  A new model for a colloidal size-asymmetric binary mixture is proposed: The
Square-Shoulder-Asakura-Oosawa. This belongs to the larger class of
non-additive hard-spheres models and has the property that its effective pair
formulation is exact whenever the solvent particle fits inside the interstitial
region of three touching solute particles. Therefore one can study its
properties from the equivalent one-component effective problem. Some remarks on
the phase diagram of this new model are also addressed.
