Kinetically driven ordered phase formation in binary colloidal crystals

TL;DR

This study uses simulations to show that kinetic effects, rather than thermodynamic stability, predominantly determine the formation of ordered NaCl-like colloidal crystals in binary suspensions, following a two-step aggregation mechanism.

Contribution

It reveals that kinetic factors and the formation of dense fluid aggregates govern the crystal phase, challenging the assumption that thermodynamic stability dictates the final structure.

Findings

NaCl phase forms preferentially despite thermodynamic stability of CsCl

Crystallization occurs via a two-step process involving dense fluid aggregates

Hydrodynamic effects are not significant in the crystallization process

Abstract

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.