On the inhibitory effect of neutral organic surfactants upon simple electrode reactions at high surface coverages

Abstract

A treatment of the inhibitory effect of neutral organic surfactants at high surface coverages is provided, based on the following assumptions:o(I)The activated complex for the electrode reaction is specifically adsorbed and is the same independent of whether it is formed by displacement of adsorbed solvent molecules or of adsorbed surfactant molecules.(II)The theory of absolute reaction rates applies.(III)The surfactant is adsorbed under equilibrium conditions. With these assumptions, a statistical mechanical treatment of different models of the adsorbed monolayer leads to a general expression for the ratio (kvS=1/kvS=0) of the rate constant at high surface coverages to the corresponding rate constant in the absence of the surfactant. This expression consists of a factor depending upon the bulk concentration of the surfactant, which is identical to that proposed by Lipkowski and Galus [1], a factor depending upon the standard free energy of adsorption of the surfactant at zero coverage, as well as a factor accounting for the lateral interaction energies of adsorbed solvent molecules, surfactant molecules and reacting particles between themselves. The inhibitory effect of aliphatic alcohols upon the kinetics of Cd2+ and Cu2+ electroreduction is examined in the light of this theoretical expression. It is suggested that the energy spent in overcoming the ion-dipole interactions between the charged activated complex and the neighbouring adsorbed water molecules as we pass from a solvent-covered to a surfactant-covered electrode surface is responsible for a decrease in the (kvS=1/kvS=0) ratio by some orders of magnitude. The activated complex for the electrode reaction is specifically adsorbed and is the same independent of whether it is formed by displacement of adsorbed solvent molecules or of adsorbed surfactant molecules. The theory of absolute reaction rates applies. The surfactant is adsorbed under equilibrium conditions. With these assumptions, a statistical mechanical treatment of different models of the adsorbed monolayer leads to a general expression for the ratio (kvS=1/kvS=0) of the rate constant at high surface coverages to the corresponding rate constant in the absence of the surfactant. This expression consists of a factor depending upon the bulk concentration of the surfactant, which is identical to that proposed by Lipkowski and Galus [1], a factor depending upon the standard free energy of adsorption of the surfactant at zero coverage, as well as a factor accounting for the lateral interaction energies of adsorbed solvent molecules, surfactant molecules and reacting particles between themselves. The inhibitory effect of aliphatic alcohols upon the kinetics of Cd2+ and Cu2+ electroreduction is examined in the light of this theoretical expression. It is suggested that the energy spent in overcoming the ion-dipole interactions between the charged activated complex and the neighbouring adsorbed water molecules as we pass from a solvent-covered to a surfactant-covered electrode surface is responsible for a decrease in the (kvS=1/kvS=0) ratio by some orders of magnitude.