Inclusion of phthalate esters by a self-assembled monolayer of thiolated cyclodextrin on a gold electrode

Abstract

Complexation of phthalic acid esters (PAEs) by a self-assembled monolayer (SAM) of thiolated alpha-cyclodextrin (6-(2-mercaptoethylamino)-6-deoxy-alpha-cyclodextrin, MEA-alpha-CD) on a gold electrode was examined by a cyclic voltammetry using hydroquinone (HQ) as a probe. From the inhibitory effect of the phthalate esters on the inclusion of HQ by the surface-confined cyclodextrin (CD), the association constants (Kasn) of the esters with the immobilized CD were estimated. For comparison, the association of PAEs with free alpha-CD was examined spectrophotometrically using methyl orange as a probe. It was concluded that, in both free and surface-confined CD systems, the Kasn value increased with an increase in the length of aliphatic alcohols conjugated to phthalic acid. Furthermore, the Kasn values for PAEs in the SAM system were much larger than those in a free CD system. This could be intuitively ascribed to the steric factor for the PAEs to come out from the cavity of surface-confined CD, whose rim was in contact with the PAEs. Thermodynamic parameters indicated that the inclusion of PAEs in the SAM system was entropy-driven, which is different from the free CD system where the inclusion was favored by both enthalpy and entropy. This is partly due to the difficulties in cancellation of strain energy by the inclusion into the cavity of the densely fixed CD (97% of the calculated maximum) and partly due to the reduction of hydrogen bonding between the PAEs and the surface-confined CD. Desolvation of the PAEs and CD by the friction at the penetration into the cavity of CD, which was rigidly fixed to the electrode, might also contribute to the positive entropy change. These factors might emphasize the apolar factor of binding to be characterized by a favorable entropy change in the immobilized CD system.