Packing polydisperse colloids into crystals: when charge-dispersity matters

TL;DR

This study combines simulations and experiments to explore how charge dispersity influences the phase behavior of polydisperse colloids, revealing a sequence of phase transitions including fcc, bcc, and glassy states.

Contribution

It provides new insights into the role of charge polydispersity in determining colloidal crystal structures and phase stability, supported by combined experimental and simulation data.

Findings

Observation of fcc to bcc transition with increasing volume fraction.

Simulation accurately predicts phase sequence and stability.

Charge dispersity and entropy drive phase stabilization.

Abstract

Monte-Carlo simulations and small-angle x-ray scattering experiments were used to determine the phase diagram of aqueous dispersions of titratable nano-colloids with a moderate size polydispersity over a broad range of monovalent salt concentrations, 0.5 mM $\leq c_s \leq$ 50 mM and volume fractions, $\phi$. Under slow and progressive increase in $\phi$, the dispersions freeze into a face-centered-cubic (fcc) solid followed unexpectedly by the formation of a body centered cubic (bcc) phase before to melt in a glass forming liquid. The simulations are found to predict very well these observations. They suggest that the stabilization of the bcc solid at the expense of the fcc phase at high $\phi$ and $c_s$ results from the interaction (charge) polydispersity and vibrational entropy.