Ingrid Kohl

A second distinct structural “state” of high-density amorphous ice at 77 K and 1 bar

High-density amorphous ice (HDA), further densified on isobaric heating from 77 K to 165 (177) K at 1.1 (1.9) GPa, relaxes at 77 K and 1 bar to the same structural “state” with a density of 1.25 ± 0.01 g cm−3. Its density is higher by ≈9% than that of HDA, and thus it is called very-high-density amorphous ice (VHDA). X-ray diffractogram and Raman spectrum of VHDA clearly differs from that of HDA, and the hydrogen-bonded O–O distance increases from 2.82 A in HDA to 2.85 A in VHDA. Implications for the polyamorphism of the amorphous forms of water are discussed.

The local and intermediate range structures of the five amorphous ices at 80 K and ambient pressure: A Faber-Ziman and Bhatia-Thornton analysis

Using isotope substitution neutron scattering data, we present a detailed structural analysis of the short and intermediate range structures of the five known forms of amorphous ice. Two of the lower density forms--amorphous solid water and hyperquenched glassy water--have a structure very similar to each other and to low density amorphous ice, a structure which closely resembles a disordered, tetrahedrally coordinated, fully hydrogen bonded network. High density and very high density amorphous ices retain this tetrahedral organization at short range, but show significant differences beyond about 3.1 A from a typical water oxygen. The first diffraction peak in all structures is seen to be solely a function of the intermolecular organization. The short range connectivity in the two higher density forms is more homogeneous, while the hydrogen site disorder in these forms is greater. The low Q behavior of the structure factors indicates no significant density or concentration fluctuations over the length scale probed. We conclude that these three latter forms of ice are structurally distinct. Finally, the x-ray structure factors for all five amorphous systems are calculated for comparison with other studies.

Towards the Experimental Decomposition Rate of Carbonic Acid (H2CO3) in Aqueous Solution

Dry carbonic acid has recently been shown to be kinetically stable even at room temperature. Addition of water molecules reduces this stability significantly, and the decomposition (H2CO3 + nH2O --> (n+1)H2O + CO2) is extremely accelerated for n = 1, 2, 3. By including two water molecules, a reaction rate that is a factor of 3000 below the experimental one (10 s(-1)) at room temperature was found. In order to further remove the gap between experiment and theory, we increased the number of water molecules involved to 3 and took into consideration different mechanisms for thorough elucidation of the reaction. A mechanism whereby the reaction proceedes via a six-membered transition state turns out to be the most efficient one over the whole examined temperature range. The determined reaction rates approach experimental values in aqueous solution reasonably well; most especially, a significant increase in the rates in comparison to the decomposition reaction with fewer water molecules is found. Further agreement with experiment is found in the kinetic isotope effects (KIE) for the deuterated species. For water-free carbonic acid, the KIE (i.e., kH2CO3/kD2CO3) for the decomposition reaction is predicted to be 220 at 300 K, whereas it amounts to 2.2-3.0 for the investigated mechanisms including three water molecules. This result is therefore reasonably close to the experimental value of 2 (at 300 K). These KIEs are in much better accordance with the experiment than the KIE for decomposition with fewer water entities.

The glassy water–cubic ice system: a comparative study by X-ray diffraction and differential scanning calorimetry

Mixtures of various ratios of cubic ice and glassy water were obtained by so-called hyperquenching of micrometer-sized water droplets at cooling rates of ≈106–107 K s−1 on a substrate held at selected temperatures between 130 and 190 K. These samples were characterized by differential scanning calorimetry (DSC) and X-ray diffraction. The minimum deposition temperature to obtain almost entirely vitrified samples is ≈140 K. Glassy water prepared at this temperature exhibits on heating an endothermic step assignable to a glass→liquid transition, without the requirement for previous annealing. Cubic ice samples obtained by deposition at 160 and 170 K undergo on heating two distinct exothermic processes of comparable intensity. One centered at ≈230 K is caused by the phase transition to hexagonal ice. The other is centered at ≈201 K in a sample deposited at 170 K, and it shifts to ≈193 K on deposition at 160 K. The latter process is attributed to the increase in particle size, relief of non-uniform strain and/or healing of different kinds of defects. Since the temperature of this second exotherm depends on the deposition temperature of the sample, it merges on sample deposition at 190 K with the exotherm from the cubic→hexagonal ice phase transition. Therefore, this can lead to an overestimation of the heat of the cubic→hexagonal phase transition. For samples deposited at ⩽150 K, the low temperature exotherm merges with the intense exotherm due to glassy water→cubic ice phase transition. X-ray diffractograms and DSC scans of cubic ice samples of different thermal history show, after annealing at the same temperature of 183 K for 5 min, essentially identical patterns. Likewise, X-ray diffractograms of cubic ice made on heating hyperquenched glassy water or vapor-deposited amorphous solid water up to 183 K are indistinguishable. Cubic ice deposited at 190 K, or annealed at 183 K, contains at most 20% amorphous component which persists up to the cubic to hexagonal ice phase transition. This is in contrast to recent claims of Jenniskens et al. (J. Chem. Phys. 1997, 107, 1232) that cubic ice obtained by heating thin films of vapor-deposited amorphous water contains more than 50% of amorphous, or even liquid, water.

(Meta-)stability domain of ice XII revealed between 158-212 K and 0.7-1.5 GPa on isobaric heating of high-density amorphous ice

High-density amorphous ice was heated at constant pressures of between 0.52 to 1.9 GPa from 77 K up to 240 K. The formed phases were characterized by x-ray diffractograms of samples recovered under liquid N2 at 1 bar. The (meta)stability domain of ice XII thus revealed extends between ≈158–212 K from ≈0.7 to ≈1.5 GPa. We further discuss whether ice XII has a low-temperature region of stability within the ice VI domain. Our (meta)stability domain of ice XII is in a different region of water’s phase diagram than that shown by Koza et al. [Phys. Rev. Lett. 84, 4112 (2000)].

Water Behaviour: Glass transition in hyperquenched water?

Arising from: Y.-Z. Yue & C. A. Angell 427, 717–720 (2004); Yue & Angell reply.It has been unclear whether amorphous glassy water heated to around 140–150 K remains glassy until it crystallizes or whether instead it turns into a supercooled and very viscous liquid. Yue and Angell compare the behaviour of glassy water under these conditions to that of hyperquenched inorganic glasses, and claim that water stays glassy as it heats up to its crystallization point; they also find a ‘hidden’ glass-to-liquid transition at about 169 K. Here we use differential scanning calorimetry (DSC) heating to show that hyperquenched water deposited at 140 K behaves as an ultraviscous liquid, the limiting structure of which depends on the cooling rate — as predicted by theoretical analysis of the liquid-to-glass transition. Our findings are consistent with a glass-to-liquid transition-onset temperature (Tg) in the region of 136 K (refs 3,4), and they indicate that measurements of the liquids properties may clarify the anomalous properties of supercooled water.

Spectroscopic Observation of Matrix-Isolated Carbonic Acid Trapped from the Gas Phase**

) is of fundamental importance, forexample,forregulationofbloodpH,intheacidificationoftheoceans, and in the dissolution of carbonates. This six-atommolecule commonly found in carbonated drinks in submicro-molar concentrations has so far eluded most attempts atisolation and direct detection. Despite the widespread beliefthat it is a highly instable molecule, the pure solid could beprepared previously,

Raman Spectroscopic Study of the Phase Transition of Amorphous to Crystalline β‐Carbonic Acid

Whats the matter? The laboratory Raman spectra for carbonic acid (H(2)CO(3)), both for the beta-polymorph and its amorphous state, are required to detect carbonic acid on the surface of the pole caps of Mars in 2009, when the Mars Microbeam Raman Spectrometer lands on the planet. The picture shows a martian crater with ice of unknown composition, possibly containing carbonic acid (image adapted from DLR, with permission from ESA, DLR, and FU Berlin--G. Neukum).

Cryoflotation: Densities of Amorphous and Crystalline Ices

We present an experimental method aimed at measuring mass densities of solids at ambient pressure. The principle of the method is flotation in a mixture of liquid nitrogen and liquid argon, where the mixing ratio is varied until the solid hovers in the liquid mixture. The temperature of such mixtures is in the range of 77-87 K, and therefore, the main advantage of the method is the possibility of determining densities of solid samples, which are instable above 90 K. The accessible density range (~0.81-1.40 g cm(-3)) is perfectly suitable for the study of crystalline ice polymorphs and amorphous ices. As a benchmark, we here determine densities of crystalline polymorphs (ices I(h), I(c), II, IV, V, VI, IX, and XII) by flotation and compare them with crystallographic densities. The reproducibility of the method is about ±0.005 g cm(-3), and in general, the agreement with crystallographic densities is very good. Furthermore, we show measurements on a range of amorphous ice samples and correlate the density with the d spacing of the first broad halo peak in diffraction experiments. Finally, we discuss the influence of microstructure, in particular voids, on the density for the case of hyperquenched glassy water and cubic ice samples prepared by deposition of micrometer-sized liquid droplets.

The low-temperature dynamics of recovered ice XII as studied by differential scanning calorimetry: a comparison with ice V

Samples of ice XII made on isobaric heating of high-density amorphous ice (HDA) were recovered at 77 K and characterized by X-ray diffraction. Studies by differential scanning calorimetry (DSC) revealed, in addition to the intense exotherm from the ice XII → cubic ice transition, on heating at 30 K min−1 a reversible endothermic step with an onset temperature (Tonset) of 130 ± 1 K, an increase in heat capacity (ΔCp) of 1.9 ± 0.2 J K−1 mol−1, and a width of 10 ± 1 K. The effects of the annealing temperature (Tanneal) for a fixed annealing time (tanneal), and of tanneal for a fixed Tanneal on the enthalpy and entropy relaxations and recovery have been ascertained, and these are phenomenologically similar to those of a glass. Ice XII samples made at 77 K on pressurizing of hexagonal ice via HDA show an additional thermal effect, namely an irreversible exotherm centered at 133 (125) K on heating at 30 (10) K min−1, which is attributed to stress/strain release. For comparison, recovered ice V samples were studied by DSC in the same manner. These also show, in addition to the exotherm from the ice V → cubic ice transition, on first heating at 30 K min−1 a reversible endothermic step with Tonset of 130 ± 1 K, ΔCp of 1.7 ± 0.2 J K−1 mol−1, and width of 20 ± 1 K, and annealing effects similar to those of ice XII. For ice V the existence of a temperature-dependent equilibrium between proton order and disorder, with proton order increasing with decreasing temperature, had been ascertained by Lobban et al. (J. Chem. Phys., 2000, 112, 7169). We thus interpret the endothermic step in unannealed ice V to kinetic unfreezing of proton order–disorder, the equilibrium line being attained by proton order → disorder transition, and the effects of annealing to enthalpy and entropy loss during annealing by approaching the proton order–disorder equilibrium, and on their subsequent recovery on reheating, in line with Handa et al. (J. Phys. 1987, 48, C1-435). The endothermic DSC features of ice XII are interpreted in the same manner, by relaxation of the frozen-in proton order–disorder towards equilibrium via proton order → disorder transition. For ice XII (ice V) the maximal value of recovered configurational entropy corresponds to 0.17 J K−1 mol−1 (0.22 J K−1 mol−1) after annealing at 115 K for 120 min (111 K for 90 min) which is 5.0% (6.5%) of the maximal value of 3.37 J K−1 mol−1 for complete proton order → disorder transition. We further report the DSC features of low-density amorphous ice recorded on heating at 30 K min−1, with Tonset of an endothermic step at 134 ± 2 K and ΔCp of 0.7 ± 0.1 J K−1 mol−1, and we show the similarities of this endothermic feature to those of ice XII.