Solvation Energy of Ions in Polymers: Effects of Chain Length and Connectivity on Saturated Dipoles near Ions

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 solutions in polar solvents. Our investigation involves a comparative analysis of the dumbbell model and the Stockmayer model, focusing on ion solvation and ion-ion correlations. We examine electrolytes containing symmetric monovalent salts dissolved in polar solvents while varying the ion density and solvent polarity. Both models predict an augmented solvent coordination number around ions as the 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 nonmonotonic 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 the solvent molecular orientation and dipole rotation. Both models qualitatively predict ion pairing and clustering behaviors across varying solvent dipole strengths and salt concentrations. The Stockmayer solvent generally provides stronger electrostatic screening than the dumbbell solvent due to its neglect of the coupling between molecular orientation and dipole rotation. What's more, at a high dipole moment regime, ion-ion correlations in Stockmayer solvent can become stronger with increasing dipole moment due to stronger solvent-solvent correlations. This study underscores the effectiveness of the dumbbell solvent model in systematically elucidating the fundamental principles governing electrolytes and offers potential applications in the rational design of electrolyte systems.

The effect of explicit polarity on the conformational behavior of a single polyelectrolyte chain

In this work using dissipative particle dynamics simulations with explicit treatment of polar species we demonstrate that the molecular nature of dielectric media has a significant impact on swelling and collapse of a polyelectrolyte chain in a dilute solution. We show that the small-scale effects related to the presence of polar species lead to the intensification of the electrostatic interactions when the charges are close to each other and/or their density is high enough. As a result, the electrostatic strength , usually regarded as the main parameter governing the polyelectrolyte chain collapse, does not have a universal meaning: the value of λ at which the coil-to-globule transition occurs is found to be dependent on the specific fixed value of the solvent bulk permittivity ε while varying the monomer unit charge Q and vice versa. This effect is observed even when the backbone and the counterions have the same polarity as the solvent beads, i.e. no dielectric mismatch is present. The reason for such behavior is rationalized in terms of the "effective" dielectric permittivity ε^eff which depends on the volume fraction φ of charged units inside the polymer chain volume; using ε^eff instead of ε collapses all data onto one master curve describing the chain shrinking with λ. Furthermore, it is shown that a polar chain adopts less swollen conformations in the polyelectrolyte regime and collapses more easily compared to a non-polar chain.

Structure and dynamics of ions in dipolar solvents: a coarse-grained simulation study

We employ the coarse-grained molecular dynamics simulation to investigate the fundamental structural and dynamic properties of the ionic solution with and without the application of an external electric field. Our simulations, in which the solvent molecules are treated as Stockmayer fluids and the ions are modeled as spheres, can effectively account for the multi-body correlations between ion-ion, ion-dipole, and dipole-dipole interactions, which are often ignored by the mean-field theories or coarse-grained simulations based on a dielectric continuum. By focusing on the coupling between effects of ion solvation, electrostatic correlations and applied electric field, we highlight some nontrivial microscopic molecular features of the systems, such as the reorganization of the dipolar solvent, clustering of the ions, and diffusions of ions and dipolar solvent molecules. Particularly, our simulation indicates the nonmonotonic dependence of the ionic clustering and ion diffusion rates on the dipolar nature of the solvent molecules, as well as the amplification of these tendencies caused by the electric field application. This work provides insights into the fundamental understanding of physicochemical properties for ion-containing liquids and contributes to the design and development of ion-containing materials.

Transport Mechanisms Underlying Ionic Conductivity in Nanoparticle-Based Single-Ion Electrolytes

Recent studies have demonstrated the potential of nanoparticle-based single-ion conductors as battery electrolytes. In this work, we introduce a coarse-grained multiscale simulation approach to identify the mechanisms underlying the ion mobilities in such systems and to clarify the influence of key design parameters on conductivity. Our results suggest that for the experimentally studied electrolyte systems, the dominant pathway for cation transport is along the surface of nanoparticles, in the vicinity of nanoparticle-tethered anions. At low nanoparticle concentrations, the connectivity of cationic surface transport pathways and conductivity increase with nanoparticle loading. However, cation mobilities are reduced when nanoparticles are in close vicinity, causing conductivity to decrease for sufficiently high particle loadings. We discuss the impacts of cation and anion choice as well as solvent polarity within this picture and suggest means to enhance ionic conductivities in single-ion conducting electrolytes based on nanoparticle salts.

Solvation Energy of Ions in a Stockmayer Fluid

We calculate the solvation energy of monovalent and divalent ions in various liquids with coarse-grained molecular dynamics simulations. Our theory treats the solvent as a Stockmayer fluid, which accounts for the intrinsic dipole moment of molecules and the rotational dynamics of the dipoles. Despite the simplicity of the model, we obtain qualitative agreement between the simulations and experimental data for the free energy and enthalpy of ion solvation, which indicates that the primary contribution to the solvation energy arises mainly from the first and possibly second solvation shells near the ions. Our results suggest that a Stockmayer fluid can serve as a reference model that enables direct comparison between theory and experiment and may be invoked to scale up electrostatic interactions from the atomic to the molecular length scale.

Similarities and differences between potassium and ammonium ions in liquid water: a first-principles study

Understanding ion solvation in liquid water is critical in optimizing materials for a wide variety of emerging technologies, including water desalination and purification.

Influence of Host Polarity on Correlating Salt Concentration, Molecular Weight, and Molar Conductivity in Polymer Electrolytes

We use coarse-grained molecular dynamics simulations to study the effect of salt concentration and host polymer molecular weight on ion transport in polymer electrolytes. We find that increasing salt concentration or molecular weight similarly slows polymer dynamics across a wide range of host polarities, and that the resulting relaxation times display a correlation to the product of the salt concentration and polymer molecular weight. However, we find that molar conductivity only decreases with polymer dynamics at high polarities but is uncorrelated with the latter at low polarities. We attribute such differences to the variation in ionic aggregation between high and low polarity electrolytes. At low polarity, ionic dissociation significantly increases with molecular weight and salt concentration, offsetting the slowdown in polymer dynamics and yielding the observed insensitivity of molar conductivity. However, at high polarity, ions are mostly dissociated, independent of either molecular weight or salt concentration, thereby strongly coupling molar conductivity to polymer dynamics.

Effect of Polymer Polarity on Ion Transport: A Competition between Ion Aggregation and Polymer Segmental Dynamics

In this work, we use computer simulations to demonstrate that there may be limits to which polymer polarity alone can be used to influence the ionic conductivity of salt-doped polymer electrolytes. Specifically, we use coarse-grained molecular dynamics simulations to probe the effect of the polarity of the polymer electrolyte upon ion mobilities and conductivities of dissolved salts. At low polymer polarities, increasing the polymer dielectric constant reduces ionic aggregation and the resultant correlated ionic motion, and increases the ionic conductivity. At higher polymer polarities, polymer-polymer and polymer-ion interactions slows polymer segmental dynamics, leading to a reduction in the conductivity of the electrolyte. As a consequence, ionic conductivity achieves an optimum at an intermediate polymer polarity.