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

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, C-18 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.