  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.
