 
Summary: Hydration interactions: Aqueous solvent effects in electric double layers
Yoram Burak and David Andelman
School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69 978, Israel
Received 4 April 2000
A model for ionic solutions with an attractive shortrange pair interaction between the ions is presented. The
shortrange interaction is accounted for by adding a quadratic nonlocal term to the PoissonBoltzmann free
energy. The model is used to study solvent effects in a planar electric double layer. The counterion density is
found to increase near the charged surface, as compared with the PoissonBoltzmann theory, and to decrease
at larger distances. The ion density profile is studied analytically in the case where the ion distribution near the
plate is dominated only by counterions. Further away from the plate the density distribution can be described
using a PoissonBoltzmann theory, with an effective surface charge that is smaller than the actual one.
PACS number s : 61.20.Qg, 82.45. z, 61.20.Gy, 68.45. v
I. INTRODUCTION
Electrolytes, in contact with charged surfaces or macro
ions, play an important role in determining the properties of
many biological and chemical systems. One of the most
widely used tools for studying ions in aqueous solutions is
the PoissonBoltzmann PB theory 16 . The mathematical
and conceptual simplicity of this theory makes it very ap
pealing both for numerical computation 7 and for gaining
