The equilibrium constant of beta-cyclodextrin-phenolphtalein complex; influence of temperature and tetrahydrofuran addition.

The temperature influence on creation of a supramolecular complex in which beta-cyclodextrin (beta-CD) is the host molecule and phenolphtalein (PP) is the guest has been studied in aqueous solution by UV-visible absorption spectroscopy. The decrease of temperature of beta-cyclodextrin-phenolphtalein system resulted in a decrease in absorbance of the UV-vis spectrum. Under favourable conditions (0.1 mM beta-CD, 30 microM PP) the termochromic effect is very significant (approximately =0.1 U of absorbance/10 degrees C). The formation constant of inclusion complex was determined at various temperatures (from 10 to 70 degrees C) using Scott's equation. The association constants (K11) for the binding in 0.02 M sodium carbonate (pH 10.5) at 10 and 70 degrees C are 7.44 and 0.26 x 10(4) M(-1) respectively. The stoichiometric ratio of investigated complex was found to be 1:1 on wide range of beta-cyclodextrin:phenolphtalein concentration ratio (from 0.8:1 to 427:1). Additionally, strong interaction between cyclodextrin and tetrahydrofuran (THF) was observed and the inhibitory effect of tetrahydrofurane on the association of beta-CD PP complex was studied. From linear Van't Hoff plots thermodynamic parameters such as: the change of enthalpy (deltaH(o)) and change of entropy (deltaS(o)) were estimated and interpreted.