Solubility, diffusion coefficient and ionic conductivity of alkyl viologens in poly(ethylene oxide) and its derivatives

A series of alkyl viologens RV (R denotes ethyl, butyl, hexyl, heptyl, and dodecyl) was dissolved in poly(ethylene oxide) (PEO) oligomers (average molar masses of 200, 300, 400, 600 and 1000 g mol⁻¹). The solubility of RV in PEO oligomers decreased with increasing alkyl chain length of RV and the molar mass of PEO. Cyclic voltammograms of RV in PEO containing 0.50 M LiClO₄ clearly show two redox waves. The ionic conductivity of PEO oligomers containing RV decreased with increasing alkyl chain length, suggesting the migration of RV itself in the PEO oligomers. Potential step chronoamperometry was used to obtain the apparent diffusion coefficient of RV in the PEO oligomers. The ionic conductivity has a linear relationship with the apparent diffusion coefficient regardless of the RV species, the PEO molar mass and the temperature. RV was shown to act as a redox mediator in PEO oligomers as long as the ionic conductivity of the PEO was high. Poly(oligo(oxyethylene) methacrylate) (PMEO) was used as a solid solvent for a series of alkyl viologens. Since PMEO is an excellent ion-conducting polymer, RV was confirmed to be an effective redox mediator in this PMEO. It was concluded in this study that ionic conductivity in the polymer matrix could be used as an effective parameter for prediction of the diffusion coefficient of charged organic molecules.