VOLTAGE-DEPENDENT ORDERING OF WATER-MOLECULES AT AN ELECTRODE-ELECTROLYTE INTERFACE

THE arrangement of water molecules at charged, aqueous interfaces is a n important question in electrochemistry, geochemistry and biology. Th eoretical studies1-11 suggest that the molecules become arranged in se veral layers adjacent to a solid interface. with densities similar to that in the bulk, and that the molecules in the first layer are reorie nted from oxygen-up to oxygen-down as the electrode charge changes fro m negative to positive. Few of these predictions have been verified ex perimentally12-16, however. Using X-ray scattering, we have measured t he water density profile perpendicular to a silver (111) surface at tw o applied voltages. We find that the water molecules are ordered in la yers extending about three molecular diameters from the electrode, and that the spacing between the electrode and first water layer indicate s an oxygen-up (oxygen-down) average orientation for negative (positiv e) charge. Contrary to current models, however, we find that the first layer has a far greater density than that in bulk water. This implies that the hydrogen-bonding network is disrupted in this layer, and tha t the properties of the water in the layer are likely to be very diffe rent from those in the bulk.