Stationary-Phase Contributions to Surface Diffusion in Reversed-Phase Liquid Chromatography: Chain Length versus Ligand Density
journal contributionposted on 2019-08-22, 19:35 authored by Julia Rybka, Alexandra Höltzel, Nicole Trebel, Ulrich Tallarek
Fast surface diffusion in reversed-phase liquid chromatography (RPLC) describes a complex phenomenon that exists in a narrow ditch region where the silica-tethered alkyl chains of the stationary phase meet the water–acetonitrile (ACN) mobile phase. The lateral mobility of analyte molecules in the ACN-rich ditch can exceed their bulk diffusivity in the mobile phase. Through molecular dynamics simulations using an established RPLC mesopore model and analyte set we study how chain length (C18 vs C8) and ligand (C8) density of the stationary phase contribute to the lateral mobility gain from surface diffusion at low and high ACN content of the mobile phase. The simulations show that C8 chains are better solvated and more often in an upright and stretched conformation than C18 chains, which leads to a higher maximum ACN excess in the ditch. High ligand density reinforces this effect. The ACN-excess advantage of C8 phases translates not necessarily into faster surface diffusion, because the shorter chains have lower bonded-phase mobility. Surface diffusion on a C8 phase is generally slower than that on a C18 phase, but surface diffusion on a high-density C8 phase can be faster than on a C18 phase when the ACN content of the mobile phase is low.