A new view on surface diffusion from molecular dynamics simulations of solute mobility at chromatographic interfaces

The behavior of solutes at the interface formed between a stationary surface and a bulk liquid lies at the heart of liquid chromatography. The diffusion coefficient of a solute in the interfacial region is an important component of the solute’s mass transfer through the chromatographic bed, but a detailed understanding of solute diffusion at chromatographic interfaces, particularly of solutes in an adsorbed state, has not been reached so far. We investigate this issue through molecular dynamics simulations of two solutes (n-butane and 1-propanol) at two complementary interfaces that represent conditions in reversed-phase and hydrophilic interaction liquid chromatography (RPLC and HILIC, respectively). The RPLC model consists of a planar silica surface covered with a hydrophobic bonded phase (C18 chains) and a 90/10 (v/v) water–acetonitrile mixture; the HILIC model consists of a planar silica surface bearing isolated, single silanol groups and a 10/90 (v/v) water–acetonitrile mixture. Our data show that the solute diffusion coefficient is linked to the local solvent composition and the structural organization in the interfacial region. In the HILIC system, the solute diffusion coefficient decreases continuously over the interfacial region, ending in a quasi-frozen state at the hydrophilic silica surface. In the RPLC system, the solute diffusion coefficient goes through a maximum in the interface region, where the C18 chains meet the liquid mobile phase, before decreasing towards the silica surface. Our results explain the experimentally observed phenomenon of surface diffusion in RPLC by the presence of an ACN ditch between bonded phase and W-rich mobile phase, in contrast to the structured W-rich layer at a bare-silica surface in HILIC which prohibits surface diffusion.