Electrified Nanogaps\nunder an AC Field: A Molecular\nDynamics Study

The organization\nand dynamics of ions and water molecules\nat electrified\nsolid–liquid interfaces are generally well understood under\nstatic fields, especially for macroscopic electrochemical systems.\nIn contrast, studies involving alternating (AC) fields tend to be\nmore challenging. In nanoscale systems, added complexity can arise\nfrom interfacial interactions and the need to consider ions and molecules\nexplicitly. Here we use molecular dynamics (MD) simulations to investigate\nthe behavior of NaCl aqueous solutions at different concentrations\nconfined in nanogaps under AC fields ranging from 10 MHz to 10 GHz.\nWe explore the impact of the gap size (2–60 nm) and of the\nsolid material composing the electrode (silica, charged silica, or\ngold). Analysis of the transient and stable responses of the system\nshows that the total transverse dipole <i>M</i><sub><i>z</i>,total</sub> formed by the water molecules and the ions\nacross the gap is always able to counter the applied field regardless\nof AC frequency, NaCl concentration, or electrode material. As expected,\nthe ions lag at higher frequencies, leading to a capacitive behavior.\nThis effect is fully compensated by water dipoles that lead the field,\nreaching a maximum lead at a specific frequency which depends on salt\nconcentration and gap size. Changing the gap size affects the magnitude\nof <i>M</i><sub><i>z</i>,total</sub>. Finally,\nthe electrode material is shown to affect the electrolyte behavior\nin the gap region. We anticipate these results to be useful for nanoscale\ndielectric spectroscopy, including scanning probes.