Short-Range Solvent-Solvent and Ion-Solvent Correlations at Metal-Electrolyte Interfaces: Parameterization and Benchmarking

Abstract

Short-range correlations in electrolyte solutions lead to oscillatory profiles of water polarization and ionic concentration at electrode-electrolyte interfaces. The recently developed density-potential-polarization functional theory (DPPFT) provides a comprehensive framework to incorporate these short-range correlation effects. In the present work, the parameters describing short-range solvent-solvent and ion-solvent correlations in DPPFT are determined from the wavenumber-dependent dielectric susceptibility spectrum of pure water and from ion solvation energies derived within nonlocal electrostatics, respectively. The experimental ionic-radius-dependent hydration energies of alkali metal cations and halide anions are well reproduced by the solvation model. The charge hydration asymmetry is explained as the stronger short-range repulsion between cations and water molecules compared to that between anions and water molecules. Using these parameters, DPPFT is then applied to investigate short-range correlation effects at the Ag(111)-NaF aqueous electrolyte interface. The water polarization profiles obtained from DPPFT calculations agree with AIMD simulations. Furthermore, as the strength of short-range ion-solvent repulsion increases, the peaks of anionic/layers shift from regions near centers of positive/negative polarization charges toward those of opposite sign, thereby preserving solvation configurations similar to those in bulk solution. This work develops a consistent procedure for parameterizing short-range correlation effects within the DPPFT framework, thereby enabling a more quantitative and computationally efficient description of atomic-scale phenomena at electrochemical interfaces.

Figures (4)

• Figure 1: Comparisons of experimental hydration energies of alkali metal cations (circles) and halide anions (squares) with those calculated from Eq. \ref{['equ: solvation energy']} using SGn model for the ionic charge distribution. The dimensionless short-range ion--solvent correlation parameter is defined as $\tilde{\alpha}=\alpha\epsilon_0/a_0^2$, with the referenced distance $a_0=1\ \text{\AA}$. Other parameters used are: $K_\alpha = -0.047\ \text{\AA}^2$ (determined from Eq. \ref{['equ: K_alpha, K_beta']}), $K_\beta = 0.0026\ \text{\AA}^4$ (determined from Eq. \ref{['equ: K_alpha, K_beta']}), $\varepsilon_\text{ir}=4.9$ (Ref. kornyshev1996shape), $\varepsilon_s=78.5$ (Ref. kornyshev1996shape). Ionic crystal radii are used for $R$, and their values together with the corresponding hydration energies are taken from Ref. marcus2015ions.
• Figure 2: (a) Water polarization at the PZC of the Ag(111)--water interface from AIMD simulations (circles) and from the DPPFT model for the Ag(111)--100 mM NaF aqueous solution interface. In the DPPFT calculations, $K_\alpha$ and $K_\beta$ are obtained using $\lambda_\text{p}=2.1\ \text{\AA}$ for pure water and an adjusted $\lambda_\text{p}=1.8\ \text{\AA}$ for interfacial water, while the same decay length $\lambda_\text{d}=4.3\ \text{\AA}$ is used in both cases. All other parameters are as listed in Table. S1 in Ref.supplemental (see also Refs.johnson1972opticalamann2016x therein). The water polarization is normalized to its maximum value. (b) Water polarization simulated by the DPPFT model with $\lambda_\text{p}=1.8\ \text{\AA}$ at different electrode potentials relative to the PZC, as indicated in the figure. In both panels, the metal surface is located at $x=0$ Å.
• Figure 3: Spatial profiles of (a) water polarization, (b) anionic concentration, and (c) cationic concentration obtained from DPPFT simulations using the parameters in Table S1 in Ref.supplemental, at different electrode potentials relative to the PZC (as indicated in FIG. \ref{['fig: ion distribution']}b). The metal surface is located at $x = 0$ Å in all panels.
• Figure 4: Spatial profiles of (a) water polarization and (b) anionic concentration obtained from DPPFT simulations at an electrode potential of 0.4 V relative to the PZC, for different $\alpha_a$ values as indicated, with all other parameters fixed (Table S1 in Ref.supplemental). The metal surface is located at $x = 0$ Å in both panels. The polarization charge centers are indicated in FIG. \ref{['fig: short-range ion-solvent correlation effect']}a.