Density fluctuations in an electrolyte from generalized Debye-Hückel theory.

Near-critical thermodynamics in the hard-sphere (1, 1) electrolyte is well described, at a classical level, by Debye-Hückel (DH) theory with s1, 2d ion pairing and dipolar-pair-ionic-fluid coupling. But DH-based theories do not address density fluctuations. Here density correlations are obtained by functional differentiation of DH theory generalized to nonuniform densities of various species. The correlation length j diverges universally at low density r as sTrd21y4 (correcting the generalized meanspherical approximation). When r ­ rc one has j . j 0 yt1y2 as t ; sT 2 TcdyTc ! 01, where the amplitudes j 0 compare informatively with experimental data.