Counter-ion specific effects on charge and solvent trapping in poly(vinylferrocene) films

This study determined the effect on the electrochemical quartz crystal micr obalance's (EQCMs) response of substituting tetrafluoroborate and hexafluor ophosphate as counter ions into a poly(vinylferrocene) (PVF) film that had been prepared initially with perchlorate ion as the counter ion in methylen e chloride. This PVF+ClO4- film was redox cycled successively in aqueous 0. 1 M sodium perchlorate, 0.1 M sodium hexafluorophosphate and 0.1 M sodium t etrafluoroborate solutions. Cyclic voltammetric EQCM experiments determined the changes in film mass and charge during the first and subsequent potent ial cycles. In all bathing solutions, the total mass change and charge cons umed during the first oxidation cycle were larger than in subsequent potent ial cycles; thereafter there were negligible changes. This is rationalized by the trapping of one or more oxidized forms of PVF produced in the first oxidation cycle, and two possible detailed mechanisms are given. The extent of film oxidation was controlled kinetically, and it changed in the counte r ion order tetrafluoroborate > perchlorate > hexafluorophosphate. Less fil m oxidation occurred at high potential scan rates for all counter ions. At the highest potential scan rate (0.080 V s(-1)) the amount of water enterin g the oxidized film from solution was greater in tetrafluoroborate, than in perchlorate, or than in hexafluorophosphate solutions. At slower potential scan rates these differences still existed but were less obvious. Free vol ume restraints within the film limited the total amount of counter ion and solvent that could be introduced under kinetically controlled conditions. ( C) 2000 Elsevier Science S.A. All rights reserved.