  The aggregation of binary colloids of same size and balanced charges is
studied by Brownian dynamics simulations for dilute suspensions. It is shown
that, under appropriate conditions, the formation of colloidal crystals is
dominated by kinetic effects leading to the growth of well-ordered crystallites
of the sodium-chloride (NaCl) bulk phase. These crystallites form with very
high probability even when the cesium-chloride (CsCl) phase is more stable
thermodynamically. Global optimization searches show that this result is not
related to the most favorable structures of small clusters, that are either
amorphous or of CsCl structure. The formation of the NaCl phase is related to
the specific kinetics of the crystallization process, which takes place by a
two-step mechanism. In this mechanism, dense fluid aggregates form at first and
then crystallization follows. It is shown that the type of short-range order in
these dense fluid aggregates determines which phase is finally formed in the
crystallites. The role of hydrodynamic effects in the aggregation process is
analyzed by Stochastic Rotation Dynamics - Molecular Dynamics simulations,
finding that these effects do not play a major role in the formation of the
crystallites.
