Amorphous solid water

The preceding observations stimulated Olander and Rice 4> to search for a substance that is simultaneously simpler" than water yet a "good model of it. They suggested that amorphous solid water [H O/as)], first reported by Burton and Oliver 5> in 1935, satisfied these two requirements. Unlike the liquid, amorphous solid water can be studied at low temperature where the effects of thermal excitation and positional and orientational disorder can be separated. Moreover, it is plausible to accept as a working hypothesis that the amorphous solid is, essentially, extensively supercooled liquid water if so, the properties of the amorphous solid should be directly related to those of the liquid. 

As noted earlier, the diffraction of X-rays, unlike the diffraction of neutrons, is primarily sensitive to the distribution of 00 separations. Although many of the early studies 9> of amorphous solid water included electron or X-ray diffraction measurements, the nature of the samples prepared and the restricted angular range of the measurements reported combine to prevent extraction of detailed structural information. The most complete of the early X-ray studies is by Bon-dot 26>. Only scanty description is given of the conditions of deposition but it appears likely his sample of amorphous solid water had little or no contamination with crystalline ice. He found a liquid-like distribution of 00 separations at 83 K, with the first neighbor peak centered at 2.77 A. If the pair correlation function is decomposed into a superposition of Gaussian peaks, the area of the near neighbor peak is found to correspond to 4.23 molecules, and to have a root mean square width of 0.50 A. 

At the time the neutron diffraction experiments were carried out it was not known that there are two forms of amorphous solid water. Consequently, although the deposition system was designed to ensure elimination of crystalline ice in the sample, neither the geometry nor the deposition rate were the same as used in the X-ray experiments of Narten, Venkatesh and Rice 7>27>. We shall argue below that although the substrate temperature used by WLR was low, their data are only consistent with diffraction from high temperature low density D20(as). 

The vapour deposition method is widely used to obtain amorphous solids. In this technique, atoms, molecules or ions of the substance (in dilute vapour phase) are deposited on to a substrate maintained at a low temperature. Most vapour-deposited amorphous materials crystallize on heating, but some of them exhibit an intervening second-order transition (akin to the glass transition). Amorphous solid water and methanol show such transitions. The structural features of vapour-deposited amorphous solids are comparable to those of glasses of the same materials prepared by melt-quenching. 

Preparation from the Gas Phase Amorphous Solid Water (ASW)... 

Using different deposition rates, even a highly compacted form of amorphous solid water of density > 1 g/cm3 could be obtained at deposition temperature T < 30 K, which transforms gradually in the temperature range 38-68 K to the lower density form of density 0.94 g/cm3 [139, 140]. This transition was proposed to be at the origin of crack-formation processes in comets [141]. We note, however, that the formation of this high-density amorph at very low temperatures has been doubted [142, 143]. Only photolysis at 20 K induces a transition to a high-density amorph [143]. 

Boutron P, Alben R (1975) Structural model for amorphous solid water. J Chem Phys 62 4848 - 4853... 

Narten AH, Venkatesh CG, Rice SA (1976) Diffraction pattern and structure of amorphous solid water at 10 and 77 K. J Chem Phys 64 1106-1121... 

Giovambattista N, Stanley HE, Sciortino E. Phase diagram of 62. amorphous solid water Low-density, high-density and very-high-density amorphous ices. Phys. Rev. E 2005 72 031510. 

Figure 5 Heat capacity of doped (solid circles) and pure (open circles) amorphous solid water...

The Au sample is cooled with liquid nitrogen to 100 K and exposed to water or methanol by backfilling the chamber. Exposures are stated in Langmuirs (L) (1 L=10 Torr s). It is expected that amorphous solid water and methanol are formed during the slow deposition of the respective vapours at temperatures below 120 K (see also ref [11]). 

Giovambattista N., Stanley H., Sciortino F. (2005) Relation between the High Density Phase and the Very-High Density Phase of Amorphous Solid Water, T /2j5. Rev. Lett. 94(10), 107803-107807. 

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