Stockmayer fluid with a shifted dipole. I. Bulk behavior

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Abstract
Shifting the point dipole from the center of a Stockmayer particle is a simple geometric modification, whose implications on liquid structure, dielectric response, and phase behavior remain incompletely understood. We combine molecular dynamics simulations with analytical theory to provide a unified physical interpretation of how dipole shifts reshape microscopic correlations and propagate to macroscopic 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 in 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 state that emerges at strong coupling and can suppress ferroelectric-like ordering. Predictions from a reparameterized co-oriented fluid functional equation for electrostatic interactions theory capture these trends within its domain of validity, highlighting the connection between local 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.
Funding Information
  • Advanced Scientific Computing Research (DE-SC0022158)
  • National Science Foundation (CHE-2337602)
  • National Science Foundation (OAC-2408259)
  • Hong Kong Quantum AI Lab and AIR@InnoHK of the Hong Kong Government
  • Caltech Resnick Sustainability Institute
  • Caltech WAVE Fellowship

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