Stockmayer Fluid with a Shifted Dipole: Bulk Behavior

TL;DR

This study investigates how shifting the dipole position in Stockmayer particles affects their microscopic structure, dielectric properties, and phase behavior, revealing that geometric frustration significantly influences macroscopic observables.

Contribution

It combines molecular dynamics and analytical theory to elucidate the impact of dipole displacement on liquid structure and thermodynamics, providing new insights into geometric frustration effects.

Findings

Dipole shifting breaks electrostatic symmetry and alters angular structure.

Reduced dielectric constant with increased dipole shift, approaching Debye limit.

Disruption of ferroelectric ordering at strong coupling due to geometric frustration.

Abstract

Shifting the point dipole from the center of a Stockmayer particle is a simple geometric modification that has been explored previously, yet its implications for liquid structure, dielectric response, and phase behavior remain incompletely understood. Here, we combine molecular dynamics simulations with analytical theory to provide a unified physical interpretation of how dipole displacement reshapes microscopic correlations and propagates to macroscopic thermodynamic properties. We show that dipole shifting breaks the fore-aft symmetry of the local electrostatic field, producing only modest changes in radial packing but strong alterations in angular structure within the first solvation shell. Enhanced alignment near the dipole head is accompanied by frustrated orientational correlations near the tail, leading to broader angular distributions and a shift away from axial configurations at strong coupling. These structural asymmetries weaken cooperative ordering and result in a systematic reduction of the dielectric constant, despite locally stronger interactions. For large shifts, the dielectric response approaches the Debye limit, indicating effective suppression of dipole-dipole correlations. The same geometric frustration governs vapor-liquid equilibria: while increasing dipole strength raises the critical temperature, even modest shifts disrupt the highly polarized liquid states that emerge at strong coupling and can suppress ferroelectric-like ordering. Predictions from a reparameterized COFFEE theory capture these trends within its domain of validity, highlighting the direct connection between local orientational structure and macroscopic observables. Overall, this work demonstrates that dipole location, not only magnitude, provides a powerful control parameter in dipolar fluids and offers a clear framework for understanding geometric frustration in electrostatic liquids.