Drying of hydrates and solvates

For the drying of systems that can form hydrates or solvates, two tasks can be differentiated (i) removal of the solvent and (ii) keeping the level of solvatization or drying to a certain state of solvatization. 

The removal of the solvent of solvatization requires pressure conditions lower than the equilibrium vapor pressure relationship. Furthermore, it is kinetically limited by diffusion of the solvent through the crystals, often through channels on the molecular scale. Moreover, the solvent can be located at isolated lattice sites, necessitating a reconstruction of the lattice. 

For the hydrate of caffeine, that crystallizes in elongated platelets it was shown that the drying of this hydrate proceeds from the end faces of the platelets into the crystal. The direction is identical with the channels in which the water molecules are located in the crystal. 

The drying of solvates and hydrates is often very destructive, that is, the starting crystals appear nice and transparent, while the product after the drying appears nontransparent. This effect is also sometimes used to comminute the crop. 

The arguments for hydrates also hold for solvates, the only difference being the typically higher vapor pressures of organic solvents. 

---