Surface excess of DBS^- (dodecylbenzene sulphonate ion) in dynamic equilibrium Γ, was measured by foam separation experiments in a continuous foam column at 35°C, using SDBS (sodium dodecylbenzene sulphonate) as surfactant and NaCl and MgCl₂ as coexisting salts. The effect of coexisting salts on Γ was examined by employing various adsorption isotherms. Two methods were further proposed for calculating Γ in coexisting salts. One is the method combining the Davies adsorption isotherm with the Gouy-Chapman model of the diffuse double layer, and the other is the method using reduced concentration of surfactant by employing critical micelle concentration. The values of Γ's calculated by these methods were compared with the experimental ones in the cases of NaCl and MgCl₂ as the coexisting salts. The calculated values of Γ's were in fairly good agreement with the experimental ones.

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