CRITICALITY IN IONIC FLUIDS - DEBYE-HUCKEL THEORY, BJERRUM, AND BEYOND

Debye-Huckel (DH) theory predicts phase separation in the primitive model electrolyte: hard spheres of diameter a with charges +/-q. The coexistence curve (CC) is acceptable with T(c)* = k(B)T(c)a/q2 = 1/16, which is roughly correct, but the critical density, rho(c)* = rho(c)a3 = 1/64pi, is far too low. Allowing for association into ideal dipolar pairs, following Bjerrum, improves rho(c) and leaves T(c) unchanged but yields an unphysical CC. Extending DH theory to compute the dipole-ionic-fluid coupling yields a mean-field description with sensible CC and (T(c)*, rho(c)*) close to Monte Carlo based estimates: (0.057 +/- 1(5), 0.030 -/+ 8).