Sparsely soluble solids

Using SFC facilities, solubilities of sparsely soluble solids in supercritical fluid solvents can also be measured as a function of pressure and temperature. 

Other types of solid-state membranes include single crystals of sparsely soluble salts and are often called heterogeneous membranes, in which the insoluble salt is embedded in some inert polymer matrix. Obviously, in order for these membranes to be at equilibrium they should be in a saturated solution. In practice, these membranes are used in solutions that are below saturation. In that case, the insoluble salt slowly dissolves. 

To form a CBC, control over the dissolution of the bases is crucial. The bases that form acid-base cements are sparsely soluble, i.e., they dissolve slowly in a small fraction. On the other hand, acids are inherently soluble species. Typically, a solution of the acid is formed first, in which the bases dissolve slowly. The dissolved species then react to form the gel. When the gel crystallizes, it forms a solid in the form of a ceramic or a cement. Crystallization of these gels is inherently slow. Therefore, bases that dissolve too fast will rapidly saturate the solution with reaction products. Rapid formation of the reaction products will result in precipitates and will not form well ordered or partially ordered coherent structures. If, on the other hand, the bases dissolve too slowly, formation of the reaction products will be too slow and, hence, formation of the gel and its saturation in the solution will take a long time. Such a solution needs to be kept undismrbed for long periods to allow uninterrupted crystal growth. For this reason, the dissolution rate of the base is the controlling factor for formation of a coherent structure and a solid product. Bases should neither be highly soluble nor almost insoluble. Sparsely soluble bases appear to be ideal for forming the acid-base cements. 

The solubility of such solids is only a fraction of that of the acid phosphates that we discussed in the last chapter. A sparsely soluble oxide (or its hydroxide) dissolves in acidic solution in two steps. The first step is ionization or dissociation. When stirred in water, the oxide decomposes into its cations and anions. This decomposition occurs because of collisions between the oxide molecules and the polar molecules of water. Second is a screening step in which the two charged ions resulting from this dissociation are kept separate by the water molecules. These steps are described in detail below. 

It is logical to conclude -with the case where the new component is not appreciably soluble in either phase. At first sight it would appear unlikely that the interfacial tension could then be affected in either direction, yet this is possible. If the interfacial tension is increased by its addition to the system this substance will be adsorbed at the interface if decreased, the added body will be more sparsely distributed at the interface or not appear there at all. It is easy to observe, at any rate, the appearance of a film of non-transparent matter at the interface, and the experiment has been carried out for a number of liquid pairs and solids by Hofmann. 

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