  We propose a mean-field analytical model to account for the observed
asymmetry in the ability to form long-range attraction by the negatively
charged colloidal particles and not their equivalently charged positive
counterpart. We conjecture that this asymmetry is due to solvation effects, and
we phenomenologically capture its physics by considering the relative strength
of this water-induced short-range repulsion between the different charge
species. We then apply our model to the colloidal system of negatively charged
disks that are neutralized by a sea of counterions and strongly absorbed to an
interface in a compressible binary system. We demonstrate the resulting
coexistence between a dilute isotropic ionic phase and a condensed hexagonal
lattice phase as a function of density and interaction strength.
