Adsorption of carbonate and bicarbonate on colloidal silver particles and accompanying optical effects

Abstract

The competing adsorption of CO 3 2− and HCO 3 − ions on colloidal silver particles and the accompanying optical effects were studied. It was found that the HCO 3 − ion shows stronger adsorption on silver particles than the CO 3 2− ion due to its more stable adsorbing surface state. The adsorbate on the silver particle is almost exclusively the HCO 3 − ion at low relative concentrations of CO 3 2−. A gradual transformation of the adsorbate from HCO 3 − to CO 3 2− occurs and almost all of the HCO 3 − ion on the silver particles is eventually replaced by CO 3 2− at high relative concentrations of CO 3 2−. A mechanism is postulated to agree with the experimental results. The colour of the silver sol changes from yellow to red on addition of the anions and a new absorption band appears at longer wavelength. This phenomenon is attributed to the reduction of the free electron density at the surface layer of the silver particle. The reduction is caused by electrostatic repulsion of the adsorbed anions. A hypothesis is proposed which suggests that only the electrons at the surface layer (few atoms depth) are sensitive to excitation by light and produce surface plasmon oscillation.