Water at electrode-electrolyte interfaces: combining HOD vibrational spectra with ab initio-molecular dynamics...

We have undertaken a vibrational study of the structure of interfacial water and its potential dependence using H₂O : D₂O mixtures to explore the O-H and O-D stretching modes of HOD as well as the bending modes of HOD and H₂O. Due to the symmetry reduction, some of the complexity characteristic of the vibrational spectrum of water is removed in HOD. Coupled with potential-dependent ab initio simulations of the gold-water interface, this has enabled a deeper insight into the hydrogen-bond network of interfacial water and into how it is affected by the applied potential. Possibly the most important conclusions of our work are (i) the absence of any ice-like first layer of interfacial water at any potential and (ii) that interfacial water reorients around a stable backbone of hydrogen bonds roughly parallel to the electrode surface. At E > pzc, interfacial water molecules are oriented with the oxygen lone pairs towards the surface and form exclusively or nearly exclusively hydrogen-donating hydrogen bonds with other water molecules. At E < pzc, the oxygen lone pairs instead point away from the surface, but the population of hydrogen-donating water molecules does not vanish. In fact, the population of hydrogen-accepting water molecules only dominates at considerably negative charge densities, due to the weak interaction of the hydrogen atoms of interfacial water molecules with the Au surface.