The energy of interaction,W12(R), of two ions at separationRin an ionic medium is considered using Debyendash;Huuml;ckel (DH) theory for spheres of diametera0. The ions are treated as spheres of dielectric constantDrsquo;which may differ from that of the medium, sayD; they have radiiband carry equal or opposite point charges. The electrostatic potential Jgr;(r) in the ionic medium satisfies the DH equationnabla;2Jgr;=kgr;2Jgr;. An exact, closedhyphen;form expression is obtained forW12(R). In the limit of zero ionic strength (kgr;2rarr;0) previous asymptotic results forRrarr;infin;, indicating Fgr;4/R4+Fgr;6/R6+Fgr;7/R7+... corrections to Coulombrsquo;s law, are reproduced; by contrast for kgr;bgsim;0 the leading interaction term whenRrarr;infin; varies, as expected, likeeminus;kgr;R/DRbut the first correction decays as kgr;2eminus;2kgr;R/DR2, i.e., with a squared screening factor, and can be stated for two ions differing in charge, radii, etc. The same calculations show that, within DH theory, modeling the ions withDrsquo;ne;Dhas no effects on the predicted thermodynamics.
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