Stockmayer Fluid with a Shifted Dipole. III. Ion Solvation

Abstract

Though ion solvation is well understood for the standard Stockmayer fluid, the effect of displacing the point dipole of a Stockmayer particle, a simple geometric modification, on ion-solvent structuring and solvation thermodynamics remains less understood. We use molecular dynamics simulations to describe how a dipole shift affects radial and orientational solvent structure around centered ions, how strongly the solvent distinguishes cations and anions, and how the solvent mediates ion-ion pairing. We show that, with a centered dipole, cations and anions have equal preferences for the local solvent environment. Shifting the dipole breaks the fore-aft symmetry of the local electrostatic field, leading to a stronger preference for a Stockmayer particle's dipole to orient towards an anion and a weaker preference to orient away from a cation. These structural asymmetries propagate to the solvation free energies: increasing dipole displacement makes anion solvation more favorable, while cation solvation becomes less favorable overall across dipole strengths. The same asymmetry also manifests in the effective ion-ion interaction-increasing the dipole shift reorganizes the local solvation environment, enhancing solvent shielding of short-range cation-anion contact while weakening screening at longer range. As a result, solvent-separated configurations become comparatively more favorable, whereas increasing dipole strength produces the opposite trend. Such effects cannot be explained by bulk dielectric screening alone, indicating that molecular charge geometry plays an important role in both single-ion solvation and solvent-mediated ion-ion interactions. This work demonstrates that dipole location, not only magnitude, provides a geometric control on ion-specific solvation and ion pairing in general polar fluids.