A thermodynamic equation of state for the Pgr;ndash;Acurves of insoluble monolayers is extended to ionized surfactant species. It is shown that at infinite dilution on a pure water subphase, the limiting law Pgr;A=ngr;skT(where ngr;sis the number of ions into which the surfactant dissociates) is obtained. For monolayers spread on salt solutions with a common counterion, it is shown that the adsorption of soluble salt appears explicitly in the equation of state. If the Gibbs dividing surface is placed such that this adsorption is taken as zero, the form of the equation becomes similar to, and can be applied in the same manner as, that used for uncharged films. In some cases, this permits fitting experimental Pgr;ndash;Acurves with surfacehyphen;pressure independent activity efficients. When this is not possible, it suggests that the relative salt and water adsorptions change as the film is compressed. Additional experimental information will be required to evaluate this possibility, and to permit application of the general equation of state for multicomponent systems. The application of this formalism to the surface tension of salt solutions without spread monolayers is also considered briefly.
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