Distribution of counterions in the double layer around a cylindrical polyion

The modified Poisson-Boltzmann (MPB) equation, and the Monte Carlo (MC) method, were applied to the cell model of a polyelectrolyte solution in order to calculate the distribution of counterions around a cylindrical polyion. Both methods suggest stronger binding of counterions to the polyion than predicted by the ordinary Poisson-Boltzmann (PB) equation. The inclusion of counterion-counterion correlation being neglected in the PB equation, leads to a better agreement of the calculated osmotic coefficients with those measured.