Study on the effect of solvent on the inclusion interaction of hydroxypropyl-β-cyclodextrin with three kinds of coumarins by phase solubility method

The characteristics of host–guest complexation between hydroxypropyl-β-cyclodextrin (HP-βCD) and coumarin as well as its derivatives of 7-hydroxycoumarin (7-HC) and 6-methylcoumarin (6-MC) were investigated with phase solubility method. The investigation was carried out in water and phosphate buffer solution (pH 7.4) and at different temperatures (from 298.15 K to 313.15 K). The phase-solubility diagrams from UV spectral measurements were of the AL type. The solubilities of coumarin and its derivatives in aqueous HP-βCD solutions increased with temperature. The apparent stability constants (KC) of inclusion complexes were calculated based on the solubility data and followed the order of 7-HC > 6-MC > coumarin in the same solvent and at the same temperature. The stability constant of 7-HC in water was larger than that in the phosphate buffer solution, which was contrary to 6-MC and coumarin. The enthalpy and entropy changes of the inclusion process were negative, which showed that the inclusion interactions were enthalpy-driven process. All the results were interpreted with the driving forces occurred in the inclusion complexation.