  The Poisson-Boltzmann (PB) theory is extensively used to gain insight on
charged colloids and biological systems as well as to elucidate fundamental
properties of intermolecular forces. Many works were devoted in the past to
study PB related features and to confirm them experimentally. In this work we
explore the properties of inter-plate forces in terms of different boundary
conditions. We treat the cases of constant surface charge, constant surface
potential and mixed boundaries. The interplay between electrostatic
interactions, attractive counter-ions release, and repulsive van 't Hoff
contribution are discussed separately for each case. Finally, we discuss how
the crossover between attractive and repulsive interactions for constant
surface charge case is influenced by the presence of multivalent counter-ions,
where it is shown that the range of the attractive interaction grows with the
valency.
