Accumulation Of Counterions Research Articles

Direct Experimental Observations of Ion Distributions during Overcharging at the Muscovite-Water Interface by Adsorption of Rb+ and Halides (Cl- , Br- , I- ) at High Salinity.

Classical electric double layer (EDL) models have been widely used to describe ion distributions at charged solid-water interfaces in dilute electrolytes. However, the chemistry of EDLs remains poorly constrained at high ionic strength where ion-ion correlations control non-classical behavior such as overcharging, i. e., the accumulation of counter-ions in amounts exceeding the substrate's surface charge. Here, we provide direct experimental observations of correlated cation and anion distributions adsorbed at the muscovite (001)-aqueous electrolyte interface as a function of dissolved RbBr concentration ([RbBr]=0.01-5.8 M) using resonant anomalous X-ray reflectivity. Our results show alternating cation-anion layers in the EDL when [RbBr]≳100 mM, whose spatial extension (i. e., ~20 Å from the surface) far exceeds the dimension of the classical Stern layer. Comparison to RbCl and RbI electrolytes indicates that these behaviors are sensitive to the choice of co-ion. This new in-depth molecular-scale understanding of the EDL structure during transition from classical to non-classical regimes supports the development of realistic EDL models for technologies operating at high salinity such as water purification applications or modern electrochemical storage.

Droplet dynamics: A phase-field model of mobile charges, polarization, and its leaky dielectric approximation

This paper presents a Poisson–Nernst–Planck–Navier–Stokes–Cahn–Hillard (PNP–NS–CH) model for an electrically charged droplet suspended in a viscous fluid under an external electric field. Our model incorporates spatial variations in electric permittivity and diffusion constants, as well as interfacial capacitance. Based on a time scale analysis, we derive two approximations of the original model: a dynamic model for the net charge (assuming unchanged conductance) and a leaky-dielectric model (assuming unchanged conductance and net charge). For the leaky-dielectric model, we perform a detailed asymptotic analysis to demonstrate the convergence of the diffusive-interface leaky-dielectric model to the sharp interface model as the interface thickness approaches zero. Numerical computations are conducted to validate the asymptotic analysis and demonstrate the model's effectiveness in handling topology changes, such as electro-coalescence. Our numerical results from these two approximation models reveal that the polarization force, induced by the spatial variation in electric permittivity perpendicular to the external electric field, consistently dominates the Lorentz force arising from the net charge. The equilibrium shape of droplets is determined by the interplay between these two forces along the direction of the electric field. Moreover, in the presence of interfacial capacitance, a local variation in effective permittivity results in the accumulation of counter-ions near the interface, leading to a reduction in droplet deformation. Our numerical solutions also confirm that the leaky-dielectric model is a reasonable approximation of the original PNP–NS–CH model when the electric relaxation time is sufficiently short. Both the Lorentz force and droplet deformation decrease significantly when the diffusion of net charge increases.

Conditional existence of Donnan potential in soft particles and surfaces: Dependence on steric effects mediated by electrolyte ions and structural charges

When a charged layer decorating a particle or a macroscopic surface is equilibrated with an electrolyte solution, a constant Donnan potential is established through that layer due to charge-driven accumulation of counterions and companion exclusion of coions. This situation arises when the thickness of the surface layer well exceeds the screening Debye length, a condition derived from mean-field Poisson-Boltzmann theory within point-like charge approximation. Herein, we revisit this condition underlying the applicability of Donnan electrostatic representation with the account of steric effects mediated by the sizes of the electrolyte ions and structural layer charges. A transcendental equation is derived for the Donnan potential as a function of sizes and valences of anions and cations, electrolyte concentration and size of the layer charges, and a closed-form expression is provided for symmetrical electrolytes. Therefrom we evidence that the existence of a Donnan potential is conditioned not only to large values of the layer thickness compared to a here-defined Debye length operative within the shell, but to additional verification of a criterion that involves space charge density of the layer, solution ionic strength and electrolyte nondiluteness parameter. Illustrative computational examples show how the existence and magnitude of the Donnan potential depend on the key molecular descriptors of the electrolyte and soft interface, and they further quantify the deviations from predictions based on classical Donnan potential expression valid for dilute electrolytes.

Current Flow in a Cylindrical Nanopore with an Object-Implications for Virus Sensing.

Interest is growing in nanopores as real-time, low-cost, label-free virus size sensors. To optimize their performance, we evaluate how external electric field and ion concentrations and pore wall charges influence currents and object (disk) radius-current relationship using simulations. The physics was described using the Poisson-Nernst-Planck and Navier–Stokes equations. In a charged cylindrical nanopore with a charged disk, elevated external electric field produces higher (and polarity independent) ion concentrations and greater ion current (largely migratory). Elevated external ion concentrations also lead to higher concentrations (mainly away from the pore wall), greater axial electric field especially in the disk-pore wall space, and finally larger current. At low concentrations, current is disk radius independent. The current rises as concentrations increase. Interestingly, the rise is greater for larger disks (except when the pore is blocked mechanically). Smaller cross-sectional area for current flow or volume exclusion of electrolyte by object thus cannot be universally accepted as explanations of current blockage. Ion current rises when pore wall charge density increases, but its direction is independent of charge sign. Current-disk radius relationship is also independent of pore wall charge sign. If the pore wall and disk charges have the same sign, larger current with bigger disk is due to higher counter-ion accumulation in the object-pore wall space. However, if their signs are opposite, it is largely due to elevated axial electric field in the object-pore wall space. Finally in uncharged nanopores, current diminishes when disk radius increases making them better sensors of virus size.

Unusual properties of the electric double layer in an extremely narrow nanotube. A grand canonical Monte Carlo and classical DFT study

A grand canonical Monte Carlo (GCMC) simulation technique at a constant electrode–electrolyte potential drop and classical density functional theory (CDFT) have been employed to study the ion distribution and the differential capacitance of an electric double layer (EDL) in extremely narrow nanotubes. In the applied constant-potential GCMC method, to preserve the electroneutrality of the system after a single ion exchange, the electrode charge is suitably modified. The resulting charge fluctuations are used to calculate the differential capacitance of the EDL. Results for the ion distributions and differential capacitance in the nanotubes are reported for symmetric +1: 1 ionic valences with a common ionic diameter of 0.4 nm at electrolyte concentrations of 0.1 m, 1.0 m, and 2.5 m, a cylinder radius of 0.6 nm, potential drop varying from −0.6 V to 0.6 V, a relative permittivity of 78.5, and a temperature of 298.15 K. These results are compared with corresponding data for size-asymmetric systems with cations of diameters of 0.32 nm and 0.48 nm and ion-valence asymmetric systems of +2: 1 and +3: 1. At higher electrode surface charges, the counter-ion density distributions show an unusual accumulation along the axis of the nanotube. Differential capacitance plots have four or fewer, often three, maxima. The central minimum surrounded by two maxima undergoes a transition from a minimum to a maximum as the electrolyte concentration is increased. The positions of the side maxima indicate the electrode potential at which the rate of the unusual counter-ion accumulation is at its highest. Comparison between the results of simulations and CDFT suggests the latter to be reliable for investigation of the electrical capacitance properties of extreme nanoscale supercapacitors, even if its degree of accuracy for particle distributions decreases with increasing surface charge.

Apolar chemical environments compact unfolded RNAs and can promote folding

It is well documented that the structure, and thus function, of nucleic acids depends on the chemical environment surrounding them, which often includes potential proteinaceous binding partners. The nonpolar amino acid side chains of these proteins will invariably alter the polarity of the local chemical environment around the nucleic acid. However, we are only beginning to understand how environmental polarity generally influences the structural and energetic properties of RNA folding. Here, we use a series of aqueous-organic cosolvent mixtures to systematically modulate the solvent polarity around two different RNA folding constructs that can form either secondary or tertiary structural elements. Using single-molecule Förster resonance energy transfer spectroscopy to simultaneously monitor the structural and energetic properties of these RNAs, we show that the unfolded conformations of both model RNAs become more compact in apolar environments characterized by dielectric constants less than that of pure water. In the case of tertiary structure formation, this compaction also gives rise to more energetically favorable folding. We propose that these physical changes arise from an enhanced accumulation of counterions in the low dielectric environment surrounding the unfolded RNA.

The Interfacial Electrostatics Defines MscS Gating and Inactivation

The membrane tension that drives transitions in mechanosensitive proteins is transmitted primarily through the polar/apolar interfaces of the lipid bilayer. The mechanosensitive channel of small conductance (MscS) of E. coli has two prominent arginines, R46 and R74, in each subunit which anchor the TM1 and TM2 helices to the cytoplasmic boundary of the membrane. Alignments of 171 MscS homologs show that the presence of positive charges (R, K or H) in these two locations is highly conserved. A distinctive property of WT MscS is that it inactivates under moderate tension, thus properly terminating an osmotic permeability response. Under depolarizing voltages, MscS inactivation is accelerated; hyperpolarization, on the contrary, slows down inactivation and accelerates recovery. Therefore, MscS utilizes normal membrane potential to return back to the resting state. We generated a variety of double mutants at these positions to investigate the interactions with the lipid head groups and examine the voltage dependence of inactivation. We found that N or T substitutions for R46 and R74 produce functional channels, even preserving the voltage sensitivity. The D, A or V mutations produce channels that do not rescue the osmotically sensitive MJF465 strain and do not open in full, demonstrating 10-30% of WT conductance. L, Y and W mutants support the gating and rescue, but remove the voltage dependence of inactivation and recovery. The data indicates that polar interactions between helix-anchoring residues and interfacial lipid phosphates are critical for gating. The transition to the open or inactivated state likely increases the number of favorable contacts with phosphates. Voltage dependence of inactivation can be related to the Lippmann's electrowetting effect, accumulation of co-ions or counter-ions in the double layer on the cytoplasmic surface, which changes the hydration of the TM2-TM3 interface that transmits tension to the gate.

Electrocatalysis in confined space

The complex interplay of restricted mass transport leading to local accumulation or depletion of educts, intermediates, products, counterions and co-ions influences the reactions at the active sites of electrocatalysts when electrodes are rough, three-dimensionally mesoporous or nanoporous. This influence is important with regard to activity, and even more to selectivity, of electrocatalytic reactions. The underlying principles are discussed based on the growing awareness of these considerations over recent years.

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.

Impact of Electrostatic Interactions on Lecithin-Stabilized Model O/W Emulsions

Natural emulsifiers, particularly those extracted from plants, are highly wanted by food industry to meet consumers demand for clean label food and beverage products. The potential utilization of soy lecithin as an emulsifier in model coffee creamer was investigated in this study. The model oil-in-water (O/W) emulsions consisted of 10 wt% medium chain triglyceride were stabilized using either 1% or 5% soy lecithin (pH 7.0). The O/W emulsions were of whitish milky color (L* = 88–92) and were able to whiten black coffee solutions (L* from 5.5 for black coffee to 44–56 for white coffees). Model O/W emulsions with smaller mean droplet diameters (0.11 to 1.09 μm), higher surface potentials (ζ = −62 to −72 mV), and better stabilities in hot coffee were fabricated using higher lecithin levels because there was more emulsifier to coat the oil droplet surfaces. Alteration of the electrostatic interactions in the model O/W emulsions (5% lecithin) by pH adjustment or calcium addition led to droplet aggregation under certain conditions, which was attributed to charge reduction by protonation of lecithin head groups and electrostatic screening by counter-ion accumulation and ion-binding. In particular, phase separation of the model creamer occurred at pH value around 4.5 when the system was acidified at a slow rate. Overall, this study suggests that lecithin-stabilized O/W emulsions may become unstable in coffee solutions with high acidity or calcium levels. The information obtained from this study provides insights on the use of plant-based emulsifiers in commercial food and beverage systems.

Enhancement of differential double layer capacitance and charge accumulation by tuning the composition of ionic liquids mixtures

Evolution from fossil fuel energy to renewable energy sources and technologies is in the spotlight towards an accelerated energy transition process. One of the challenges of the intermittent renewable energy production is related to the existence of an appropriate energy storage technology in order to effectively use the renewable energy generated. Electrochemical energy storage devices rely on the key property of the electrical double layer integral capacitance. The use of mixed ionic liquids can be an effective strategy to increase the performance of electric double layer capacitors.Here, the studies on the interfacial behaviour of ionic liquids mixtures containing a common ion for a model mercury/ionic liquid interface are reported. Enhancement of the differential capacitance, nearly 3 times higher compared to ILs in the pure state, was achieved by an appropriate combination of ion size both in cation and the anion and asymmetry. The results are interpreted as a consequence of surface voids occupation and by the accumulation of more counter ions and displacement larger anion by the smaller anion in the mixture.

Electrohydrodynamics of spherical polyampholyte-grafted nanoparticles: Multiscale simulations by coupling of molecular dynamics and lattice-boltzmann method

We perform multiscale simulations based on the coupling of molecular dynamics and lattice-Boltzmann (LB) method to study the electrohydrodynamics of a polyampholyte-grafted spherical nanoparticle. The long-range hydrodynamic interactions are modeled by coupling the movement of particles to a LB fluid. Our results indicate that the net-neutral soft particle moves with a nonzero mobility under applied electric fields. We systematically explore the effects of different parameters, including the chain length, grafting density, electric field, and charge sequence, on the structures of the polymer layer and the electrophoretic mobility of the soft particle. It shows that the mobility of nanoparticles has remarkable dependence on these parameters. We find that the deformation of the polyampholyte chains and the ion distribution play dominant roles in modulating the electrokinetic behavior of the polyampholyte-grafted particle. The enhancement or reduction in the accumulation of counterions around monomers can be attributed to the polymer layer structure and the conformational transition of the chains in the electric field. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017

Counterion accumulation effects on a suspension of DNA molecules: Equation of state and pressure-driven denaturation.

The study of the effects associated with the electrostatic properties of DNA is of fundamental importance to understand both its molecular properties at the single molecule level, like the rigidity of the chain, and its interaction with other charged bio-molecules, including other DNA molecules; such interactions are crucial to maintain the thermodynamic stability of the intra-cellular medium. In the present work, we combine the Poisson-Boltzmann mean-field theory with an irreversible thermodynamic approximation to analyze the effects of counterion accumulation inside DNA on both the denaturation profile of the chain and the equation of state of the suspension. To this end, we model the DNA molecule as a porous charged cylinder immersed in an aqueous solution. These thermo-electrostatic effects are explicitly studied in the particular case of some genes for which damage in their sequence is associated with diffuse large B-cell lymphoma.

Accumulation of counterions and coions evaluated by cryogenic XPS as a new tool for describing the structure of electric double layer at the silica/water interface.

We introduce a new method of evaluating the structure of electric double layer (EDL) at the native solid/liquid interface using cryogenic X-ray photoelectron spectroscopy technique. This method is based on evaluating the atomic concentration ratio of counterions and co-ions of supporting electrolyte at the close-to-in situ state surface of colloid particles by the cryo-XPS and comparing it with analogous ratio predicted by EDL models. For silica colloids in aqueous KCl solutions at pH 6 to 8 it has been found that the latter ratio is higher than unity, as expected for the negatively charged surface of silica, but does not correspond with the prediction of the basic Gouy-Chapman EDL model for the ideal interface. However, it agrees with that deduced from experiments on electrolytic coagulation kinetics of analogous silica colloids by applying a simple EDL model of swellable ion-permeable (Donnanian) polyelectrolyte gel layer. It turns out that the traditional Stern layer-based concept of EDL at solid/liquid interfaces is not justified for metal oxides at least in KCl solutions.

Salting-Out and Salting-In of Polyelectrolyte Solutions: A Liquid-State Theory Study

We study the phase behavior of polyelectrolyte (PE) solutions with salt using a simple liquid-state (LS) theory. This LS theory accounts for hard-core excluded volume repulsion by the Boublik–Mansoori–Carnahan–Starling–Leland equation of state, electrostatic correlation by the mean-spherical approximation, and chain connectivity by the first-order thermodynamic perturbation theory. We predict a closed-loop binodal curve in the PE concentration-salt concentration phase diagram when the Bjerrum length is smaller than the critical Bjerrum length in salt-free PE solution. The phase-separated region shrinks with decreasing Bjerrum length, and disappears below a critical Bjerrum length. These results are qualitatively consistent with experiments, but cannot be captured by the Voorn–Overbeek theory. On the basis of the closed-loop binodal curve and the lever rule, we predict three scenarios of salting-out and salting-in phenomena with addition of monovalent salt into an initially salt-free PE solution. The Galvani potential—the electric potential difference between the coexisting phases—is found to depend nonmonotonically on the overall salt concentration in some PE concentration range, which is related to the partition of the co-ions in the coexisting phases. Free energy analysis suggests that phase separation is driven by a gain in the electrostatic energy, at the expense of a large translational entropy penalty, due to significant counterion accumulation in the PE-rich phase.

Ionic Effects on VEGF G-Quadruplex Stability.

In a potassium solution, a modified 22-meric DNA sequence Pu22-T12T13 from a region proximal to the transcription initiation site of the human VEGF gene adopts a single parallel-stranded G-quadruplex conformation with a 1:4:1 loop-size arrangement. We measured the thermal stability, TM, of the K(+)-stabilized Pu22-T12T13 G-quadruplex as a function of stabilizing K(+) ions and nonstabilizing Cs(+) and TMA(+) ions. The thermal stability, TM, of the Pu22-T12T13 G-quadruplex increases with the concentration of the stabilizing potassium ions, while it sharply decreases upon the addition of the nonstabilizing cations. We interpret these results as underscoring the opposing effects of internal binding and counterion condensation on the stability of the Pu22-T12T13 G-quadruplex. While centrally bound ions stabilize the G-quadruplex conformation, counterion condensation destabilizes it, favoring the coil conformation. From the initial slopes of the dependences of TM on the concentration of Cs(+) and TMA(+) cations, we estimate that the deleterious effect of counterion condensation stems from roughly one extra counterion associated with the coil relative to the G-quadruplex state of Pu22-T12T13. The reduced accumulation of counterions around the G-quadruplex state of Pu22-T12T13 relative to its coil state is due to the low surface charge density of the G-quadruplex reflecting its structural characteristics. On the basis of the analysis of our data along with the results of a previous study, we propose that the differential effect of internally (stabilizing) and externally (destabilizing) bound cations may be a general feature of parallel intramolecular G-quadruplexes.

Charge reversal at a planar boundary between two dielectrics.

Despite the ubiquitous character and relevance of the electric double layer in the entire realm of interface and colloid science, very little is known of the effect that surface heterogeneity exerts on the underlying mechanisms of ion adsorption. Herein, computer simulations offer a perspective that, in sharp contrast to the homogeneously charged surface, discrete groups promote multivalent counterion binding, leading to charge reversal but possibly having not a sign change of the electrophoretic mobility. Counterintuitively, the introduction of dielectric images yields a significantly greater accumulation of counterions, which further facilitates the magnitude of charge reversal. The reported results are very sensitive to both the degree of ion hydration and the representation of surface charges. Our findings shed light on the mechanism for charge reversal over a broad range of coupling regimes operating the adsorption of counterions through surface group bridging attraction with their own images and provide opportunities for experimental studies and theoretical development.

Asymmetric size of ions and orientational ordering of water dipoles in electric double layer model - an analytical mean-field approach

Electric double layer is theoretically described within the mean-field approach by taking into account the orientational ordering of water dipoles and asymmetric size of cations and anions. Analytical expressions for the spatial distribution of ions and water dipoles are derived. The effect of asymmetric ionic size on accumulation of counterions and partial depletion of water molecules near the charged surface, on spatial dependence of relative permittivity and on differential capacitance of electric double layer are presented.

Adsorption of charged and neutral polymer chains on silica surfaces: the role of electrostatics, volume exclusion, and hydrogen bonding.

We develop an off-lattice (continuum) model to describe the adsorption of neutral polymer chains and polyelectrolytes to surfaces. Our continuum description allows taking excluded volume interactions between polymer chains and ions directly into account. To implement those interactions, we use a modified hard-sphere equation of state, adapted for mixtures of connected beads. Our model is applicable to neutral, charged, and ionizable surfaces and polymer chains alike and accounts for polarizability effects of the adsorbed layer and chemical interactions between polymer chains and the surface. We compare our model predictions to data of a classical system for polymer adsorption: neutral poly(N-vinylpyrrolidone) (PVP) on silica surfaces. The model shows that PVP adsorption on silica is driven by surface hydrogen bonding with an effective maximum binding energy of about 1.3k(B)T per PVP segment at low pH. As the pH increases, the Si-OH groups become increasingly dissociated, leading to a lower capacity for H bonding and simultaneous counterion accumulation and volume exclusion close to the surface. Together these effects result in a characteristic adsorption isotherm, with the adsorbed amount dropping sharply at a critical pH. Using this model for adsorption data on silica surfaces cleaned by either a piranha solution or an O(2) plasma, we find that the former have a significantly higher density of silanol groups.

Ion permeation inside microgel particles induced by specific interactions: from charge inversion to overcharging.

In this work we have performed a theoretical study of a system formed by ionic microgels in the presence of monovalent salt with the help of Ornstein-Zernike integral equations within the hypernetted-chain (HNC) approximation. We focus in particular on analysing the role that the short-range specific interactions between the polymer fibres of the microgel and the incoming ions have on the equilibrium ion distribution inside and outside the microgel. For this purpose, a theoretical model based on the equilibrium partitioning effect is developed to determine the interaction between the microgel particle and a single ion. The results indicate that when counterions are specifically attracted to the polymer fibres of the microgel, an enhanced counterion accumulation occurs that induces the charge inversion of the microgel and a strong increase of the microgel net charge (or overcharging). In the case of coions, the specific attraction is also able to provoke the coion adsorption even though they are electrostatically repelled, and so increasing the microgel charge (true overcharging). Moreover, we show that ion adsorption onto the microgel particle is very different in swollen and shrunken states due to the competition between specific attraction and steric repulsion. In particular, ion adsorption occurs preferentially in the internal core of the particle for swollen states, whereas it is mainly concentrated in the external shell for de-swollen configurations. Finally, we observe the existence of a critical salt concentration, where the net charge of the microgels vanishes; above this inversion point the net charge of the microgels increases again, thus leading to reentrant stability of microgel suspensions.