Theoretical Aspects of a Surface Electrode Reaction Coupled with Preceding and Regenerative Chemical Steps: Square‐wave Voltammetry of a Surface CEC’ Mechanism

Abstract

Large number of lipophilic substances, whose electrochemical transformation takes place from adsorbed state, belong to the class of so‐called “surface‐redox reactions”. Of these, especially important are the enzymatic redox reactions. With the technique named “protein‐film voltammetry” we can get insight into the chemical features of many lipophilic redox enzymes. Electrochemical processes of many redox adsorbates, occurring at a surface of working electrode, are very often coupled with chemical reactions. In this work, we focus on the application of square‐wave voltammetry (SWV) to study the theoretical features of a surface electrode reaction coupled with two chemical steps. The starting electroactive form Ox(ads) in this mechanism gets initially generated via preceding chemical reaction. After undergoing redox transformation at the working electrode, Ox(ads) species got additionally regenerated via chemical reaction of electrochemically generated product Red(ads) with a given substrate Y. The theory of this so‐called surface CEC’ mechanism is presented for the first time under conditions of square‐wave voltammetry. While we present plenty of calculated voltammograms of this complex electrode mechanism, we focus on the effect of rate of regenerative (catalytic) step to simulated voltammograms. We consider both, electrochemical reactions featuring moderate and fast electron transfer. The obtained voltammetric patterns are very specific, having sometime hybrid‐like features of voltammograms as typical for CE, EC and EC’ mechanisms. We give diagnostic criteria to recognize this complex mechanism in SWV, but we also present hints to access the kinetic and thermodynamic parameters relevant to both chemical steps, and the electrochemical reaction, too. Indeed, the results presented in this work can help experimentalists to design proper experiments to study chemical features of important lipophilic systems.

Diffusional Electrochemical Catalytic (EC’) Mechanism Featuring Chemical Reversibility of Regenerative Reaction-Theoretical Analysis in Cyclic Voltammetry

“…It is important to stress that the magnitude of limiting currents of steady-state voltammograms calculated under such circumstances is function of Keq and Kcatalytic, but it is not affected by the parameters related to electron transfer step of the electrode reaction (ks o , α). [7,17,19] As reported elsewhere, [3,7,[17][18][19] the magnitude of "limiting current" (Ψlimiting) of steady-state cyclic voltammograms at EC' mechanisms is important parameter that can be explored for quantitative analytical purposes, but also for the kinetic determinations. We analyzed in this work the magnitudes of Ψlimiting as a function of equilibrium constant (measured at various rates of catalytic chemical reaction), and as a function of rate of regenerative chemical reaction (evaluated at several Keq values).…”

“…[7,17] Briefly, the magnitude of limiting currents of the steady-state cyclic voltammograms of diffusional EC'rev mechanism (for Keq > 3000) is a linear function of the square-root of dimensionless catalytic parameter Kcatalytic. [4,7,9,17] [1][2][3]18], then we can evaluate the value of Kcatalytic. We should keep constant during the experiments only the τ parameter, and the concentration of substrate "S" as well.…”

“…[4,7,9,14] Cyclic voltammetry is recognized as the most valuable voltammetric technique, suitable for studying electrochemical and chemical features of many substances. [1][2][3][4][5][6][7][8][9]11,[14][15][16][17][18] The cyclic voltammetry allows us to get insight into chemical changes that occur during an electrochemical transformation of given "electroactive" substances at working electrode. [7] Due to many efforts focused on the designing of sustainable voltammetric experimental methods for studying redox chemistry of enzymes and drugs, in the past three decades we witness great resurgence in the development of theories of cyclic voltammetry for studying various electrode mechanisms.…”

“…[7] Due to many efforts focused on the designing of sustainable voltammetric experimental methods for studying redox chemistry of enzymes and drugs, in the past three decades we witness great resurgence in the development of theories of cyclic voltammetry for studying various electrode mechanisms. [1,3,4,7,[9][10][11][12][13][14][15][16][17][18][19] The theoretical models in cyclic voltammetry can be explored to develop relevant methods for the determination of important kinetic and thermodynamic parameters related to the chemistry of many physiologically important substances. [4,7] In the family of electrode mechanisms that are complicated with chemical reactions, the socalled electrochemical-catalytic EC' mechanism is recognized as the most suitable model that provides insight into kinetic parameters relevant to interactions of many drugs, and the enzyme-substrate interactions as well.…”

“…[3][4][5][6][7] Until recently, theoretical models developed for the EC' mechanisms considered an irreversible chemical reaction involved in the regeneration of initial electroactive reactant. [3,7,9] Only recently, our group [18] and others, [19] assumed a chemical reversibility of the regenerative steps at the EC' mechanism. This scenario brings more complexity in theoretical considerations, since there is one more degree of freedom to be considered in the mathematical models, i.e.…”