Mesoscopic theory for size- and charge-asymmetric ionic systems: I. The case of extreme asymmetry

A mesoscopic theory for the primitive model of ionic systems is developed for arbitrary size, λ = σ + /σ − , and charge, Z = e + /| e − |, asymmetry. Our theory is an extension of the theory that we developed earlier for the restricted primitive model. The case of extreme asymmetries and is studied in some detail in a mean field approximation. The phase diagram and correlation functions are obtained in the asymptotic regime and , and for infinite dilution of the larger ions (volume fraction n p ∼1/ Z or less). We find a coexistence between a very dilute 'gas' phase and a crystalline phase in which the macroions form a bcc structure with the lattice constant ≈3.6σ + . Such coexistence was observed experimentally in deionized aqueous solutions of highly charged colloidal particles.