Basic requirements for potential differences across solid-fluid interfaces

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 V across the first few molecular layers. Here, we calculate surface potentials at interfaces between 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. Furthermore, 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.