Bonding strength of water

Figure A3.5.10. Bond strengths of water clustering to various core ions as a fiinction of the number of water molecules.

In comparison with riboflavine, the quantum yield for the photoreduction of lumiflavine is very low, indicating that the n-ribityl group also contributes to the lowering of the bond strength of water. Although the structure of the riboflavine-water-activator complex has not yet been elucidated in detail, there is evidence that the 2-hydroxyl group of the D-ribitol residue may be involved. 

The amount of water present in foods has only a very vague relationship with their resistance to microbial attack and biochemical and chemical reactions, which occur in foods. A more important factor than the actual water content is its availability. Water availability is related to interactions of water with food components, with the bond strength of water bound by physical adsorption or chemisorption. Tightly bound water is less available than weakly bound water, which is again less available than free water. The measure of water availability is water activity. The water activity of a food is not the same thing as its moisture content. 

Pentaerythritol in rosin ester form is used in hot-melt adhesive formulations, especially ethylene—vinyl acetate (EVA) copolymers, as a tackifier. Polyethers of pentaerythritol or trim ethyl ol eth an e are also used in EVA and polyurethane adhesives, which exhibit excellent bond strength and water resistance. The adhesives maybe available as EVA melts or dispersions (90,91) or as thixotropic, one-package, curable polyurethanes (92). Pentaerythritol spko ortho esters have been used in epoxy resin adhesives (93). The EVA adhesives are especially suitable for cellulose (paper, etc) bonding. 

If the principal cohesive forces between solute molecules are London forces, then the best solvent is likely to be one that can mimic those forces. For example, a good solvent for nonpolar substances is the nonpolar liquid carbon disulfide, CS2-It is a far better solvent than water for sulfur because solid sulfur is a molecular solid of S8 molecules held together by London forces (Fig. 8.19). The sulfur molecules cannot penetrate into the strongly hydrogen-bonded structure of water, because they cannot replace those bonds with interactions of similar strength. 

Cooking oil, a non-polar liquid, has a boiling point in excess of 200 C. Water boils at lOO C. How can you explain these facts, given the strength of water s hydrogen bonding ... 

Fig. 5.7. Effect of immersion in hot water on interfacial bond strength of silane treated glass fiber-epoxy matrix composite. After Koenig and Emadipotir (1985).

Any cation in liquid water will be surrounded by water molecules oriented with the oxygen atom facing the cation (Fig. 5.2). The number of water molecules in the coordination sphere will normally be close to the ideal coordination number expected for the cation (Appendix 4) but if the hydrated complex is to be stable, the bonding strength of the cation must match that of the water molecule. This is best illustrated by examples. 

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