Phase diagrams of charged colloidal rods: can a uniaxial charge distribution break chiral symmetry?

TL;DR

This study models phase diagrams of charged colloidal rods, revealing complex phase coexistence and showing that uniaxial charge distributions do not spontaneously break chiral symmetry, as the twist elastic constant remains positive.

Contribution

It provides a detailed phase diagram analysis of charged rods and demonstrates that uniaxial charge distributions cannot induce chiral symmetry breaking.

Findings

Identifies isotropic-nematic and nematic-nematic coexistence regimes.

Finds the twist elastic constant remains positive, ruling out spontaneous cholesteric phases.

Reveals complex phase behavior influenced by charge and aspect ratio.

Abstract

We construct phase diagrams for charged rodlike colloids within the second-virial approximation as a function of rod concentration, salt concentration, and colloidal charge. Besides the expected isotropic-nematic transition, we also find parameter regimes with a coexistence between a nematic and a second, more highly aligned nematic phase including an isotropic-nematic-nematic triple point and a nematic-nematic critical point, which can all be explained in terms of the twisting effect. We compute the Frank elastic constants to see if the twist elastic constant can become negative, which would indicate the possibility of a cholesteric phase spontaneously forming. Although the twisting effect reduces the twist elastic constant, we find that it always remains positive. In addition, we find that for finite aspect-ratio rods the twist elastic constant is also always positive, such that there is no evidence of chiral symmetry breaking due to a uniaxial charge distribution.