Solvent-induced phase transformations of hydrates

An increase of the specific surface area of solid phases is often desirable e.g. for the bioavailability of pharmaceuticals or in chemical processes. Such an increase can a.o. be achieved by suspending crystalline substances in a solvent that induces phase transformations. Hence, the original substa nce has to be in a metastable state in the solvent. If the stable phase aft er transformation has in addition a very low solubility in the solvent, a d endritic growth is forced to occur because of the high local supersaturatio ns during the phase change. This dendritic growth of the stable phase in te rm leads to needle- or whisker-like crystals, which have the desired larger specific surface area in comparison to the initial crystalline substance. In order to investigate this phenomenon several hydrates of salts were chos en, which undergo phase transformations to their anhydrates accompanied by a corresponding loss of crystal water when suspended in excess in lower alc ohols. Consequently, anhydrous forms were created by dehydrating these hydr ates. The transformation rate or in this case the dehydration level can thu s be indirectly measured by Karl-Fischer titration. The thermodynamic backg round of the dehydration phenomena can be clarified by solubility studies o f the hydrates and anhydrates in water/alcohol-mixtures.