Gas-Solid Coexistence in Highly Charged Colloidal Suspensions

TL;DR

This study investigates the phase behavior of highly charged colloidal suspensions, revealing gas-solid coexistence and inhomogeneous structures through experiments and simulations, challenging previous notions of reentrant disordered states.

Contribution

It provides experimental evidence of gas-solid coexistence in highly charged colloids, supported by simulations with long-range attractive pair potentials, clarifying the nature of reentrant disordered states.

Findings

High charge density suspensions remain disordered under deionized conditions.

Crystallization occurs in low charge density suspensions, showing iridescence.

Inhomogeneous coexistence of voids with dense ordered or disordered regions was observed.

Abstract

Aqueous suspensions of highly charged polystyrene particles with different volume fractions have been investigated for structural ordering and phase behavior using static light scattering (SLS) and confocal laser scanning microscope (CLSM). Under deionized conditions, suspensions of high charge density colloidal particles remained disordered whereas suspensions of relatively low charge density showed crystallization by exhibiting iridescence for the visible light. Though for unaided eye crystallized suspensions appeared homogeneous, static light scattering measurements and CLSM observations have revealed their inhomogeneous nature in the form of coexistence of voids with dense ordered regions. CLSM investigations on disordered suspensions showed their inhomogeneous nature in the form coexistence of voids with dense disordered (amorphous) regions. Our studies on highly charged colloids confirm the occurrence of gas-solid transition and are in accordance with predictions of Monte Carlo simulations using a pair-potential having a long-range attractive term [Mohanty and Tata, Journal of Colloid and Interface Science 2003, 264, 101]. Based on our experimental and simulation results we argue that the reported reentrant disordered state [Yamanaka et al Phys. Rev. Lett. 1998, 80, 5806 and Toyotama et al Langmuir, 2003, 19, 3236] in charged colloids observed at high charge densities is a gas-solid coexistence state.