  We consider binary mixtures of soft repulsive spherical particles and
calculate the depletion interaction between two big spheres mediated by the
fluid of small spheres, using different theoretical and simulation methods. The
validity of the theoretical approach, a virial expansion in terms of the
density of the small spheres, is checked against simulation results. Attention
is given to the approach toward the hard-sphere limit, and to the effect of
density and temperature on the strength of the depletion potential. Our results
indicate, surprisingly, that even a modest degree of softness in the pair
potential governing the direct interactions between the particles may lead to a
significantly more attractive total effective potential for the big spheres
than in the hard-sphere case. This might lead to significant differences in
phase behavior, structure and dynamics of a binary mixture of soft repulsive
spheres. In particular, a perturbative scheme is applied to predict the phase
diagram of an effective system of big spheres interacting via depletion forces
for a size ratio of small and big spheres of 0.2; this diagram includes the
usual fluid-solid transition but, in the soft-sphere case, the metastable
fluid-fluid transition, which is probably absent in hard-sphere mixtures, is
close to being stable with respect to direct fluid-solid coexistence. From
these results the interesting possibility arises that, for sufficiently soft
repulsive particles, this phase transition could become stable. Possible
implications for the phase behavior of real colloidal dispersions are
discussed.
