P. Nikitas et S. Andoniou, SIMPLE-MODELS FOR ADSORPTION ON ELECTRODES .2. AGGREGATION PROCESSES AND PHASE-TRANSITIONS, Journal of electroanalytical chemistry [1992], 375(1-2), 1994, pp. 339-356
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.