Coulombic reactions of polyelectrolytes with counterions of different sizes

Implicit equations were derived for interactions between a charged rod polyelectrolyte model and counterions of different size. The selectivity of the polymer for the smaller of two counterions as a function of its charge density, distances of approach and annular volume was calculated. Ion binding (site binding) was calculated by assigning a finite thickness to the Stern layer so as to include a finite number of closely held counterions. Further refinements included the consideration of counterion polarizability as a function of the electric field.