Minimal coarse-grained models of polar solvent for electrolytes: Stockmayer versus dumbbell

This study explores the potential of the dumbbell solvent as a minimal model for understanding electrolyte behavior in polar solvents, in contrast to the widely-used Stockmayer model. The distinguishing feature of the dumbbell model lies in its straightforward capture of the coupling between molecular orientation and dipole rotation. Our investigation involves a comparative analysis of the dumbbell and Stockmayer solvent models, focusing on ion solvation and ion-ion correlations. We examine electrolytes containing symmetric monovalent salts dissolved in polar solvents while varying ion density and solvent polarity. Both models predict an augmented solvent coordination number around ions as solvent polarity increases, with the dumbbell solvent displaying a more pronounced effect. Notably, radial distribution functions (RDFs) between solvent and ions yield differing trends; Stockmayer models exhibit a non-monotonic relationship due to strong dipole-dipole interactions at higher polarity, while RDFs for ions and dumbbell solvents consistently rise. In response to increased solvent polarity, Stockmayer solvents within the ion's solvation shell undergo continuous dipole orientation shifts, whereas the dumbbell solvent predominantly adopts pointing-away dipole orientations, diminishing pointing-to orientations. This underscores the significance of the interplay between solvent molecular orientation and dipole rotation. Under non-polar conditions, the Stockmayer solvent model inadequately replicates ion-ion correlations of electrolytes based on hard-sphere solvent, attributed to potential numerical issues in electrostatic energy calculations. Conversely, the dumbbell solvent model effectively reproduces these correlations, underscoring its utility. Moreover, it reasonably predicts ion pairing and clustering behaviors across varying solvent dipole strengths and salt concentrations. This study underscores the effectiveness of the dumbbell solvent model in systematically elucidating the fundamental principles governing electrolytes, offering potential applications in the rational design of electrolyte systems in the future.