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 (C 18 vs C 8 ) and ligand (C 8 ) 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 C 8 chains are better solvated and more often in an upright and stretched conformation than C 18 chains, which leads to a higher maximum ACN excess in the ditch. High ligand density reinforces this effect. The ACN-excess advantage of C 8 phases translates not necessarily into faster surface diffusion, because the shorter chains have lower bondedphase mobility. Surface diffusion on a C 8 phase is generally slower than that on a C 18 phase, but surface diffusion on a highdensity C 8 phase can be faster than on a C 18 phase when the ACN content of the mobile phase is low.

Control over the competition between an organometallic hexamer macrocycle and oligomer chains formed from the non-alternant aromatic 1,3-dibromoazulene (DBAz) precursor has been achieved in surface-assisted synthesis on a copper(111) surface. In contrast to kinetic reaction control via the high-dilution principle, the ring formation is achieved here by thermodynamic control, which is based on two-dimensional (2D) confinement and reversible bonds.

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The interfacial phenomena behind analyte separation in a reversed-phase liquid chromatography column take place nearly exclusively inside the silica mesopores. Their cylindrical geometry can be expected to shape the properties of the chromatographic interface with consequences for the analyte density distribution and diffusivity. To investigate this topic through molecular dynamics simulations, we introduce a cylindrical pore inside a slit pore configuration, where the inner curved and outer planar silica surface bear the same bonded phase. The present model replicates an average-sized (9 nm) mesopore in an endcapped C 18 column equilibrated with a mobile phase of 70/30 (v/v) water/acetonitrile. Simulations performed for ethylbenzene and acetophenone show that the surface curvature shifts the bonded phase and analyte density toward the pore center, decreases the solvent density in the bonded-phase region, increases the acetonitrile excess in the interfacial region, and considerably enhances the surface diffusivity of both analytes. Overall, the cylindrical pore provides a more hydrophobic environment than the slit pore. Ethylbenzene density is decidedly increased in the cylindrical pore, whereas acetophenone density is nearly equally distributed between the cylindrical and slit pore. The cylindrical pore geometry thus sharpens the discrimination between the apolar and moderately polar analytes while enhancing the mass transport of both.