Generalized Onsager theory for strongly anisometric patchy colloids

TL;DR

This paper develops a generalized Onsager theory to analyze how soft patchy interactions influence the phase transitions of strongly elongated colloidal rods and disks, revealing electrostatic effects on nematic order and phase stability.

Contribution

It introduces a generic density functional framework for studying liquid crystal phases of highly anisometric colloids with surface-specific soft interactions, including electrostatic effects.

Findings

Electrostatic interactions destabilize nematic order in charged colloids.

Reduced screening narrows the isotropic-nematic coexistence region.

High density electrostatic repulsion suppresses nematic phases, favoring inhomogeneous states.

Abstract

The implications of soft `patchy' interactions on the orientational disorder-order transition of strongly elongated colloidal rods and flat disks is studied within a simple Onsager-van der Waals density functional theory. The theory provides a generic framework for studying the liquid crystal phase behaviour of highly anisometric cylindrical colloids, which carry a distinct geometrical pattern of repulsive or attractive soft interactions localised on the particle surface. In this paper, we apply our theory to the case of charged rods and disks for which the local electrostatic interactions can be described by a screened-Coulomb potential. We consider infinitely thin rod-like cylinders with a uniform line charge and infinitely thin discotic cylinders with several distinctly different surface charge patterns. Irrespective of the backbone shape, the isotropic-nematic phase diagrams of charged colloids feature a generic destabilization of nematic order, a dramatic narrowing of the biphasic density region and a reentrant phenomenon upon reducing the electrostatic screening. At higher particle density the electrostatic repulsion leads to a complete suppression of nematic order in favour of spatially inhomogeneous liquid crystals.