  We theoretically investigate electrostatic properties between two charged
surfaces with a grafted polyelectrolyte layer in an aqueous electrolyte
solution by using the Poisson-Boltzmann approach accounting for ion
partitioning. In order to consider the ion partitioning effect, we focus on
changes of electrostatic properties due to the difference in dielectric
permittivity of the polyelectrolyte layer and the aqueous electrolyte solution.
We find that ion partitioning enhances electrostatic potential in the region
between two charged soft surfaces and hence increases the electrostatic
interaction between two charged soft surfaces. Ion partitioning effect on
osmotic pressure is enhanced not only by an increase in the thickness of
polyelectrolyte layer and Debye length but also by a decrease in ion radius.
