Basic requirements for potential differences across solid--fluid interfaces

TL;DR

This work probes the minimal molecular requirements for interfacial charge ordering and a nonzero surface potential $\chi$ at solid–fluid interfaces using MD simulations of five model fluids (FA, MA, FS, SD, STM) confined between planar walls. A key finding is that a nonzero $\chi$ requires the dipolar center $\mathbf{r}_d$ to be offset from the geometric center $\mathbf{r}_g$ of the molecule ($\mathbf{r}_d \neq \mathbf{r}_g$); the wall–fluid interaction strength $\zeta$ has little influence on $\chi$, while steric differences can flip the sign of $\chi$. Temperature and density modulate interfacial structure, with $\chi$ generally weakening as $T$ increases and fluids become less dense; subcritical fluids display stronger, more distinct $\chi$ contrasts than supercritical ones, where different fluids converge to similar $\chi$ values. Collectively, the results highlight geometric and dipolar asymmetry as the main determinants of interfacial electrostatics, offering a framework to interpret charge ordering in real polar fluids like water.

Abstract

At model water--vapor and water--solid interfaces, molecular ordering leads to charge oscillations and, thereby, to a spatially varying electrostatic potential. Atomistic simulations indicate that such ordering leads to an electric potential difference $χ$, the surface potential, of about $-0.5\,\mathrm{V}$ across the first few molecular layers. Here, we calculate surface potentials at interfaces between a simple model fluids and a solid, with Molecular Dynamics simulations. The fluids are made up of either diatomic, dipolar molecules or a single Lennard-Jones particle with a dipole moment. All fluids show some structuring near the interface, but charge oscillations and a non-zero surface potential are present only for asymmetric molecules (unequal diameters of the atoms) or molecules with an off-center dipole. We condense this finding into the criterion that the geometric and dipolar centers of a molecule must differ for the fluid to exhibit a surface potential. Remarkably, while the solid--fluid interaction strength strongly affects the magnitude of charge oscillations, it hardly affects the potential drop $χ$. Further, our results demonstrate that changing the diameter of the smaller atom can flip the sign of the surface potential, thus highlighting the importance of steric effects.

Figures (11)

• Figure 1: Molecular geometry and charge placement ($+$/$-$) with the respective labels employed in this study: fully asymmetric (FA), moderately asymmetric (MA), fully symmetric (FS), shifted dipole (SD), and Stockmayer-like (STM).
• Figure 2: Interactions and molecular parameters, illustrated for the moderately asymmetric (MA) molecules.
• Figure 3: Half-cell of the simulation box. Grey, blue, and white spheres correspond to solid atoms, atoms $\alpha$ and $\beta$ of the FA molecules respectively. The atoms are not depicted to scale.
• Figure 4: (a) Excess density $\rho_{\mathrm{ex}}$, (b) charge density $q$, and (c) electric potential $\psi$ profiles as a function of the distance $z$ from the solid--fluid interface, for various solid--fluid interaction parameter $\zeta$ values, for the FA fluid at ${T/T_{\mathrm{c}}=0.75}$. The surface potential $\chi$ is indicated with an arrow. The inset in (b) shows $q(z)$ results for $\zeta=0.01$ and $0.1$.
• Figure 5: (a) Excess density $\rho_{\mathrm{ex}}$, (b) charge density $q$, and (c) electric potential $\psi$ profiles as a function of the distance $z$ from the solid--fluid interface for the FA, SD, and STM fluids [see \ref{['fig:particles']}] at reduced temperature $T/T_{\mathrm{c}}=0.75$.