Electrified Nanogaps under an AC Field: A Molecular Dynamics Study

The organization and dynamics of ions and water molecules at electrified solid-liquid interfaces are 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 behavior of NaCl aqueous solutions at different concentrations confined in nanogaps under AC fields ranging from 10 MHz to 10 GHz. We explore the impact of the gap size (2-60 nm) and of the solid material composing the electrode (silica, charged silica, or gold). Analysis of the transient and stable responses of the system shows that the total transverse dipole <i>M</i> _{<i>z</i>,total} formed by the water molecules and the ions across the gap is always able to counter the applied field regardless of AC frequency, NaCl concentration, or electrode material. As expected, the ions lag at higher frequencies, leading to a capacitive behavior. This effect is fully compensated by water dipoles that lead the field, reaching a maximum lead at a specific frequency which depends on salt concentration and gap size. Changing the gap size affects the magnitude of <i>M</i> _{<i>z</i>,total}. Finally, the electrode material is shown to affect the electrolyte behavior in the gap region. We anticipate these results to be useful for nanoscale dielectric spectroscopy, including scanning probes.