Electrochemical quantification of the thermodynamic equilibrium constant of the tenoxicam-β-cyclodextrin inclusion complex formed on the surface of a poly-β cyclodextrin-modified carbon paste electrode

Abstract

In this work it is shown that when a carbon paste electrode, CPE, is modified with a β-cyclodextrin polymer, the tenoxicam oxidation becomes an adsorption controlled process due to formation of a surface inclusion complex with the β-CD molecules comprising the surface of the polymer. It was found that such surface inclusion complex can be formed independently of the tenoxicam predominant species, Tenox’, in the aqueous solution namely: H2Tenox+, HTenox or Tenox−, depending on the solution pH. The electrochemical quantification of the thermodynamic constant of the equilibrium Tenox’+β-CD (polymer)=Tenox’-β-CD (polymer) was estimated as log Kincl.=4.26±0.01. Furthermore, from the analyses of the experimental voltammograms according with Laviron's equation for an irreversible surface reaction [E. Laviron, J. Electroanal. Chem. 52 (1974) 355-393] it is shown that the surface concentration, ΓR, of tenoxicam increases as its concentration in solution does, reaching a maximum value of 1.51×10−10molcm−2 at 64μM.