EQCM Study of the Adsorption of Polyethyleneglycol With Different Molecular Weights and its Coadsorption With Cl- ions on Pt in Perchloric Acid Solution

The adsorption of polyethyleneglycol with different molecular weights (PEGX; x = 400, 3000, 8000, or 20000), and its coadsorption with Cl ions, was investigated using cyclic voltammetry (CV) in conjunction with simultaneous measurements of the frequency changes of an electrochemical quartz crystal microbalance (EQCM). First we investigated the adsorption of PEGX on the surface of the Pt electrode at the open circuit potential. The results obtained showed that the mass of PEGX adsorbed on the Pt surface reaches a stationary value with time, and that this stationary value increases in proportion to the PEG molecular weight. The quantity of PEGX adsorbed on the Pt surface increased when Cl ions were present. In addition, the adsorption of PEGX did not interfere with the oxidation of the Pt surface to form a PtO film. Analysis of simultaneously recorded voltammograms and massograms revealed that during the potential scan in the negative direction in solutions without Cl three processes involving the partial desorption of PEGX from the Pt surface. In the presence of Cl, by contrast, one adsorption process and two partial desorption processes of PEGX occur. On the basis of the CV and EQCM data, we propose that in the absence of Cl ions, the oxidation of PEGX in the potential interval from 0.5 to 0.8 V form the corresponding aldehyde adsorbed on the Pt surface. In the presence of Cl ions, however, the oxidation of PEGX was almost completely suppressed