Investigations of simple double layers

The authors interpreted the spectra in terms of the adsorption of the acetonitrile and the movement of the anion into the double layer, on positive polarization of the platinum electrode. Thus, the increasing adsorption of acetonitrile at potentials positive of -0.5 V vs. Ag/Ag+ (the base potential was chosen for its proximity to the pzc of platinum in the chosen electrolyte [98]) is shown by the positive feature at 2350 cm-1 whose intensity increases 

The broad negative feature at 3350 cm-1 was tentatively attributed to the gain of water hydrogen-bonded to the anion. The authors assigned the positive bands at 3625, 3550, and 1625 cm-1 to the loss, at the more anodic potentials, of water symmetrically hydrogen-bonded between two CH3CN molecules. This is a well-characterised species for which the v4, v2 and v3 modes occur at the observed frequencies [99]. 

These results were used by the authors to postulate a model for the changing population of species in the optical path. Thus, rendering the electrode potential anodic of — 0.5 V vs. Ag/Ag+ causes the anions to move into the double layer and acetonitrile to adsorb. These species are in equilibrium with those in the thin layer, which are in turn replenished from the electrolyte solution outside the thin layer. Hence, the movement of anions into the double layer leads to a net increase in the total number of anions in the optical path and of any associated water. This leads to a displacement of some surface MeCN, both free and complex with water, particularly at the higher positive potentials. 

These first papers boded well for the new technique, indicating the considerable amount of information on the composition of the double layer that 

Increasing the concentration of p-difluorobenzene in the thin layer up to 0.5 mM causes no change in the spectra. At solution concentrations 0.8 mM, however, there is an appreciable increase in the intensities of the bands and negative bands appeared for all the modes. The authors attributed 

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