AN ION-EXCHANGE REACTION AS MEASURED BY SURFACE-ENHANCED RAMAN-SPECTROSCOPY ON SILVER COLLOIDS

Extensive measurements of surface-enhanced Raman (SER) spectra of benzoic acid (BA) in silver colloidal solutions and in the presence of various sodium salts were performed. It was observed that the intensity of Raman spectra of the SERS active species, i.e., the benzoate anion, was quenched on the addition of these salts. The observed decrease of the SERS intensity depends on the nature and on the concentration of the added salts, and this was assigned to an ion-exchange reaction, taking place at the outer Helmholtz surface (OHS) of the double-layer colloidal system. It is shown that a simple model, developed to interpret the reaction mechanism at the OHS, is able to account for the experimental results with all the sodium salts except the halides. Thus, this model gives important SERS parameters for the Ag metal surface, such as the number of ''active sites'' and the enhancement factor. The inadequacy of the model in the case of halide anions is attributed to their specific adsorption at the inner Helmholtz surface (IHS). Lastly, on the basis of the exchange constant values, a scale of anion reactivity for silver sols is tentatively proposed.