The Influence of β - and γ -Cyclodextrin Cavity Size on the Association Constant with Decanoate and Octanoate Anions

Abstract

This work evaluates the influence of the β- and γ-cyclodextrin (CD) cavity size on the association constant (KCDA) with decanoate (C10) and octanoate (C8) anions. The spectral displacement technique with phenolphthalein was used to obtain the 1:1 association constant (KCDA) in NaHCO3/NaOH buffer pH 10.5 at 25 °C. The KCDA value obtained were 2.6 (±0.2) × 103, 2.5 (±0.5) × 102, for beta;CD–C10 and γCD–C10 inclusion complexes, and 5.1 (±0.2) × 102 and 4.7 (±0.2) × 101 for βCD–C8 and γCD–C8 inclusion complexes, respectively. The KCDA values of either acid with βCD is approximately 10 times higher than for the same acid with γCD, where as for the same cyclodextrin, the KCDA value is 5 times higher for the C10 association than for the C8. The data demonstrate that the cyclodextrin cavity size exerts a greater influence on the association constant than the chain length of the acid for these compounds. 1H NMR studies show that fatty acid protonation has a distinct effect on the chemical shift of CD protons.