Determination of interfacial solvent structuring of binary mixtures on silica surfaces by Vibrationally Resolved Sum Frequency Generation

Hydrophilic interaction liquid chromatography (HILIC) is a separation mode that is used to separate polar organic compounds such as peptides, proteins, carbohydrates, and pharmaceuticals. This technique has become increasingly popular because of the selectivity it provides. Even though this separation modality is being used more extensively, the retention mechanisms involved in a HILIC separation are still debated. The general consensus is that retention is caused by the formation of an enriched water layer at the silica surface and analytes partition between the two phases. Sum frequency generation (SFG) is a nonlinear optical process that provides surface specific vibrational spectra without any interference from the bulk molecules of a sample. This technique was employed to observe the surface ordering of several binary solvent mixtures used in HILIC (acetonitrile/water, acetone/water, and methanol/water) at different silica surfaces (quartz and silica particle). The OH stretching modes of surface bound water suggest a disordering of the hydrogen bonding network as acetonitrile or acetone are introduced. A red shift is also apparent in the methyl symmetric stretch of acetonitrile and acetone as the bulk organic concentration increases, suggesting an increased interaction between the methyl groups of neighboring acetonitrile or acetone. Data also suggest acetonitrile forms a bilayer like structure at the silica surface. The similarities between the acetonitrile/water and acetone/water spectra suggest that acetone can be substituted in place of acetonitrile in HILIC mobile phases and produce comparable separations. These similarities remain consistent whether quartz or a silica particle film are used as the solid substrate. Differences between spectral features of methanol/water mixtures further support this claim.