Self-assembled monolayers at electrode metal surfaces

Neutral organic compounds, dissolved in an electrolyte in contact with an electrode, adsorb and form different monolayers which may range from dilute to compact films. In some instances, non-electroactive organic molecules are highly associated and form 2D condensed phases which are characterized by the presence of phase transitions. The occurence of these self-assembled monolayers is discussed on the basis of experimental results obtained at equilibrium as well as under dynamic conditions. Self-assembling depends on the relative magnitude of the interactions involving the surfactant, the solvent and the electrode. Adequate potential-step programmes have been successfully used to trigger the formation of the ordered phase. It is found that the kinetics are controlled by a nucleation and growth mechanism. According to the experimental conditions, a deterministic or stochastic behaviour is observed. The amplitude of the supersaturation, given by the surface free energy gap between the final and metastable states, is independently controlled by the potential, temperature and surfactant concentration. The classical nucleation theory allows the determination of key parameters such as the line tension, the radius and free energy of formation of the critical nucleus. Ion and electron transfer processes through condensed monolayers are also briefly described.