DISTRIBUTION OF WATER-MOLECULES AT AG(111)/ELECTROLYTE INTERFACE AS STUDIED WITH SURFACE X-RAY-SCATTERING

The spatial distribution of water molecules at solid-electrolyte interfaces has received extensive theoretical study, due to the importance of this interface in electrochemistry and other sciences. Such studies suggest that adjacent to the interface water is arranged in several layers, that the molecular arrangements in the inner layer is similar to bulk water, and that the inner-layer molecules have an oxygen-up (oxygen-down) average orientation for negative (positive) electrode charge (or, equivalently, potential). However, little of this has been verified by experimental measurements. In this paper we report surface X-ray scattering measurements of the water distribution perpendicular to a Ag(111)-electrolyte interface in 0.1M NaF at two potentials: +0.52 and -0.23 V from the potential of zero charge (PZC) on the electrode. We find that, first, the water is ordered in layers extending about three molecular diameters from the electrode. Second, the extent of ordering and the distance between the electrode and first water layer depend on potential, the latter being consistent with an oxygen-up (oxygen-down) average molecular orientation for negative (positive) electrode potential. Third, the inner water layer contains 1.55 X 10(15) (at -0.23 V) and 2.6 X 10(15) (at +0.52 V) water molecules per cm(-2), remarkably more than expected from the bulk water density (i.e., similar to 1.15 x 10(15) cm(-2)). Such a large compression shows that the molecular arrangements in the inner layer are significantly different from bulk, which has not been anticipated in current models of charged, aqueous interfaces. We give a qualitative explanation of this large density as resulting from the strong electric field at the charged Ag(111) electrode and present a tentative model of the molecular arrangements.