Cyclic voltammetric, EQCM and impedance study of the 1-octanol-beta-cyclodextrin-electrodeposited platinum system in perchloric medium

Abstract

The system of I-octanol-beta-cyclodextrin in 0.5 M HClO4 at Pt electrodeposited on an Au/quartz crystal (Pt/Au/Q) has been studied by cyclic voltammetry (CV), the electrochemical quartz crystal microbalance (EQCM) and impedance measurements. beta-Cyclodextrin (beta-CD), a cyclic oligomer of seven alpha-D-glucopyranose units, was chosen for this study because it forms an inclusion complex with primary alcohols and because of its hydrophilic exterior. Previous cycling in I-octanol profoundly affected ("aged") the electrodeposited Pt, as is readily seen in subsequent CVs and mass curves in 0.5 M HClO4. Since adsorbed I-octanol and beta-CD would tend to render the Pt surface hydrophobic and hydrophilic, respectively, these opposite tendencies should be detectable by the EQCM. Effectively, in a positive potential scan, 1-octanol and beta-CD added at -0.22 V produced a mass decrease and increase, respectively, of aged Pt/Au/Q. owing to a decrease and increase of the amount of adsorbed water and/or ions, which was attributed to physisorption of these compounds. With freshly deposited Pt/Au/Q, I-octanol added at open circuit produced a mass decrease over the whole potential cycle, again evidence of adsorption of a hydrophobic compound. The potential at which the aged Pt/Au/Q electrode immersed in an 1-octanol solution was held while beta-CD was added to the electrolyte crucially affected the subsequent voltammogram and mass Curve. If beta-CD and I-octanol were added together at -0.22 V, the current in a subsequent positive scan was the same as in I-octanol added at -0.22 V, but the mass was higher, both at -0.22 V and over the whole positive scan. This mass increase was probably due to physisorption (since the current was unaffected.) of the inclusion complex, since beta-CD alone did not affect the mass at -0.22 V. On the contrary, if beta-CD was added at open circuit to an electrolyte already containing I-octanol, also added at open circuit, both the H desorption and Pt oxidation currents were lower than in I-octanol. indicating a strong interaction of beta-CD with the Pt Surface, and the mass was also lower over most of the positive scan. Most probably this strong interaction of beta-CD involves adsorbed residues formed in the dissociative chemisorption of I-octanol at open circuit.