Electron and Mass Transport in Hybrid Redox Polyether Melts Contacted with Carbon Dioxide
2002-07-13T00:00:00Z (GMT) by
Films of neat metal salts with covalently attached oligoether side chains ([Co(bpy(CO2MePEG-350)2)3](ClO4)2; bpy is 2,2‘-bipyridine, and MePEG-350 is methyl-terminated oligomeric ethylene oxide with an average molecular weight of 350 Da) undergo marked changes in physical and electrochemical properties upon contact with CO2. Electrochemical measurements indicate that the physical diffusion coefficient (DPHYS) of the Co(II) species, the observed rate constant for Co(II/I) self-exchange (kEX), and the physical diffusion coefficient of the perchlorate counterion (DClO4) increase from 2.4 × 10-11 to 7.0 × 10-10 cm2/s, 6.8 × 105 to 4.5 × 106 M-1 s-1, and 3.4 × 10-10 to 4.3 × 10-9 cm2/s, respectively, as CO2 pressure is increased from 0 to 2000 psi at 23 °C. A reduction in activation energy accompanies the enhancement of each of these properties over this pressure range. Increasing CO2 pressure from ambient to 1000 psi causes the films to swell 13%, and free-volume theory explains the enhanced mass transport properties of the films. The origin of increases in electron-transfer kinetics is considered. Plots of log(kEX) versus log(DPHYS) and log(kEX) versus log(DClO4) are both linear. This suggests that electron self-exchange is controlled by factors that also affect log(DPHYS) or log(DClO4). One explanation is based on plasticization of the oligoether side-chain motions by CO2 that affect ether dipole repolarization and Co complex diffusion rates. A second explanation for the changes in kEX is based on control of electron transfer by relaxation of counterions neighbor to the Co complexes, which is measured by DClO4. Both explanations represent a kind of solvent dynamics control of kEX.