Direct Observation of Confinement-Induced Charge Inversion at a Metal Surface

Surface interactions across water are central to areas from nanomedicine to colloidal stability. They are predominantly a combination of attractive but short-ranged dispersive (van der Waals) forces, and long-ranged electrostatic forces between the charged surfaces. Using a surface force balance, we showed that electrostatic forces between two surfaces across water, one at constant charge (a dielectric) while the other (a molecularly-smooth metal surface) is at constant potential of the same sign, may revert smoothly from repulsion to attraction on progressive confinement of the aqueous intersurface gap. This remarkable effect, long predicted theoretically in the classic Gouy-Chapman (Poisson-Boltzmann) model but never previously experimentally observed, unambiguously demonstrates surface charge reversal at the metal-water surface.

Its experimental demonstration emphasizes the importance of taking such charge reversal – and the accompanying cross-over from repulsion to attraction – into account, for interactions between dielectrics and metal surfaces in aqueous media in similar circumstances. These include phenomena such as colloidal interactions, adsorption of proteins, cells or nanoparticles on metal surfaces, imaging of metal surfaces by ceramic AFM tips and tribology of dielectric-metal interfaces in aqueous surroundings.