Structure of adsorbed films on dilute solutions

Structure of adsorbed films on dilute solutions. Gibbs s equations (7.1 to 7.7) permit the calculations of the area per molecule, if the surface tension is determined over a range of concentration of the solutions. The area per molecule is proportional to the slope of the curve relating the lowering of surface tension to the activity /2c2 (the vapour pressure of component 2 if the vapour obeys the simple gas laws). This assumes 

Robinson, Wetting and Detergency, (1937), 141 150, traced a minimum in interfacial tension to very small amounts of polyvalent electrolytes present in ordinary distilled water.  

In every case yet worked out, the adsorbed films of soluble substances are of the gaseous type there are always corrections, except in very dilute solutions, to the perfect gas equation due to the area actually occupied by the adsorbed molecules and in many cases also there are corrections due to the lateral adhesion between them. 

Traube,1 in an extended investigation of the surface tension of aqueous solutions of many organic compounds, established several important points, of which the significance for the structure of the adsorbed films has been pointed out by Langmuir.2 

For very dilute solutions of any one solute, the depression of surface tension F caused by the solute is proportional to the concentration, i.e. 

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