: 99%

The vapor pressure over nano-crystalline ice

2018

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Abstract. The crystallization of amorphous solid water (ASW) is known to form nano-crystalline ice. The influence of the nanoscale crystallite size on physical properties like the vapor pressure is relevant for processes in which the crystallization of amorphous ices occurs, e.g., in interstellar ices or cold ice cloud formation in planetary atmospheres, but up to now is not well understood. Here, we present laboratory measurements on the saturation vapor pressure over ice crystallized from ASW between 135 and 190 K. Below 160 K, where the crystallization of ASW is known to form nano-crystalline ice, we obtain a saturation vapor pressure that is 100 to 200 % higher compared to stable hexagonal ice. This elevated vapor pressure is in striking contrast to the vapor pressure of stacking disordered ice which is expected to be the prevailing ice polymorph at these temperatures with a vapor pressure at most 18 % higher than that of hexagonal ice. This apparent discrepancy can be reconciled by assuming that nanoscale crystallites form in the crystallization process of ASW. The high curvature of the nano-crystallites results in a vapor pressure increase that can be described by the Kelvin equation. Our measurements are consistent with the assumption that ASW is the first solid form of ice deposited from the vapor phase at temperatures up to 160 K. Nano-crystalline ice with a mean diameter between 7 and 19 nm forms thereafter by crystallization within the ASW matrix. The estimated crystal sizes are in agreement with reported crystal size measurements and remain stable for hours below 160 K. Thus, this ice polymorph may be regarded as an independent phase for many atmospheric processes below 160 K and we parameterize its vapor pressure using a constant Gibbs free energy difference of (982 ± 182) J mol −1 relative to hexagonal ice.

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“…Furthermore, it has been shown that faster deposition enhances the porosity and leads to a slower sintering process [15], which raises the question whether the pores in our sample could remain intact even when annealing to temperatures where significant sample desorption already takes place?…”

Section: Resultsmentioning

confidence: 99%

“…One obvious candidate for such defects in ASW are molecules in the vicinity of pores. ASW can be a highly porous solid, with a pore size distribution which strongly depends on the deposition conditions [15,16]. Pores in ASW are characterized by dangling OH bonds of molecules located close to the pores, hence, the different slow relaxation rates or the appearance of a fast relaxing component (or both phenomena) seen in figure 2, could perhaps be related to a high density of pores when growing at 52K, which reduces for higher growth temperatures of ASW and disappears when growing well above the crystallization temperature.…”

Section: Resultsmentioning

confidence: 99%

“…The 200K annealing temperature is well above the nominal crystallization 8 temperature. Furthermore, a few studies have shown that the porous structure collapses already at lower temperatures (< 160K) [15,16,18][23] . Consequently, if indeed the relative intensity of the faster relaxing component was related to the number of molecules located at, or close to, the surface of pores, one could expect the relative intensity of this component to vanish or at least significantly decrease after annealing, similarly to what was observed in the dielectric measurements [17].…”

Section: Resultsmentioning

confidence: 99%

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In Situ NMR Measurements of Vapor Deposited Ice

et al. 2016

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In-situ NMR spin-lattice relaxation measurements were performed on several vapor deposited ices. The measurements, which span more than 6 orders of magnitude in relaxation times, show a complex spin-lattice relaxation pattern that is strongly dependent on the growth conditions of the sample. The relaxation patterns change from multi-timescale relaxation for samples grown at temperatures below the amorphous-crystalline transition temperature to single exponential recovery for samples grown above the transition temperature. The slow-relaxation contribution seen in cold-grown samples exhibits a temperature dependence, and becomes even slower after the sample is annealed at 200K. The fast-relaxation contribution seen in these samples, does not seem to change or disappear even when heating to temperatures where the sample is evaporated. The possibility that the fast relaxation component is linked to the microporous structures in amorphous ice samples is further examined using an environmental electron scanning microscope. The images reveal complex meso-scale microporous structures which maintain their morphology up to their desorption temperatures. These findings, support the possibility that water molecules at pore surfaces might be responsible for the fast-relaxation contribution. Furthermore, the results of this study indicate that the pore-collapse dynamics observed in the past in amorphous ices using other experimental techniques, might be effectively inhibited in samples which are grown by relatively fast vapor deposition. * Electronic address: ga232@tx.technion.ac.il Tel: +97248295563 1 arXiv:1711.07228v1 [cond-mat.mtrl-sci]

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Small-angle neutron scattering study of micropore collapse in amorphous solid water

Cited by 38 publications

References 73 publications

Pore evolution in interstellar ice analogues

Pore evolution in interstellar ice analogues

The vapor pressure over nano-crystalline ice

In Situ NMR Measurements of Vapor Deposited Ice

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