Structure and dielectric properties of polar fluids with extended dipoles: results from numerical simulations

TL;DR

This study uses molecular dynamics simulations to explore how the structure and dielectric properties of polar fluids change with extended dipoles, revealing limitations of the point-dipole model and identifying phase transitions.

Contribution

It provides a detailed analysis of the effects of dipole extension on polar fluid properties, highlighting the range where the point-dipole approximation remains valid and identifying a phase transition to a hexagonal columnar phase.

Findings

Point dipole model agrees with extended dipoles up to d/σ ≈ 0.3.

Dielectric constant ε varies non-trivially for larger d/σ.

Transition to a hexagonal columnar phase occurs when d/σ > 0.6.

Abstract

The strengths and short-comings of the point-dipole model for polar fluids of spherical molecules are illustrated by considering the physically more relevant case of extended dipoles formed by two opposite charges $\pm q$ separated by a distance $d$ (dipole moment $\mu=q d$). Extensive Molecular Dynamics simulations on a high density dipolar fluid are used to analyse the dependence of the pair structure, dielectric constant $\eps$ and dynamics as a function of the ratio $d/\sigma$ ($\sig$ is the molecular diameter), for a fixed dipole moment $\mu$. The point dipole model is found to agree well with the extended dipole model up to $d/\sig \simeq 0.3$. Beyond that ratio, $\eps$ shows a non-trivial variation with $d/\sig$. When $d/\sig>0.6$, a transition is observed towards a hexagonal columnar phase; the corresponding value of the dipole moment, $\mu^2/\sig^3 k T=3$, is found to be substantially lower than the value of the point dipole required to drive a similar transition.