Solvent exchange between dimethylsulfoxide and water in the dexamethasone acetate structure

Dexamethasone acetate can crystallize as a DSMO (dimethylsulfoxide) solvate with a stoechiometry (1:1), and its crystal structure shows the presence of channels containing the DMSO molecules. When single crystals of this solvate are immersed in water according to the methodology proposed by Nordhoff & Ulrich [9], the exchange between DMSO and water occurs, associated with the formation of needle-like crystals (whiskers). The phase obtained is a sesquihydrate, instead of the monohydrated variety already described, and the whiskers are formed preferentially on high energy sites (defects, edges). These observations are discussed with the aim to propose a mechanism describing both the desolvation step and the formation of the whiskers, and seem to be consistent with a mechanism of destructive-reconstructive transformation.