(548f) Adsorption and Diffusion of Water-Acetonitrile Mixtures At a Bare Silica Surface: A Molecular Dynamics Study

A bare silica surface is the most popular stationary phase for separation of polar analytes in hydrophilic liquid chromatography (HILIC). Knowledge of adsorption behaviour and dynamic features of water-acetonitrile mixtures (the most common choice of the mobile phase in HILIC) at the surface is a key prerequisite for entire understanding of separation mechanism in HILIC. In practice adsorption is often characterized by excess adsorption isotherms. However, this information is essentially an integral over the whole chromatographic support, and it requires further interpretations to get an actual spatial distribution of the solvent mixture in the adsorpted phase. In this work we employ molecular dynamics (MD) simulation to make a link between experimentally obtainable (macro) characteristics of adsorption and a picture at the micro-level (which is hardly can be measured directly in experiment).

We have performed MD simulation of the water-acetonitrile mixture (for whole range of compositions) at the bare flat silica surface. There have been studied three types of idealized surfaces: a surface covered only by single (isolated) silanol groups, a surface covered only by geminal (partially bridged) silanol groups and the last one with only siloxane bridges. Iterative simulation runs have been preliminary performed to adjust necessary amount of water and acetonitrile molecules in that adsorption/desorption system. Our results demonstrate that at mixture compositions with the low water fraction water shows the positive surface excess for all three considered types of silica surface. Acetonitrile appears to have the positive surface excess mixtures with the high water content only at the surface with siloxane bridges. In our presentation we will also discuss how these three studied types of silica surfaces, and especially their combinations, affect diffusion of the solvent molecules at the surface.