Binary Solvent Organization at Silica/Liquid Interfaces: Preferential Ordering in Acetonitrile-Methanol Mixtures.
Nonlinear vibrational spectroscopy experiments examined solvent organization at the silica/binary solvent interface where the binary solvent consisted of methanol and acetonitrile in varying mole fractions. Data were compared with surface vibrational spectra acquired from silica surfaces exposed to a vapor phase saturated with the same binary solvent mixtures. Changes in vibrational band intensities suggest that methanol ideally adsorbs to the silica/vapor interface but acetonitrile accumulates in excess relative to vapor-phase composition. At the silica/liquid interface, acetonitrile's signal increases until a solution phase mole fraction of ∼0.85. At higher acetonitrile concentrations, acetonitrile's signal decreases dramatically until only a weak signature persists with the neat solvent. This behavior is ascribed to dipole-paired acetonitrile forming a bilayer with the first sublayer associating with surface silanol groups and a second sublayer consisting of weakly associating, antiparallel partners. On the basis of recent simulations, we propose that the second sublayer accumulates in excess.
Department of Chemistry and Biochemistry, Montana State University, P.O. Box 173400, Bozeman, Montana 59717, United States.
Eric A
EA
Gobrogge
Department of Chemistry and Biochemistry, Montana State University, P.O. Box 173400, Bozeman, Montana 59717, United States.
Robert A
RA
Walker
acetonitrile; competitive adsorption; liquid chromatography; methanol; silica; vibrational sum frequency generation