Volume and heat capacity studies to evidence interactions between cyclodextrins and nicotinic acid in water

Density and heat capacity of the water+cyclodextrin (CD), water+nicotinic acid (NA) and water+CD+NA mixtures were determined at 298.15 K. CDs with different cavity size and alkylation were selected. From the experimental data the apparent molar properties were calculated. Assuming the formation of inclusion complexes of 1:1 stoichiometry, these properties were modeled and provided the stability constants of CD/NA inclusion complexes and the corresponding property change. The binding of NA with the smallest sized alpha-cyclodextrin (alpha-CD) generates more stable complexes accompanied by the lower volume and the heat capacity changes. These results are in agreement with earlier proposed binding mode according to which deep insertion of NA into alpha-CD takes place and it is governed by the hydrophobic-hydrophilic forces. The volume and the heat capacity changes caused by the interactions of CDs with NA were interpreted in terms of cosphere overlap model and the release of water molecules from the CD cavity due to the NA incorporation.