Surfactant Concentration Tight Monolayer Packing

A review of early studies is presented in the Harkins monograph. The author writes  

This conclusion was based on a careful study of OAV emulsions stabilized by sodium oleate. The particle size distributions of the emulsions wgre determined by optical microscopy, and the-surfactant concentration in the bulk solution was measured prior to and after emulsification. This allowed the evaluation of the area per surfactant molecule in the adsorbed layer of the emulsion droplets. The authors showed that the area per surfactant molecule at the interface of stable ( 1 h) emulsion droplets falls within the range 20-45 A. Fischer and Harkins note that this range of areas per molecule corresponds to a dense molecular packing, but is not sufficiently small to allow more than one molecule for the thickness of the film, and conclude that one densely packed surfactant monolayer is necessary and sufficient for stabilization. 

The tight monolayer packing concept applies not only to emulsions, but also to foams. The stability of foam films formed by a homologous series of potassium carboxylates (Cg-Cu) shows a sharp increase in the vicinity of the CMC, after which it levels off or slightly decreases. The data covering several orders of magnitude in surfactant concentration are very convincing. 

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