Dehydration classification schemes

Several schemes for the classification of hydrates have been proposed. Galwey describes a dehydration classification based on the thermal behavior of the sample (http://dx.doi.org/10.1016/S0040-6031(00)00448-2). He mainly focuses on the water release and divides the mechanisms in six water evolution types (WET). These describe patterns of dehydration defined by structural, observational and kinetic criteria. Three major classes can be distinguished: WET1 and WET2 define dehydration with no or only small changes in the crystal structure while the water leaves by diffusion. These classes are descriptive for non-stoichiometric hydrates. WET3 and WET4 present dehydration associated with larger structural changes in the product due to nucleation and growth mechanisms after the water release. WET5 and WET6 describe dehydration behavior resulting in melting of the crystal.

Classification scheme of dehydration reactions proposed by Galwey. From http://dx.doi.org/10.1016/S0040-6031(00)00448-2.Classification scheme of dehydration reactions proposed by Galwey. From http://dx.doi.org/10.1016/S0040-6031(00)00448-2.

The Rouen 96 model by Petit and Coquerel is based purely on the premises and subsequent changes of the crystal structures during dehydration (http://dx.doi.org/10.1021/cm9600438). It is assumed that during every dehydration process a new anhydrous material (NAM) emerges and thus the process is divided into two steps; the release of water, which is affected by the crystal structure of the hydrated phase, and the optional reorganization of the NAM. Four criteria are applied for the classification, two (C1 and C2) on the dehydration process and two (C3 and C4) on the reorganization process. Two general classes of dehydration products are distinguished: class I which has no more information of the hydrate, and class II which is closely related to the hydrate.

General desolvation scheme proposed by Petit and Coquerel (Rouen 96 model). From http://dx.doi.org/10.1021/cm9600438.General desolvation scheme proposed by Petit and Coquerel (Rouen 96 model). From http://dx.doi.org/10.1021/cm9600438.