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

5 years ago

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

Alexandra Höltzel, Julia Rybka, Ulrich Tallarek

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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b04746

DOI: 10.1021/acs.jpcc.7b04746