Water at the electrode-electolyte interface Andrew Burley, Pavithra Gunaeskaran and Angel Cuesta School of Chemistry, University of Aberdeen, UK
Water plays a crucial role in determining the properties of the electric double layer in aqueous electrolytes. Despite this, its behaviour is poorly understood. Surface enhanced infrared absorption spectroscopy in the attenuated total reflection configuration (ATR-SEIRAS) is highly sensitive to species within a few nm of the electrode surface, thus, it is highly suited to probe the dynamics of interfacial water. Osawa and co-workers were the first to apply this technique to explore the interfacial region 1 . We have used ATR-SEIRAS to improve our understanding of the structure of interfacial water at the gold-electrolyte interface by examining the O-H stretching of HOD molecules in 0.1 M HClO 4 dissolved in a 25% H 2 O / 75% D 2 O mixture. Examination of the ν OH band of HOD diluted in D 2 O shows significant changes from potentials below the point of zero charge (pzc) to potentials more positive than the pzc. In particular, deconvolution resolves two distinct contributions at potentials negative to the pzc, whilst three are observed at potentials positive of the pzc. The high frequency of this third contribution to the O-H stretching mode of interfacial HOD – around 3580 cm -1 – indicates a distinct population of interfacial water which experiences significant disruption to its hydrogen bonding network above the pzc. The intensity of this band, and therefore this population of disrupted water, increases as the potential becomes more positive. As perchlorate is known to be a chaotrope, this band is attributed to the effect of this ion migrating into the double layer. The δ HOD band shows a stepwise increase in frequency at the pzc, appearing at 1440 cm -1 below the pzc and 1460 cm -1 at potentials more positive to the pzc. Unexpectedly, the ratio of the intensities of the ν OH to ν D2O band differs for interfacial water versus the bulk. This allows us to hypothesise that water molecules have a tendency to adopt an orientation at the interface with one of the O-H bonds perpendicular to the surface, pointing either to ( E < pzc) or away ( E > pzc) of the gold surface. References 1. K. Ataka, T Yotsuyanagi and M Osawa, J. Phys. Chem . 1996 , 100 , 10664-10672
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