The adsorption of surface active electrolytes at the air/water interface

It is shown that the adsorption of surface active electrolytes at the air/water interface cannot be calculated from surface tension data by applying the Gibbs adsorption isotherm, except when excess neutral electrolyte is present in the detergent solution. Certain non-thermodynamic assumptions can be employed to estimate the adsorption from simple aqueous ionic detergent solutions in the absence of excess neutral salt. In the presence of excess electrolyte the simple form dπ = kTΓd log c applies to binary electrolyte detergents. The theory is applied to the calculation of the salt effects on the force-area characteristics of sodium dodecyl sulphate (SDS) at the air/water interface. It is shown that spread and adsorbed monolayers of this detergent are of the same character. Approximate equations of state are derived from electrostatic theory.