Time and Length Scales of Eddy Dispersion in Chromatographic Beds

Time and length scales as well as the magnitude of individual contributions to eddy dispersion in chromatographic beds are resolved. We address this issue by a high-resolution numerical analysis of flow and mass transport in computer-generated bulk (unconfined) packings of monosized, nonporous, incompressible, spherical particles and complementary confined cylindrical packings with a cylinder-to-particle diameter ratio of dc/dp = 20. The transient behavior of longitudinal and transverse dispersion is analyzed and correlated with the spatial scales of heterogeneity in the bulk and confined packings. Simulations were carried out until complete transcolumn equilibration in the confined packings was achieved to facilitate a quantitative study of the geometrical wall effect. Longitudinal plate height data calculated over a wide range of reduced velocities (0.1 ≤ ν ≤ 500) were fitted to the comprehensive Giddings equation. The determined transition velocities for individual contributions to eddy dispersion were found to be widely disparate. As a consequence, the total effect of eddy dispersion on the plate height curves can be approximated in the practical range of chromatographic operational velocities (5 ≤ ν ≤ 20) by a composite expression in which only the short-range interchannel contribution retains its coupling characteristics, while transchannel and transcolumn contributions appear as simple mass transfer velocity-proportional terms. Copyright © 2009 American Chemical Society. [accessed November 25, 2009

Composition, Structure, and Mobility of Water−Acetonitrile Mixtures in a Silica Nanopore Studied by Molecular Dynamics Simulations

To investigate the effect of the nanoscale confinement on the properties of a binary aqueous−organic solvent mixture, we performed molecular dynamics simulations of the equilibration of water−acetonitrile (W/ACN) mixtures between a cylindrical silica pore of 3 nm diameter and two bulk reservoirs. Water is enriched, and acetonitrile is depleted inside the pore with respect to the bulk reservoirs: for nominal molar (~volumetric) ratios of 1/3 (10/90), 1/1 (25/75), and 3/1 (50/50), the molar W/ACN ratio in the pore equilibrates to 1.5, 3.2, and 7.0. Thus, the relative accumulation of water in the pore increases with decreasing water fraction in the nominal solvent composition. The pore exhibits local as well as average solvent compositions, structural features, and diffusive mobilities that differ decidedly from the bulk. Water molecules form hydrogen bonds with the hydrophilic silica surface, resulting in a 0.45 nm thick interfacial layer, where solvent density, coordination, and orientation are independent of the nominal W/ACN ratio and the diffusive mobility goes toward zero. Our data suggest that solute transport along and across the nanopore, from the inner volume to the interfacial water layer and the potential adsorption sites at the silica surface, will be substantially different from transport in the bulk. Copyright © 2011 American Chemical Society [accessed April 15th 2011

Concentration polarization and nonequilibrium electroosmotic slip in hierarchical monolithic structures

This article illustrates the appearance and electrohydrodynamic consequences of concentration polarization (CP) in hierarchically structured monolithic fixed beds used as stationary phases in CEC and related electrical-field-assisted separation techniques. Subject of the investigation are silica-based monoliths in capillary format with a bimodal pore size distribution. Ion-permselectivity in the intraskeleton pore space together with diffusive and electrokinetic transport induces depleted and enriched CP zones at the anodic and cathodic interfaces, respectively, of the cation-selective mesoporous skeleton. The extent of electrical-field-induced CP is shown to be governed by the fluid phase ionic strength, which tunes the ion-permselectivity of the mesoporous monolith skeleton via local electrical double layer overlap, and by the applied electrical field strength, which determines local transport. The analysis of quantitative confocal laser scanning microscopy data, resolving CP on the local skeleton scale, indicates that at sufficiently high field strength a transition from intraskeleton to interskeleton boundary-layer-dominated transport of charged species occurs. This transition is correlated to the onset of macroscopically measured, nonlinear EOF velocities, whose occurrence is explained in the framework of a nonequilibrium electroosmotic slip. It is shown that the onset of nonlinear electrokinetics in the system can be tuned by properties of the BGE, particularly buffer pH, which modulates the pH-dependent surface charge density and consequently the ion-permselective skeleton's charge selectivity. Finally, the CP dynamics of monolithic and particulate fixed beds are compared, and the observed differences are related to the specific morphologies of the two hierarchical fixed bed structures. Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [accessed June 6, 2008