Equation of state of classical systems of charged particles

Abstract

Recent developments in the classical theory of fully ionized gases and strong electrolyte solutions are reviewed and used to discuss the equation of state at high temperatures and low densities. The pressure is calculated with the ring-integral approximation, and quantitative estimates of higher correction terms are given. The effect of short-range repulsive forces is shown by comparing the results with two kinds of potential functions: hard spheres of diameter a, and "soft" spheres for which the short-range potential cancels the Coulomb potential at the origin and decreases exponentially with distance. It is found that the use of either type of potential extends the range of validity of the ring-integral approximation to considerably greater densities and lower temperatures. Since there is little difference in the results for the hard spheres and the soft spheres in this range, and since the soft-sphere system is more easily handled by analytical methods, it is investigated more extensively. The expressions derived for the free energy of a system of charged particles can also be used in ionization equilibrium calculations, and the effect of electrostatic interactions on the equilibrium concentrations of various kinds of ions is indicated.