Phase Transition and Miscibility in Mixed Adsorbed Films of Sodium Chloride and Sodium Dodecyl Sulfate at the Water/Air Interface

The phase transition in adsorbed films of sodium dodecyl sulfate (SDS) at the water/air interface in the presence of sodium chloride NaCl can be studied from the thermodynamic viewpoint of mixed adsorption of NaCl and SDS. The miscibility of NaCl and SDS in the adsorbed film is clarified by applying the thermodynamic treatment of mixed adsorption to the NaCl-SDS mixture. The surface tension of aqueous solutions of a NaCl-SDS mixture was measured as a function of the total concentration of the mixture at constant mixture composition at 298.15 K. The total surface density of the mixture discontinuously changes at a break in the surface tension vs total concentration curve. Judging from the surface pressure-area curve for the mixture, a first-order phase transition takes place in the mixed adsorbed film of NaCl and SDS between the gaseous and expanded states. The mole fraction of SDS in the mixed adsorbed film is higher than unity due to the negative adsorption of NaCl, enhanced by the adsorption of SDS. Phase diagrams of adsorption and phase transition are created for the mixed adsorption and the equilibrium between the gaseous and expanded states, respectively. The mean activity coefficient of SDS in the adsorbed film is greater than unity, indicating a positive deviation from ideal mixing in the adsorbed film caused by the electrostatic repulsion between the dodecyl sulfate and chloride ions. The phase transition and miscibility of the NaCl-SDS system are compared to those of NaCl-nonionic and binary ionic surfactant systems.