Resonance Raman spectroscopic study of Methyl Red and Ethyl Red adsorbed on silica

Resonance Raman spectra have been recorded from Methyl Red and Ethyl Red adsorbed from toluene solutions on to Aerosils 90, 130, 200 and 300. The spectra were obtained from samples with surface coverages down to θ= 0.01 and show clear evidence for the formation of protonated species. Adsorption data for both indicators, over a wide range of equilibrium concentrations, give satisfactory fits to Langmuir isotherms. Together with the spectroscopic data, the isotherms enable a determination of the densities of Brønsted acidic sites on the silica surface. Densities of acidic sites increase with silica surface area and after calcination at 300 °C.