Osmotic coefficient of a polyelectrolyte solution with a mixture of monovalent counterions

An analytical solution of the Poisson-Boltzmann equation is presented for the cell model of a polyelectrolyte solution with two species of monovalent counterions of different size. On this basis the osmotic coefficient is calculated as a function of the mole fraction of counterions and their radii. It is shown that the degree of binding of the smaller ion species increases as its mole fraction decreases.