Surface stoichiometry of ionic surfactants at interfaces: A new thermodynamic model

A new theoretical development based on the concept that the air-water inter facial layer of adsorbed ionic surfactant constitutes a nonautonomous phase is presented. This adsorbed layer can thus be described by a state functio n of variables whose values are dependent on the adjacent bulk phase. This thermodynamic analysis allows demonstration that, for a given interfacial a mount of adsorbed ionic surfactant, a Corrin-Harkins-like relationship appl ies when varying the concentration of an inorganic salt having the same cou nterion in common with the ionic surfactant. Experimental validation of thi s analysis is done using the data of Tajima, who studied with a radiotracer method the effect of sodium chloride on the adsorption of sodium dodecyl s ulfate (Na+DS-) at the air-water interface, The Corrin-Harkin-like relation , Log(a(DS-)) + beta Log(n(Na+)) = K, is verified with excellent correlatio n coefficients for various amounts of adsorbed DS- at the air-water interfa ce. This result suggests that the behavior of the ionic surfactant layer at the air-water interface is similar to that of micelles. In particular, bet a may constitute a scale for quantifying the degree of counterion binding t o the interfacial layer of surfactant. Finally, the presented model allows the concept of surface stoichiometry for the calculation of the surface are a occupied by the polar headgroup of a ionic surfactant at an air-water int erface to be introduced.