Polarizable Surface Research Articles

Electrosorption of alkaline earth metal ions onto activated carbon as affected by pH and applied potential (pE)

The electrosorption of alkaline earth metal ions from aqueous solution was studied using a graphite supported activated carbon electrode (NSA@G). Zeta potential measurement revealed a low pHzpc of 3.0 for the NSA electrode, suggesting a negatively charged L-type carbon surface. The electrosorption behavior of Ca2+ ion followed the Langmuir adsorption isotherm and pseudo-first-order rate law. The initial Ca2+ ion concentration, solution pH, and applied potential affected the electrosorption capacity. Results showed that both reversible surface charge (regulated by the potential determining ions, i.e., H+ and OH−, or pH) and polarizable surface charge (controlled by the applied potential, or pE) contributed to the overall Ca2+ ion removal process. The contribution of reversible and polarizable surface charge varied with pH and pE, respectively. Specifically, the reversible surface charge played a more significant role in Ca2+ electrosorption at high pH value and low pE (at 60–83 % of the total Ca2+ uptake), while the polarizable surface charge dominated at low pH and high pE (at 60–62 % of the total Ca2+ uptake). Factors, such as ionic radius, hydration ratio, and hydration enthalpy, significantly affected the electrosorption capacity of divalent alkaline earth metals, i.e., Ca2+, Mg2+, Sr2+, and Ba2+, over NSA@G electrode. NoveltyThis work elucidated the mechanisms of electrode charging and Ca2+ ion uptake via electrosorption. Previous research often attributed ion electrosorption capacity solely to the surface charge derived from a polarizable electrode. In addition to polarizable surface charge, which is controlled by the applied potential (or pE), this study demonstrated that reversible surface charge, regulated by the potential determining ions, i.e., H+ and OH− ions (or pH), also played a significant role in total Ca2+ ion removal. The novelty of this work lies in quantifying the contribution of reversible and polarizable surface charge to overall Ca2+ ion electrosorption. Furthermore, this study investigated the factors affecting the electrosorption behavior of alkaline earth metals, i.e., Mg2+, Ca2+, Sr2+, and Ba2+. A rational approach to predicting electrosorption performance is also proposed, using capacitance characterization from cyclic voltammetry measurements based on Lipmann's electrocapillarity theory.

A Novel Electrokinetic-Based Technique for the Isolation of Circulating Tumor Cells.

The separation of rare cells from complex biofluids has attracted attention in biological research and clinical applications, especially for cancer detection and treatment. In particular, various technologies and methods have been developed for the isolation of circulating tumor cells (CTCs) in the blood. Among them, the induced-charge electrokinetic (ICEK) flow method has shown its high efficacy for cell manipulation where micro-vortices (MVs), generated as a result of induced charges on a polarizable surface, can effectively manipulate particles and cells in complex fluids. While the majority of MVs have been induced by AC electric fields, these vortices have also been observed under a DC electric field generated around a polarizable hurdle. In the present numerical work, the capability of MVs for the manipulation of CTCs and their entrapment in the DC electric field is investigated. First, the numerical results are verified against the available data in the literature. Then, various hurdle geometries are employed to find the most effective geometry for MV-based particle entrapment. The effects of electric field strength (EFS), wall zeta potential magnitude, and the particles' diameter on the trapping efficacy are further investigated. The results demonstrated that the MVs generated around only the rectangular hurdle are capable of trapping particles as large as the size of CTCs. An EFS of about 75 V/cm was shown to be effective for the entrapment of above 90% of CTCs in the MVs. In addition, an EFS of 85 V/cm demonstrated a capability for isolating particles larger than 8 µm from a suspension of particles/cells 1-25 µm in diameter, useful for the enrichment of cancer cells and potentially for the real-time and non-invasive monitoring of drug effectiveness on circulating cancer cells in blood circulation.

Recent developments in porous metal chalcogenides for environmental remediation and sustainable energy

AbstractPorous metal chalcogenides have emerged as promising materials for environmental remediation and sustainable energy generation. Their tunable optical band gap (from infrared to the visible range), highly polarizable surface, chemical activity, and adjustable structure make them attractive for various applications. This review summarizes the recent developments concerning the synthesis and characterization of multifunctional porous chalcogenide materials. It explores their remarkable potential in addressing environmental and energy challenges. Moreover, we discuss the several factors that affect the performance of porous metal chalcogenides, such as their microstructure, morphology, and chemical composition, to gain deeper insights into these materials. Finally, we highlight some of the key challenges and future research directions in the development of porous metal chalcogenides as effective and efficient materials for environmental remediation and sustainable energy generation.image

Prediction of protonation constants of hydrazones and Schiff bases derived from pyridoxal 5′-phosphate, pyridoxal, 3-hydroxyisonicotinaldehyde and salicylic aldehyde

Hydrazones and Schiff bases are potent metal chelators applied in various areas such as analytical chemistry and medicine because of their various biological activities, including antimicrobial, antitumor, analgesic, and anti-inflammatory action. The hydrazones and Schiff bases derived from aldehyde forms of the B6 vitamin are arguably the most biologically active representatives of these classes. Protonation constants associated with pharmacokinetic characteristics are among the most important properties of drug-like molecules as they affect their binding to proteins, membrane transport, excretion, etc. The present paper accumulates data on protonation constants of hydrazones and Schiff bases derived from pyridoxal, pyridoxal 5′-phosphate, 3-hydroxyisonicotinaldehyde, and salicylic aldehyde available in the literature. A novel simple chemometric model is constructed based on these data to describe the protolytic equilibria constants. The model proposed takes into account such molecular descriptors as molecular weight, number of hydrogen bond donors and acceptors, polarizable surface area, and log P. The contribution of ionic strength is also included in the model. The adequacy of the model and the statistical significance of separate parameters are studied using standard mathematical procedures. The protonation constants of five hydrazones derived from pyridoxal and hydrazides of isonicotinic, 2- and 3-furoic, 2- and 3-thiophenecarboxylic acids are determined experimentally in aqueous solution at T = 298.2 K, p = 0.1 MPa and I = 0.50 mol/L (NaCl) and compared with the model predictions. Protonation constants determined can be used further in studies of coordination equilibria of the hydrazones.

Constant chemical potential-quantum mechanical-molecular dynamics simulations of the graphene-electrolyte double layer.

We present the coupling of two frameworks-the pseudo-open boundary simulation method known as constant potential molecular dynamics simulations (CμMD), combined with quantum mechanics/molecular dynamics (QMMD) calculations-to describe the properties of graphene electrodes in contact with electrolytes. The resulting CμQMMD model was then applied to three ionic solutions (LiCl, NaCl, and KCl in water) at bulk solution concentrations ranging from 0.5 M to 6 M in contact with a charged graphene electrode. The new approach we are describing here provides a simulation protocol to control the concentration of electrolyte solutions while including the effects of a fully polarizable electrode surface. Thanks to this coupling, we are able to accurately model both the electrode and solution side of the double layer and provide a thorough analysis of the properties of electrolytes at charged interfaces, such as the screening ability of the electrolyte and the electrostatic potential profile. We also report the calculation of the integral electrochemical double layer capacitance in the whole range of concentrations analyzed for each ionic species, while the quantum mechanical simulations provide access to the differential and integral quantum capacitance. We highlight how subtle features, such as the adsorption of potassium graphene or the tendency of the ions to form clusters contribute to the ability of graphene to store charge, and suggest implications for desalination.

Surface chemistry on a polarizable surface: Coupling of CO with KTaO3(001).

Polarizable materials attract attention in catalysis because they have a free parameter for tuning chemical reactivity. Their surfaces entangle the dielectric polarization with surface polarity, excess charge, and orbital hybridization. How this affects individual adsorbed molecules is shown for the incipient ferroelectric perovskite KTaO3. This intrinsically polar material cleaves along (001) into KO- and TaO2-terminated surface domains. At TaO2 terraces, the polarity-compensating excess electrons form a two-dimensional electron gas and can also localize by coupling to ferroelectric distortions. TaO2 terraces host two distinct types of CO molecules, adsorbed at equivalent lattice sites but charged differently as seen in atomic force microscopy/scanning tunneling microscopy. Temperature-programmed desorption shows substantially stronger binding of the charged CO; in density functional theory calculations, the excess charge favors a bipolaronic configuration coupled to the CO. These results pinpoint how adsorption states couple to ferroelectric polarization.

Effects of membrane polarization, steric repulsion and ion-solvent interactions on electroosmosis through a conical nanopore

The impact of solid polarization of a dielectric membrane on ion transport in a conical nanopore drilled through a membrane is studied by considering ions as finite-sized dielectric charged spheres. The excess polarization of the hydrated ions creates a dielectric decrement. A mean-field-based approach is adopted by modifying the Nernst-Planck equation to incorporate the ion steric interactions, the Born force, and the dielectrophoretic force arising due to the spatial variation of electrolyte permittivity. The viscosity of the medium is considered to vary with the local ionic concentration through the Bachelor-Green equation, which leads to a reduction in the mobility of ions near a charged surface. Governing equations are solved numerically through the control volume method along with a pressure-correction-based iterative algorithm. The solid polarization of the membrane, which is often neglected in existing studies, is found to have a strong impact on the volume flux, conductance, selectivity, and ion current rectification in the conical nanopore. The ion saturation in the vicinity of the charged surface, as governed by the present modified model, creates a pronounced alteration in the electrokinetics compared to that predicted by the standard Poisson-Nernst-Planck (PNP-model) based on the point charge consideration of ions. This discrepancy due to ion steric repulsion and ion-solvent interactions is pronounced at a higher ionic concentration as well as a highly polarizable surface. A close agreement between the predictions of the present modified model and the existing experimental data, compared with that of the standard PNP model, is encouraging. The present modified model provides an accurate analysis of the electroosmotic flow and ion transport in a dielectric charged conical nanopore beyond the low charge density and dilute solution limits.

Exploring the polarization window during fluoride electrosorption in two activated carbons with significant differences in their pore-size distribution

This work reports the electrosorption of fluoride by two commercial activated carbons (coconut-shell and bituminous carbons) with significant differences in their pore-size distribution but with a similar surface area, surface chemistry and charging processes. The coconut-shell carbon is 95% microporous, while the bituminous carbon presents micro (75%) and mesoporosity (25%). Fluoride electrosorption was evaluated in sacrifice cells by polarizing at several potentials (Ecell = 0, 1.2, 1.6, 2 and 2.4 V). The fluoride removal capacity and rate without polarizing was really low and increased at different degrees as a function of increasing the applied potential for both carbons. At 2 Vcell, the bituminous and coconut-shell carbons increased their capacity from 0.073 to 0.431 mg g−1 and from 0.047 to 0.256 mg g−1, respectively, compared to conventional adsorption. On the other hand, the removal rate of the bituminous carbon was ∼ 4 times faster than that of the coconut-shell carbon at short times. This was associated to a higher presence of mesopores, as they enhance the mass-transport and the accessibility of the anion to the polarizable surface, and present less overlapping of the electrical double-layer (EDL). In this regard, a first approximation of the decrease of the EDL thickness was obtained from mathematical approximations and the obtained experimental results at 2 Vcell, in which the overlapping decreased by half, from a pore width of 18.67 nm to ∼ 9.5 nm. Parasitic faradaic reactions were induced at all the applied potentials, which decreased the charge efficiency of the process, although these reactions were more significant at 2.4 Vcell.

Evaluation of aqueous Cd2+ and Pb2+ removal by natural loess using spectral induced polarization and microscopic characterization.

Mining and landfill activities can cause serious soil and groundwater contamination with lead (Pb) and cadmium (Cd). Loess soils are common and have been reported as effective for the removal of heavy metals. The spectral induced polarization (SIP) technique has been approved for its nondestructive ability to characterize the contaminant transport process and surface geochemical properties in porous media. In the present study, SIP was applied to monitor Pb2+ and Cd2+ removal processes using loess through column flow-through experiments. The outflow aqueous geochemical analyses indicated a better retention capability of loess for Pb2+, which was through precipitation induced by calcite dissolution and aqueous pH increment, as confirmed by SEM-EDS and XRD results. Cd retention took place mainly through ion exchange with Ca2+ and Mg2+ on the loess surface. The SIP signals showed a continuous decrement on the magnitude of imaginary conductivity during both Pb2+ and Cd2+ flow-through, which was attributed to the total surface area and decrement of polarizable surface charges. The SIP signals differentiated the interactions between loess and Pb2+/Cd2+ by displaying a peak shift to a higher frequency on the imaginary conductivity spectra during Pb2+ flow-through, which was attributed to calcite dissolution and proved by the high correlation (R2 = 0.9366) between the estimated dissolved calcite mass and the peak of imaginary conductivity. The above results suggest that loess has a great potential for field heavy metal remediation applications, and the SIP technique displays a promising capability of monitoring the remediation performance.

Fluid pumping by liquid metal droplet utilizing ac electric field.

We report a unique phenomenon in which liquid metal droplets (LMDs) under a pure ac electric field pump fluid. Unlike the directional pumping that occurs upon reversing the electric field polarity under a dc signal, this phenomenon allows the direction of fluid motion to be switched by simply shifting the position of the LMD within the cylindrical chamber. The physical mechanism behind this phenomenon has been termed Marangoni flow, caused by nonlinear electrocapillary stress. Under the influence of a localized, asymmetric ac electric field, the polarizable surface of the position-offset LMD produces a net time-averaged interfacial tension gradient that scales with twice the field strength, and thus pumps fluid unidirectionally. However, the traditional linear RC circuit polarization model of the LMD/electrolyte interface fails to capture the correct pump-flow direction when the thickness of the LMD oxide skin is non-negligible compared to the Debye length. Therefore, we developed a physical description by treating the oxide layer as a distributed capacitance with variable thickness and connected with the electric double layer. The flow profile is visualized via microparticle imaging velocimetry, and excellent consistency is found with simulation results obtained from the proposed nonlinear model. Furthermore, we investigate the effects of relevant parameters on fluid pumping and discuss a special phenomenon that does not exist in dc control systems. To our knowledge, no previous work addresses LMDs in this manner and uses a zero-mean ac electric field to achieve stable, adjustable directional pumping of a low-conductivity solution.

Fluoride electrosorption by hybrid La(III)-activated carbon electrodes under the influence of the La(III) content and the polarization profile

Chronic and excessive fluoride consumption has important implications for human health. Several methods have been implemented to decrease its concentration to acceptable levels. This work studies fluoride electrosorption by an activated carbon impregnated with 3 La(III) percentages: La-0.5%, La-1.5% and La-2.0%. Lanthanum clusters decreased the surface area and pore volume of the adsorbents, which also decreased their polarizable surface. The deprotonation of lanthanum hydroxyls at the working conditions increased the input of negative surface charges, reflected in the acidification and anodization of their points (pHPZC < 5) and potentials (EPZC > 0.1 V) of zero charge, respectively, compared to the pristine carbon (pHPZC= 8.5 and EPZC= 0 V). Electrosorption was evaluated by applying 0.8 V (vs. Ag/AgCl/3 M NaCl) in two profiles: (i) polarizing from the beginning and (ii) polarizing after the adsorption equilibrium. For La-0.5%, the same removal (~3 mg g-1) was reached independently of the profile, although the first profile (i) promoted a faster rate. For La-1.5%, the second profile (ii) increased by 50% its removal (~9 mg g-1), compared to the first (i) profile (~6 mg g-1). The lanthanum hydroxyls are exchanged by fluoride during conventional adsorption, which decreased their input of negative charges and thus their competition when polarized at anodic potentials after adsorption. This was demonstrated by obtaining more alkaline pHPZC, less anodic EPZC and better charging processes after fluoride adsorption. Then, polarizing after adsorption could benefit the performance of electrodes, especially those that undergo synergistic changes in their surface charges during adsorption.

The role of reversible and polarizable surface charge in the electro-sorption of NaCl electrolyte onto activated carbon-graphite electrode

An electrode made of activated carbon that had high-specific-surface area and low pHzpc supported on graphite sheet was fabricated. The electrode, NSA/G, was used to study the effect of reversible surface charge on the adsorption of simple electrolyte, i.e., NaCl, on polarizable electrode. Polarizable surface charge is originated from externally applied potential whereas reversible surface charge is derived from change of activity of potential-determining ions, i.e., H+ or OH– owning to hydrous surface, which undergoes protonation and deprotonation of surface hydroxyl groups. Ion adsorption density was obtained from direct analytical measurement of corresponding ion studied instead of conductivity measurement. Langmuir adsorption isotherm was applied to estimate the monolayer ion adsorption density. Electrosorption/desorption kinetics was fitted well by the first-order rate law. In the applied potential range of −0.3 to −1.0 V (vs. SCE) and pH of 3 to 10, the pH-determining surface, i.e., reversible charge, accounted for Na+ sorption intensity was estimated based on the Gouy-Chapman version of electrical double layer theory. Results revealed that the two major surface charges, i.e., polarizable (σE) and reversible (σpH), contributed equally to the total charge density (σT). At high pH (pH 10) and low applied voltage (-0.3 V vs. SCE), the fraction of pH-determining contribution to total surface charge with respect to Na+ sorption, i.e., σpH/σT, was around 0.50. Results reaffirmed the importance of reversible surface charge together with polarizable surface charge on the design and operation of CDI system for electrosorption of ions from water.

Adsorption and Chromatographic Separation of Thiophene Derivatives on Graphitized Thermal Carbon Black

The thermodynamic characteristics of adsorption (TCA) of thiophene and its derivatives are determined under the conditions of equilibrium gas adsorption chromatography (GAC) on columns with graphitized thermal carbon black (GTCB) of Carbopack C HT grade. It is shown that the TCA values ​​ depend largely on the number and nature of the substituents in the main structural fragment. It is found that the surface of the GTCB is characterized by low structural selectivity for positional isomers in the series of thiophene derivatives. The effect of polar retention of an adsorbate on graphite from the gas phase due to additional intermolecular specific interactions of polar groups of the adsorbate with an easily polarizable surface of the base face of graphite is established and studied. Conclusions are reached on the applicability of the two-dimensional ideal gas model for describing the mobility of strongly polar molecules of thiophene derivatives on surfaces of graphite, and on the considerable limitations of this model when applied to the adsorption of thiophene molecules, along with its methyl and halogen derivatives.

Gauge transformation in macroscopic quantum electrodynamics near polarizable surfaces

To describe charged particles interacting with the quantized electromagnetic field, we point out the differences of working in the so-called generalized and the true Coulomb gauges. We find an explicit gauge transformation between them for the case of the electromagnetic field operators quantized near a macroscopic boundary described by a piece-wise constant dielectric function. Starting from the generalized Coulomb gauge we transform operators into the true Coulomb gauge where the vector potential operator is truly transverse everywhere. We find the generating function of the gauge transformation to carry out the corresponding unitary transformation of the Hamiltonian and show that in the true Coulomb gauge the Hamiltonian of a particle near a polarizable surface contains extra terms due to the fluctuating surface charge density induced by the vacuum field. This extra term is represented by a second-quantised operator on equal footing with the vector field operators. We demonstrate that this term contains part of the electrostatic energy of the charged particle interacting with the surface and that the gauge invariance of the theory guarantees that the total interaction energy in all cases equals the well known result obtainable by the method of images when working in generalized Coulomb gauge. The mathematical tools we have developed allow us to work out explicitly the equal-time commutation relations and shed some light on typical misconceptions regarding issues of whether the presence of the boundaries should affect the field commutators or not, especially when the boundaries are modelled as perfect reflectors.

Magic numbers, quantum delocalization, and orientational disordering in anionic hydrogen and deuterium clusters.

The Diffusion Monte Carlo (DMC) method was applied to anionic hydrogen clusters H-(H2)n (n = 1-16, 32) and their deuterated analogs using a polarizable all-atom potential energy surface (PES) developed by Calvo and Yurtsever. For the hydrogen clusters, the binding energy ΔEn appears to be a smooth function of the cluster size n, thus contradicting the previous claim that n = 12 is a "magic number" cluster. The structures of the low energy minima of the PES for these clusters belong to the icosahedral motif with the H2 molecules aligned toward the central H- ion. However, their ground state wavefunctions are highly delocalized and resemble neither the structures of the global nor local minima. Moreover, the strong nuclear quantum effects result in a nearly complete orientational disordering of the H2 molecules. For the deuterium clusters, the ground state wavefunctions are localized and the D2 molecules are aligned toward the central D- ion. However, their structures are still characterized as disordered and, as such, do not display size sensitivity. In addition, DMC simulations were performed on the mixed H-(H2)n(D2)p clusters with (n, p) = (6, 6) and (16, 16). Again, in contradiction to the previous claim, we found that the "more quantum" H2 molecules prefer to reside farther from the central H- ion than the D2 molecules.

Porous graphite as platform for the separation and characterization of synthetic polymers – an overview

Porous graphite as sorbent differs significantly from all other HPLC column packings. It stands out due to its chemically extremely homogeneous surface, which moreover is planar on an atomic level. This sorbent, according to its non-polar but polarizable surface, is able to adsorb polar as well as non-polar small molecules as well as macromolecules. Moreover, it enables their separation induced by minute differences in their molecular architecture, which includes the aspects of planarity, branching or tacticity of macromolecules. Although graphite had already been used many years for the separation of small molecules, the application of porous graphite for separations in the domain of synthetic polymers has been rare. In 2009 it was found that porous graphite enables the separation of polyethylene and polypropylene on the basis of their full adsorption and desorption, when suitable solvents are used. This approach has led to the fast elaboration of HPLC systems for separations of various polar modified as well as non-polar polyolefins. Due to pronounced adsorptive interactions, porous graphite is applicable even at temperatures as high as 160 °C. The results presented in this paper manifest that porous graphite enables to obtain important information about the composition distribution of various synthetic polymers, the architecture of macromolecules (i.e., branching) or their tacticity, and underlines its enormous application potential.

Sensitive untargeted identification of short hydrophilic peptides by high performance liquid chromatography on porous graphitic carbon coupled to high resolution mass spectrometry

The combination of an efficient chromatographic separation with post-column addition of a supercharging agent was evaluated for the determination of small peptides. The procedure takes advantage of porous graphitic carbon (PGC) ability in retaining very polar and ionic molecules to overstep the poor retention of small peptides on conventional reversed phase (RP) columns. The method was developed specifically for the most hydrophilic di-, tri- and tetrapeptides, which are not identified in ordinary peptidomics experiments. In addition to retention mechanisms acting on conventional RP, the method exploited the charge induced interactions generated by the charges on the peptides with the polarizable surface of PGC. This results in efficient retention of very short and highly polar peptides using classical RP mobile phases. The effects of varying mobile phase composition (organic solvent and ion-pairing additives) as well as column temperature have been thoroughly investigated using short peptide standards. Under optimized conditions (water and acetonitrile/tetrahydrofuran 99:1 (v/v), both with 0.15% trifluoroacetic acid, as phase A and B, respectively, 0.5 mL min−1 flowrate at 50 °C) the effect of post-column addition of 3-nitrobenzylic alcohol was also investigated allowing effective coupling of the chromatographic system with high resolution mass spectrometry. Finally, an untargeted approach for peptide identification was pursued, based on precursor identification in database with all possible combinations of the 20 natural amino acids and fragmentation spectra matching to in silico generated spectra. The method was then applied to investigation of the short endogenous peptides in human serum from healthy individuals resulting in the identification of 30 short peptides.

Induced-charge electrokinetics in rotating electric fields: A linear asymptotic analysis

Concerning the electroconvective analyte manipulation in microfluidics, we describe the basic physics of fluid flow driven by rotating induced-charge electro-osmosis (ROT-ICEO), which occurs on the planar surface of a single floating electrode in an external rotating electric field. First, based on a linear asymptotic analysis, the dynamic flow stagnation line in ROT-ICEO induced on the bipolar electrode from a rotary Debye screening charge revolves synchronously with the applied rotating fields. A net hydrodynamic torque is then generated that acts on any fluid or particle sample to produce either a synchronous or asynchronous co-field rotation depending on the frequency of the ac signal. Next, from the synergy between the hydrodynamic and electrochemical ion relaxations, an analytical solution of the sample rotation rate subject to ROT-ICEO slipping on an ideally polarizable surface is obtained for different frequency ranges and determined by the transient nature of the rotating electro-osmotic flow oscillating at twice the field frequency. To visualize the flow field in ROT-ICEO, experiments were performed with fluorescent tracer nanoparticles; they exhibited concentric rotational behavior at the polarized phase interface. Formed like the arms of a nebula disk, the four twisted tails of nanoparticles can be arbitrarily directed under voltage-phase rectification. These experimental results are in good agreement with our mathematical simulations using the Debye–Hückel approximation on ROT-ICEO.

On the origin of membrane potential in membranes with polarizable nanopores

We report a new mechanism for the generation of membrane potential in polarizable nanoporous membranes separating electrolytes with different concentrations. The electric field generated by diffusion of ions with different mobilities induces a non–uniform surface charge, which results in charge separation inside the nanopore. The corresponding Donnan potentials appear at the pore entrance and exit leading to a dramatic enhancement of membrane potential in comparison with an uncharged non–polarizable membrane. At high concentration contrast, the interaction between electric field and uncompensated charge at a low concentration side results in the development of electrokinetic vortices. The theoretical predictions are based on the Space–Charge model, which is extended to nanopores with polarizable conductive surface for the first time. This model is validated against full Navier–Stokes, Nernst–Planck, and Poisson equations, which are solved in a high aspect ratio nanopore connecting two reservoirs. The experimental measurements of membrane potential of dielectric and conductive membranes in KCl and NaCl aqueous solutions confirm the theoretical results. The membranes are prepared from Nafen nanofibers with ∼10nm in diameter and modified by depositing a conductive carbon layer. It is shown theoretically that the membrane potential enhancement becomes greater with decreasing the electrolyte concentration and pore radius. A high sensitivity of membrane potential to the ratio of ion diffusion coefficients is demonstrated. The described phenomenon may find applications in precise determination of ion mobilities, electrochemical and bio–sensing, as well as design of nanofluidic and bioelectronic devices.

Liquid Adsorption of Organic Compounds on Hematite α-Fe2O3 Using ReaxFF.

ReaxFF-based molecular dynamics simulations are used in this work to study the effect of the polarity of adsorbed molecules in the liquid phase on the structure and polarization of hematite (α-Fe2O3). We compared the adsorption of organic molecules with different polarities on a rigid hematite surface and on a flexible and polarizable surface. We show that the displacements of surface atoms and surface polarization in a flexible hematite model are proportional to the adsorbed molecule's polarity. The increase in electrostatic interactions resulting from charge transfer in the outermost solid atoms in a flexible hematite model results in better-defined adsorbed layers that are less ordered than those obtained assuming a rigid solid. These results suggest that care must be taken when parametrizing empirical transferable force fields because the calculated charges on a solid slab in vacuum may not be representative of a real system, especially when the solid is in contact with a polar liquid.