Vibrational sum frequency generation (VSFG) spectroscopy was used to examine the air-silica interface before, during, and after adsorption of water, chloroform, acetonitrile, and atrazine, an s-triazine molecule. Adsorption via the surface silanol sites was observed for all the compounds. Adsorption of chloroform and acetonitrile was weaker compared to water. Binding to the surface silanol groups was observed to be reversible for chloroform, acetonitrile, and atrazine. The standard free energy of adsorption, Delta G degrees(abs), of atrazine to the silica surface was determined to be -35 kJ/mol at 23 degrees C. (C) 2009 Elsevier B.V. All rights reserved.