  The Poisson-Boltzmann mean-field description of ionic solutions has been
successfully used in predicting charge distributions and interactions between
charged macromolecules. While the electrostatic model of charged fluids, on
which the Poisson-Boltzmann description rests, and its statistical mechanical
consequences have been scrutinized in great detail, much less is understood
about its probable shortcomings when dealing with various aspects of real
physical, chemical and biological systems. These shortcomings are not only a
consequence of the limitations of the mean-field approximation per se, but
perhaps are primarily due to the fact that the purely Coulombic model
Hamiltonian does not take into account various additional interactions that are
not electrostatic in their origin. We explore several possible
non-electrostatic contributions to the free energy of ions in confined aqueous
solutions and investigate their ramifications and consequences on ionic
profiles and interactions between charged surfaces and macromolecules.
