Surfactants adsorbed from aqueous solution onto mineral oxide surfaces are known to form micellelike aggregates at the interface. It has long been the view that at low coverages these surfactant aggregates were principally in the form of monolayers, formed on patches of the surface. Monomers in these aggregates are viewed as being oriented in such a manner that the hydrophilic groups of the surfactants are next to the surface, with the surfactant tail groups forming a hydrophobic film in contact with the aqueous solution. Aggregates of this structure are commonly referred to as hemimicelles. It has recently been proposed that bilayered aggregates, termed admicelles, were more consistent with the known role of the hydrophobic effect in the aggregate formation. This paper presents the results of a study that examines whether the hydrophobic interactions involved in the aggregation of adsorbed surfactants are large enough to induce admicelle formation at a lower chemical potential than that at which a hemimicelle would form. It is shown that while there are conditions under which a second layer of surfactant will always form simultaneously with the first layer of surfactant (admicelle formation), there are also wide ranges of conditions under which hemimicelles will form; i.e., under certain conditions surfáctants will aggregate to form a first layer without the simultaneous formation of a second layer. This is shown to be because the electrostatic contributions to the free energy of transferring a monomer from a hemimicelle to an admicelle are potentially much larger in magnitude than the largest possible hydrophobic contributions to the free energy of transfer. It is also concluded that when admicelles do form, there is little if any interpenetration of the hydrocarbon tails of the second layer of surfactant monomers between the hydrocarbon tails of the first layer, because the electrostatic repulsion between the two layers of head groups is larger than the largest possible gain in hydrophobic bonding that could result from the interpenetration.

The behavior of adsorbed surfactant aggregates, admicelles, was found to be very similar to that of micelles in the presence of alkanes. However, peculiar phenomena were observed in the incorporation of alcohols into admicelles: (i) very high ratios of alcohol to surfactant adsorption at lower coverages, (ii) remarkable increases of surfactant adsorption below the cmc, and (iii) a slight decrease of plateau adsorption. A two-site adsolubilization model is presented to interpret this complicated system. The hypothesis is that while the alkanes are adsolubilized only in a hydrocarbon core of the admicelle, there are two sites at which alcohols are adsolubilized, at the core and at the hydrophobic perimeter of the admicelle. The resulting model seems to be reasonable not only in explaining general trends in the alcohol and surfactant adsorption data but also in its estimates of aggregation numbers and the variation of aggregation numbers with alcohol and surfactant concentrations.

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