Competitive Adsorption of Toluene and n-Alkanes at Binary Solution/Silica Interfaces

The competitive adsorption of toluene and n-alkanes at binary solution/silica interfaces was studied at room temperature using IR-visible sum frequency generation vibrational spectroscopy. The Surface coverage of toluene for toluene-pentane, toluene-heptane, and toluene-tetradecane mixtures was measured over the complete mole fraction range from 0 to 1. The competitive adsorption process was reversible, and the toluene coverage only depended oil the bulk mole fraction, not on the history of the system. The estimated molar adsorption free energy of toluene is 3.4 +/- 0.3, 1.8 +/- 0.3, and 0.84 +/- 0.3 kJ/mol higher than pentane, heptane, and tetradecane, respectively. Overall, toluene competes favorably on silica, and the molar adsorption free energy of alkanes increases as the chain length increases. It is consistent with the observed SFG spectra, indicating that the alkanes lie flat on the silica surface.