Characteristics of Surface Diffusion and Effective Pore Diffusion in Reversed-Phase Liquid Chromatography from Molecular Dynamics Simulations

Through the use of molecular dynamics (MD) simulations, solute distribution and mass transport at solid-liquid interfaces can be elucidated on the molecular level. In reversed-phase liquid chromatography (RPLC), retained analyte molecules can diffuse faster in the interfacial region between a hydrophobic stationary phase and a water–acetonitrile (ACN) mobile phase. ACN accumulates on top of the hydrophobic, alkyl-modified stationary phase (a C $$_{18}$$ 18 - or C $$_8$$ 8 -modified silica support) and forms an ACN-rich layer (the “ACN ditch”) on top of the bonded-phase chains. Because the high content of the organic compound is more conducive to analyte mobility, lateral (surface-parallel) analyte diffusivity goes through a maximum in the ACN ditch. In this project, we investigate the characteristics of surface diffusion in RPLC by MD simulations using GROMACS with respect to the influence of the applied mobile-phase composition, chain length and grafting density of the bonded phase on the lateral mobility gain for a set of four typical aromatic hydrocarbon analytes. The simulated spatially-dependent analyte mobilities serve as input parameters to calculate the effective macroscopic bed diffusivities in hierarchical macro–mesoporous structures.