Ice phases

The effects of pressure on the phase transition of liquid water to ice (and within the ice phase itself) are complicated by the formation of several pressure-dependent ice polymorphs (Chaplin, 2004 Franks, 1984, 2000 Kalichevsky et al., 1995 Ludwig, 2001). Thirteen crystalline forms of ice have been reported to date Ih (hexagonal or normal or regular ice), Ic (cubic... 

Before proceeding, it is worth noting that at temperatures below freezing (i.e., below the freezing point of food, in the presence of an ice phase), the definition of aw changes to (Fennema, 1996)... 

The freeze concentration process is based on the partial solidification of water into ice in a fluid food product followed by the removal of the solid ice phase from the concentrated liquid phase. This process has some inherent advantages over evaporation and reverse osmosis for concentrating fluid foods as well as other process streams (1). One advantage is that essentially none... 

The DNA solvation shell consists of about 20-22 water molecules per nucleotide of these, — 15-17 waters associate with the nucleoside and —5 waters associate with the phosphate group [13,14]. Water outside the solvation layer is termed bulk water. Upon freezing, the DNA solvation water forms two primary phases the ice phase, consisting of one or more of the crystalline forms of ice, and a DNA-associated phase, consisting of ordered water which comes in direct contact with the DNA (primary layer) and disordered water in the secondary layer. DNA hydration is expressed in terms of F, the number of water molecules per nucleotide. 

Studies of the direct effect have been largely confined to DNA samples in the solid state. This is done in order to maximize direct-type damage and minimize indirect-type damage. In addition, low temperatures are often employed both as a means of sequestering the DNA from the bulk water and as a means of stabilizing free radical intermediates. In frozen DNA samples, the mobility of holes and excess electrons differs for the different sample components ice, solvation shell, DNA backbone, and base stacks. We start with the ice phase. 

During irradiation, the holes produced in the ice phase are sequestered by the ice. This is not strictly true for the excess electrons some escape the ice phase and are selectively scavenged by DNA. The degree to which this increases the yield of electrons trapped by DNA is considered to be relatively small and therefore when computing yields of direct damage in DNA, the mass of the ice phase is usually excluded from the target mass. While ice is a relatively passive component of the system, it does provide an important reference point with respect to product yields. Electrons and holes are quite mobile in ice [87,88] it is a decent conductor. Consequently, even at low temperatures, the free radical yields are quite low, e.g., the yield of HO in ice at 77 K is 0.037 gmol/J [83]. This is just one example of a... 

Fig. 2 Scheme depicting the radiological behavior of the waters of hydration in DNA. For the first nine waters, ESR evidence suggests that both holes and electrons efficiently transfer to the DNA. For samples with additional waters from F=9 to F=21 (F is defined as the number of water molecules per nucleotide), holes form hydroxyl radicals with ESR parameters characteristic of a glassy environment and electrons efficiently transfer to the DNA. For samples with F>21, a crystalline ice phase forms holes in the ice phase form hydroxyl radicals with parameters characteristic of a crystalline ice environment. There is no ESR evidence that electrons from this phase transfer to the DNA. For samples with F>21, the glassy phase is reduced to about 14 waters per nucleotide with the remainder ice phase [3c]... 

Fig. 9 Plots of the transfer rates of electrons, holes, and overall DNA radicals at 77 K vs hydration levels lower axis) as well as vs the distance between DNA ds s (upper axis). Values of D s are estimated from the work of Lee et al. [39]. The results show that as amorphous (glassy) hydration increases up to T=22 D20/nucleotide, D s increases and transfer rate decreases. At T=30 D20/nucleotide, the ice is formed, and leaves the actual amorphous hydration level at around 14 D20/nucleotide with the remainder in the ice phase. The plot clearly shows equivalent transfer rates for both hydration levels at 14 and 30 D20/nucleotide. This result suggests that Djs plays an important role in hydration-dependent hole and electron transfer in DNA [7dj. Reprinted with permission from the J. Phys. Chem. Copyright (2001) American Chemical Society...

Ice refers to frozen aqueous solutions which form an crystalline ice phase and regions of DNA with ca. 14 hydration waters... 

More specifically, the H20 triple point is the specific state in which the liquid, gaseous, and ice phases of H20 (of terrestrial isotopic composition in the absence of air) coexist in equilibrium. (Why such a triple point corresponds to a unique specification of temperature and other particulars will be discussed in Section 7.1.)... 

Surprising behavior of liquid and ice phases is found if we follow various 7, P paths in this extended phase diagram. Sidebar 7.4 illustrates how to determine the expected phase transitions and properties of H20 for various temperatures and pressures far outside the realm of ordinary experience. It is remarkable that such multiplicity of forms and properties can result from a pure substance composed of only a single type of molecule. 

At very high pressures (0.3-2.1 GPa), gas hydrates undergo structural transitions to other hydrate phases and filled ice phases. Guests can multiply occupy the large cages of these high-pressure hydrate phases. 

However, engineers normally can access only the concentrations of the water-free hydrocarbon phase, and that of the hydrocarbon-free water/ice phase, rather than the total phase concentrations. Other concentrations are generally difficult to measure, except by unusual techniques. While the concentration of the water-free hydrocarbon phase is readily measured (via chromatography, for example) the water concentration of the hydrocarbon phase is generally so low (typically parts per thousand or less) that sophisticated techniques (dual chromatographic columns, etc.) are required for reliable measurement. 

What is the time required to fully dissociate (hydrate and ice phases dissociated)... 

The time for the annulus spacing to achieve 2 in. can be determined by opening the file two.dat (see below) in Excel and looking at the time for the diameter of the ice phase to reach 14 in. 

Hydration of the DNA has also a strong influence on the radical yield at 77K (Wang et al. 1993). The G values increase by over fourfold upon addition of the primary hydration layer, that is about 20 water molecules per nucleotide. Upon further water addition, the excess water freezes into an apparently independent bulk ice phase which steals about five water molecules from the hydration layer and thus reduces the DNA radical yield. It has been concluded that efficient hole... 

Fig 1.10 shows log J J = nuclei per unit time and volume) as a function of the temperature of the water-ice phase transition at different pressures of 1 and 2100 bar according to Riehle. At 2100 bar, J is comparable to J at an approximately 35 °C higher temperature. Under pressure, water can be subcooled further, with a delayed formation of nuclei. 

Fig. 1.10. Nucleation rate J (nuclei/volume time) as a function of the temperature of the water-ice phase transformation (Figure 4 from [1.6])...

Abstract A simplified quintuple model for the description of freezing and thawing processes in gas and liquid saturated porous materials is investigated by using a continuum mechanical approach based on the Theory of Porous Media (TPM). The porous solid consists of two phases, namely a granular or structured porous matrix and an ice phase. The liquid phase is divided in bulk water in the macro pores and gel water in the micro pores. In contrast to the bulk water the gel water is substantially affected by the surface of the solid. This phenomenon is already apparent by the fact that this water is frozen by homogeneous nucleation. 

A major discontinuity in the behavior of water occurs at about 2100 bars, where the equilibrium ice phase changes from ordinary ice Ih, which is less... 

Given that interstellar ices are the building blocks of comets and comets are thought to be an important source of the species that fell on primitive Earth, the structures of molecules in comets may be related to the origin of life. It is possible that organic materials formed in the solid ice phase of interstellar materials provided raw materials used for life originating solely on Earth. If so, the deep freeze of ice in the Oort cloud would have been an excellent place to store these, especially the unstable ones, awaiting delivery to a planet. 

CH3, NH2 (see equations 8.12, 8.25, and 8.29). Once formed, the radicals remain trapped in the ice phase. Heat produced on collisions causes the radical to become mobile and react with other radicals and molecules within the ice particle, thus producing complex organic compounds ... 

From the preceding discussions, it should be clear that photochemistry within all phases of the atmosphere is a major driver of chemical transformations in relatively short time scales. With increasing knowledge of the ever-widening array of chromophoric compounds emitted and produced in the atmosphere, there is definitely room for much more fundamental research into primary and secondary photochemical reactions of relevance. In particular, the role of humic-like substances in aerosol, cloud and ice phases needs to be studied. 

---