Molecular Organization of Water at the Hydrophilic Silica/Liquid Interface Probed by Sum Frequency Generation Spectroscopy

We Investigate the orientation and structuring of strong protic solvents as a model system for separation techniques. Hydrophilic interaction liquid chromatography (HILIC) is an alternative mode to high performance liquid chromatography (HPLC) for separating highly polar and amphiphilic analytes. However, inconsistency of retention mechanism contributions has limited the advancement of the separation technique. The consensus is that both the liquid/liquid and solid/liquid interfaces may play a vital role in shedding light to these contributions. Knowing the specific molecular organization near surfaces may pave way to understanding the functional role of the liquid solvent mixture adsorbed at the interface. Surface selective vibrational sum-frequency generation (VSFG) is a second order ultrafast spectroscopic technique suitable for such studies, where the interfacial layers are selectively probed. We present here spectroscopic measurements of a double-layer formation of liquid methanol at the interface as well as probing the interfacial organization of water at the hydrophilic silica/liquid interface. Measurements of the methyl region of liquid methanol on hydrophilic surfaces have remained elusive, but here we report the first observation on the CH stretch spectrum.