Model chromatographic interfaces based on Ag surfaces modified with octadecanethiol and mixed octadecanethiol-methylthiol monolayers were created by emersion from D₂O and acetonitrile. Interfacial structures were investigated using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) in an emersion configuration. For pure octadecanethiol layers, the spectral data indicate little interaction between either solvent and these layers, even when these layers were formed under conditions known to yield disordered layers. Hence, even when disordered, these monolayers are impermeable to the aforementioned solvents. In contrast, Ag surfaces modified with mixed monolayers of octadecanethiol-methylthiol of varying mole fraction prepared in the kinetic regime clearly show evidence of strong acetonitrile interaction. These results are interpreted on terms of monolayer structure as a function of composition and solvation interactions. Detailed molecular pictures of these interfaces are proposed.