  It is widely accepted that the Poisson-Boltzmann (PB) theory provides a valid
description for charged surfaces in the so-called weak coupling limit. Here, we
show that the image charge repulsion creates a depletion boundary layer that
cannot be captured by a regular perturbation approach. The correct
weak-coupling theory must include the self-energy of the ion due to the image
charge interaction. The image force qualitatively alters the double layer
structure and properties, and gives rise to many non-PB effects, such as
nonmonotonic dependence of the surface energy on concentration and charge
inversion. In the presence of dielectric discontinuity, there is no limiting
condition for which the PB theory is valid.
