Amorphous ices

N2 as adsorbate, was quite similar to that for N2 on a directly prepared and probably amorphous ice powder [35, 141], On the other hand, N2 adsorption on carbon with increasing thickness of preadsorbed methanol decreased steadily—no limiting isotherm was reached [139]. 

More recently, simulation studies focused on surface melting [198] and on the molecular-scale growth kinetics and its anisotropy at ice-water interfaces [199-204]. Essmann and Geiger [202] compared the simulated structure of vapor-deposited amorphous ice with neutron scattering data and found that the simulated structure is between the structures of high and low density amorphous ice. Nada and Furukawa [204] observed different growth mechanisms for different surfaces, namely layer-by-layer growth kinetics for the basal face and what the authors call a collected-molecule process for the prismatic system. 

Fig. 1.12. Phase diagram of water - glycerine. On the left hand side the dependence of the phase transition time from the ice temperature is shown At -140 °C, amorphous ice transforms into cubic ice in approx. 10 min (Fig. 8 from [1.98]).

Amorphous water (also called glassy water or amorphous ice) can form when the temperature is decreased extremely rapidly below the glass transition temperature (Tg) of water (about 130 K at 0.1 MPa) (Mishima and Stanley, 1998). There are three types of amorphous ice low-density amorphous ice (LDA), high-density amorphous ice (HDA), and very high-density amorphous ice (VHDA), with VHDA being discovered most recently (Finney et al., 2002). 

Since the work function is very sensitive to contaminants, the most reliable measurements are done in ultrahigh vacuum conditions. From the determination of the electron work functions of Fe, Co, Ni, Cu, Au, and other metals in the presence of water adsorbed from the gas phase, it follows that water molecules are oriented with oxygen atoms toward the metal surface. The method is very sensitive to the presence of water. For example, upon adsorption of 3 x lO molecules of water per square centimeter of Co film (4% of a monolayer), the work function value is decreased by ca. 0.3 eV. However, these measurements were done at 77 K, meaning that adsorbed water was likely to be in a crystalline or amorphous ice form. Hence, the quoted results are of limited value to understanding the metal-water system in electrolyte solutions. 

Figure 19 Dependence on Ei of yield collected at various temperatures from a 20-bilayer porous amorphous ice films grown at 27 K. Individual scans are offset vertically for display and are labeled with the temperature at which they were collected. (From Ref. 223.)...

The low threshold energies for the production of D( S), 0( P), and 0( D2) show the importance of valence excited states in the BSD of neutral fragments [47]. The pathway for D( S) desorption probably involves D O D -I- OD. Ffowever, the thresholds for producing 0( P2) and 0( D2), which are the same within experimental error, are lower than the 9.5-and 11.5-eV thermodynamic energies required to produce 0( P2) + 2D( S) and 0( D2) + 2D( S), respectively. The low threshold values therefore indicate that the formation of 0( P2) and 0( D2) must occur by a pathway which involves simultaneous formation of D2. Kimmel et al. have in fact reported [46] a threshold for the production of D2 from D2O ice at — 6 to 7 eV, which supports this conclusion. Above the ionization threshold of amorphous ice, these excited states can be formed directly or via electron-ion recombination. 

Graham, J. D., and J. T. Roberts, Interaction of HCI with Crystalline and Amorphous Ice Implications for the Mechanisms of Ice-Catalyzed Reactions, Geophys. Res. Lett., 22, 251-254 (1995). Graham, J. D J. T. Roberts, L. A. Brown, and V. Vaida, Uptake of Chlorine Dioxide by Model Polar Stratospheric Cloud Surfaces Ultrahigh-Vacuum Studies, J. Phys. Chem.., 100, 3115-3120 (1996a). 

Depending on conditions, frozen substances in comet nuclei can be crystalline ices, amorphous ices, and clathrate hydrates (compounds in which cages in the water-ice lattice can host guest molecules). Compositions of the ices and associated organic materials in comets have been determined from both telescopic and spacecraft observations. Spectral line measurements of gases in a comet s coma allow the identification of molecules and radicals. An inherent difficulty in spectral measurements is that volatiles in the coma are commonly broken... 

Cometary activity occurring at great distance from the Sun (corresponding to temperatures <100 K) is probably controlled by ices more volatile than H20. For example, comet Hale-Bopp exhibited emission of highly volatile CO at great solar distances. Trapped CO was presumably released by crystallization of amorphous ice or sublimation of ice crystals at very low temperatures. 

Recent advances in spectroscopic methods have enabled the water pentamer to be studied experimentally. Infrared cavity ringdown spectroscopy has been used to examine the intramolecular absorption features of the gas-phase water pentamer, which match the spectral features of the pentamer rings in liquid water and amorphous ice (Paul et al., 1999 Burnham et al., 2002). Vibration Rotation Tunnelling (VRT) spectroscopy has been used to provide a more direct probe of the water pentamer intermolecular vibrations and fine structure in liquid water (Liu et al., 1997 Harker et al., 2005). The water pentamer was found to average out... 

Eldrup, M., Vehanen, A., Schultz, P.J. and Lynn, K.G. (1985). Positronium formation and diffusion in crystalline and amorphous ice using a variable-energy positron beam. Phys. Rev. B 32 7048-7064. 

To summarize, amorphous ice is stable below -160 °C, until -125 °C when cubic ice is formed irreversibly from the amorphous phase above this temperature, hexagonal ice develops. Between -160 and -130 °C, cubic ice can be embedded in an amor-... 

These amorphous phases of ice can be of interest for creation on their basis of adjustable stores hydrogen fuel in the form of methane and other. Progress in understanding nature of ice amorphism has been made using developments of fine experiments. But data about formation hydrate methane in amorphous ice are scarce. For quite some time now the scientists have not been trying to identify ways to resolve this problem by studying different samples of ice and learning what combinations of pressure and temperature keep the methane locked up. Other party to problem is how the methane can be extracted. 

In a given work computer simulations devoted to study of nanostructure of abovementioned cryogenic amorphous phases of ice, mechanisms of their transformations, and properties to accumulate methane and hydrogen was realized within the theoretical concepts thermo field dynamics [5] and quantum-field chemistry [6-9]. We developed two models of nanostructures corresponding to HDA-ice and LDA-ice, respectively. Some computations of energetic barriers locking molecules CH4 and H2 inside of amorphous ice were fulfilled. 

Notice that transformation from a crystalline phase to presumably metastable amorphous phases is called amorphization. It is very promising to use for making of adjustable stores hydrogen fuel phenomena that is called polyamorphism. This term meaning that the pure material can exist in more than one amorphous state. In principle, the abovementioned mechanism of density jumps at polyamorphic transition of ice allows to obtain reversible accumulation of methane inside cellular nanostructures of cryogenic amorphous ice. It is important that the degree of accumulation can be sharp adjusted by pressure and temperature. 

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