Complete description of re-entrant phase behaviour in a charge variable colloidal model system

TL;DR

This study maps the complete phase diagram of charged colloidal spheres, revealing re-entrant melting behavior influenced by particle density, effective charge, and electrolyte concentration, aligning with simulation predictions.

Contribution

It provides the first full experimental phase diagram of charged colloids considering effective charge, including re-entrant phase behavior, consistent with theoretical models.

Findings

Crystallization occurs with increasing effective charge at constant electrolyte concentration.

Melting occurs with increasing electrolyte concentration at nearly constant effective charge.

Experimental phase boundaries match universal melting line predictions from simulations.

Abstract

In titration experiments with NaOH we have determined the full phase diagram of charged colloidal spheres in dependence on the particle density n, the particle effective charge Zeff and the concentration of screening electrolyte c using microscopy, light and Ultra Small Angle X-Ray Scattering (USAXS). For sufficiently large n the system crystallizes upon increasing Zeff at constant c and melts upon increasing c at only slightly altered Zeff. In contrast to earlier work equilibrium phase boundaries are consistent with a universal melting line prediction from computer simulation, if the elasticity effective charge is used. This charge accounts for both counter-ion condensation and many body effects.