  The behavior of electrolyte solutions close to a charged surface is studied
theoretically. A modified Poisson-Boltzmann equation which takes into account
the volume excluded by the ions in addition to the electrostatic interactions
is presented. In a formal lattice gas formalism the modified Poisson-Boltzmann
equation can be obtained from a mean-field approximation of the partition
function. In an alternative phenomenological approach, the same equation can be
derived by including the entropy of the solvent molecules in the free energy.
In order to visualize the effect of the steric repulsion, a simple case of a
single, highly charged, flat surface is discussed. This situation resembles
recent adsorption experiments of large ions onto a charged monolayer. A simple
criterion for the importance of the steric effects is expressed in terms of the
surface charge density and the size of the ions. It is shown that when these
effects are important a saturated layer is formed near the surface. A modified
Grahame equation relating the ion concentration at the surface to the surface
charge density is obtained.
