  In aqueous solutions, dissolved ions interact strongly with the surrounding
water, thereby modifying the solution properties in an ion-specific manner.
These ion-hydration interactions can be accounted for theoretically on a
mean-field level by including phenomenological terms in the free energy that
correspond to the most dominant ion-specific interactions. Minimizing this free
energy leads to modified Poisson-Boltzmann equations with appropriate boundary
conditions. Here, we review how this strategy has been used to predict some of
the ways ion-specific effects can modify the forces acting within and between
charged interfaces immersed in salt solutions.
