Raman Spectroscopy of the Liquid-Solid Interface: Monolayer and Bilayer Adsorption of Pyridine on Silica

Raman spectra of pyridine adsorbed at a silica-solution interface are reported. A flow-through sample cell is employed to control the illuminated surface area of the particulate sample and to minimize background from solution phase species. The technique is applied to study the adsorption of pyridine onto silica gel from carbon tetrachloride solution. Spectroscopic adsorption isotherms are acquired with sufficient precision to allow monolayer and bilayer forms of adsorbed pyridine to be distinguished and quantified. The monolayer form adsorbs with proton transfer decreases uniformly with increasing surface coverage. A Frumkin model fits the monolayer isotherm data and provides an estimate of the decrease in free energy of adsorption with increasing surface coverage. The bilayer isotherm exhibits a concentration threshold which could only be fit by assuming that its energy of adsorption depends strongly on monolayer coverage. The accumulation of this pyridine bilayer appears to play a role in stabilizing the charge density of ion pairs in the monolayer.