A Molecular Dynamics View on Hydrophilic Interaction Chromatography with Polar-Bonded Phases: Properties of the Water-Rich Layer at a Silica Surface Modified with Diol-Functionalized Alkyl Chains

Abstract

In hydrophilic interaction chromatography solutes are retained in a water (W)-rich layer that forms upon equilibrating the polar stationary phase with an acetonitrile (ACN)-rich aqueous mobile phase. We study the properties of the W-rich layer at a polar-bonded phase, modeled by a planar silica surface carrying diol-functionalized ligands and residual OH groups at densities of 3.1 and 4.4 µmol/m 2 , respectively. Molecular dynamics simulations performed in a 10-nm slit pore containing between 30/70 and 2/98 (v/v) W/ACN show that a rigid, single layer of W molecules adsorbed directly at the silica surface is connected through an interstitial, single layer of W molecules with a ca. 0.8 nm-thick diffuse W layer associated with the polar groups at the flexible chain ends. Hydrophilic and hydrophobic elements are largely segregated in the diffuse W layer. Hydrophilic patches are formed by W clusters (groups of hydrogen-bond connected W molecules) and diol clusters (groups of typically 2-4 directly connected diol chains), hydrophobic patches are formed by ACN molecules solvating hydrocarbon groups of the bonded phase. The W-rich layer at the diol-modified surface has lower translational mobility and can hold more excess W than the W-rich layer at an unmodified silica surface.

Single‐Molecule and Ensemble Diffusivities in Individual Nanopores with Spatially Dependent Mobility

“…The linear slope of the resulting distance‐dependent mean‐squared displacement curves analyzed for the period t= 4–16 ps of each productive 20 ps time interval was used to calculate D ∥ by using the Einstein relation [Eq. ]: …”

“…To receive the smooth D k (z)p rofiles in Figure1,m ean-squared displacements hr 2 t ðÞ iof the solvent molecules parallel to the surface were monitored for an observation time of 20 ps repeatedly throughout a4 0nst rajectory.T he linear slopeo ft he resulting distance-dependent mean-squared displacement curves analyzed for the period t = 4-16 ps of each productive 20 ps time interval was used to calculate D k by using the Einstein relation[Eq. (1)]: [40] D jj z ðÞ¼ 1 4…”

“…The residence time of a solute in each of the three regions, R =I, II, and III, was calculated according to an established approach . For a group of N 0 molecules of a given species, located in a certain region at an initial time t 0 , a variable was introduced, the value of which depends on the location of molecule i at time ( t 0+ t ): it is 1, if molecule i remains in the particular region R ; and it is 0, if molecule i leaves its initial location.…”

“…In recent years, MD simulations have become ap owerful tool in the analysis of diffusion processes for both solventa nd solute molecules in confined systems. [33][34][35][36][37][38][39][40] For example, the RPLC characteristics were elucidated from MD simulations [39,[41][42][43][44][45] and Monte Carlo computations, particularly by Siepmannand co-workers. [46] Using MD simulations,single-molecule and ensembled iffusivities can be directly recorded in the same nanopore used as am odel geometry with full flexibility on the chemical-surface modification, pore size, and choice of the solute molecules.…”

“…(This phenomenons hould not be confused with the microheterogeneity that is also observedi n water/ACN( W/ACN) mixtures in the absence of any confinement due to the amphiphilic character of acetonitrile. [53] )A ll these effects can be analyzed in detail by using MD simulations, [38][39][40][49][50][51][52] and the solute molecules can be accurately traced individually through all mobility regions. This allows ac omplete pore-level molecular exchange to be achieved and, thus, allows the long-time behavioro fD pore to be monitored.…”