A Molecular Dynamics View on Hydrophilic Interaction Chromatography with Polar-Bonded Phases: Properties of the Water-Rich Layer at a Silica Surface Modified with Diol-Functionalized Alkyl Chains

In hydrophilic interaction chromatography solutes are retained in a water (W)-rich layer that forms upon equilibrating the polar stationary phase with an acetonitrile (ACN)-rich aqueous mobile phase. We study the properties of the W-rich layer at a polar-bonded phase, modeled by a planar silica surface carrying diol-functionalized ligands and residual OH groups at densities of 3.1 and 4.4 mu mol/m(2), respectively. Molecular dynamics simulations performed in a 10 nm slit pore containing between 30/70 and 2/98 (v/v) W/ACN show that a rigid, single layer of W molecules adsorbed directly at the silica surface is connected through an interstitial, single layer of W molecules with a ca. 0.8 nm thick diffuse W layer associated with the polar groups at the flexible chain ends. Hydrophilic and hydrophobic elements are largely segregated in the diffuse W layer. Hydrophilic patches are formed by W clusters (groups of hydrogen-bond-connected W molecules) and diol clusters (groups of typically 2-4 directly connected diol chains); hydrophobic patches are formed by ACN molecules solvating hydrocarbon groups of the bonded phase. The W-rich layer at the diol-modified surface has lower translational mobility and can hold more excess W than the W-rich layer at an unmodified silica surface.