Pharmaceutical applications of isothermal microcalorimetry

4 Pharmaceutical applications of isothermal microcalorimetry 8.9.4.1 Dynamic physicochemical stability 

The difference in solution enthalpy between both forms is equal to the heat of transition AHt, where  

This relationship is valid for solvents which allow a rapid dissolution in the calorimeter. Whereas AHf and AH depend on the solvent (if no association or complexation takes place), the difference between them is independent of the solvent. The heat of transition A Ht should also equal the difference of the melting enthalpies of A and B which may be determined by DSC. 

Because the heat of solution is generally of low energy, modern, accurate microcalorimeters can deliver accurate determinations and are suitable for the study of both liquid-liquid and solid-liquid interactions. 

Previously, Lindenbaum and McGraw have used solution microcalorimetry to study drug forms. Because different crystal forms have different structures, they will inevitably have different heats of solution. However, the difference between the heats of solution of two polymorphs in different solvents should remain the same (Table 8.6) if there is no solvate formation. This difference is the heat of transition between the forms at that temperature. 

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Buckton G. Principles and Pharmaceutical Applications of Isothermal Microcalorimetry. In Craig IX M. Reading M. eds. Thermal Analysis of Pharmaceuticals. Boca Raton. USA CRC Press. 2(X)7 265-86. 

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