Dehydration and rehydration of a hydrate diclofenac salt at room temperature

The salt diclofenac/N-(2-hydroxyethyl) pyrrolidine crystallizes from water as a dihydrate, while it precipitates from organic solvents anhydrously: the two salts have different crystal structures. Dehydration of the dihydrate salt was carried out in a desiccator over silica gel at room temperature: the process occurs with the retention of the crystal structure. Slight changes observed in the diffractograms suggest, that soon after dehydration, a phase transition starts, slowly due to the low temperature of the process. The reaction was followed determining the loss of weight as a function of time and by thermal analysis, since the dihydrate and the dehydrate forms have different thermograms, but similar diffractograms. The reaction was complete after 24 h. The analysis of the experimental data suggests a kinetic process related to a one-dimensional diffusion of the crystallization water molecules outwards the solid particles. At room temperature, the dehydrate material rapidly back-absorbs the two molecules of crystallization water from the atmosphere moisture. The interaction with water of the different forms of the salt was discussed as a function of their solid structures as well as of the complex equilibria present in aqueous solution: these can explain previous apparently anomalous results.