Steric Effects in Electrolytes: A Modified Poisson-Boltzmann Equation
  The adsorption of large ions from solution to a charged surface is investigated theoretically. A generalized Poisson--Boltzmann equation, which takes into account the finite size of the ions is presented. We obtain analytical expressions for the electrostatic potential and ion concentrations at the surface, leading to a modified Grahame equation. At high surface charge densities the ionic concentration saturates to its maximum value. Our results are in agreement with recent experiments. 
I. Borukhov, D. Andelman (Tel Aviv University, Israel), H. Orland   (CE-Saclay, France)