  The possibility to induce demixing in a colloidal mixture by adding small
polymers, or other equivalent depletant agents, is theoretically investigated.
By use of Mean Field Theory, suitably generalized to deal with short range
effective interactions, the phase diagram of a binary mixture ofcolloidal
particles (modelled as hard spheres) in a solvent is determined as a function
of the polymer concentration on the basis of the Asakura-Oosawa model.The
topology of the phase diagram changes when the relative size of the colloidal
particles is reduced: the critical line connecting the liquid-vapour critical
points of the two pure fluids breaks and the branch starting from the critical
point of the bigger particles bends to higher volume fractions, where
concentration fluctuations drive the transition. The effects of a softer
colloid-polymer interaction is also investigated: Even the presence of a small
repulsive tail in the potential gives rise to a significant lowering of the
stability threshold. In this case, phase transitions may take place by adding
just a few percent of depletant in volume fraction. These results may be
relevant for the interpretation of recent experiments of solidification
kinetics in colloidal mixtures.
