Factors influencing DSC curves of hydrates and solvates

With hermetically sealed pans, the sample is sealed within a closed system such that the volatiles cannot escape and remain in the head space of the pan even after ejection from the sample. In this pan type, the melting peak of the solvate may be observed, but not the loss of volatiles. 

If a small pinhole is made in the cover of the pan, or if a crimped or open pan is used, then volatiles can escape, but the shape of the DSC curve and the temperatures involved will depend upon the rate at which the volatiles can escape, which in turn depends upon the extent of crimping or the size of a hole made in the pan together with experimental conditions, such as heating rate and sample size. As a result, data obtained may not be very reproducible. In some cases, both melting of the solvate and desolvation in the solid state may happen, which gives rise to two endothermic peaks. 

When a sealed pan has been chosen, make sure it is capable of withstanding the internal pressure generated and take care to seal properly. If the pan cannot withstand the pressure build-up, the pan may rupture and can cause serious contamination to the instrument. Also note that the presence of solvent or water as vapour aroimd the crystal in the atmosphere of the pan may induce metastable solvate-free forms (due to vapour diffusion of solvent into the solid phase and a resulting solvent-induced, solid-solid phase transformation) and also the amorphous state. 

When performing DSC experiments on solvated/hydrated compounds, the DSC thermogram can often become rather complex due to the multiple events that can take place. To help with interpretation it is essential to perform a corresponding TGA experiment, so that 

This section briefly deals with some types of DSC curves that maybe obtained when examining solvates and hydrates and also explains some of the factors that can be responsible for misinterpretation of the data. 

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