Defect Activity in ICY Solids from Isotopic Excange Rates: Implications for Conductance and Phase Transitions

Devlin, J. Paul

doi:10.1007/978-1-4615-3444-0_22

Cited by 4 publications

(3 citation statements)

References 19 publications

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“…Previously, molecular defects-vacancies and interstitials-have been investigated theoretically as possible trapping sites for Bjerrum defects (23, 24), but no clear evidence of a trapping site that differentiated between the two Bjerrum defects emerged from these studies. Another possibility is the complexation of the Bjerrum defects to the opposing charged ionic species; to explain the experimental finding that L-defect mobility is higher than that of the D-defect, the OH − species should be more efficacious at trapping the D-defect than the L-defect trapped at H 3 O þ species (6,9). Recent theoretical work of Cwiklik et al (26,27) has identified a stable hydroxide species in an "offsite" position, or interstitial trap, that corresponds to a D-defect complexing to the hydroxide ion.…”

Section: Resultsmentioning

confidence: 99%

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Point defects at the ice (0001) surface

Watkins

VandeVondele²,

Slater³

2010

Proc. Natl. Acad. Sci. U.S.A.

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Using density functional theory we investigate whether intrinsic defects in ice surface segregate. We predict that hydronium, hydroxide, and the Bjerrum L-and D-defects are all more stable at the surface. However, the energetic cost to create a D-defect at the surface and migrate it into the bulk crystal is smaller than its bulk formation energy. Absolute and relative segregation energies are sensitive to the surface structure of ice, especially the spatial distribution of protons associated with dangling hydrogen bonds. It is found that the basal plane surface of hexagonal ice increases the bulk concentration of Bjerrum defects, strongly favoring D-defects over L-defects. Dangling protons associated with undercoordinated water molecules are preferentially injected into the crystal bulk as Bjerrum D-defects, leading to a surface dipole that attracts hydronium ions. Aside from the disparity in segregation energies for the Bjerrum defects, we find the interactions between defect species to be very finely balanced; surface segregation energies for hydronium and hydroxide species and trapping energies of these ionic species with Bjerrum defects are equal within the accuracy of our calculations. The mobility of the ionic hydronium and hydroxide species is greatly reduced at the surface in comparison to the bulk due to surface sites with high trapping affinities. We suggest that, in pure ice samples, the surface of ice will have an acidic character due to the presence of hydronium ions. This may be important in understanding the reactivity of ice particulates in the upper atmosphere and at the boundary layer.DFT | first-principles | water | interface T he surface of ice acts as a catalyst in the upper atmosphere yielding halogen containing species that facilitate the destruction of ozone (1). More generally, probing the structure/activity relationship of ice with trace gases is important, topical, and yet difficult in the laboratory. Here, we use computational approaches to address the fundamental question of whether charged defects in ice show a tendency to locate at the surface in preference to the crystal interior.

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Section: Resultsmentioning

confidence: 99%

“…However, a series of experiments on various forms of ice have indicated that proton conduction is mediated by the L-defect while the D-defect is inactive and that the hydronium species is much more mobile than the hydroxide (6) (cubic ice), (7) (cubic and amorphous), (8,9) (polycrystalline ice films), (10) (amorphous ice films).…”

mentioning

confidence: 99%

Point defects at the ice (0001) surface

Watkins

VandeVondele²,

Slater³

2010

Proc. Natl. Acad. Sci. U.S.A.

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“…As a consequence, populations of the "catalytic" guest molecules can generate/change/destroy pure CH nanocrystals on an hour timescale at temperatures as low as 100 K. 13,14 There is also limited evidence that above 110 K mobile defects, generated by a guest "catalyst" within one CH structural layer, can act to form the simple CO 2 CH epitaxially in a vapor-deposited overlayer. 17,18 That is not surprising, since orientational L defects can travel macroscopic distances before recombining with the relatively very few D defects supported by the L-D equilibrium. 15 By contrast, as demonstrated multiple times, classical guest molecules, such as N 2, O 2, CO 2, CH 4 , CF 3 Br, etc., are inactive with respect to CH formation when present alone as potential guest molecules below ∼180 K. Other small guest molecules, including H 2 S and SO 2 , that have been observed to form a simple CH at low temperature (∼140 K) and to accelerate significantly the formation of ether CHs, may also act by lowering activation energies through stabilization of defects.…”

Section: Introductionmentioning

confidence: 96%

Catalytic activity of methanol in all-vapor subsecond clathrate-hydrate formation

Devlin

2014

The Journal of Chemical Physics

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Methanol's property as a catalyst in the formation of gas clathrate hydrates has been recognized for several years and was recently employed in a broad ranging study [K. Shin, K. A. Udachin, I. L. Moudrakovski, D. M. Leek, S. Alavi, C. I. Ratcliffe, and J. A. Ripmeester, Proc. Natl. Acad. Sci. U.S.A. 110, 8437 (2013)]. A new measure of that activity is offered here from comparative rates of formation of methanol (MeOH) clathrate hydrates within our all-vapor aerosol methodology for which tetrahydrofuran (THF) and other small ethers have set a standard for catalytic action. We have previously described numerous examples of the complete conversion of warm all-vapor mixtures to aerosols of gas clathrate hydrates on a sub-second time scale, generally with the catalyst confined primarily to the large cage of either structure-I (s-I) or structure-II (s-II) hydrates. THF has proven to be the most versatile catalyst for the complete subsecond conversion of water to s-II hydrate nanocrystals that follows pulsing of appropriate warm vapor mixtures into a cold chamber held in the 140-220 K range. Here, the comparative ability of MeOH to catalyze the formation of s-I hydrates in the presence of a small-cage help-gas, CO2 or acetylene, is examined. The surprising result is that, in the presence of either help gas, CH-formation rates appear largely unchanged by a complete replacement of THF by MeOH in the vapor mixtures for a chamber temperature of 170 K. However, as that temperature is increased, the dependence of effective catalysis by MeOH on the partial pressure of help gases also increases. Nevertheless, added MeOH is shown to markedly accelerate the s-II THF-CO2 CH formation rate at 220 K.

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Evidence for the surface origin of point defects in ice: Control of interior proton activity by adsorbates

Devlin

Buch

2007

The Journal of Chemical Physics

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Spectroscopic studies are presented of H-D isotopic exchange in the interior of ice nanocrystals. The exchange process is dominated by ionic and orientational defects long viewed as governing the electrical properties of ice. A new finding that interior exchange rates can be controlled by acidic and basic adsorbates is evidence that the defects originate at the ice surface. In particular, it is argued that interior isotopic exchange is a reflection of proton concentrations equilibrated at the ice surface.

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Defect Activity in ICY Solids from Isotopic Excange Rates: Implications for Conductance and Phase Transitions

Cited by 4 publications

References 19 publications

Point defects at the ice (0001) surface

Point defects at the ice (0001) surface

Catalytic activity of methanol in all-vapor subsecond clathrate-hydrate formation

Evidence for the surface origin of point defects in ice: Control of interior proton activity by adsorbates

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