Theory for surface structure of dilute electrolyte solutions

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Abstract
A theory is developed for the salt concentration profile and ion–ion correlations near surfaces of dilute electrolyte solutions. We use the RPA to study the primitive surface model previously employed by Onsager and Samaras, and others. In this model the chief technical complication is the correct treatment of image forces. We invent an exact rearrangement of the mathematical formulation of the problem which makes especially transparent the special case solutions (infinite dielectric constant mismatch) previously found by Onsager and Samaras, and by Buff and Stillinger. We then show that our reformulation allows an analytical solution for arbitrary dielectric constant mismatch between the two phases, subject to the other assumptions adopted by these previous workers. These remaining assumptions affect the internal self-consistency of the results. We indicate a systematic perturbative procedure which will be of practical use in improving the self-consistency of the solutions. Finally, we discuss the questions of whether the primitive surface model studied is sufficiently general to yield generally correct surface behavior for limiting small salt concentrations, and whether limiting laws independent of short ranged correlations exist for surface thermodynamic properties. Our results strongly suggest that both answers are no.

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