Add time:09/08/2019         Source:sciencedirect.com

Processing of crystalline pharmaceuticals often involves removal of the solvent of crystallization. Several cases are documented where removal of solvated alcohol is much faster when the sample is simply exposed to ambient humidity than when the sample is vacuum-dried. The most dramatic example of this phenomenon occurs with the methanolate of cefamandole sodium. Cefamandole sodium may exist in 1 of 3 crystalline forms giving different X-ray powder patterns: methanolate, hydrate, or anhydrate. This research is a detailed phenomenological study of the kinetics and thermodynamics of the desolvation and solvation reactions of cefamandole sodium. The effects of water vapor, temperature, sample history, and crystal size are experimentally evaluated. The phenomenological generalizations are employed to develop a mechanistic interpretation of the data. The methanolate of cefamandole sodium exhibits some characteristics of a non-stoichiometric solvate and has a solvation enthalpy 3.7 kcal/mol greater than the enthalpy of condensation of the solvent. The rate of vacuum demethanolation increases sharply as the crystal size decreases; is as much as a factor of 200 slower than demethanolation in the presence of water vapor; yields an apparent activation energy equal to the enthalpy of reaction; and is apparently limited by mass transfer of the methanol through the low permeability anhydrate phase. Demethanolation in the presence of water vapor yields a hydrate which, as a first approximation, is a non-stoichiometric solvate. Dehydration of the hydrate is rapid. The much faster rate of conversion of methanolate to hydrate is largely a result of the extensive crystal cracking and subsequent reduction of effective crystal size which occurs during the early stages of hydrate formation. However, it also appears that the hydrate phase is more permeable to methanol than is the anhydrate phase.

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