Electrosorption of tetraalkylammonium ions at the silver iodide—electrolyte interface

Abstract

In this paper, the electrosorption of tetraalkylammonium ions at the silver iodide-electrolyte interface is discussed. The adsorption of TPrA+ and TBuA+ ions was calculated from charge—potential curves using Gibbs' law. The computed values agree with those obtained directly from depletion measurements, proving the applicability of equilibrium thermodynamics to the silver iodide—electrolyte interface. For the charge—potential behaviour of AgI in the presence of TAA+ ions a model was developed in which, at a constant surface charge, a linear dependence between the number of adsorbed TAA+ ions and the Galvani potential difference across the solid-liquid interface was assumed. The double layer in the absence of organic adsorbate followed a patchwise hydration model developed earlier. The point of zero charge shifts to more positive potentials with increasing adsorption of TAA+ ions. For TBuA+ ions the maximum shift amounts to 265 mV, of which 188 mV can be attributed to displaced water dipoles and 77 mV to specifically adsorbed TBuA+ charges. For a homologous series of TAA+ ions, the affinity increases sharply with size. The charge—potential curves at different TAA+ concentrations show a more or less well—defined common intersection point (cip), which shifts to more positive potentials with increasing alkyl chain length. The cip corresponds to an adsorption maximum for TAA+ ions. Our model related the surface charge at the cip to the radius of the TAA+ ion, yielding good agreement with experimental data. At low coverage the charge potential curves of AgI in the presence of TBuA+ ions could be fitted satisfactorily with our model, however at higher coverages deviations were observed.