SIMPLE-MODELS FOR ADSORPTION ON ELECTRODES .2. AGGREGATION PROCESSES AND PHASE-TRANSITIONS

The aggregation of neutral organic molecules on electrode surfaces to form oligomers or surface micelles and the phase transformations of an adsorbed monolayer were studied by means of statistical mechanics. Th e model developed for aggregation phenomena predicts that the formatio n of oligomers is not depicted in the capacitance plots, which exhibit the same features as those for adsorption of monomers under all circu mstances. The equilibrium between monomers and oligomers extends throu ghout the polarization range where these two states of the adsorbate c oexist. As the number of monomer units in the aggregates increases, mo nomers and aggregates tend to separate, occupying different polarizati on regions. In addition, the equilibrium between monomers and large ag gregates leads in general to complicated capacitance plots characteriz ed by the appearance of very sharp capacitance peaks. The same feature s characterize the formation of mixed two-dimensional micellar films o n electrode surfaces. Phase transformations of adsorbed monolayers on electrode surfaces, which may lead to the formation of either two immi scible concentrated surface solutions of adsorbate in solvent and vice versa or pure adsorbate precipitate, are examined within the framewor k of the molecular models developed in this series of papers. The sepa ration of the adsorbed layer to immiscible surface solutions is relate d to the short-range particle-particle interactions, whereas the surfa ce precipitation process is considered as an aggregation process where the aggregation number tends to infinity. It is shown that in general there is an acceptable agreement between theory and experiment.