Influence of size,ellipticity and charge separation on the free energy of some aqueous bolaform electrolyte models

The free energy of some models of aqueous bolaform electrolytes have been calculated at the Debye-Hückel limiting law plus B ₂ level of approximation. The repulsive forces are modeled by hard spheres or hard ellipsoids. The charges are placed either at the center of the sphere or at the foci of the ellipsoid. Parameters were chosen to approximate the size and shape of sodium and calcium p-benzenedisufonate and sodium 4,4-biphenyldisulfonate. The results show that contrary to the standard explanations for the unusual properties of bolaform electrolytes, separating the charges has little effect on the excess free energy. It is also shown that changing from a sphere to an ellipse has little effect on the excess free energy. The most important determinates of the properties of these salts are the sizes of the ions. Agreement of the model calculations with experimental results is reasonable considering the simplicity of the models.