Ice lattices

Two cell models of water have been reported. Weissman and Blum 63> considered the motion of a water molecule in a cell generated by an expanded but perfect ice lattice. Weres and Rice 64> developed a much more detailed model, based on a more sophisticated description of the cell and a good, nonparametric water-water interaction namely the Ben-Naim-Stillinger potential 60>. The major features of the WR model are the following ... 

The underlying metal lattice structure also has a significant effect on the water structure. As pointed out by Spohr," although the Pt-Pt nearest-neighbor distance is ao/ Jl = 0.277 nm, which is very close to the 0-0 distance in ice, the cubic symmetry of the 100 surface is incompatible with the hexagonal symmetry of the ice lattice. As a result, the water molecules cannot form a uniform monolayer and occupy all adsorption sites. On the other hand, Berkowitz and co-workers showed that the hexagonal Pt (111) surface is able to support a more complete layer of adsorbed water molecules, and one can identify patches of an icelike structure in the first layer. This freezing is further enhanced by an external electric field, as will be discussed later. 

Methane hydrates form when methane molecules become trapped within an ice lattice as water freezes. They can form in very cold conditions or under high-pressure conditions. Both of these conditions are met in deep oceans and in permafrost. In Canada, hydrates have already been found in large quantities in the Canadian Arctic. Methane hydrate has a number of remarkable properties. For example, when brought into an oxygen atmosphere, the methane fumes can be ignited, making it appear that the ice is burning ... 

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... 

Illustrative examples of substances which can behave as porous hosts in one of the above ways are also given. For instance, water readily forms open ice lattices which incorporate guests in clathrate hydrates of types I and II (see later text). Ordinary ice also possesses considerable porosity so that, as shown in Table I, He and Ne can readily diffuse through it. Ice below 0°C is zeolite-like in that it has a permanent, somewhat porous structure which (unlike the open-ice frameworks of the clathrate hydrates) does not require guest molecules for stabilization. 

The water framework seems ice-like because of comparable heats of formation. However, the crystal lattice is somewhat different than ice since an ice lattice provides no space for even the smallest hydrocarbon molecule. 

One must, however, determine whether the increase in th with water adsorption is due to the formation of an ice lattice inside the resin or instead results only from the interactions of Fe3+ with the water molecules within its coordination sphere 187). A study of partially hydrated resins provides the answer, the results of which are shown in Fig. 24. The final form of the... 

Mixture Models Broken-Down Ice Structures. Historically, the mixture models have received considerably more attention than the uniformist, average models. Somewhat arbitrarily, we divide these as follows (1) broken-down ice lattice models (i.e., ice-like structural units in equilibrium with monomers) (2) cluster models (clusters in equilibrium with monomers) (3) models based on clathrate-like cages (again in equilibrium with monomers). In each case, it is understood that at least two species of water exist—namely, a bulky species representing some... 

Before proceeding, it is important to recall the significant feature which appears to distinguish the cluster model from the two other prominent mixture models—i.e., the broken-down ice lattice and the clathrate hydrate cage structures. The latter two theories allow for the existence of discrete sites in water, owing to the cavities present either in the ice... 

The importance of carefully considering anomalies when studying the behavior of solute-solvent interactions has been stressed. For aqueous solutions, many anomalous results presented in the literature suggest the existence of some type of discreteness in water structure. Discreteness is consistent with a view of water structure providing distinct sites such as those found in the models of water, implying a broken down ice lattice structure or clathrate hydrate cage-like structures. 

FIGURE 2.2 (a) Two-dimensional proton-disordered ice lattice (Bjerrum defect illustrated... 

The plain five membered rings of clathrates may have a lesser degree of freedom as the non-plain six membered ring of the normal ice structure. We can expect that the entropy of this structure is smaller than in the ice lattice. This model could interpret the entropy effect of aqueous mixtures without structure increase. 

Fig. 28. Below a model of the ice lattice. Above a peptide group in a conformation with H-bond angles (3 = 0 to the water molecules 27 ...

The peptide group can be fixed at the ice lattice with p = 0127. Ice-like distances of the H-bond acceptors are in different molecules for example Biotin. Thymine, Triglyceride, 1,4-Quinone etc.141. Hechter gave a model for biologic membranes too142. In the membrane surface Hechter assumes a water double hydrate lamella of 4.9 A thickness in which K+ could be placed, but Na+ would be too big. 

The modelling just described of the direct HC1 + CIONO2 reaction on an ice lattice to produce molecular chlorine and ionized nitric acid portrayed a relatively facile coupled proton transfer/SN2 mechanism, evidently supported in subsequent calculations.26 These results also reinforced the idea of an ionic pathway involving ionized HC1,912>21 as opposed to molecular HC1. 

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