Water hydrated crystal lattices

Methane condenses at lower temperatures as methane clathrate hydrate (CH4 ALUO) at 78 K and CH4 ice. A clathrate hydrate is a cage compound in which a gas molecule is trapped inside the water ice crystal lattice. One gas atom is trapped for every seven water molecules. Clathrate hydrates form via reactions exemplified by... 

AHs may be either positive or negative. That principally depends on the relative magnitudes of the terms that figure on the right-hand side of the equation. In some cases heat is evolved on the dissolution of a salt in water. It is mainly due to the fact that heat evolved when the gaseous ions are hydrated (AHX) is more than the heat absorbed in rupturing the crystal lattice. In the majority of cases, however, there is an absorption ofheat when a salt dissolves in water. 

The composition and properties of the ions contained in the solution are not the same as those of ions contained in the ionic crystal lattice. It is already known that anhydrous copper sulfate (CuS04) is colorless. This implies that Cu2+ and SCT ions that make up the crystal lattice of the sulfate are colorless. When the Cu2+ ions combine with water molecules during dissolution they turn blue (the color characteristic of copper salt). This color is therefore due to hydrated ions of copper, i.e., ions connected with the water molecules. 

When an ionic compound is dissolved in a solvent, the crystal lattice is broken apart. As the ions separate, they become strongly attached to solvent molecules by ion-dipole forces. The number of water molecules surrounding an ion is known as its hydration number. However, the water molecules clustered around an ion constitute a shell that is referred to as the primary solvation sphere. The water molecules are in motion and are also attracted to the bulk solvent that surrounds the cluster. Because of this, solvent molecules move into and out of the solvation sphere, giving a hydration number that does not always have a fixed value. Therefore, it is customary to speak of the average hydration number for an ion. 

The inclusion of solvent molecules as part of the crystal lattice is another common phenomena in both organic and inorganic systems. Calcium phosphate used in modem building plaster, sets when it reacts with water and crystallizes as a stable deca-hydrate. In pharmaceutical systems it is common... 

The ability of an ionic solid to dissolve depends on its lattice energy, as well as the degree to which its ions can become hydrated. The lattice energy of an ionic crystal is a measure of the strength of its three-dimensional network of bonds. If these interactions are weaker than the solute-solvent attractions, the ionic bonds will be easily disrupted by water molecules. 

Hydration Incorporation of water molecule(s) into a molecule or crystal lattice of a mineral without hydrolysis... 

An important advantage of the inclusion complexes of the cyclodextrins over those of other host compounds, particularly in regard to their use as models of enzyme-substrate complexes, is their ability to be formed in aqueous solution. In the case of clathrates, gas hydrates, and the inclusion complexes of such hosts as urea and deoxycholic acid, the cavity in which the guest molecule is situated is formed by the crystal lattice of the host. Thus, these inclusion complexes disintegrate when the crystal is dissolved. The cavity of the cyclodextrins, however, is a property of the size and shape of the molecule and hence it persists in solution. In fact, there is evidence that suggests that the ability of the cyclodextrins to form inclusion complexes is dependent on the presence of water. Once an inclusion complex has formed in solution, it can be crystallized however, in the solid state, additional cavities appear in the lattice, as in the case of the hosts previously mentioned, which enable the inclusion of further guest molecules. ... 

In Chapters 2 and 3 we have described basic structural properties of the components of an interphase. In Chapter 2 we have shown that water molecules form clusters and that ions in a water solution are hydrated. Each ion in an ionic solution is surrounded predominantly by ions of opposite charge. In Chapter 3 we have shown that a metal is composed of positive ions distributed on crystal lattice points and surrounded by a free-electron gas which extends outside the ionic lattice to form a surface dipole layer. 

Terrace Ion-Transfer Mechanism, In the terrace siteion-transfer mechanism a metal ion is transferred from the solution (OHP) to the flat face of the terrace region (Fig. 6.15). At this position the metal ion is in the adion (adsorbed-like) state, having most of its water of hydration. It is weakly bound to the crystal lattice. From this position it diffuses on the surface, seeking a position of lower energy. The final position is a kink site. 

Electrolytic oxidation of cobalt(II) fluoride in 40% hydrofluoric acid yields hydrated cobalt(III) fluoride, CoF3 3.5H20 (3.5 is the stoichiometric amount of water per C0F3 molecule in the crystal lattice). 

The water lattice may be an important element in forming the ordered thymine structure necessary for dimerization, as pointed out by Beukers and Berends.37 Thymine can crystallize from solution as a monohydrate (a real hydrate)38 in whose crystal lattice one thymine is directly above another. The influence of humidity upon dimer yield in dry films may be connected with monohydrate formation, and monohydrate formation in frozen solutions may be the reason for the almost theoretically maximum quantum yields for dimer formation.31 The possible existence of aggregates in frozen aqueous solutions is supported by a tenfold increase in purine phosphorescence at 44°K produced by the presence of 1% ethanol and by a blue shift of excitation and emission spectra.39... 

Water can interact with ionic or polar substances and may destroy their crystal lattices. Since the resulting hydrated ions are more stable than the crystal lattice, solvation results. Water has a very high dielectric constant (80 Debye units [D] versus 21 D for acetone), which counteracts the electrostatic attraction of ions, thus favoring further hydration. The dielectric constant of a medium can be defined as a dimensionless ratio of forces the force acting between two charges in a vacuum and the force between the same two charges in the medium or solvent. According to Coulomb s law. 

A notable feature of carbohydrate crystals is the absence (or small proportion) of water of hydration. As monosaccharides are hydrophilic and water-soluble, considerable deposition of water (association) in a sugar crystal may be expected. The absence of water from the crystal lattice of a sugar must, therefore, be explained by supposing that the sugar is able to form sufficiently strong and numerous hydrogen bonds without the involvement of molecules of water this has been observed, although there are some exceptions. 

Water of hydration and other solvents found in crystal lattice are not added into formulas of the compounds listed, e.g., C36H46N2O4 (C2H5)20. 

Gas hydrates behave as solutions of gases in crystalline solids rather than as chemical compounds. The main framework of the hydrate crystal is formed with water molecules. The hydrocarbon molecules occupy void spaces within the lattice of water molecules. 

The term lattice water" is commonly applied to cases in which the water molecules are occupying definite positions in the crystal lattice but arc apparently not coordinated with either cations ur anions. Again, clear-cut cases are those in which the compound is so highly hydrated that both lattice water and "ion water" arc present... 

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