Potential-Dependent Water Orientation: An in Situ Spectroscopic Study

This paper concerns the possible orientation of water molecules at the surface of platinum electrodes in aqueous NaF solutions. The work was carried out by using in situ FTIR spectroscopy in SNIFTIRS (substractively normalized interfacial Fourier transform infrared spectroscopy) mode. The intensity of the IR spectral band around 3300 cm^-1, characteristic of O-H vibration, was found to increase from about 0.0 V on the NHS when the potential was shifted anodically. Corresponding bands were observed for the O-D stretch in a 50–50 H₂0-D₂0 mixture. However, the intensity of these were 4–8 times less than those for the O-H vibration. This difference is explained in terms of their differences in hydrogen bonding. The dependence of the intensity of OH and OD vibration upon potential fits a model for water orientation in which water molecules lie flat (i.e., both oxygen and hydrogen atom of a water molecule lie on the surface) near the pzc and gradually orient with their oxygen end toward the surface with the increase of electrode potential in the anodic direction. The results would also probably fit other models in which the water dipoles change their direction on change of electrode charge.