Electrified Nanogaps under AC Field: A Molecular Dynamics Study [dataset]

The organisation and dynamics of ions and water molecules at electrified solid-liquid interfaces is generally well understood under static fields, especially for macroscopic electrochemical systems. In contrast, studies involving alternating (AC) fields tend to be more challenging. In nanoscale systems, added complexity can arise from interfacial interactions and the need to consider ions and molecules explicitly. Here we use molecular dynamics (MD) simulations to investigate the behaviour of NaCl aqueous solutions at different concentrations confined inside nanogaps under AC fields ranging from 10 MHz to 10 GHz. We explore the impact of the gap size (2 nm - 60 nm) and of the solid material composing the electrode (silica, charged silica, gold). Analysis of the transient and stable responses of the system show that the total transverse dipole MZ,total formed by the water molecules and the ions across the gap is always able to counter applied field regardless of AC frequency, NaCl concentration, gap size or electrode material. As expected, the ions lag at higher frequencies, leading to capacitive behaviour. This effect is fully compensated by water dipoles which lead the field, reaching a maximum lead around 300 MHz. Changing the gap size affects the magnitude of MZ,total with a sharp maximum around 6 nm. Finally, the electrode material is shown to affect the electrolyte behaviour in the gap region. We anticipate these results to be useful for nanoscale dielectric spectroscopy, including with scanning probes.