  We report that binary dispersions of like-charged colloidal particles with
large charge asymmetry but similar size exhibit phase separation into crystal
and fluid phases under very low salt conditions. This is unexpected because the
effective colloid-colloid pair interactions are accurately described by a
Yukawa model which is stable to demixing. We show that colloid-ion interactions
provide an energetic driving force for phase separation, which is initiated by
crystallization of one species.
