Electrohydrodynamics in hierarchically structured monolithic and particulate fixed beds

Organic-solvent ditch overlap in reversed-phase liquid chromatography: A molecular dynamics simulation study in cylindrical 6-12 nm-diameter pores

Mass transport through the mesopore space of a reversed-phase liquid chromatography (RPLC) column depends on the properties of the chromatographic interface, particularly on the extent of the organic-solvent ditch that favors the analyte surface diffusivity. Through molecular dynamics simulations in cylindrical RPLC mesopore models with pore diameters between 6 and 12 nm we systematically trace the evolution of organic-solvent ditch overlap due to spatial confinement in the mesopore space of RPLC columns for small-molecule separations. Each pore model of a silica-based, endcapped, C18-stationary phase is equilibrated with two mobile phases of comparable elution strength, namely 70/30 (v/v) water/acetonitrile and 60/40 (v/v) water/methanol, to consider the influence of the mobile-phase composition on the onset of organic-solvent ditch overlap. The simulations show that, as the pore diameter decreases from 9 to 6 nm, the bonded-phase density extends and compacts towards the pore center, which leads to increased accumulation of organic-solvent excess and thus enhanced organic-solvent diffusivity in the ditch. Because the acetonitrile ditch is more pronounced than the methanol ditch, acetonitrile ditch overlap sets in at less severe spatial confinement than methanol ditch overlap. The pore-averaged methanol and acetonitrile diffusivities are considerably raised by ditch overlap in the 6 nm-diameter pore, but also benefit from the ditch (without overlap) in the 7 to 12 nm-diameter pores, whereby local and pore-averaged effects are generally larger for acetonitrile than methanol.

Scanning Ion Conductance Microscopy of Nafion-Modified Nanopores

Single nanopores in silicon nitride membranes are asymmetrically modified with Nafion and investigated with scanning ion conductance microscopy, where Nafion alters local ion concentrations at the nanopore. Effects of applied transmembrane potentials on local ion concentrations are examined, with the Nafion film providing a reservoir of cations in close proximity to the nanopore. Fluidic diodes based on ion concentration polarization are observed in the current-voltage response of the nanopore and in approach curves of SICM nanopipette in the vicinity of the nanopore. Experimental results are supported with finite element method simulations that detail ion depletion and enrichment of the nanopore/Nafion/nanopipette environment.

Confinement Effects on Distribution and Transport of Neutral Solutes in a Small Hydrophobic Nanopore

Molecular dynamics simulations are used to study confinement effects in small cylindrical silica pores with extended hydrophobic surface functionalization as realized, for example, in reversed-phase liquid chromatography (RPLC) columns. In particular, we use a 6 nm cylindrical and a 10 nm slit pore bearing the same C₁₈ stationary phase to compare the conditions inside the smaller-than-average pores within an RPLC column to column-averaged properties. Two small, neutral, apolar to moderately polar solutes are used to assess the consequences of spatial confinement for typical RPLC analytes with water (W)-acetonitrile (ACN) mobile phases at W/ACN ratios between 70/30 and 10/90 (v/v). The simulated data show that true bulk liquid behavior, as observed over an extended center region in the 10 nm slit pore, is not recovered within the 6 nm cylindrical pore. Instead, the ACN-enriched solvent layer around the C₁₈ chain ends (the ACN ditch), a general feature of hydrophobic interfaces equilibrated with aqueous-organic liquids, extends over the entire pore lumen of the small cylindrical pore. This renders the entire pore a highly hydrophobic environment, where, contrary to column-averaged behavior, neither the local nor the pore-averaged sorption and diffusion of analytes scales directly with the W/ACN ratio of the mobile phase. Additionally, the solute polarity-related discrimination between analytes is enhanced. The consequences of local ACN ditch overlap in RPLC columns are reminiscent of ion transport in porous media with charged surfaces, where electrical double-layer overlap occurring locally in smaller pores leads to discrimination between co- and counterionic species.

Characterization and comparison of mixed-mode and reversed-phase columns; interaction abilities and applicability for peptide separation

In this work, two mixed-mode columns from a different manufacturers and one marketed as a reversed-phase column were characterized and compared in the terms of their interaction abilities, retentivity, peak symmetry, and applicability for peptide separation. All the tested columns contain octadecyl ligand and positively charged modifier, i.e. pyridyl group for the reversed-phase column XSelect CSH C18, quaternary alkylamine for mixed-mode column Atlantis PREMIER BEH C18 AX, and permanently charged moiety (details not available from the manufacturer) for mixed-mode column Luna Omega PS C18. For detailed characterization and comparison of their interaction potential, several approaches were used. First, a simple Walters test was performed to estimate hydrophobic and silanophilic interactions of the tested columns. The highest values of both parameters were observed for column Atlantis PREMIER BEH C18 AX. To investigate the effect of pH and buffer concentration on retention, mobile phases composed of acetonitrile and buffer (ammonium formate, pH 3.0; ammonium acetate pH 4.7 and pH 6.9) in various concentrations (5mM; 10mM; 15mM and 20mM) were used. The analysis of permanently charged compounds was used to describe the electrostatic interaction abilities of the stationary phases. The most significant contribution of electrostatic interactions to the retention was observed for Atlantis PREMIER BEH C18 AX column in the mobile phase with buffer of pH 3.0. A set of ten dipeptides, three pentapeptides and one octapeptide was used to investigate the effects of pH and buffer concentration on retention and peak symmetry. Each of the tested columns provides the optimal peak shape under different buffer pH and concentration. The gradient separation of the 14 tested peptides was used to verify the application potential of the tested columns for peptide separation. The best separation was achieved within 4 minutes on column Atlantis PREMIER BEH C18 AX.

A numerical study on ion concentration polarization and electric circuit performance of an electrokinetic battery

Ion concentration polarization (ICP) imposes remarkable adverse effects on the energy conversion performance of the pressure-driven electrokinetic (EK) flows through a capillary system that can be equivalently treated as a battery. An optimized dimensionless numerical method is proposed in this study to investigate the causes and the effects of the ICP. Results show that remarkable ICP phenomena are induced under certain conditions such as high applied pressure, high surface charge density, and small inversed Debye length at dimensionless values of 6000, -10, and 0.5. Meanwhile, different factors influence the ICP and the corresponding electric properties in different ways. Particularly for the overall electric resistance, the applied pressure and the surface charge density mainly affect the variation amplitude and the level of the overall electric resistance when varying the output electric potential, respectively. Differently, the Debye length affects the overall electric resistance in both aspects. Ultimately, the induced ICP leads to significant nonlinear current-potential curves.

Electrophoresis of a pH-regulated zwitterionic nanoparticle in a pH-regulated zwitterionic capillary

We consider the electrophoresis of a rigid sphere along the axis of a narrow cylindrical capillary; both are pH-regulated and zwitterionic. This extends available analyses in the literature to a more general and realistic case. Adopting a titanium oxide (TiO2) particle in a silicon dioxide (SiO2) capillary as an example, we examine the capillary radius, the solution pH, and the electrolyte concentration (or double-layer thickness) for their influences on the electrophoretic behavior of a particle. Because the pH solution is adjusted by HCl and NaOH, the presence of four kinds of ionic species, namely, H(+), OH(-), Na(+), and Cl(-), should be considered if NaCl is the background electrolyte. This also extends conventional electrophoresis analyses to the case of multiple ionic species. The interactions of the electroosmotic flow, the properties of the particle and the solution, and the capillary wall yield complicated electrophoretic behavior that can be regulated by the solution pH and the background electrolyte concentration. The results gathered are necessary for the future design of nanopore-based electrophoresis devices.

Importance of electroosmotic flow and multiple ionic species on the electrophoresis of a rigid sphere in a charge-regulated zwitterionic cylindrical pore

The influence of electroosmotic flow (EOF) on the electrophoretic behavior of a particle is investigated by considering a rigid sphere in a charge-regulated, zwitterionic cylindrical pore filled with an aqueous solution containing multiple ionic species. This extends conventional analyses to a more general and realistic case. Taking a pore with pK(a) = 7 and pK(b) = 2 (point of zero charge is pH = 2.5) filled with an aqueous NaCl solution as an example, several interesting results are observed. For instance, if pH < 5.5, the particle mobility is influenced mainly by boundary effect, and is influenced by both EOF and boundary effects if pH ≥ 5.5. If pH is sufficiently high, the particle behavior is dominated by EOF, which might alter the direction of electrophoresis. The ratio of (pore radius/particle radius) influences not only the boundary effect, but also the strength of EOF. If the boundary effect is insignificant, the mobility varies roughly linearly with log(bulk salt concentration). These findings are of practical significance to both the interpretation of experimental data and the design of electrophoresis devices.

Concentration polarization of interface and non-linear electrokinetic phenomena

The review addresses the peculiarities of concentration polarization caused by an electric current passing through conducting and around nonconducting charged materials. The conditions of emergence of an induced space charge of large density and thickness behind an electrical double layer, leading to strong non-linearity of electroosmosis and electrophoresis, are analyzed. Basic findings about concentration polarization, its theoretical modeling and experimental investigations, as well as its influence on electrokinetic phenomena and mass transfer through ion-exchange materials are discussed from the point of view of the fundamental knowledge about polarization processes and from the perspective of their practical application. The analysis focuses on the main properties of concentration polarization, electroosmotic flow of liquid around single fixed particles and through the system of particles, and electrophoresis of particles suspended in aqueous medium and current through flat, spherical and cylindrical interfaces and membranes with heterogeneous conductivity. The paper also presents the general ideas of concentration polarization and non-linear electrokinetic phenomena in case of nonconducting particles and their dependence on particle surface electroconductivity. Existing theoretical models describing polarization of nonconducting particles at high and low Peclet numbers are analyzed, with appropriate experimental data being provided to validate the theory. A joint analysis of polarization of conducting and nonconducting particles completes the review.

The use of scanning contactless conductivity detection for the characterisation of stationary phases in micro-fluidic chips

The use of scanning capacitively coupled contactless conductivity detection for the evaluation of the structural homogeneity and density of both packed and monolithic stationary phases in micro-fluidic chips is presented here for the first time.

Inherent peak compression of charged analytes in electrochromatography

This work resolves peak compression of charged analytes in CEC with strong cation-exchange stationary phase particles. By combining electrochromatographic peak shape analysis with the results of numerical simulations and confocal laser scanning microscopy in the packed capillaries, we identify electrical field-induced concentration polarization as the key physical phenomenon responsible for the inherent existence of local electrical field gradients on the scale of an individual support particle. Consequently, positive and negative field gradients exist between and inside the particles along the whole packing. Their intensity depends on the particles cation-selectivity (governed by the particles volume charge density and the mobile phase ionic strength) and the applied field strength. The interplay of these local field gradients with the analytes retention (intraparticle adsorption) determines whether fronting, tailing, or spiked analyte peaks are observed, and it provides a mechanism by which strongly retained analytes can be eluted over long distances with little zone dispersion. Our analysis explains the "anomalous" peak compression effects with strong cation-exchange particles, which have been reported more than a decade ago (Smith, N. W., Evans, M. B., Chromatographia 1995, 41, 197-203) and since then remained largely unresolved.

CEC: selected developments that caught my eye since the year 2000

During the last decade, a number of new developments have emerged in the field of CEC. This paper focuses only on monolithic columns prepared from synthetic polymers. Monolithic columns have become a well-established format of stationary phases for CEC immediately after their inception in the mid-1990s. They are readily prepared in situ from liquid precursors. Also, the control over both porous properties and surface chemistries is easy to achieve. These advantages make the monolithic separation media an attractive alternative to capillary columns packed with particulate materials. Since the number of papers concerned with just this single topic of polymer-based monolithic CEC columns is large, this overview describes only those approaches this author found interesting.

Ionic current rectification at a nanofluidic/microfluidic interface with an asymmetric microfluidic system

A nanofluidic-microfluidic interface is reported that rectifies ionic current using uncoated symmetric nanocapillaries. Previously, ionic current rectification has been achieved by other groups with nanochannels with differential coatings and in nanopores that are conical in shape. This simple device uses nanocapillary membranes (NCMs) with uncoated symmetric channels to connect a microfluidic channel and a larger solution reservoir. The conductivity of the solution in the microchannel appears to be critical in the formation of the low "off" state current and the high "on" state current. It is hypothesized that the "off" state current is low due to the formation of an ion depletion zone in the microchannel while the higher "on" state currents are produced by a zone of enhanced ionic concentration in the microchannel.

Ionic conductance of nanopores in microscale analysis systems: where microfluidics meets nanofluidics

In this tutorial review we illustrate the origin and dependence on various system parameters of the ionic conductance that exists in discrete nanochannels as well as in nanoporous separation and preconcentration units contained as hybrid configurations, membranes, packed beds, or monoliths in microscale liquid phase analysis systems. A particular complexity arises as external electrical fields are superimposed on internal chemical and electrical potential gradients for tailoring molecular transport. It is demonstrated that the variety of geometries in which the microfluidic/nanofluidic interfaces are realized share common, fundamental features of coupled mass and charge transport, but that phenomena behind the key steps in a particular application can be significantly tuned, depending on the morphology of a material. Thus, the understanding of morphology-related transport in internal and external electrical potential gradients is critical to the performance of a device. This addresses a variety of geometries (slits, channels, filters, membranes, random or regular networks of pores, etc.) and applications, e. g., the gating, sensing, preconcentration, and separation in multifunctional miniaturized devices. Inherently coupled mass and charge transport through ion-permselective (charge-selective) microfluidic/nanofluidic interfaces is analyzed with a stepwise-added complexity and discussed with respect to the morphology of the charge-selective spatial domains. Within this scenario, the electrostatics and electrokinetics in microfluidic and nanofluidic channels, as well as the electrohydrodynamics evolving at microfluidic/nanofluidic interfaces, where microfluidics meets nanofluidics, define the platform of central phenomena.

Concentration polarization and nonequilibrium electroosmotic slip in dense multiparticle systems

Electrical field-induced concentration polarization (CP) and CP-based nonequilibrium electroosmotic slip are studied in fixed beds of strong cation-exchange particles using confocal laser scanning microscopy (CLSM) and the macroscopic electroosmotic flow (EOF) dynamics. A key property of the investigated fixed beds is the coexistence of quasi-electroneutral macroporous regions between the micrometer-sized particles and the ion-permselective (here, cation-selective) intraparticle mesopores with a mean size of 10 nm. The application of an external electrical field to the particles induces depleted and enriched CP zones along their anodic and cathodic interfaces, respectively, by the local interplay of diffusive and electrokinetic transport. The intensity and dimension of the CP zones depend on the applied electrical field strength and the fluid-phase ionic strength. With increasing field strength a limiting current density through a particle is approached, meaning that charge transport locally through a particle becomes controlled by the dynamics in the adjoining extraparticle convective-diffusion boundary layer (depleted CP zone). In this regime a nonequilibrium electrical double layer can be induced electrokinetically in the depleted CP zone and intraparticle pore space, resulting in nonlinear EOF in the interparticle macropore space. The local CP dynamics analyzed by CLSM is successfully correlated with the onset of nonlinearity in the macroscopic EOF dynamics. We further demonstrate that multiparticle effects arising in fixed beds (random close packings) of ion-permselective particles modulate significantly the local pattern of CP and intensity of the nonequilibrium electroosmotic slip with respect to the undisturbed single-particle picture.

Influence of the hydrothermal treatment on the chromatographic properties of monolithic silica capillaries for nano-liquid chromatography or capillary electrochromatography

In the last decade, silica monolithic capillaries have focused more and more attention on miniaturized separation techniques like capillary electrochromatography (CEC), nano-liquid chromatography (nano-LC) and chip electrochromatography owing to their unique chromatographic properties and their simplified preparation compared with packed columns. They are synthesized according to a sol-gel multi-step process that includes, after a gelation step at 40 degrees C leading to the formation of the macropores network and the silica skeleton, a post-gelation step (hydrothermal treatment at 120 degrees C in basic medium) that allows to tailor the mesopores and finally a calcination or a washing step to remove remaining polymers. In order to reduce the synthesis time, the number of synthesis steps and above all the temperature synthesis, to adapt the synthesis of such silica monoliths in polymeric microsystem devices, we extensively studied the influence of the hydrothermal treatment and its duration on textural (pore size distribution) and chromatographic properties (retention, efficiency) of in situ-synthesized capillary monoliths in nano-LC and CEC. This study was performed on pure silica and octyl chains grafted silica monoliths. Untreated monoliths show small pores (<6 nm), whereas hydrothermally treated monoliths exhibit medium and large mesopores (8-17 nm). It was demonstrated that the hydrothermal treatment at 120 degrees C was not necessary for pure silica monolithic capillaries dedicated to normal phase liquid chromatography or hydrophilic interaction liquid chromatography (HILIC) and electrochromatography: the suppression of the hydrothermal treatment did not impair efficiencies in CEC and in nano-LC but contributed to increase in retention factors. Minimal plate heights of ca. 5 microm in CEC and 6 microm in nano-LC were obtained with or without hydrothermal treatment with bare silica. In the same way, the hydrothermal treatment was not necessary for grafted silica monoliths only dedicated to CEC. However, the results clearly indicate that the hydrothermal treatment becomes essential before grafting in order to preserve the efficiency of the monolithic silica capillaries dedicated to nano-LC: in this particular case, the suppression of the hydrothermal treatment leads approximately to a loss of a factor two in efficiency.

Investigation of Anion Retention and Cation Exclusion Effects for Several C18 Stationary Phases

When mobile-phase salt content is increased, cationic analytes often show increased retention. This effect is generally attributed to chaotropic or ion pairing effects. However, a cation exclusion mechanism could explain the same effects. In this study, experimental conditions were manipulated to enhance cation exclusion effects and reduce chaotropic/ion pairing effects by using (1) low ionic strength mobile phases to reduce electrostatic screening, (2) a buffer anion (dihydrogen phosphate) that exhibits minimal chaotropic/ion pairing effects, and (3) columns that show evidence of a weak positive charge. Urea was used as neutral void marker and glycinamide (in protonated form) as cationic void marker. It was assumed the difference in retention volumes between void markers would reflect an "excluded volume", inaccessible to cationic analytes. As ionic strength was lowered, it appeared as much as 80% of the pore volume became inaccessible to the glycinamide cation at the lowest ionic strength tested (1.4 mM). Three model cationic analytes showed retention loss approximately proportional to the excluded volume as ionic strength was decreased. This suggests that, under certain conditions, cation exclusion may become the dominant mechanism in mediating the retention of cationic analytes as the mobile-phase salt content is varied.

Fluid dynamics in capillary and chip electrochromatography

This review is concerned with the phenomenological fluid dynamics in capillary and chip electrochromatography (EC) using high-surface-area random porous media as stationary phases. Specifically, the pore space morphology of packed beds and monoliths is analyzed with respect to the nonuniformity of local and macroscopic EOF, as well as the achievable separation efficiency. It is first pointed out that the pore-level velocity profile of EOF through packed beds and monoliths is generally nonuniform. This contrasts with the plug-like EOF profile in a single homogeneous channel and is caused by a nonuniform distribution of the local electrical field strength in porous media due to the continuously converging and diverging pores. Wall effects of geometrical and electrokinetic nature form another origin for EOF nonuniformities in packed beds which are caused by packing hard particles against a hard wall with different zeta potential. The influence of the resulting, systematic porosity fluctuations close to the confining wall over a distance of a few particle diameters becomes aggravated at low column-to-particle diameter ratio. Due to the hierarchical structure of the pore space in packed beds and silica-based monoliths which are characterized by discrete intraparticle (intraskeleton) mesoporous and interparticle (interskeleton) macroporous spatial domains, charge-selective transport prevails within the porous particles and the monolith skeleton under most general conditions. It forms the basis for electrical field-induced concentration polarization (CP). Simultaneously, a finite and -- depending on morphology -- often significant perfusive EOF is realized in these hierarchically structured materials. The data collected in this review show that the existence of CP and its relative intensity compared to perfusive EOF form fundamental ingredients which tune the fluid dynamics in EC employing monoliths and packed beds as stationary phases. This addresses the (electro)hydrodynamics, associated hydrodynamic dispersion, as well as the migration and retention of charged analytes.

Considerations on influence of charge distribution on determination of biomolecules and microorganisms and tailoring the monolithic (continuous bed) materials for bioseparations

The importance of continuous beds (monoliths) as separation materials is connected with their better chromatographic properties and easier preparation in comparison to particulate-packed columns. Moreover the tuning of porosity as well as surface chemistry can lead to obtaining of highly selective materials, especially useful in separation of biologically important compounds or even microorganisms. To obtain high selectivity for such analytes as e.g. proteins, it is often important to have a knowledge about their shape, size, charge and finally charge distribution. This article presents our considerations on the charge distribution on the monolithic stationary phase and surface of such species as proteins or microorganisms as well as its eventual influence on the separation or sample preparation processes and tuning of their selectivity.

Thin film electro-osmotic pumps for biomicrofluidic applications

Electro-osmotic flow (EOF) pumps are attractive for fluid manipulation in microfluidic channels. Open channel EOF pumps can produce high pressures and flow rates, and are relatively easy to fabricate on-chip or integrate with other microfluidic or electrical components. An EOF pump design that is conducive to on-chip fabrication consists of multiple small channel arms feeding into a larger flow channel. We have fabricated this type of pump design using a thin film deposition process that avoids wafer bonding. We have evaluated pumps fabricated on both silicon and glass substrates. Consistent flow rate versus electric field were obtained. For the range of 40-400 V, flow rates of 0.19-2.30 muLmin were measured. Theoretical calculations of pump efficiency were made, as well as calculations of the mechanical power generated by various pump shapes, to investigate design parameters that should improve future pumps.

Pore-Scale Dispersion in Electrokinetic Flow through a Random Sphere Packing

The three-dimensional velocity field and corresponding hydrodynamic dispersion in electrokinetic flow through a random bulk packing of impermeable, nonconducting spheres are studied by quantitative numerical analysis. First, a fixed bed with interparticle porosity of 0.38 is generated using a parallel collective-rearrangement algorithm. Then, the interparticle velocity field is calculated using the lattice-Boltzmann (LB) method, and a random-walk particle-tracking method is finally employed to model advection-diffusion of an inert tracer in the LB velocity field. We demonstrate that the pore-scale velocity profile for electroosmotic flow (EOF) is nonuniform even under most ideal conditions, including a negligible thickness of the electrical double layer compared to the mean pore size, a uniform distribution of the electrokinetic potential at the solid-liquid interface, and the absence of applied pressure gradients. This EOF dynamics is caused by a nonuniform distribution of the local electrical field strength in the sphere packing and engenders significant hydrodynamic dispersion compared to pluglike EOF through a single straight channel. Both transient and asymptotic dispersion behaviors are analyzed for EOF in the context of packing microstructure and are compared to pressure-driven flow in dependence of the average velocity through the bed. A better hydrodynamic performance of EOF originates in a still much smaller amplitude of velocity fluctuations on a mesoscopic scale (covering several particle diameters), as well as on the microscopic scale of an individual pore.

Key to Analyte Migration and Retention in Electrochromatography

This work identifies electrical field-induced concentration polarization (CP) as a key physical mechanism influencing the retention behavior of charged analytes in electrochromatography with fixed beds of porous adsorbent particles. Due to an insufficient screening of intraparticle surface charge, under most general conditions the porous (permeable) particles become charge-selective. CP is caused by coupled mass and charge transport normal to the charge-selective external surface of the permeable particles, which leads to concentration gradients of ionic species in the adjoining interparticle electrolyte solution. Cation-exchange (cation-selective) particles were employed to investigate the influence of applied voltage on the retention factor of counterionic, i.e., positively charged, analytes. It is demonstrated by macroscopic retention data and microscopic studies resolving the CP phenomenon on a particle scale that the dependence of CP on electrical field and mobile-phase ionic strengths is directly reflected in concomitant changes of analyte retention. The CP zones that develop at the interface between interparticle and intraparticle pore space are recognized by charged, but not electroneutral analytes while entering or leaving the particles. The intensity of these convective-diffusion boundary layers (CP zones) depends on the applied field strength and charge selectivity of a particle. Thus, it is the charge-selective transport between the interparticle and intraparticle pore space in packed beds that prevails under typical experimental conditions in electrochromatography and that forms the physical basis for a general electrical field dependence of the retention factor of charged analytes.