Electrostatic Screening Length Research Articles

Cluster Formation Induced by Local Dielectric Saturation in Restricted Primitive Model Electrolytes.

Experiments using the Surface Force Apparatus (SFA) have found anomalously long-ranged charge-charge underscreening in concentrated salt solutions. Meanwhile, theory and simulations have suggested ion clustering to be a possible origin of this behavior. The popular Restricted Primitive Model of electrolyte solutions, in which the solvent is represented by a uniform relative dielectric constant, εr, is unable to resolve the anomalous underscreening seen in experiments. In this work, we modify the Restricted Primitive Model to account for local dielectric saturation within the ion hydration shell. The dielectric "constant" in our model locally decreases from the bulk value to a lower saturated value at the ionic surface. The parameters for the model are deduced so that typical salt solubilities are obtained. Our simulations for both bulk and slit geometries show that our model displays strong cluster formation and these give rise to long-ranged density correlations between charged surfaces, at distances similar to what has been observed in SFA measurements. An electrolyte model wherein the dielectric constant remains uniform does not display similar clusters, even with εr equal to the low saturated value at ion contact. Hence, the observed behaviors are not simply due to an enhanced Coulomb interaction. In the latter case, cluster growth is counteracted by long-ranged repulsions between like-charged ions within clusters; this is an effect that is considerably reduced when the dielectric response drop is local. Our results imply that long-ranged interactions in these systems are mainly due to cluster-cluster correlations, rather than large electrostatic screening lengths.

Disjoining Pressure Decay Length in Room-Temperature Ionic Liquids: A Molecular Simulation Study.

We present all-atom molecular simulations to investigate the behavior of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) in negatively charged carbon nanopores of different widths (h = 5÷15 nm) and lengths (l = 4÷10 nm). The goal of our study was to determine how the disjoining pressure varies as a function of the pore width at different lengths and to understand the influence of edge effects. Our results show that the edge effect decreases as the pore length increases. Using an exponential function, we can approximate the disjoining pressure at large pore widths and use this approximation to estimate the decay length that can correlate with the electrostatic screening length. The results agreed well with those of previous experimental studies.

An efficient method to establish electrostatic screening lengths of restricted primitive model electrolytes.

We present a novel, and computationally cheap, way to estimate electrostatic screening lengths from simulations of restricted primitive model (RPM) electrolytes. We demonstrate that the method is accurate by comparisons with simulated long-ranged parts of the charge density, at various Bjerrum lengths, salt concentrations and ion diameters. We find substantial underscreening in low dielectric solvent, but with an "aqueous" solvent, there is instead overscreening, the degree of which increases with ion size. Our method also offers a possible path to (future) more accurate classical density functional treatments of ionic fluids.

The mystery of anomalously long-ranged screening in concentrated ionic systems

1. Ionic liquids behave as dilute electrolyte solutions Authors: M. A. Gebbie, M. Valtiner, X. Banquy, E. T. Fox, W. A. Henderson, and J. N. Israelachvili Proc. Natl. Acad. Sci. U.S.A 110, 9674 (2013); DOI: 10.1073/pnas.1307871110 2. The electrostatic screening length in concentrated electrolytes increases with concentration Authors: A. M. Smith, A. A. Lee, and […]

Disjoining pressure of room temperature ionic liquid in charged slit carbon nanopore: Molecular dynamics study

We present a study of the behavior of the room-temperature ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate) in negatively charged carbon slit nanopores of various widths (1 ÷ 15 nm) using all-atom and coarse-grained molecular dynamics simulations. For the room temperature ionic liquids, we calculate and analyze the disjoining pressure as a function of pore width from the all-atom and coarse-grained molecular dynamics simulations. We discuss the damped oscillations of the disjoining pressure for sufficiently small pore widths and elucidate their microscopic nature. We also analyze radial distribution functions, and angle distributions charge, mass, and number densities of the ionic liquid inside the pore. We find that a confined ionic liquid has a layering structure inside the pore of <5 nm in width. We show that the cations in the layer closest to the charged surfaces are in parallel orientation and form a lamellar-like structure. We observe charge overcompensation near the graphene surfaces at σ = −12.8 μC/cm2 and the lattice saturation effect at σ = −41.6 μC/cm2. Based on the disjoining pressure behavior, we estimate the effective electrostatic screening length.

Electrokinetic Janus micromotors moving on topographically flat chemical patterns

Ionic and molecular selectivity are considered unique for the nanoscale and not realizable in microfluidics. This is due to the scale-matching problem—a difficulty to match the dimensions of ions and electrostatic potential screening lengths with micron-sized confinements. Here, we demonstrate a microscale realization of ionic transport processes closely resembling those specific to ionic channels or in nanofluidic junctions, including selectivity, guidance and flow focusing. As a model system, we explore electrokinetic spherical Janus micromotors moving over charged surfaces with complex charge distribution and without any topographical wall. We discuss peculiarities of the long-range electrostatic interaction on the behavior of the system including interface crossing and reflection of positively charged particles from negatively charged interfaces. These results are crucial for understanding the electrokinetic transport of biochemical species under confinement, have the potential to increase the precision of lab-on-chip-based assays, as well as broadening use cases and control strategies of nano-/micromachinery.

Tuning Ionic Screening To Accelerate Electrochemical CO2 Reduction in Ionic Liquid Electrolytes

Electric double layer formation often governs the rate and selectivity of CO2 electrochemical reduction. Ionic correlations critically define double layer properties that are essential to electrocatalytic performance, including capacitance and localization of potential gradients. However, the influence of ionic correlations on CO2 electroreduction remains unexplored. Here, we use electrochemical conversion of CO2 to CO in ionic liquid-based electrolytes to investigate how the emergence of ionic correlations with increasing ion concentration influences reaction rates and selectivity. Remarkably, we find substantial acceleration of potential-dependent CO2 reduction rates and enhancement of faradaic efficiency to CO at intermediate concentrations of 0.9 M ionic liquid in acetonitrile, a concentration regime that has not been studied previously. We find that onset potentials for CO2 reduction remain relatively unchanged at −2.01 V vs Ag/Ag+ from 0.025 M up to 1.1 M and increase to −2.04 V vs Ag/Ag+ in the limit of neat ionic liquids. Hence, the acceleration of CO2 reduction we observe originates from the amplification of potential-dependent driving forces, as opposed to changes in onset potential. Importantly, our findings are general across cocatalytic and noncatalytic ions. We propose that concentrations of maximum reactivity correspond to conditions where electric double layers exhibit the strongest screening, which would localize electric fields to stabilize polar intermediates. Our study demonstrates that tuning bulk electrostatic screening lengths via modulation of ionic clustering provides a general approach to accelerating both inner-sphere and outer-sphere electrochemical reactions.

Screening length in concentrated electrolytes

The law of increase of the electrostatic screening length with increasing concentration, recently reported in highly-concentrated electrolytes (ionic liquids, inorganic salts), References (Smith et al., 2017; Lee et al., 2017; Lee et al., 2017; Gaddam and Ducker, 2019), is derived from the dispersive plasmons generated by Coulomb and short-range interactions, the latter arising from screened-charge electrostatic interactions.

Voltage-Controlled Switching of Magnetic Anisotropy in Ambipolar Mn2CoAl/Pd Bilayers

An ultrahigh electric field induced by ionic liquid gating (ILG) can be employed to manipulate ferromagnetism with low Joule heating dissipation, showing great potential for spintronics applications. In ferromagnetic/heavy metal thin films, however, typical materials used in both layers are electron-carrier dominant, which significantly suppresses the ILG effect due to the short electrostatic screening length in metal. Here, we employ ${\mathrm{Mn}}_{2}\mathrm{Co}\mathrm{Al}$, a spin gapless semiconductor with hole carriers, as the ferromagnetic layer and investigate the ILG effect in $\mathrm{Mg}\mathrm{O}/{\mathrm{Mn}}_{2}\mathrm{CoAl/Pd}$ ultrathin films with perpendicular magnetic anisotropy. Reversible change of the magnetic anisotropy from the out-of-plane to the in-plane direction is achieved, induced by electrostatic charge accumulation. Moreover, ambipolar transport behavior has been observed and explained by a two-carrier model. Finally, we find that skew scattering is the mechanism of the anomalous Hall effect and can be enhanced at a positive gate voltage in our system. Our results strongly demonstrate that a significant ILG effect on magnetism can be easily achieved in two-carrier dominant ultrathin films.

Underscreening and hidden ion structures in large scale simulations of concentrated electrolytes.

The electrostatic screening length predicted by Debye-Hückel theory decreases with increasing ionic strength, but recent experiments have found that the screening length can instead increase in concentrated electrolytes. This phenomenon, referred to as underscreening, is believed to result from ion-ion correlations and short-range forces such as excluded volume interactions among ions. We use Brownian Dynamics to simulate a version of the Restrictive Primitive Model for electrolytes over a wide range of ion concentrations, ionic strengths, and ion excluded volume radii for binary electrolytes. We measure the decay of the charge-charge correlation among ions in the bulk and compare it against scaling trends found experimentally and determined in certain weak coupling theories of ion-ion correlation. Moreover, we find that additional large scale ion structures emerge at high concentrations. In this regime, the frequency of oscillations computed from the charge-charge correlation function is not dominated by electrostatic interactions but rather by excluded volume interactions and with oscillation periods on the order of the ion diameter. We also find the nearest neighbor correlation of ions sharing the same charge transitions from negative at small concentrations to positive at high concentrations, representing the formation of small, like-charge ion clusters. We conclude that the increase in local charge density due to the formation of these clusters and the topological constraints of macroscopic charged surfaces can help explain the degree of underscreening observed experimentally.

Flexible, charged biopolymers in monovalent and mixed-valence salt: Regimes of anomalous electrostatic stiffening and of salt insensitivity.

The conformations of biological polyelectrolytes (PEs), such as polysaccharides, proteins, and nucleic acids, affect how they behave and interact with other biomolecules. Relative to neutral polymers, PEs in solution are more locally rigid due to intrachain electrostatic repulsion, the magnitude of which depends on the concentration of added salt. This is typically quantified using the Odijk-Skolnick-Fixman (OSF) electrostatic-stiffening model, in which salt-dependent Debye-Hückel (DH) screening modulates intrachain repulsion. However, the applicability of this approach to flexible PEs has long been questioned. To investigate this, we use high-precision single-molecule elasticity measurements to infer the scaling with salt of the local stiffness of three flexible biopolymers (hyaluronic acid, single-stranded RNA, and single-stranded DNA) in both monovalent and mixed-valence salt solutions. In monovalent salt, we collapse the data across all three polymers by accounting for charge spacing, and find a common power-law scaling of the electrostatic persistence length with ionic strength with an exponent of 0.66±0.02. This result rules out simple OSF pictures of electrostatic stiffening. It is roughly compatible with a modified OSF picture developed by Netz and Orland; alternatively, we posit the exponent can be explained if the relevant electrostatic screening length is the interion spacing rather than the DH length. In mixed salt solutions, we find a regime where adding monovalent salt, in the presence of multivalent salt, does not affect PE stiffness. Using coarse-grained simulations, and a three-state model of condensed, chain-proximate, and bulk ions, we attribute this regime to a "jacket" of ions surrounding the PE that regulates the chain's effective charge density as ionic strength varies. The size of this jacket in simulations is again consistent with a screening length controlled by interion spacing rather than the DH length. Taken together, our results describe a unified picture of the electrostatic stiffness of polyelectrolytes in the mixed-valence salt conditions of direct relevance to cellular and intercellular biological systems.

Ionic gating in metallic superconductors: A brief review

Ionic gating is a very popular tool to investigate and control the electric charge transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, and has been proven to be capable of tuning even the properties of metallic systems. In this short review, I summarize the main results which have been achieved so far in controlling the superconducting (SC) properties of thin films of conventional metallic superconductors by means of the ionic gating technique. I discuss how the gate-induced charge doping, despite being confined to a thin surface layer by electrostatic screening, results in a long-range ‘bulk’ modulation of the SC properties by the coherent nature of the SC condensate, as evidenced by the observation of suppressions in the critical temperature of films much thicker than the electrostatic screening length, and by the pronounced thickness-dependence of their magnitude. I review how this behavior can be modelled in terms of proximity effect between the charge-doped surface layer and the unperturbed bulk with different degrees of approximation, and how first-principles calculations have been employed to determine the origin of an anomalous increase in the electrostatic screening length at ultrahigh electric fields, thus fully confirming the validity of the proximity effect model. Finally, I discuss a general framework—based on the combination of ab-initio Density Functional Theory and the Migdal-Eliashberg theory of superconductivity—by which the properties of any gated thin film of a conventional metallic superconductor can be determined purely from first principles.

Correction to “Confinement Induced Dilution: Electrostatic Screening Length Anomaly in Concentrated Electrolytes in Confined Space”

Correction to “Confinement Induced Dilution: Electrostatic Screening Length Anomaly in Concentrated Electrolytes in Confined Space”

Chiral motion in colloidal electrophoresis.

Asymmetrically charged, nonspherical colloidal particles in general perform complex rotations and oblique motions under an electric field. The interplay of electrostatic and hydrodynamic forces complicates the prediction of these motions. We demonstrate a method of calculating the body tensors that dictate translational and rotational velocity vectors arising from an external electric field. We treat insulating rigid bodies in the linear-response regime, with indefinitely small electrostatic screening length. The method represents the body as an assembly of point sources of both hydrodynamic drag and surface electric field. We demonstrate agreement with predicted electrophoretic mobility to within a few percent for several shapes with uniform and nonuniform charges. We show that even symmetric shapes can have strong chiral twisting motions. The method applies more generally to active colloidal swimmers.

Electrostatic Screening Length in “Soft” Electrolyte Solutions

Using the Surface Forces Apparatus on solutions of polymeric ions, the effect of specific ion-ion correlations is evaluated on the characteristic decay length, λ, of screened electrostatic interactions between charged surfaces. Electrolyte solutions composed of point charges surrounded by repulsive polymeric shells were used to elucidate the role of ions size and size asymmetry between co- and counterions on the screening of electrostatic forces. In electrolytes composed of large polymeric cations and small point-charge anions, because of the steric and excluded volume effects, the screening length follows the simple scaling relation λ ∼ d, where d is the characteristic size of the large cation. It is also reported that both co- and counterion sizes affect the thickness of the electrical double layer and influence the screened electrostatic interactions. In solutions of polymeric cation/anion pairs, the screening length is shown to depend on an asymmetry factor. These results provide insights into correlation effects in electrolytes, which were so far unreachable experimentally and help elucidate such effects in electronics, energy storage devices, and biomedical systems.

Solution Conditions Tune and Optimize Loading of Therapeutic Polyelectrolytes into Layer-by-Layer Functionalized Liposomes.

Layer-by-layer (LbL) nanoparticles offer great potential to the field of drug delivery, where these nanocomposites have been studied for their ability to deliver chemotherapeutic agents, small molecule inhibitors, and nucleic acids. Most exciting is their ability to encapsulate multiple functional elements, which allow nanocarriers to deliver complex combination therapies with staged release. However, relative to planar LbL constructs, colloidal LbL systems have not undergone extensive systematic studies that outline critical synthetic solution conditions needed for robust and efficient assembly. The multistaged process of adsorbing a series of materials onto a nanoscopic template is inherently complex, and facilitating the self-assembly of these materials depends on identifying proper solution conditions for each synthetic step and adsorbed material. Here, we focus on addressing some of the fundamental questions that must be answered in order to obtain a reliable and robust synthesis of nucleic acid-containing LbL liposomes. This includes a study of solution conditions, such as pH, ionic strength, salt composition, and valency, and their impact on the preparation of LbL nanoparticles. Our results provide insight into the selection of solution conditions to control the degree of ionization and the electrostatic screening length to suit the adsorption of nucleic acids and synthetic polypeptides. The optimization of these parameters led to a roughly 8-fold improvement in nucleic acid loading in LbL liposomes, indicating the importance of optimizing solution conditions in the preparation of therapeutic LbL nanoparticles. These results highlight the benefits of defining principles for constructing highly effective nanoparticle systems.

Electrostatic Screening Length in Concentrated Salt Solutions

Thin films (0-30 nm) of very concentrated aqueous monovalent salt solutions (2-10 M of LiCl, NaCl, and CsCl) were examined to determine how ionic strength affects the screening length of the electrostatic potential. Measurements were consistent with a screening length in the range of 3-12 nm. The screening length increased monotonically as a function of salt concentration, and the rate of increase was a function of the monovalent salt type. The results were incompatible with the Debye length of Poisson-Boltzmann theory but consistent with previous measurements of surface forces. The screening length was determined from the surface excess of fluorescein, a dianion under basic conditions, which was present in trace amounts in the thin film and detected via its fluorescence emission. That is, we directly observed that the ion concentration in very concentrated solutions is perturbed far from an interface.

The Intrinsic Stability of Metal Ion Complexes with Nanoparticulate Fulvic Acids.

The electrostatic contributions to metal ion binding by fulvic acids (FAs) are characterized in light of recent theoretical developments on description of the net charge density of soft nanoparticles. Under practical electrolyte concentrations, the radius of the small, highly charged soft nanoparticulate FAs is comparable to the electrostatic screening length and their electric potential profile has a bell shape that extends into the surrounding aqueous medium. Consequently, accumulation of counterions in the extraparticulate zone can be significant. By comparison of experimentally derived Boltzmann partitioning coefficients with those computed on the basis of (i) the structural FA particle charge and (ii) the potential profile for a nanoparticulate FA entity equilibrated with indifferent electrolyte, we identify the thickness of the extraparticulate counter charge accumulation shell in 1-1 and 2-1 electrolytes. The results point to the involvement of counterion condensation phenomena and call into question the approaches for modeling electrostatic contributions to ion binding that are invoked by popular equilibrium speciation codes. Overall, the electrostatic contributions to Cdaq2+ and Cuaq2+ association with FA are weaker than those previously found for much larger humic acids (HA). The intrinsic chemical binding strength of CdFA is comparable to that of CdHA, whereas CuFA complexes are weaker than CuHA ones.

Gate‐Induced Massive and Reversible Phase Transition of VO2 Channels Using Solid‐State Proton Electrolytes

AbstractThe use of gate bias to control electronic phases in VO2, an archetypical correlated oxide, offers a powerful method to probe their underlying physics, as well as for the potential to develop novel electronic devices. Up to date, purely electrostatic gating in 3‐terminal devices with correlated channel shows the limited electrostatic gating efficiency due to insufficiently induced carrier density and short electrostatic screening length. Here massive and reversible conductance modulation is shown in a VO2 channel by applying gate bias VG at low voltage by a solid‐state proton (H+) conductor. By using porous silica to modulate H+ concentration in VO2, gate‐induced reversible insulator‐to‐metal (I‐to‐M) phase transition at low voltage, and unprecedented two‐step insulator‐to‐metal‐to‐insulator (I‐to‐M‐to‐I) phase transition at high voltage are shown. VG strongly and efficiently injects H+ into the VO2 channel without creating oxygen deficiencies; this H+‐induced electronic phase transition occurs by giant modulation (≈7%) of out‐of‐plane lattice parameters as a result of H+‐induced chemical expansion. The results clarify the role of H+ on the electronic state of the correlated phases, and demonstrate the potentials for electronic devices that use ionic/electronic coupling.

Magnetotransport measurements on Bi2Te3 nanowires electrodeposited in etched ion-track membranes

We report on the magneto-transport properties of individual Bi2Te3 nanowires with diameters ranging from 25 nm to 120 nm fabricated by electrodeposition in etched ion-track membranes. The metallic behavior of all nanowires is typical for highly-degenerate semiconductors and the weak temperature dependence of the resistivity suggests a weak electron-phonon coupling, as expected for topological surface states with quantum confinement. The diameter dependence of the resistivity further confirms the quasi-ballistic nature of charge carriers, in agreement with the reduction of the number of surface conductance channels in disordered quantum wires with a stronger quantum confinement. Although the bulk conductance is not negligible, both the transport length of bulk carriers and their electrostatic screening length remain shorter than the nanowire diameter, probably leading to a small contribution to the resistivity change. Quantum transport measurements give further evidence of finite-size effects, as shown by the weak anti-localization correction to the conductance, below 15K, and reveal the coexistence of surface Aharonov-Bohm oscillations and bulk universal conductance fluctuations, below 5K.