Determination of the Activity of the Solid phase of Molecular Compounds
(English)Article in journal (Other academic) Submitted
The activity of the solid phase of an organic molecular compound is often defined using the melt at the same temperature as the thermodynamic reference. However, far below the melting temperature, the properties of this reference state cannot be determined experimentally, and different simplifications and approximations are normally adopted. In the present work, a novel method is presented to determine the activity of the solid phase (= ideal solubility) and the heat capacity difference between the supercooled melt and solid. The approach is based on rigorous thermodynamics, using standard experimental thermodynamic data at the melting temperature of the pure compound and solubility measurements in different solvents at various temperatures. The method is illustrated using data for ortho-, meta-, and para-hydroxybenzoic acid, salicylamide and paracetamol. The results show that complete neglect of the heat capacity terms may lead to estimations of the activity that are incorrect by a factor of 12. Other commonly used simplifications may lead to estimations that are only one third of the correct value.
Solid-state activity, ideal solubility, heat of fusion, heat capacity, supercooled liquid
IdentifiersURN: urn:nbn:se:kth:diva-8418OAI: oai:DiVA.org:kth-8418DiVA: diva2:13732
QS 201203282008-05-122008-05-122012-03-28Bibliographically approved