Surface Diffusion of Aromatic Hydrocarbon Analytes in Reversed-Phase Liquid Chromatography

Abstract

In reversed-phase liquid chromatography (RPLC), retained analytes can diffuse faster along the hydrophobic surface of the stationary phase than when dissolved in the water (W)–acetonitrile (ACN) mobile phase. We investigate the surface diffusion of four typical aromatic hydrocarbon analytes in RPLC through molecular dynamics simulations in a slit-pore RPLC model consisting of a silica-supported, end-capped, C18 stationary phase and a 70/30 (v/v) W/ACN mobile phase. Our data show that the lateral (surface-parallel) diffusive mobility of the analytes goes through a maximum in the ACN ditch, an ACN-rich border layer around the terminal part of the bonded-phase chains, because the solvent composition there is more conducive to analyte mobility than the W-rich mobile phase. At their lateral mobility maximum, analytes have contacts with 12–15 bonded-phase groups, 5–6 ACN and 1–2 W molecules. The lateral mobility gain from surface diffusion decreases with analyte polarity first and size second (like and unlike retention in RPLC, respectively). The lateral diffusive mobility of analytes at the ACN density maximum in the RPLC system can be approximated by their bulk molecular diffusion coefficient in a W–ACN mixture that matches the local solvent composition at the ACN density maximum. On the basis of data received from analyte-free simulations of the RPLC system with mobile phases between 10/90 and 90/10 (v/v) W/ACN and from simulations of the bulk molecular diffusion coefficients of the analytes over this range of W/ACN ratios, we predict that enhanced surface diffusion persists under gradient elution conditions.

PoreMS: a software tool for generating silica pore models with user-defined surface functionalisation and pore dimensions

“…As of version 0.2.0, only the structure of β-cristobalite SiO 2 has been implemented. Simulations in slit-pore models have proved that the faces of β-cristobalite SiO 2 , particularly its (111) face, support the envisioned ligand density and residual hydroxylation of functionalised and bare silica materials used in chromatographic columns [10,[12][13][14]24,25,34,40,41]. Moreover, MD simulations have shown that non-functionalised, cylindrical pores carved from β-cristobalite SiO 2 replicate the relevant properties of the bare silica surface in liquid chromatography well [9,11].…”

“…These typically include gas or solvent molecules, solute molecules, and the ligands and surface atoms of the functionalised or bare silica surface. Molecular simulations, particularly atomistic molecular dynamics (MD) simulations, have already been used to elucidate the mass transport characteristics of different liquid chromatography modes [10][11][12][13][14][15][16] and recently to model catalytic [17][18][19] and drug delivery systems [20]. This suggests that molecular simulations are increasing in importance as an essential building block in multiscale modelling of heterogeneous catalysis [21] and the selection and development of chromatographic columns [22][23][24][25].…”

Molecular dynamics simulations of amino acid adsorption and transport at the acetonitrile–water–silica interface: the role of side chains

“…32 Drobny et al studied the interaction between the amino acids and water-TiO 2 interface with both experiments and MD simulations, and their data indicated basic amino acids were adsorbed onto rutile TiO 2 surfaces in multiple forms that depends on the surface hydroxyl group density. 33 In spite of recent progress on simulations of amino acid adsorption at water-solid interfaces as well as the retention and transport of small organic molecules on interfaces in LC, [34][35][36][37][38][39] the microscopic simulation studies of amino acid solvation and transport at the liquid-solid (LS) interfaces in LC are still lacking.…”

Retention and effective diffusion of model metabolites on porous graphitic carbon

“…Contrary to what was observed with the parallel-diffusion model, it has also been shown using peak-parking experiments with EMT models and molecular dynamics simulations, that the rate of surface diffusion increases as analytes become more retained due to the presence of an acetonitrile-rich “ditch” present in the interfacial region near the ends of the C18 chains. This region has a lower viscosity than the bulk mobile phase and results in faster surface diffusion [37–40]. Due to the differences in retention mechanism and surface chemistry (C18 bonded layer vs. flat graphite surface) it is thought that Hypercarb might exhibit different effective diffusion properties from a C18 bonded material.…”