Thermodynamic parameters of the inclusion complexes of 2-methylnaphthoate and α- and β-cyclodextrins

Steady-state fluorescence has been used to study the inclusion complexes of 2-methylnaphthoate (MN) with alpha- and beta-cyclodextrins (CDs). Emission spectra of MN show two bands that are very sensitive to the CD concentration and temperature. The stoichiometry and formation constants of these complexes were investigated by obtaining the ratio of the two bands. Results showed identical stoichiometry (1:1) for both MN/alpha-CD and MN/beta-CD complexes. However, the estimated formation constants were quite different. The thermodynamic parameters Delta H and Delta S were obtained from vant'Hoff plots. Results showed that MN/alpha-CD complexation is accompanied by an high enthalpy change, while MN/beta-CD complexation is mainly entropically favored. Anisotropy measurements were used to interpret the different signs of Delta S upon inclusion for both complexes. In addition, 2-methylnaphthoate seems to be a good probe for estimating microenvironmental polarity. Effective dielectric constants of inner alpha- and beta-CD cavities were evaluated.