  The classical Debye-Huckel (DH) theory clearly accounts for the origin of
screening in electrolyte solutions and works rather well for dilute electrolyte
solutions. While the Debye screening length decreases with the ion
concentration and is independent of ion size, recent surface-force measurements
imply that for concentrated solutions, the screening length exhibits an
opposite trend; it increases with ion concentration and depends on the ionic
size. The screening length is usually defined by the response of the
electrolyte solution to a test charge, but can equivalently be derived from the
charge-charge correlation function. By going beyond DH theory, we predict the
effects of ion size on the charge-charge correlation function. A simple
modification of the Coulomb interaction kernel to account for the excluded
volume of neighboring ions yields a non-monotonic dependence of the screening
length (correlation length) on the ionic concentration, as well as damped
charge oscillations for high concentrations.
