Hydrate Layer Structures

Such an example is pyridine trihydrate [793] shown in Fig. 21.10, where there are 4-, 5-, and 6-membered hydrogen-bonded rings formed by water molecules. Of 

The hydrate layer structures which display more regular buckled pentagonal nets belong to two main types. In one, the pentagonal nets consist exclusively of water molecules and are three-dimensionally connected by functional groups of the enclosed molecules. Thus, in piperazine hexahydrate [815], and in pinacol hexa-hydrate [816], the H2N- and HO-groups respectively form hydrogen bonds to the 

Hydrates of Small Biological Molecules Carbohydrates, Amino Acids, Peptides, Purines, Pyrimidines, Nucleosides and Nucleotides 

In contrast to the ices, the clathrate hydrates and layer structures described in the preceding chapter, where four-coordinated water is the majority species present in the structure, the water molecules in low hydrates may be three- or four-coordinated. While the water molecule rarely fails to donate two hydrogen bonds, it may accept one or two, but rarely none. This applies both to the organic hydrates discussed in this chapter and the inorganic salt hydrates where the water oxygen can be coordinated to one or two cations (cf. Thble 7.8). 

The relatively large number of crystal structure analyses of hydrates is, of course, a consequence of the use of water as the least expensive and most common solvent. The water molecules are small enough that they can stabilize crystal structures by filling voids between molecules for more efficient packing, while also contributing to the lattice energy by additional hydrogen bonds. 

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Leboda, R., Turov, V.V., Marciniak, M., Malygin, A.A., and Maikov, A.A. 1999b. Characteristics of the hydration layer structure in porous titania-silica obtained by the chemical vapor deposition method. Langmuir 15 8441-8446. 

This strnctnring of liqnids into discrete layers when confined by a solid surface has been more readily observable in liquid systems other than water [1,55]. In fact, such solvation forces in water, also known as hydration forces, have been notoriously difficult to measure due to the small size of the water molecule and the ease with which trace amounts of contamination can affect the ordering. However, hydration forces are thought to be influential in many adhesive processes. In colloidal and biological systems, the idea that the hydration layer mnst be overcome before two molecules, colloidal particles, or membranes can adhere to each other is prevalent. This implies that factors affecting the water structure, such as the presence of salts, can also control adhesive processes. 

In the compound with water, continuous layers of water alternate with bilayers of host molecules, defining two distinct regions in the solid (Fig. 7). Within the bilayers, the structure is stabilized mainly by dipolar interactions between the C-Cl groups turning inward. All the oxygen-containing functions of the host point outward on both sides of the bilayer, and are linked efficiently to the adjacent hydration layers. 

The second approach is to test catalysts as layers in full MEA sfrucfures. This has the advantage of testing catalysts under realistic conditions and in realistic environments. However, this approach depends on creating a near-optimal catalyst layer structure that shows high utilization of fhe cafalysf, fogefher wifh a structure that allows adequate hydration and reactant/product transport. 

Takata, T., Shinohara, K., Tanaka, A., Kara, M., Kondo, J.N., Domen, K. 1997a. A highly active photocatalyst for overall water splitting with a hydrated layered perovskite structure. J Pho-tochem PhotobiolA Chem 106 45 9. 

A glass membrane in an electrolyte solution cannot be taken to be a homogeneous system in the direction perpendicular to the surface. When the membrane is in contact with the solution, water molecules can enter it and form a 5-100 nm thick hydrated layer [319]. The formation of this hydrated layer is actually a condition for good functioning of the glass electrode. The basic characteristics of the glass structure probably do not change in the hydrated layer, but the cation mobility increases considerably compared with the compact membrane interior... 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Electron and Hole Transfer from the Hydration Layer to DNA. - Debije et al. investigated the transfer of X-radiation-induced electrons and holes (H20 +) from the hydration layer of crystalline oligonucleotides into the oligo itself.57 Results from these Q-band ESR experiments at 4 K suggest that ionization of hydration water results in both electron and hole transfer to the DNA model structures. The proton transfer reaction from H20 + to form the hydroxyl radical occurs on the timescale of a few molecular vibrations, thus it limits hole transfer distance from a water hole to DNA. 

The water molecules that are in immediate contact with dissolved nonpolar groups are partially oriented. They form a cagelike structure around each hydrophobic group. When particles surrounded by such hydration layers are 1-2 nm apart, they sometimes experience either a fairly strong repulsion or an enhanced attraction caused by these hydration layers.21 64 66,72 Direct experimental measurements have shown that these effects extend to distances of 10 nm21,63 and can account for the previously mentioned long-range van der Waals forces. 

This oxide has a layered structure consisting of [Nb60i7]4 macropolyanion sheets interleaved by K+ ions, as shown in Fig. 16.3. The K+ ions can be replaced by various mono- and multivalent metal cations,22,37,38 protons6,12,38,395 and organic cations.405 An important aspect of the structure is the presence of two. types of interlayer regions,5,95 referred to as interlayer I ana interlayei II, with differing interlayer reactivity.375 This makes the [Nb60i7]4 sheets nonsymmetric with respect to mirror reflection about the sheet planes. Besides the anhydrous compound of the above formula, it also forms two hydrates, one with about three... 

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