Re-entrant melting and freezing in a model system of charged colloids

TL;DR

This study investigates the complex phase transitions of charged colloids in a low-polarity solvent, revealing re-entrant melting and multiple crystal structures driven by volume fraction-dependent particle interactions.

Contribution

It demonstrates how colloid charge variations with volume fraction influence phase behavior, including re-entrant melting and different crystal formations, in a low-polarity system.

Findings

Transition from fluid to BCC crystal at 0.0415 volume fraction

Re-entrant melting occurs at 0.1165 volume fraction

Formation of HCP crystal at high volume fractions

Abstract

We studied the phase behavior of charged and sterically stabilized colloids using confocal microscopy in a less polar solvent (dielectric constant 5.4). Upon increasing the colloid volume fraction we found a transition from a fluid to a body centered cubic crystal at 0.0415+/-0.0005, followed by re-entrant melting at 0.1165+/-0.0015. A second crystal of different symmetry, random hexagonal close-packed, was formed at a volume fraction around 0.5, similar to that of hard spheres. We attribute the intriguing phase behavior to particle interactions that depend strongly on volume fraction, mainly due to changes in the colloid charge. In this low polarity system the colloids acquire charge through ion adsorption. The low ionic strength leads to fewer ions per colloid at elevated volume fractions and consequently a density-dependent colloid charge.