Unveiling Crystalline Order from Glassy Behavior of Charged Rods at Very Low Salt Concentrations

TL;DR

This study combines experiments and simulations to reveal how long-range electrostatic interactions in charged rods lead to direct nematic to smectic-B phase transitions at very low salt concentrations, challenging traditional phase sequences.

Contribution

It uncovers a novel direct nematic to smectic-B transition driven by electrostatics, bypassing the usual smectic-A phase, and demonstrates the significant role of Coulomb repulsion in phase behavior.

Findings

Direct nematic to smectic-B transition observed

Electrostatic interactions alter phase sequences

Long-range repulsion drives self-organization

Abstract

Charged colloids can form ordered structures like Wigner crystals or glasses at very low concentrations due to long-range electrostatic repulsions. Here, we combine small-angle x-ray scattering (SAXS) and optical experiments with simulations to investigate the phase behavior of charged rodlike colloids across a wide range of salt concentrations and packing fractions. At ultra low ionic strength and packing fractions, we reveal both experimentally and numerically a direct transition from a nematic to a crystalline smectic-B phase, previously identified as a glass state. This transition, bypassing the smectic-A intermediate phase, results from minimizing Coulomb repulsion and maximizing entropic gains due to fluctuations in the crystalline structure. This demonstrates how long-range electrostatic repulsion significantly alters the phase behavior of rod-shaped particles and highlights its key-role in driving the self-organization of anisotropic particles.