Pressure perturbation Calorimetry of solvation changes in cyclodextrin complexes

Solvation changes occurring during cyclodextrin host: guest complex formation were investigated using the new calorimetric technique of Pressure Perturbation Calorimetry (PPC). This can determine the thermal expansion coefficient of molecules in solution. PPC was used to measure the change in heat (DeltaQ) that occurs upon application of pressure to three different solutions: guest (1-adamantanecarboxylic acid or 1-adamantanamine), beta-cyclodextrin, and beta-cyclodextrin/guest mixture. DeltaQ for the complex in solution was found to be smaller than anticipated from the sum of the heat changes of the separate components. Since DeltaQ is directly related to thermal expansivity (alpha), the results imply that the complex expands less with temperature than expected. This reduction is most likely due to the removal of the solvation shell around the ligand and, to a lesser extent, expulsion of water molecules from the cyclodextrin cavity during complex formation.