Specific counterion effect on the adsorption of alkali decyl sulfate surfactants at air/solution interface

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Abstract
Experimental results are presented on the counterion dependence of the adsorption of alkali (Li+, Na+, K+, Rb+ and Cs+) decyl sulfates at the air/solution interface. The adsorption isotherms calculated from equilibrium surface tension vs. concentration data by means of the Gibbs equation show significant counterion dependence. We propose a theoretical model based on the Gouy–Chapman–Stern theory for the description of the ionic surfactant adsorption at the air/solution interface. The model is based on the physical picture that the counterions can enter among the surfactant headgroups if the hydrated counterion size is smaller than that of the headgroups. In this case, the diffuse part of the double layer starts from the plane of the headgroups. If the size of the counterions is larger than the size of the headgroups then the closest approach of the counterions in the diffuse layer is assumed to be equal to the difference between the size of the hydrated surfactant headgroup and that of the counterion. The model correctly describes the counterion dependence of the adsorption isotherms measured for the alkali alkyl sulfates using the electrical capacity of the Stern-layer and the hydrophobic driving force as fitting parameters. The results indicate that the fine structure of the double layer can play an important role on the counterion specificity of ionic surfactant adsorption. Beyond the description of the Stern-layer the description of the hydrophobic driving force was also modified in order to reflect recent experimental and theoretical results about the structure of the monolayer.

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