A NEW THEORETICAL APPROACH TO ADSORPTION OF IONIC SURFACTANTS AT WATER OXIDE INTERFACES - STUDIES OF THE MECHANISM OF CATIONIC SURFACTANT ADSORPTION

The adsorption isotherms of cationic surfactants adsorbed from nearly neutral solutions on polar surfaces show, as a rule, two steps: one at small surface coverages and the other one terminating at a plateau corresponding to the crictical micelle concentration. The second step is generally believed to be due to the formation of bilayered surface aggregates (admicelles). The first step corresponds to the formation of a monolayer-like hydrophobic phase which may well be composed of single noncondensed monomers or to the creation of monolayered aggregates (hemimicelles). In order to study such two-step isotherms and the corresponding heats of adsorption, we have generalized our new theoretical approach for the case when the two kinds of surface aggregates coexist on a solid surface. The extended theoretical treatment can be applied also to monomer-admicelle surface equilibria by putting equal to one the aggregation number of hemimicelles in appropriate theoretical expressions. The obtained theoretical expressions for adsorption isotherms and heats of adsorption were next fitted to the experimental data obtained at CNRS Laboratory in Montpellier. Our computer exercises showed that when agreement with experiment was achieved, the calculated aggregation number of hemimicelles was unity. This suggests that the first step on the isotherms of cationic surfactants is due to single monomer adsorption, in accordance with such an assumption often expressed in literature, but not sufficiently documented. The aggregation numbers determined by computer are smaller than those found previously by us for zwitterionic surfactants and are much smaller than the surface aggregation numbers found experimentally for anionic surfactants adsorbed at the water/oxide interface.