Adsorption of Water-Acetonitrile Mixtures to Model Silica Surfaces

The retention mechanism of hydrophilic interaction chromatography relies on analyte partitioning from a mostly organic water acetonitrile (W/ACN) mobile phase into an extended W-rich layer at the stationary phase. The formation of the W-rich layer is driven by the surface hydrophilicity of the stationary phase. We study the origin of the hydrophilicity of bare-silica stationary phases by molecular dynamics simulations of the adsorption of W/ACN mixtures from 99/1 to 2/98 (v/v) at three surfaces that each model one type of surface functional group: single silanol groups, geminal silanol groups, and siloxane bridges. Initiated by surface W hydrogen bonding, the two silanol surfaces accumulate a dense W layer; their W surface excess adsorption isotherms remain positive over the whole W/ACN range. The siloxane bridges surface coordinates with an exceptionally tight layer of alternate W and ACN clusters; the mixed W/ACN surface layer is reflected in an S-shaped adsorption isotherm with W surface excess below 54/46 (v/v) W/ACN and ACN surface excess above 54/46 (v/v) W/ACN. Our results suggest that surface hydrophilicity is created largely by silanol groups, whereas siloxane bridges by offering adsorption sites to ACN molecules contribute in a different way to the retentive properties of bare-silica stationary phases.