A close look into the excluded volume effects within a double layer
  We explore the effect of steric interaction on the ionic density distribution near a charged hard wall. For weakly charged walls, small particles, and monovalent ions the mean-field Poisson-Boltzmann equation provides an excellent description of the density profiles. For large ions and large surface charges, however, deviations appear. To explore these, we use the density functional theory. We find that local density functionals are not able to account for steric interactions near a wall. Based on the weighted density approximation we derive a simple analytical expression for the contact electrostatic potential which allows us to analytically calculate the differential capacitance of the double layer. 
Derek Frydel and Yan Levin
cond-mat.soft