“…Despite this observation, the predicted difference in formal potentials of 59 mV (eq 7) between the 0.01 and 0.1 M solutions is somewhat higher than the observed values. Our results show a smaller deviation from Nernstian behavior for PVF than reported previously. , We believe this is caused by differences in film history and experimental conditions. 5 6 …”
Section: Discussioncontrasting
confidence: 66%
“…Our results show a smaller deviation from Nernstian behavior for PVF than reported previously. 6,7 We believe this is caused by differences in film history and experimental conditions.…”
Section: Discussionmentioning
confidence: 99%
“…The simple model usually fails to fit experimental data well. Evans et al modified the simple model and introduced corrections for volume changes caused by redox switching and the mechanical energy connected with these changes. − Bard et al. , used a different model, assuming not one but a set of discrete E ° values to fit cyclic voltammetric data. Lang, Bacskai, and Inzelt used a transmission line model based upon two double-layer capacitances at each boundary of the film to explain anomalies in impedance spectra.…”
Cyclic voltammetry was used to create nonequilibrium populations of different solvation and configurational states of partially oxidized polyvinylferrocene (PVF). Oxidation levels were established by scanning either from a fully reduced state to the desired oxidation level or from a fully oxidized state to the desired level. Coulostatic conditions were then established by opening the external circuit, and subsequent mass and potential changes were followed. The film's approach to equilibrium involves configurational changes within the polymer and simultaneous and subsequent solvent transfer. At very short times (t e 0.2 s) the approach to equilibrium is limited by both solvation and reconfiguration processes. For a short time afterward (0.2 < t/s < 1.0) reconfiguration alone is rate limiting. At intermediate times (1 < t/s < 5) both processes play comparable roles. At long times (t > 5 s) solvation is the controlling step. The electroactive polymer film does not completely reach equilibrium even after 1 h at open circuit as evidenced by continuous small mass changes. These mass changes are the result of water transfer between the polymer film and the bathing electrolyte. A scheme of cubes model rationalizes mechanistic pathways leading to equilibrium. In particular, the observed extrema in mass (solvent population) are predicted. The electrode potential, after 1 h at open circuit, shows nearly Nernstian dependence on the redox composition for film states produced by either anodic or cathodic cyclic voltammetric scans. These Nernst plots are displaced by only a few millivolts because only a weak Nernstian dependence on film water content exists. Films that are 50% oxidized exhibit a sub-Nernstian response with respect to the perchlorate concentration in the bathing solution under permselective conditions.
“…Despite this observation, the predicted difference in formal potentials of 59 mV (eq 7) between the 0.01 and 0.1 M solutions is somewhat higher than the observed values. Our results show a smaller deviation from Nernstian behavior for PVF than reported previously. , We believe this is caused by differences in film history and experimental conditions. 5 6 …”
Section: Discussioncontrasting
confidence: 66%
“…Our results show a smaller deviation from Nernstian behavior for PVF than reported previously. 6,7 We believe this is caused by differences in film history and experimental conditions.…”
Section: Discussionmentioning
confidence: 99%
“…The simple model usually fails to fit experimental data well. Evans et al modified the simple model and introduced corrections for volume changes caused by redox switching and the mechanical energy connected with these changes. − Bard et al. , used a different model, assuming not one but a set of discrete E ° values to fit cyclic voltammetric data. Lang, Bacskai, and Inzelt used a transmission line model based upon two double-layer capacitances at each boundary of the film to explain anomalies in impedance spectra.…”
Cyclic voltammetry was used to create nonequilibrium populations of different solvation and configurational states of partially oxidized polyvinylferrocene (PVF). Oxidation levels were established by scanning either from a fully reduced state to the desired oxidation level or from a fully oxidized state to the desired level. Coulostatic conditions were then established by opening the external circuit, and subsequent mass and potential changes were followed. The film's approach to equilibrium involves configurational changes within the polymer and simultaneous and subsequent solvent transfer. At very short times (t e 0.2 s) the approach to equilibrium is limited by both solvation and reconfiguration processes. For a short time afterward (0.2 < t/s < 1.0) reconfiguration alone is rate limiting. At intermediate times (1 < t/s < 5) both processes play comparable roles. At long times (t > 5 s) solvation is the controlling step. The electroactive polymer film does not completely reach equilibrium even after 1 h at open circuit as evidenced by continuous small mass changes. These mass changes are the result of water transfer between the polymer film and the bathing electrolyte. A scheme of cubes model rationalizes mechanistic pathways leading to equilibrium. In particular, the observed extrema in mass (solvent population) are predicted. The electrode potential, after 1 h at open circuit, shows nearly Nernstian dependence on the redox composition for film states produced by either anodic or cathodic cyclic voltammetric scans. These Nernst plots are displaced by only a few millivolts because only a weak Nernstian dependence on film water content exists. Films that are 50% oxidized exhibit a sub-Nernstian response with respect to the perchlorate concentration in the bathing solution under permselective conditions.
“…Similar nonideal behavior has been described for other redox molecules that are specifically adsorbed to electrode surfaces, contained in self-assembled monolayer structures, or incorporated into polymeric redox films. The nonidealities associated with these fixed-site redox reactions have been attributed to a variety of factors and include concentration-dependent surface activity coefficients, inhomogeneity or a distribution of redox sites, excess chemical potentials, molecular dipole effects, Donnan equilibria, mechanical stress, and surface electrostatic effects . With the exception of inhomogeneity effects, most of these factors are thought to be relatively unimportant in the experiments described here and would not appreciably contribute to the observed non-Nernstian behavior.…”
The effects of J-aggregation on the redox thermodynamics
for a cationic cyanine spectral sensitizing dye are
investigated. When adsorbed to the surfaces of cubic AgBr
microcrystals, both the monomeric and J-aggregate
forms of the cyanine dye can be reversibly oxidized with redox
solutions containing ferricyanide or
molybdicyanide complex. Diffuse reflectance spectra recorded for
thin gelatin coatings of the dyed
microcrystals after treatment with redox solution show distinct
spectral bands associated with the dye and a
stable, monooxidized dye radical ion. The fraction of dye oxidized
can be accurately varied by simple
adjustment of the electrochemical potential of the redox buffer
solution. For the J-aggregated dye, the
reflectance band for the dye shifts to shorter wavelengths and becomes
significantly broadened with increasing
fractional degree of oxidation. The formal oxidation potential for
the adsorbed dye can be obtained from a
Nernstian plot of redox-solution potential E vs log
[oxidized dye]/[dye] as constructed from the
reflectance
spectral data. The results indicate that the one-electron
oxidation potential of the monomer on cubic AgBr
to be lower than that of the J-aggregate by 74 mV. The energy of
the singlet excited state of the J-aggregate
is calculated to be lower than that of the monomer by a much larger
amount. For the cationic dye in this
study, the aggregation-induced changes in redox potential result in
dramatic differences in the comparative
photoresponses of the monomer and aggregate dye systems.
“…[17][18][19][20][21] One of the most important conclusions of the work of Mauk and Moore 17 is that ''the possibility of redox state-dependent conformational changes should be considered''. Precisely, the idea of a redox potential being dependent on the state of tension (electron transfer/ deformation coupling) was introduced by Evans et al, [22][23][24] to explain the redox potential distribution of poly(vinylferrocene). The coupling of electron transfer, deformation, binding and electrostatic screening effects, was introduced to explain the redox potential distribution during the redox switching of arylamine substituted polymers.…”
Experimental data are presented demonstrating that electrochemically active macromolecules show a coupling among electron transfer, deformation, screening and binding. The work includes dependence of the redox potential of synthetic and natural electrochemically active polymers on the electrolyte pH (electron transfer-binding coupling), the changes in volume during the redox switching of synthetic electrochemically active polymers (deformation-electron transfer coupling) and the changes in the macromolecular conformation during the acid-base titration of polyelectrolytes and proteins (deformation-binding coupling). A simple equilibrium statistical thermodynamic model is presented that allows explaining these couplings effects. The model is based on the assumption that a macromolecule is composed of segments of different length that may bind species present in the external solution and that also contain redox centers that may be oxidized and reduced. The partition function of the system is obtained, and from it the expressions for the redox potentials, the total length and the chemical potential of the bound species are obtained. Simple calculations shows that the model satisfactorily explains the qualitative behavior of the experimental results.
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