Exact description of aqueous electrical double layers

WE describe the exact steps needed to reduce the problem of planar electric al double layers in aqueous solvent to two simpler problems: (i) a referenc e system consisting of solvent between the surfaces and (ii) ions interacti ng with other ions and with surfaces via the potentials of mean force evalu ated in the reference system. The mapping is achieved by extending the McMi llan-Mayer transformation to the nonuniform fluid between the surfaces. The results indicate approximate additivity of short- and long-range effects i n ion density and pressure, as observed earlier in numerical work. After tw o approximation steps, we arrive at a practical and accurate method for cal culating properties of planar aqueous double layers.