Study of oleate adsorption at the mercury/electrolyte solution interface as a function of electrode potential and time

The adsorption of sodium oleate from electrolyte solutions on a polarized mercury electrode can be studied by monitoring the differential capacitance oft he electrode double layer. Its variation with potential and time can provide qualitative information on the state of the oleate adsorbate. A new experimental methodology, three-dimensional phase-sensitive ac voltammetry, is used to provide safer information on the equilibrium transition characteristics and also to follow the time evolution of the phenomenon. The steady-state differential capacitance vs electrode potential curve is interpreted by means of recently developed thermodynamic criteria for phase transitions of surfactants at charged interfaces. At concentrations below the bulk critical micelle concentration monolayer adsorption takes place at moderate and negative polarizations, whereas there is some evidence of multilayer adsorption at more positive potentials. At higher concentrations, multilayer adsorption occurs throughout the potential range studied. In both cases, the formation of a close-packed structure is likely to be restricted to the first layer and the phase transitions of the latter to be realized via aggregates formed by its interaction with the outer layers. In addition, a close-packed polylayer is also observed at extreme positive potentials for high bulk oleate concentrations. Analysis of the capacitance transients based on simple diffusion, adsorption, or Avrami formalisms cannot describe successfully the time dependence of oleate adsorption on mercury, indicating that the latter is under mixed control or/and a complex process.