Quantitative analysis of the influence
of intermolecular interactions
on the response of electroactive binary monolayers, incorporating
the presence of electroinactive coadsorbed species and Marcus–Hush
kinetic formalism, has been deduced as an extension of previous models.
An expression for the current–potential relationship in terms
of two global phenomenological interaction parameters and the apparent
charge-transfer rate constant and formal potential has been obtained
and applied to multipotential chronoamperometry for the analysis of
intermolecular interaction effects on the kinetics of the charge transfer.
Experimental verification of the theoretical framework has been performed
with binary ferrocenylundecanethiol/decanethiol monolayers on gold
and platinum substrates for different coverage degrees of the redox
probe. The increase of the ferrocene coverage gives rise to a slowing
down of the charge-transfer process through a decrease of the apparent
rate constant above a 90% for gold and platinum electrodes when the
surface coverage increases from 1–2 to 15–30%. Significant
differences in the magnitude of intermolecular interactions and the
symmetry degree of the charge-transfer process are observed between
gold and platinum electrodes. Moreover, for high surface coverages
of ferrocene, two different domains appear, from which it is possible
to carry out a kinetic discrimination of the two responses. Values
of the interaction and kinetic parameters have been obtained for the
different monolayers under study, and the limit of applicability of
this treatment is discussed.
The analysis of effects of intermolecular interactions on square wave voltcoulommetric (SWVC) responses of electroactive monolayers under Nernstian conditions is presented. Theoretical expressions are given for the Faradaic and non‐Faradaic charge potential curves in terms of the potential perturbation variables and of the phenomenological interaction parameter G. Simple methods are proposed for determining the total excess of electroactive species, the apparent formal potential and the interaction parameter. The application of these methods to the analysis of the electrochemical behaviour of binary monolayers of ferrocenylundecanethiol/decanethiol at gold and platinum electrodes in two different non‐aqueous solvents allows us to obtain reliable values of the above parameters that clearly improve those determined by using Cyclic Voltammetry. Electrostatic insights into the interaction parameters is provided on the basis of a previously reported model. The importance of the presence of ion pairing processes in the value of the parameter G is discussed. The results obtained with SWVC technique point out the fact that intermolecular interactions are not avoided at low surface excesses of the redox probe, indicating that the impact of electrostatics in the appearance of non‐ideal electrochemical responses depends on the nature of the electrolytic media and on the structure of the monolayer in a complex way.
A theoretical model for the response of bidirectional surface confined molecular electrocatalysts in the presence of intermolecular interactions is presented. Particular cases corresponding to Nernstian and fully irreversible redox behaviour are discussed, with the main features of the stationary current‐potential response being analysed in detail. This analysis has revealed important nuances in the understanding of the overall process relative to the limits of validity of some frequent assumptions, as for example, a Nernstian behaviour of the charge transfer step and therefore of the complete catalytic mechanism. The presence of interactions leads to a distortion of the current‐potential responses for which, in the case of repulsive interactions, higher overpotential values are required for a given conversion rate. In the case of non‐Nernstian charge transfer processes, high catalytic rates give rise to an increase of the overall irreversibility of the catalytic currents. The experimental verification of the model corresponding to the response of binary 11‐(ferrocenyl)undecanethiol/decanethiol in presence of ferro/ferricyanide is presented and discussed. The impact of the experimental conditions on the apparent values of the formal potential and redox rate constants of the molecular electrocatalyst is also analysed.
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