An EQCM electroacoustic study of poly(vinylferrocene) modified electrodes in different aqueous electrolytes

Abstract

In this paper we report the study of poly(vinylferrocene) films cast on Au electrodes by the electrochemical quartz crystal microbalance (EQCM) electroacoustic impedance technique. The simultaneous recording of the quartz resonant frequency and the surface electro-acoustic impedance of the quartz-film-liquid electrolyte composite system and a viscoelastic analysis of these parameters provide an estimate of the viscoelastic changes as a function of redox conversion and the mass changes as a result of solvent and ion exchange with the electrolyte. The changes in mass obtained by this method have been compared to the values calculated with the Sauerbrey approximation. By comparison of this information with results of probe beam deflection (PBD) technique under similar conditions for different electrolytes, the number of water molecules exchanged per electron/ion transferred, the ionic exchange mechanism and the shear modulus could be obtained. It has been found that they depend on the nature of the anion in the electrolyte. While the EQCM is sensitive to the total mass of ions and solvent exchanged with the electrolyte, the PBD detects the flux of ions exchanged but it is insensitive to the flux of solvent. We also describe for the first time the viscoelastic changes that take place during film conditioning by incorporation of solvent and ions during ‘break in’. Solvation of PVF + produces an increase in the shear modulus at 10 MHz as a result of viscoelastic losses in the swollen polymer film with segmental motion of polyion strands and electrolyte trapped in the polymer structure.