Solid phase hydration effect

The alkali cations in pfa normally occur almost entirely in the glass, and when the latter reacts may be presumed to enter the alkali-rich silicate that appears to be the initial product. When this phase is decomposed by reaction with Ca, they will be distributed, like alkali cations from any other source, between the solution and the solid hydration products, on which they are probably adsorbed (Section 7.3.2). The C-S-H tends to take them up more strongly as its Ca/Si ratio decreases (BI58,G63) consequently, the alkali cations released from the pfa are less effective in raising the OH concentration of the pore solution than are those released from the cement. The method outlined in Section 7.5.2 for calculating the OH concentration in the pore solution of Portland cement mix was extended to cover Portland-pfa cement mixes taking this into account (T37). 

Solid hydrate research of the last fifteen years is critically evaluated with regard to bonding and structure of water molecules. This review focusses on new results of structure determination and infrared and Raman studies in terms of hydrogen bonding and other intermolecular bonding interactions, distortion and disorder of water molecules, intermolecular and intramolecular coupling and anharmonicity of water bands, isotopic effects, and phase transitions. The techniques used for structure determination and spectroscopic measurements of solid hydrates are discussed. 

The hydrogen ion H" " cannot exist as a free species in condensed phases its hydration has long fascinated chemists and physicists. Existence of the hydrated proton was first postulated to explain the catalytic effect of the proton in esterification and later to rationalize the conduction of aqueous sulphuric acid solutions , The concept of electrolytic dissociation and consequent conduction in aqueous solutions is a forerunner of the modern notion of the salts themselves as solid electrolytes in the absence of any solvating medium. The parallel is particularly clear for strong mineral acid hydrates where several acid/water compositions of ionic character exist, many of which are proton conducting, and in which proton hydrates and H502 have been identified . 

C. Formation of Chlorine Hydrate. Because of the presence of traces of water in compressed chlorine, the chlorine hydrate discussed in Section 9.1.3.5 again becomes a problem. As chlorine condenses, some of the water accompanies it. Depending on the temperature, a certain amount of water is soluble in the chlorine. So long as this solubility is not exceeded, the condensate remains homogeneous and solid hydrate does not form. Below we develop an estimate of the solubility of water in liquid chlorine and show that, because of its very low solubility in chlorine and therefore its very high activity coefficient in solution, it behaves as a volatile component. The practical effect of this is that water tends to concentrate in the gas phase in most first-stage liquefiers. 

Metal carbonate decompositions proceed to completion in one or more stages which are generally both endothermic and reversible. Kinetic behaviour is sensitive to the pressure and composition of the prevailing atmosphere and, in particular, to the availability and ease of removal of C02. The structure and porosity of the solid product and its relationship with the reactant phase controls the rate of escape of volatile product by inter-and/or intragranular diffusion, so that rapid and effectively complete withdrawal of C02 from the interface may be difficult to achieve experimentally. Similar features have been described for the removal of water from crystalline hydrates and attention has been drawn to comparable aspects of reactions of both types in Garner s review [ 64 ]. 

The phenomenon of pseudopolymorphism is also observed, i.e., compounds can crystallize with one or more molecules of solvent in the crystal lattice. Conversion from solvated to nonsolvated, or hydrate to anhydrous, and vice versa, can lead to changes in solid-state properties. For example, a moisture-mediated phase transformation of carbamazepine to the dihydrate has been reported to be responsible for whisker growth on the surface of tablets. The effect can be retarded by the inclusion of Polyoxamer 184 in the tablet formulation [61]. 

This chapter describes some of the properties of solids that affect transport across phases and membranes, with an emphasis on biological membranes. Four aspects are addressed. They include a comparison of crystalline and amorphous forms of the drug, transitions between phases, polymorphism, and hydration. With respect to transport, the major effect of each of these properties is on the apparent solubility, which then affects dissolution and consequently transport. There is often an opposite effect on the stability of the material. Generally, highly crystalline substances are more stable but have lower free energy, solubility, and dissolution characteristics than less crystalline substances. In some situations, this lower solubility and consequent dissolution rate will result in reduced bioavailability. 

Lamivudine is an example of the effect of hydrates in nonaqueous solvents (Jozwiakowski et al., 1996). In distilled water at 25D, the anhydrate free base (form II) is 1.2 times as soluble as the 0.2 hydrate (form I). In ethanol at 26, the hydrate is 1.6 times as soluble as the anhydrate. The maximum solubility in ethanol-water mixtures was found to be at 40-60% water in ethanol, when form I is the most stable solid phase. The transition composition was with 18-20% water in ethanol in binary mixtures with more than 20% water, only the hydrate was found at equilibrium, and with less than 18% water, only the anhydrate was found at equilibrium. 

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