Film mass and volume changes accompanying redox-driven solvent and salt transfer during redox switching of polyvinylferrocene films

Abstract

We describe electrochemical quartz crystal microbalance (EQCM) measurements of salt and solvent transfers that accompany redox switching of poly(vinylferrocene) (PVF) films exposed to aqueous solutions of NaClO₄. At low salt concentration (c_salt ⩽ 0.1 mol dm⁻³), oxidized and reduced PVF films are both permselective. At high salt concentration, salt partitions into the film upon film oxidation; this effect is significant for c_salt ⩾ 1 mol dm⁻³. In order to explain salt partition quantitatively, we develop a new thermodynamic model that directly incorporates the variation of electroactive polymer film volume with film redox state and salt concentration. Application of the model to the PVF–NaClO₄ system shows that oxidized PVF films shrink appreciably (typically 35%) under non-permselective conditions. The salt and solvent partition coefficients into oxidized PVF are K_salt,Ox≈ 1.0 and K_solv,Ox ≈ 0.15, respectively.