  In this paper, we study electrostatic properties between two similar or
oppositely charged surfaces immersed in an electrolyte solution by using
mean-field approach accounting for solvent polarization and non-uniform size
effect. Applying a free energy formalism accounting for unequal ion sizes and
orientational ordering of water dipoles, we derive coupled and self-consistent
equations to calculate electrostatic properties between charged surfaces.
Electrostatic properties for similarly charged surfaces depend on counterion
size but not coion size. Moreover, electrostatic potential and osmotic pressure
between similarly charged surfaces are found to be increased with increasing
counterion size. On the other hand, the corresponding ones between oppositely
charged surfaces are related to both sizes of positive and negative ions. For
oppositely charged surfaces, the electrostatic potential, number density of
solvent molecules and relative permittivity of an electrolyte having unequal
ion sizes are not symmetric about the centerline between the charged surfaces.
For either case, the consideration of solvent polarization results in an
decrease in the electrostatic potential and the osmotic pressure compared to
the case without the effect.
