Stockmayer Fluid with a Shifted Dipole: Bulk Behavior

TL;DR

This paper addresses how displacing a Stockmayer fluid's dipole from the particle center reshapes microscopic structure and macroscopic properties. It combines molecular dynamics simulations with the COFFEE-PeTS theory to connect local orientational arrangements within the first solvation shell to bulk observables such as the dielectric constant $\epsilon_r$ and vapor–liquid equilibria, using a shifted dipole model with parameters $\mu^*$ and $d^*$. The major findings show that dipole shift breaks fore–aft symmetry, broadens angular distributions, and induces orientational frustration that weakens cooperative dipolar correlations, causing $\epsilon_r$ to decrease and approach the Debye limit for large shifts, while increasing dipole strength still raises Tc and liquid density but with shift-modulated effects. Overall, the work establishes dipole location as a potent parameter for tuning structure and thermodynamics in dipolar fluids, offering a physical framework for geometric frustration in electrostatic liquids and guiding coarse-grained representations of polar solvents and ionic systems.

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.

Figures (10)

• Figure 1: Model of a shifted Stockmayer fluid (sSF). Compared to the regular Stockmayer fluid, the particle has a dipole $\vec{\mu}$ shifted from the center by a distance $d$ in the direction of the dipole (depicted as the blue arrow). The excluded-volume interaction between the particles is described by a regular Lennard-Jones potential with characteristic length scale $\sigma$, and the electrostatic interaction is the dipole-dipole interaction between the shifted dipoles.
• Figure 2: Slab system of shifted-dipole Stockmayer particles. The Lennard-Jones beads are visualized in light purple and the shifted dipoles in dark blue. Ghost particle have been hidden.
• Figure 3: Schematic illustration of angular descriptors for the specific particle pair. The angles $\varphi_1$ and $\varphi_2$ denote the orientation of each dipole moment $\boldsymbol{\mu}_1$ and $\boldsymbol{\mu}_2$ relative to the dipole-displacement vector $\mathbf{r}^{d}$, while $\Theta$ is the angle between the two dipoles. The scalar center-to-center distance is $r$, and $d$ represents the shift of each dipole from the particle center.
• Figure 4: Radial distribution functions $g(r)$ in panels (a)–(c) and angular-radial distribution functions $\log P(r,\varphi)$ in panels (d)–(i) for the shifted Stockmayer fluid. In (a) and (b), $d^*=0.00$ and $d^*=0.25$, respectively, with varying dipole strengths; panel (c) fixes $\mu^*=1.500$ and varies $d^*$. Angular–radial distribution functions $\log P(r,\varphi)$ in polar coordinates. Color denotes the log-normalized probability of finding a neighbor LJ center at separation $r/\sigma$ and angle $\varphi$ between a reference particle dipole and its neighbor.
• Figure 5: Average angle between dipoles of neighboring particles, $\langle\cos\Theta\rangle$, in the first solvation shell as a function of angular position $\varphi$ between a reference particle dipole and its neighboring particles for (a) $d=0.001$ with $\mu^*=1.000,\,1.500,\,2.000$ and (b) $d=0.250$ with the same $\mu^*$.