Freezing in flat monolayers of soft spherocylinders

TL;DR

This study uses molecular dynamics simulations to explore phase transitions in flat membranes of soft spherocylinders, revealing a high nematic-crystal transition packing fraction and the effects of electrostatic interactions on phase stability.

Contribution

It provides new insights into the phase behavior of 2D rod monolayers, including a theoretical explanation for the high packing fraction at freezing and the impact of electrostatics.

Findings

Nematic-crystal transition occurs at packing fraction ~0.82

Electrostatic interactions reduce lateral compressibility but do not hinder positional order

Residual orientational entropy influences the high packing fraction transition

Abstract

Lamellar or smectic phases often have an intricate intralamellar structure that remains scarcely understood from a microscopic viewpoint. In this work, we use molecular dynamics simulations to study the effect of volume exclusion and electrostatic repulsion on the phase transitions of a flat membrane of soft spherocylinders. With increasing rod packing, we identify nematic and solid phases and find that the nematic-crystal phase transition happens at a uniform packing fraction ($\eta_c \approx 0.82$), independent of the spherocylinder aspect ratio. This value is considerably higher than the well-known critical freezing transition of a hard disk fluid ($\eta_c \approx 0.7$) to which one could naively map a system of near-parallel rods with co-planar mass centers. We attribute this difference to a non-vanishing residual orientational entropy per rod. Our findings are corroborated by a simple theory based on a simple microscopic density functional theory of freezing of a two-dimensional rod fluid. Introduction of electrostatic interactions between the rods reduces the lateral compressibility of the monolayer fluid but keeps the positional order unhindered, which in turn maintains the packing fraction at the nematic-crystal transition. The strength of the orientational fluctuations of the individual rods in our membranes exhibits a density scaling that differs from 3D bulk smectics. Our findings contribute to a qualitative understanding of liquid crystal phase stability in strong planar confinement and engage with recent experimental explorations involving nanorods on 2D substrates.