Atomistic analysis of nematic phase transition in 4-cyano-4$^{\prime}$-$n$-alkyl biphenyl liquid crystals: Sampling for the first-order phase transition and the free-energy decomposition

TL;DR

This study uses advanced molecular dynamics simulations with gREM to analyze the first-order nematic-isotropic phase transition in 4-cyano-4'-n-alkylbiphenyl liquid crystals, focusing on free-energy contributions and sampling challenges.

Contribution

It introduces the application of gREM for enhanced sampling of first-order phase transitions and employs free-energy decomposition to elucidate transition mechanisms.

Findings

gREM effectively samples near the NI phase transition temperature.

Free-energy decomposition reveals the balance of entropic and energetic contributions.

The transition involves a significant change in free-energy components.

Abstract

Molecular dynamics (MD) simulations were conducted using the generalized replica exchange method (gREM) on the 4-cyano-4$^{\prime}$-$n$-alkylbiphenyl ($n$CB) system with $n=5$, 6, 7, and 8, which exhibits a nematic-isotropic (NI) phase transition. Sampling near the phase transition temperature in systems undergoing first-order phase transitions, such as the NI phase transition, is demanding due to the substantial energy gap between the two phases. To address this, gREM, specifically designed for first-order phase transitions, was utilized to enhance sampling near the NI phase transition temperature. Free-energy calculations based on the energy representation (ER) theory were employed to characterize the NI phase transition. ER evaluates the insertion free energy of $n$CB molecule for both nematic and isotropic phases, revealing a change in the temperature dependence across the NI phase transition. Further decomposition into intermolecular interaction energetic and entropic terms shows quantitatively the balance between these contributions at the NI phase transition temperature.