Thermodynamics of complexation of dimethyl esters of tere-, iso-, and phthalic acids with alpha- and beta-cyclodextrins.

Thermodynamics of inclusion of alpha- and beta-cyclodextrins (CDs) with three diester precursors of polyesters, dimethyl terephthalate (DMT), dimethyl isophthalate (DMI), and dimethyl phthalate (DMP) were studied with different fluorescence spectroscopy techniques. The stoichiometry, association constants, and other thermodynamics parameters were investigated from the fluorescence intensity decrease that takes place upon CD addition and temperature changes. Apart from DMP, which did not form a complex with alphaCD, all other systems showed 1:1 stoichiometry complexes with different stabilities. The complexes formed with the DMT guest are the most stable, whereas the stability constant for DMP:betaCD is the lowest. Binding constants are also larger for complexes formed with betaCD than for those formed with alphaCD. Fluorescence polarization, quenching, and lifetime measurements provide information about the structure of the complexes formed. This geometry explains the values of enthalpy and entropy changes during complexation. Van der Waals interactions seem to be involved in the formation of these complexes.