Air/Solution Interface and Adsorption – Solution for the Gibbs Paradox*

Several experimental evidences differentiating an insoluble monomolecular film just on the air/ water interface from an adsorbed film of soluble amphiphile solution are presented in order to suggest that the adsorbed film is not located at the air/solution interface. The difference between the two films can be observed by evaporation rates of water across the air/solution interface of three kinds of surfactant solutions and across an insoluble film or monolayer, by the corresponding activation energies, and by the kinetic theory of evaporation across the air/solution interface. The difference was further substantiated by the Brewster angle microscopy, BAM, image of the air/water interface. Surface tension vs. concentration curve for soluble surfactant solution was divided into three regions to solve the Gibbs paradox, and then the adsorbed film turned out to be concentrated as aggregates at some distance beneath the air/solution interface. The new concept of adsorbed film is consistent with several interfacial phenomena of surfactant solutions.