Apparent self-exchange rate

Fig. 5. Plot of apparent electron self exchange rate constants kf P, derived from polymer De values for films containing the indicated metals, mixed valent states, and ligands, all in acetonitrile, using Equation 2, vs. literature heterogeneous electron transfer rate constants k° for the corresponding monomers in nitrile solvents. See Ref. 6 for details. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)...

The electrostatics-corrected self-exchange rate for plastocyanin based on Co(phen)33+ is 2.6 X 103 M 1 sec 1. The kncorr value for plastocyanin based on the cytochrome c cross reaction, 5 X 105 M"1 sec 1, is substantially smaller than the uncorrected value (3 X 107 M 1 sec 1). Taking either value, however, it is apparent that both cytochrome c and Co-(phen)33+ are better electron transfer agents for plastocyanin than is Fe(EDTA)2". 

The values for the rates of oxidation of Eu(II) by a series of Ru(II) amines have been reported by Poon and Tang (1984). These authors used the results that had been previously reported for the reduction of these same amines by Cr(II) and V(II) as the basis for the postulate that the Eu(II) exchange reactions were outer-sphere. They then calculated self-exchange reactions for the Ru(III) complexes from the data obtained with V(II), and used the data to calculate apparent self-exchange reactions for Eu(II)/Eu(III). The authors concluded that the values so calculated (within the range 2.3 x 10 to 1.7 x 10 M" s" ) were in reasonable agreement with other estimates. 

Zelsmann and co-workers [88] have reported tracer diffusion coefficients for water in Nafion membranes exposed to water vapor of controlled activity. These were determined by various techniques, including isotopic exchange across the membrane. They reported apparent self-diffiision coefficients of water much lower than those determined by Zawodzinski et al. [64], with a weaker dependence on water content, varying from 0.5 x 10 cm to 3 x 10 cm /s as the relative humidity is varied from 20 to 100%. It is likely that a different measurement method generates these large differences. In the experiments of Zelsmaim et al., water must permeate into and through the membrane from vapor phase on one side to vapor phase on the other. Since the membrane surface in contact with water vapor is extremely hydrophobic (see Table 7), there is apparently a surface barrier to water uptake from the vapor which dominates the overall rate of water transport in this type of experiment. 

The fact that some self-exchange occurs may be seen as difficult to reconcile with the apparent irreversibility of the adsorption. If one takes the point of view that the adsorption is inherently reversible but that desorption occurs at a vanishingly slow rate, the question immediately arises as to why self-exchange is rapid. Since self-exchange presunably involves desorption of one molecule followed by adsorption of another, then it would be expected that desorption into buffer would occur at a similar rate to self-exchange. The major difference between the self-exchange experiment and the desorption experiment is the complete absence of solution fibrinogen in the latter. It may therefore be speculated that desorption is facilitated by the participation of protein from solution perhaps via impact collision or complex formation. 

The ease of preparation and the variety of electron active ions make ion-exchange polymers amenable to fundamental studies of electron transfer mechanisms. The systematic variation of the concentration (effective density) of redox sites within a coating has been useful in the construction of electron hopping models (4). These models are based on the apparent rate of electron diffusion through the film. Because the redox centers are able to diffuse within the polymer, the apparent rate is related to two parameters redox molecule diffusion and the rate of electron self-exchange. 

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