Overstoichiometric hydrates

In this category of models, the water molecules are localized. They may either be in their normal position (in which case we would have substoichiometric hydrates of the limited hydrate S,( + p) H2O), or in an interstitial position, in which case we are dealing with an overstoichiometric hydrate of the inferior hydrate S,... 

The calculation tells us that this view of things also yields expression [4.24] for the isotherm. In fact, this means that a zone of divariance with localized water molecules can be viewed either as a substoichiometric hydrate in relation to a superior hydrate, or as an overstoichiometric hydrate in relation to an inferior hydrate (or an anhydrous salt). The two models are absolutely equivalent. 

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