Modeling electrokinetics in ionic liquids

Using direct numerical simulations, we provide a thorough study regarding the electrokinetics of ionic liquids. In particular, modified Poisson–Nernst–Planck equations are solved to capture the crowding and overscreening effects characteristic of an ionic liquid. For modeling electrokinetic flows in an ionic liquid, the modified Poisson‐Nernst‐Planck equations are coupled with Navier–Stokes equations to study the coupling of ion transport, hydrodynamics, and electrostatic forces. Specifically, we consider the ion transport between two parallel charged surfaces, charging dynamics in a nanopore, capacitance of electric double‐layer capacitors, electroosmotic flow in a nanochannel, electroconvective instability on a plane ion‐selective surface, and electroconvective flow on a curved ion‐selective surface. We also discuss how crowding and overscreening and their interplay affect the electrokinetic behaviors of ionic liquids in these application problems.     

Electrophoretic mobility measurements of fluorescent dyes using on-chip capillary electrophoresis

We present an experimental study of the effect of pH, ionic strength, and concentrations of the electroosmotic flow (EOF)-suppressing polymer polyvinylpyrrolidone (PVP) on the electrophoretic mobilities of commonly used fluorescent dyes (fluorescein, Rhodamine 6G, and Alexa Fluor 488). We performed on-chip capillary zone electrophoresis experiments to directly quantify the effective electrophoretic mobility. We use Rhodamine B as a fluorescent neutral marker (to quantify EOF) and CCD detection. We also report relevant acid dissociation constants and analyte diffusivities based on our absolute estimate (as per Nernst-Einstein diffusion). We perform well-controlled experiments in a pH range of 3-11 and ionic strengths ranging from 30 to 90 mM. We account for the influence of ionic strength on the electrophoretic transport of sample analytes through the Onsager and Fuoss theory extended for finite radii ions to obtain the absolute mobility of the fluorophores. Lastly, we briefly explore the effect of PVP on adsorption-desorption dynamics of all three analytes, with particular attention to cationic R6G.     

Modeling of electroosmotic and electrophoretic mobilization in capillary and microchip isoelectric focusing

Our dynamic capillary electrophoresis model which uses material specific input data for estimation of electroosmosis was applied to investigate fundamental aspects of isoelectric focusing (IEF) in capillaries or microchannels made from bare fused-silica (FS), FS coated with a sulfonated polymer, polymethylmethacrylate (PMMA) and poly(dimethylsiloxane) (PDMS). Input data were generated via determination of the electroosmotic flow (EOF) using buffers with varying pH and ionic strength. Two models are distinguished, one that neglects changes of ionic strength and one that includes the dependence between electroosmotic mobility and ionic strength. For each configuration, the models provide insight into the magnitude and dynamics of electroosmosis. The contribution of each electrophoretic zone to the net EOF is thereby visualized and the amount of EOF required for the detection of the zone structures at a particular location along the capillary, including at its end for MS detection, is predicted. For bare FS, PDMS and PMMA, simulations reveal that EOF is decreasing with time and that the entire IEF process is characterized by the asymptotic formation of a stationary steady-state zone configuration in which electrophoretic transport and electroosmotic zone displacement are opposite and of equal magnitude. The location of immobilization of the boundary between anolyte and most acidic carrier ampholyte is dependent on EOF, i.e. capillary material and anolyte. Overall time intervals for reaching this state in microchannels produced by PDMS and PMMA are predicted to be similar and about twice as long compared to uncoated FS. Additional mobilization for the detection of the entire pH gradient at the capillary end is required. Using concomitant electrophoretic mobilization with an acid as coanion in the catholyte is shown to provide sufficient additional cathodic transport for that purpose. FS capillaries dynamically double coated with polybrene and poly(vinylsulfonate) are predicted to provide sufficient electroosmotic pumping for detection of the entire IEF gradient at the cathodic column end.     

An improved hybrid continuum‐atomistic four‐way coupled model for electrokinetics in nanofluidics

In this study, an efficient hybrid continuum‐atomistic method is proposed to study electrokinetic transport of aqueous solutions in nanofluidics. The aqueous phase is considered as a continuous phase containing immersed ion particles. The behavior of the system is then simulated through utilization of an improved hybrid continuum‐atomistic four‐way coupled approach, including the MultiPhase Particle‐In‐Cell method for the short‐ranged interaction between the ion particles, the Brownian force for the collision between the aqueous phase molecules and the ion particles, and a wall force accounting for the short‐ranged interaction of ions and walls. The validation of the proposed model with the results of Molecular Dynamics simulations suggests that this model can be a promising approach for studying the electrokinetic phenomena in more complicated geometries where the Molecular Dynamics approach is computationally prohibitive. Finally, the effects of electrokinetic parameters, such as the height of the channel, the external electric field, and bulk ionic concentration, on the electroosmotic flow in a nanochannel are investigated and discussed.     

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.     

Comparison of the Nernst-Planck model and the Poisson-Boltzmann model for electroosmotic flows in microchannels

In the analysis of electroosmotic flows, the internal electric potential is usually modeled by the Poisson-Boltzmann equation. The Poisson-Boltzmann equation is derived from the assumption of thermodynamic equilibrium where the ionic distributions are not affected by fluid flows. Although this is a reasonable assumption for steady electroosmotic flows through straight microchannels, there are some important cases where convective transport of ions has nontrivial effects. In these cases, it is necessary to adopt the Nernst-Planck equation instead of the Poisson-Boltzmann equation to model the internal electric field. In the present work, the predictions of the Nernst-Planck equation are compared with those of the Poisson-Boltzmann equation for electroosmotic flows in various microchannels where the convective transport of ions is not negligible.     

Porous glass electroosmotic pumps: theory

This paper presents an analytical study of electroosmotic (EO) pumps with porous pumping structures. We have developed an analytical model to solve for electroosmotic flow rate, pump current, and thermodynamic efficiency as a function of pump pressure load for porous-structure EO pumps. The model uses a symmetric electrolyte approximation valid for the high-zeta-potential regime and numerically solves the Poisson-Boltzmann equation for charge distribution in the idealized pore geometry. Generalized scaling of pumping performance is discussed in the context of a parameterization that includes porosity, tortuosity, pore size, bulk ionic density, and the nonuniform conductivity distribution over charge layers. The model also incorporates an approximate ionic-strength-dependent zeta potential formulation.     

毛细管电泳分离I^—和IO3^—中的电渗流的控制与调节

毛细管电泳分离是基于带电粒子在电场下的电动现象，当电场加到带电粒子所处的介质上时，存在电泳和电渗两种电动现象，其中电泳与带电粒子的大小、电荷和形状有关，而电渗与毛细管的表面及电泳介质的组成有关．由于电渗流能显著地影响分析时间和分离效率，因此，控制电渗流在毛细管电泳分离中是一个重要的环节［‘·‘j．此外，调节PH值、离子强度和缓冲液的粘度也是常用的控制电渗流的方法［’j．Ewing［‘与1入一个附加电场来控制电渗流．在缓冲液中加入足够浓度的阳离子表面活性剂将改变电渗流的方向．与通常的电渗流相比，人们对反向…     

Co‐electroosmotic capillary electrophoresis of basic proteins with 1‐alkyl‐3‐methylimidazolium tetrafluoroborate ionic liquids as non‐covalent coating agents of the fused‐silica capillary and additives of the electrolyte solution

The paper reports the results of a study carried out to evaluate the use of three 1‐alkyl‐3‐methylimidazolium‐based ionic liquids as non‐covalent coating agents for bare fused‐silica capillaries and additives of the electrolyte solutions (BGE) for CE of basic proteins in the co‐EOF separation mode. The three ionic liquids are differentiated from each other by the length of the alkyl group on the imidazolium cation, consisting of either an ethyl, butyl or octyl substituent, whereas tetrafluoroborate is the common anionic component of the ionic liquids. Coating the capillary with the ionic liquid resulted in improved peak shape and protein separation, while the EOF was maintained cathodic. This indicates that each ionic liquid is effective at masking the protein interaction sites on the inner surface of the capillary, also when its adsorption onto the capillary wall has not completely neutralized all the negative charges arising from the ionization of the silanol groups and the ionic liquid is not incorporated into the BGE employed for separation. Using the coated capillaries with BGE containing the ionic liquid employed for the coating, at concentration low enough to maintaining the EOF cathodic, both peak shape and protein separation varied to different extents, based on the particular ionic liquid used and its concentration. Fast and efficient separation of the model basic protein mixture in co‐electroosmotic CE is obtained with the 1‐butyl‐3‐methylimidazolium tetrafluoroborate coated capillary and 100 mM acetate buffer (pH 4.0) containing 4.4 mM 1‐butyl‐3‐methylimidazolium tetrafluoroborate as the BGE.     

β‐cyclodextrin‐ionic liquid polymer based dynamically coating for simultaneous determination of tetracyclines by capillary electrophoresis

Tetracyclines are a group of broad spectrum antibiotics widely used in animal husbandry to prevent and treat diseases. However, the improper use of tetracyclines may result in the presence of their residues in animal tissues or waste. Recently, great attention has been drawn towards the green solvents ionic liquids. Ionic liquids have been employed as a coating material to modify the electroosmotic flow in capillary electrophoresis. In this study, a functionalized ionic liquid, mono‐6‐deoxy‐6‐(3‐methylimidazolium)‐β‐cyclodextrin tosylate, was synthesized and used for the simultaneous separation and quantification of tetracyclines by capillary electrophoresis. Good separation efficiency could be achieved due to the multiple functions of β‐cyclodextrin derived ionic liquid, including the electrostatic interaction, the hydrogen bonding, and the cavity structure in β‐cyclodextrin ionic liquid which can entrap the tetracyclines to form inclusion complex. After optimization, baseline separation achieved in 25 min with the running buffer consisted of 10 mmol/L, pH 7.2 phosphate buffer and 20 mmol/L β‐cyclodextrin ionic liquid. The satisfied result demonstrated that the β‐cyclodextrin ionic liquid is an ideal background electrolyte modifier in the separation of tetracyclines with high stability and good reproducibility. And it is an effective strategy to design and synthesize specific ILs as additive applied in separation.     

Zeta-potential measurement using the Smoluchowski equation and the slope of the current-time relationship in electroosmotic flow

The zeta -potential of a solid-liquid interface is an important surface characterization quantity for applications ranging from the development of biomedical polymers to the design of microfluidic devices. This study presents a novel experimental technique to measure the zeta -potentials of flat surfaces. This method combines the Smoluchowski equation with the measured slope of current-time relationship in electroosmotic flow. This method is simple and accurate in comparison with the traditional streaming potential and electrophoresis techniques. Using this method the zeta -potentials of glass and poly(dimethylsiloxane) (PDMS) coated surfaces in KCl and LaCl3 aqueous solutions were measured using several flow channels ranging from 200 to 300 microm in height. The zeta -potential was found to vary from -88 to -66 mV for glass surface and -110 to -68 mV for PDMS surfaces depending on the electrolyte and the ionic concentration. The measured values of the zeta -potential are found to be independent of the channel size and the applied driving voltage and generally are repeatable within +/-6%.     

Characterization of the surface charge property and porosity of track-etched polymer membranes

As an important property of porous membranes, the surface charge property determines many ionic behaviors of nanopores, such as ionic conductance and selectivity. Based on the dependence of electric double layers on bulk concentrations, ionic conductance through nanopores at high and low concentrations is governed by the bulk conductance and surface charge density, respectively. Here, through the investigation of ionic conductance inside track-etched single polyethylene terephthalate (PET) nanopores under various concentrations, the surface charge density of PET membranes is extracted as ∼−0.021 C/m² at pH 10 over measurements with 40 PET nanopores. Simulations show that surface roughness can cause underestimation in surface charge density due to the inhibited electroosmotic flow. Then, the averaged pore size and porosity of track-etched multipore PET membranes are characterized by the developed ionic conductance method. Through coupled theoretical predictions in ionic conductance under high and low concentrations, the averaged pore size and porosity of porous membranes can be obtained simultaneously. Our method provides a simple and precise way to characterize the pore size and porosity of multipore membranes, especially for those with sub-100 nm pores and low porosities.     

Magnetic ionic liquid‐assisted synthesis of polypyrrole/AgCl nanocomposites

The synthesis of polypyrrole (PPy)/AgCl nanocomposites with their size ranging around 70–100 nm are achieved by using magnetic ionic liquid as the oxidant in the interface polymerization system. The interface polymerization leads to the formation of uniform and unaggregated nanocomposites with a relatively narrow size distribution confined to submicrometer‐sized domains. The morphology and structure of the nanocomposites are characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X‐ray diffraction (XRD). The potential application of PPy/AgCl nanocomposites as a H₂O₂ biosensor is also reported. Copyright © 2010 John Wiley & Sons, Ltd. Erratum: Magnetic ionic liquid‐assisted synthesis of polypyrrole/AgCl nanocomposites     

ITP in dynamically double-coated fused-silica capillaries

Bidirectional ITP in fused-silica capillaries double-coated with Polybrene and poly-(vinylsulfonate) is a robust approach for analysis of low-molecular-mass compounds. EOF towards the cathode is strong (mobility >4.0 x 10(-8) m(2)/Vs) within the entire pH range investigated (2.40-8.08), dependent on ionic strength and buffer used and, at constant ionic strength, higher at alkaline pH. Electrokinetic separations and transport in such coated capillaries can be described with a dynamic computer model which permits the combined simulation of electrophoresis and electroosmosis in which the EOF is predicted either with a constant (i.e. pH- and ionic strength-independent) or a pH- and ionic strength-dependent electroosmotic mobility. Detector profiles predicted by computer simulation agree qualitatively well with bidirectional isotachopherograms that are monitored with a setup comprising two axial contactless conductivity detectors and a UV absorbance detector. The varying EOF predicted with a pH- and ionic strength-dependent electroosmotic mobility can be regarded as being realistic.     

Liquid transport in rectangular microchannels by electroosmotic pumping

The characteristics of electroosmotic flow in rectangular microchannels were investigated in this paper. A 2D Poisson–Boltzmann equation and the 2D momentum equation were used to model the electric double layer field and the flow field. The numerical solutions show significant influences of the channel cross-section geometry (i.e. the aspect ratio) on the velocity field and the volumetric flow rate. Also, the numerical simulation of the electroosmotic flow reveals how the velocity field and the volumetric flow rate depend on the ionic concentration, zeta potential, channel size and the applied electrical field strength.     

Numerical analysis of the thermal effect on electroosmotic flow and electrokinetic mass transport in microchannels

Joule heating is present in electrokinetically driven flow and mass transport in microfluidic systems. Nowadays, there is a trend of replacing costly glass-based microfluidic systems by the disposable, cheap polymer-based microfluidic systems. Due to poor thermal conductivity of polymer materials, the thermal management of the polymer-based microfluidic systems may become a problem. In this study, numerical analysis is presented for transient temperature development due to Joule heating and its effect on the electroosmotic flow (EOF) and mass species transport in microchannels. The proposed model includes the coupling Poisson-Boltzmann (P-B) equation, the modified Navier-Stokes (N-S) equations, the conjugate energy equation, and the mass species transport equation. The results show that the time development for both the electroosmotic flow field and the Joule heating induced temperature field are less than 1 s. The Joule heating induced temperature field is strongly dependent on channel size, electrolyte concentration, and applied electric field strength. The simulations reveal that the presence of the Joule heating can result in significantly different characteristics of the electroosmotic flow and electrokinetic mass transport in microchannels.     

Electroosmosis in homogeneously charged micro- and nanoscale random porous media

Electroosmosis in homogeneously charged micro- and nanoscale random porous media has been numerically investigated using mesoscopic simulation methods which involve a random generation-growth method for reproducing three-dimensional random microstructures of porous media and a high-efficiency lattice Poisson-Boltzmann algorithm for solving the strongly nonlinear governing equations of electroosmosis in three-dimensional porous media. The numerical modeling and predictions of EOF in micro- and nanoscale random porous media indicate that the electroosmotic permeability increases monotonically with the porosity of porous media and the increasing rate rises with the porosity as well; the electroosmotic permeability increases with the average solid particle size for a given porosity and with the bulk ionic concentration also; the proportionally linear relationship between the electroosmotic permeability and the zeta potential on solid surfaces breaks down for high zeta potentials. The present predictions agree well with the available experimental data while some results deviate from the predictions based on the macroscopic theories.     

Scaling of electrokinetic transport in nanometer channels

Electrokinetic transport is a popular transport mechanism used in nanofluidic systems, and understanding its scaling behavior is important for the design and optimization of nanofluidic devices. In this article, we report on the scaling of electroosmotic flow and ionic conductivity in positively charged slit nanochannels by using continuum and atomistic simulations. The effects of confinement and surface charge are discussed in detail. In particular, we found that the viscosity of the interfacial water increases substantially as the surface charge density increases and the electrophoretic mobility of the interfacial ions decreases. We show that such effects can influence the scaling of the electrokinetic transport in confined nanochannels significantly.     

Nanofluidic Ion Transport and Energy Conversion through Ultrathin Free‐Standing Polymeric Carbon Nitride Membranes

Ions transport through confined space with characteristic dimensions comparable to the Debye length has many applications, for example, in water desalination, dialysis, and energy conversion. However, existing 2D/3D smart porous membranes for ions transport and further applications are fragile, thermolabile, and/or difficult to scale up, limiting their practical applicability. Now, polymeric carbon nitride alternatively allows the creation of an ultrathin free‐standing carbon nitride membrane (UFSCNM), which can be fabricated by simple CVD polymerization and exhibits excellent nanofluidic ion‐transport properties. The surface‐charge‐governed ion transport also endows such UFSCNMs with the function of converting salinity gradients into electric energy. With advantages of low cost, facile fabrication, and the ease of scale up while supporting high ionic currents, UFSCNM can be considered as an alternative for energy conversion systems and new ionic devices.     

Correlating backbone‐to‐backbone distance to ionic conductivity in amorphous polymerized ionic liquids

The morphology and ionic conductivity of poly(1‐n‐alkyl‐3‐vinylimidazolium)‐based homopolymers polymerized from ionic liquids were investigated as a function of the alkyl chain length and counterion type. In general, X‐ray scattering showed three features: (i) backbone‐to‐backbone, (ii) anion‐to‐anion, and (iii) pendant‐to‐pendant characteristic distances. As the alkyl chain length increases, the backbone‐to‐backbone separation increases. As the size of counterion increases, the anion‐to‐anion scattering peak becomes apparent and its correlation length increases. The X‐ray scattering features shift to lower angles as the temperature increases due to thermal expansion. The ionic conductivity results show that the glass transition temperature (T_g) is a dominant, but not exclusive, parameter in determining ion transport. The T_g‐independent ionic conductivity decreases as the backbone‐to‐backbone spacing increases. Further interpretation of the ionic conductivity using the Vogel–Fulcher–Tammann equation enabled the correlation between polymer morphology and ionic conductivity, which highlights the importance of anion hoping between adjacent polymer backbones. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012