Interactions admicelles

When sur-f actants adsorb on metal oxide sur-f aces (e.g., minerals), at low concentrations, the adsorbate molecules are widely dispersed enough that no signi-ficant interactions between adsorbed sur-f actants occurs. Above a certain critical concentration, dense sur-factant aggregates form on the surface (72). These are called admicelles. For ionic surfactants, the admicelles are bilayered structures (72). Above the CMC, the total adsorption of surfactant can increase or decrease slowly. 

The relative tendency for surfactants or surfactant mixtures to form a micelle compared to a monolayer is approximately the same. However, the relative tendency to form an admicelle can be substantially different from that to form micelles or monolayers. This is because there can be specific interactions between the solid surface and the surfactants as well as intersurfactant interactions in the aggregate. The surfactant technologist can take advantage of this to design... 

The explanation For this eFFect lies in the steric constraints oF the two-dimensional admicelle. In the admicelle, the thickness oF the hydrophobic portion oF the admicelle is probably dictated by the length oF the longest alkyl chain present (i.e., the dodecyl chain). In the mixed admicelle, the decyl chain is almost completely surrounded by other alkyl chains and so is exposed to approximately the same environment as in a pure CioS0 admicelle. However, the two additional methylene groups on the dodecyl chain are not in as hydrophobic an environment as in the pure Ci SO admicelle, since neighboring decyl chains cannot interact... 

The agreement between the mixture adsorption data and ideal solution theory is excellent. It is important to remember that while looking at various constant leyels of adsorption in Region II, we are looking at the CAC of the mixed admicelle that has just formed on a particular patch. By looking at different adsorption levels, we are looking at how the two surfactants interact on different energy level patches on the surface. 

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