On the inhibitory effect of normal aliphatic alcohols upon Cd 2+, Pb(OH) −3 and Bi 3+ electroreductions on mercury at high surface coverages

Abstract

A detailed investigation of the inhibitory effect of normal aliphatic alcohols, from n-butanol to n-octanol, upon Cd 2+ and Bi 3+ reductions and Cd o (Hg) oxidation in perchloric medium, as well as upon Pb(OH) − 3 reduction and Pb o (Hg) oxidation in (NaF + NaOH) mixtures, was carried out at different ionic strengths on dropping mercury and amalgam electrodes. Experiments were conducted under conditions in which the coverage θ of the electrode surface by the surface-active alcohols is almost complete (θ ≅ 1). The experimental values of the ratio (k θ=1/k θ=0) of the rate constant for the electrode reaction at high surface coverages to the corresponding rate constant in the absence of the alcohols, were compared with a theoretical expression of the form [1] ▪ Here c * s and C * W are the bulk concentrations of the alcohol(S) and of water (W), n ≠ and n S are the numbers of adsorbed water molecules displaced from the electrode surface by one adsorbing particle of the activated complex for the electrode reaction (≠) and by one adsorbing molecule of the surfactant S, respectively, and Δ G o ads is the standard free energy of adsorption of S. Moreover, c is the coordination number in the adsorbed monolayer and Δ G lat measures the energy involved in the process ▪ where the hyphen denotes lateral interactions between contiguous particles in the adsorbed monolayer. The derivation of eqn. (1) relies on the following assumptions: ( 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. The expression for Δ G lat depends both on the particular model of the adsorbed monolayer and on the particular mechanical statistical treatment of this model. For all systems investigated the quantity (cn ≠ Δ G lat/RT) turns out to be notably positive. This behaviour can be explained by the ability of adsorbed water molecules to establish strong ion-dipole interactions - and, possibly, further interactions of a more specific nature - with the charged activated complex, no such strong interactions with this complex being established by adsorbed aliphatic molecules. The experimental dependence of k θ=1 for Pb(OH) − 3 and Bi 3+ reduction and for Pb o (Hg) oxidation upon the bulk concentration of n-butanol, under certain experimental conditions, suggests a gradual passage from the mechanism previously outlined to an alternative mechanism in which the rate determining step involves the tunnelling of the transferring electron for the Me(II) + e ⇌ Me(I) charge transfer step, across the adsorbed monolayer of the alcohol. This gradual passage takes place as the n-butanol bulk concentration is progressively decreased. The dependence of the rate constant k θ=1, suitably corrected for diffuse-layer effects, upon the applied potential in the presence of the various normal aliphatic alcohols, is in qualitative agreement with an expression which accounts for the progressive increase in the compact-layer thickness with an increase in the length of the hydrocarbon chain.