Electric Double Layer Research Articles

Mechanism of overscreening breakdown by molecular-scale electrode surface morphology in asymmetric ionic liquids

The interfacial nature of the electric double layer (EDL) assumes that electrode surface morphology significantly impacts the EDL properties. Since molecular-scale roughness modifies the structure of EDL, it is expected to disturb the overscreening effect and alter differential capacitance (DC). In this paper, we present a model that describes EDL near atomically rough electrodes with account for short-range electrostatic correlations. We provide numerical and analytical solutions for the analysis of conditions for the overscreening breakdown and DC shift estimation. Our findings reveal that electrode surface structure leads to DC decrease and can both break or enhance overscreening depending on the relation of surface roughness to electrostatic correlation length and ion size asymmetry.

Non-isothermal electroosmotic flow of a viscoelastic fluid through a porous medium in a microchannel

The influence of the temperature-dependent viscosity on the electroosmotic flow through a porous medium in a microchannel of parallel flat plates. At the end, an imposed electric field induces Joule heating and temperature gradients, modifying the viscosity of the fluid. The Phan-Thien–Tanner and Darcy–Forchheimer models describe the viscoelastic fluid through the porous media. Consequently, the electric double layer (EDL) overlaps at the center plane of the microchannel, causing an electrically charged porous matrix and modifying the zeta potential along the microchannel walls. Therefore, we define a zeta potential ratio of the porous matrix and that at the microchannel walls. Unlike similar investigations, the boundary condition for the zeta potential varies locally due to the temperature field, modifying the interaction among hydrodynamics, electrical, viscoelastic, thermal and porosity effects. A modified Péclet number is defined as a function of the porosity and thermal conductivity of the fluid and an effective charge density is determined based on the Poisson–Boltzmann equation, accounting for the pore size and the zeta potential ratio. The set of equations are solved numerically considering the lubrication approximation theory. We found that the influence of the temperature-dependent viscosity increases the shear stresses at the EDL region, particularly where the temperature rises to maximum values. Also, the overlapping of the EDL occurs mainly for low values of permeability. Finally, the pressure gradient involved in the Darcy law can be altered through the zeta potential ratio.

Mechanical and physicochemical characteristics of sub-millimeter and macrobubbles: Insights from AFM analysis, zeta potential measurements and rise velocity

This study presents the change in mechanical and physicochemical behaviour of macrobubbles and sub-millimeter bubbles with respect to pH. While macrobubbles conventionally range from 100 μm to 2 mm in diameter, this study focuses on the comparatively less-explored sub-millimeter bubble range, centered around a median diameter of 500 μm. The difference in sub-millimeter bubbles (500 μm) compared to their larger counterparts (1100 μm) within macrobubble size range were evidenced through atomic force microscopy (AFM) analysis, zeta potential measurements, and rise velocity experiments. The adhesion force and energy between bubbles and the silicon nitride cantilever tip showed a decreasing trend with increasing pH, notably diminishing by about 50% beyond pH 9. Although no significant differences were statistically revealed between the two bubble sizes at lower pH, the adhesion force and energy were consistently smaller for macrobubble compared to sub-millimeter bubble at higher pH beyond pH 9. This finding suggests that the electrical double layer (EDL) interactions are influenced by the specific surface area, particularly in the alkaline region. Zeta potential measurements exhibited a cubic trend, with an isoelectric point at pH 3.5 and a minimum zeta potential at pH 11. The study also depicted a similar trend in the rise velocity of bubbles, suggesting a direct relationship between bubble stiffness and its rise velocity. Therefore, changes in pH affected the bubbles' properties, while on the other hand, certain physicochemical properties are also influenced by bubble sizes.

Interfacial Electrochemistry of Catalyst-Coordinated Graphene Nanoribbons.

The immobilization of molecular electrocatalysts on conductive electrodes is an appealing strategy for enhancing their overall activity relative to those of analogous molecular compounds. In this study, we report on the interfacial electrochemistry of self-assembled two-dimensional nanosheets of graphene nanoribbons (GNR-2DNS) and analogs containing a Rh-based hydrogen evolution reaction (HER) catalyst (RhGNR-2DNS) immobilized on conductive electrodes. Proton-coupled electron transfer (PCET) taking place at N-centers of the nanoribbons was utilized as an indirect reporter of the interfacial electric fields experienced by the monolayer nanosheet located within the electric double layer. The experimental Pourbaix diagrams were compared with a theoretical model, which derives the experimental Pourbaix slopes as a function of parameter f, a fraction of the interfacial potential drop experienced by the redox-active group. Interestingly, our study revealed that GNR-2DNS was strongly coupled to glassy carbon electrodes (f = 1), while RhGNR-2DNS was not (f = 0.15). We further investigated the HER mechanism by RhGNR-2DNS using electrochemical and X-ray absorption spectroelectrochemical methods and compared it to homogeneous molecular model compounds. RhGNR-2DNS was found to be an active HER electrocatalyst over a broader set of aqueous pH conditions than its molecular analogs. We find that the improved HER performance in the immobilized catalyst arises due to two factors. First, redox-active bipyrimidine-based ligands were shown to dramatically alter the activity of Rh sites by increasing the electron density at the active Rh center and providing RhGNR-2DNS with improved catalysis. Second, catalyst immobilization was found to prevent catalyst aggregation that was found to occur for the molecular analog in the basic pH. Overall, this study provides valuable insights into the mechanism by which catalyst immobilization can affect the overall electrocatalytic performance.

Engineering battery corrosion films by tuning electrical double layer composition

Engineering battery corrosion films by tuning electrical double layer composition

(Sm3+/Eu3+/Tm3+)-TiO2 heterostructures for electrochemical and photovoltaic applications

This study reports on the synthesis of lanthanide oxides triply doped titania, forming a novel hetero-system known as (Sm3+/Eu3+/Tm3+)-TiO2. An environmentally friendly sol gel method was used to synthesize the energy-efficient (Sm3+/Eu3+/Tm3+)-TiO2, and thin films were deposited by spin coating. Electrode and solar cell devices were fabricated under ambient conditions, and rare earth doping enhanced the opto-electronic aspects of the host by narrowing the band gap up to 3.92, 3.85, and 3.56 eV. Particle sizes for the pure and doped materials were 85.15 and 82.72 nm, respectively. With a specific capacitance of 667.34 F g−1, the lanthanide-doped ornamented nickel foam electrode outperformed the pristine electrode, which had a specific capacitance of 169.59 F g−1. Moreover, this electrode exhibited tiny equivalent series resistance (Rs) of 0.13 Ω, indicating quicker response kinetics at the interface, in addition to its electrical double layer capacitance behavior. Electrochemical studies revealed that (Sm3+/Eu3+/Tm3+)-TiO2 semiconductor has a high specific power density of 8157.03 W kg−1, compared to 4079.73 W kg−1 for undoped titania. This improved specific power density demonstrates its suitability for practical scale applications. The produced materials were an excellent HER electro-catalyst with remarkably lower overpotential and Tafel slopes i.e., of 137 mV and 87.6 mV dec−1, in a respective manner, in comparison to the reference TiO2 with 147 mV and 104.5 mV dec−1, respectively. The electro-catalyst's response to the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was enhanced upon lanthanide doping. Additionally, this substance successfully retrieved electrons from a perovskite solar cell device, achieving efficiency levels of 16.49 and 16.75 % in both forward and reverse bias with fill factor increase.

Interface engineering enabling non-corrosive sulfonimide salt for 4.4 V class lithium batteries

Lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) with outstanding electrochemical and thermal stability has promising applications in electrolytes for lithium batteries. However, a low concentration of the sulfonimide salt electrolyte can corrode a commonly used aluminum (Al) current collector, posing a perennial conundrum that hinders its wide application. In this work, we find that pure H2O as an additive in the 1 M LiTFSI electrolyte effectively protects the Al current collector from corrosion by TFSI−. The interface engineering by specific adsorption of H2O plays a pivotal role in regulating the structure of the electrical double layer on the Al foil surface, blocking the interaction of TFSI− with Al. Meanwhile, hydrogen bonds are verified between H2O and TFSI−, weakening the attack of TFSI− on the Al collector and improving the Li+ conductivity. The mechanism has been confirmed by theoretical calculations and a variety of characterizations. Benefiting from the addition of H2O, the lithium batteries exhibit excellent cycling performance at a high cut-off voltage of 4.4 V even at −20 °C and a 3 Ah pouch full cell shows long cycling performance. The results demonstrate the high compatibility of the H2O-added electrolyte with high voltage cathode, lithium metal and graphite anodes. This work introduces a strategy for the utilization of sulfonimide salt at high voltage and wide temperature even at a low concentration.

Mechanism of sodium lauryl sulfate reducing froth stability of dodecylamine in hematite flotation: Experiments and molecular dynamics simulation

In this study, the influence of sodium lauryl sulfate (SLS) on the stability of dodecylamine (DDA) froth during the reverse flotation of hematite was investigated due to the frequent issue of excessively viscous foam leading to suboptimal reverse flotation outcomes. The research involved conducting flotation tests and two/three-phase froth stability assessments to explore how a specific molar ratio combination of DDA and SLS could enhance the flotation performance while simultaneously mitigating froth stability. The investigation into the mechanism by which SLS reduces the stability of DDA froth was aided by molecular dynamics simulations. The findings revealed that in the collector system comprising DDA and SLS, several key changes occurred at the molecular level. These included a decrease in the thickness of the electric double layer at the interface, a reduction in the thickness of the gas–liquid interface layer, weakening of the interaction strength between the head group and water molecules, a decrease in the coordination numbers of water molecules in the first hydration layer, and an enhanced migration ability of water molecules. Furthermore, there was an increased inclination angle between the head groups or molecular chains of the collector and the Z-axis, resulting in a diminished interaction depth between the head group and water and a closer positioning of the molecular tail chain to the gas–liquid interface with a more pronounced curvature. Consequently, these molecular adjustments led to a sparser arrangement of DDA molecules at the gas–liquid interface, lower coverage of the interface adsorption layer, and increased gas permeation, ultimately reducing DDA froth stability. The aforementioned conclusions provide a comprehensive molecular-level analysis of the principal mechanisms that can effectively reduce foam stability, including the attenuation of molecular head group-water interactions, reduction in gas–liquid interface layer thickness, decrease in gas–liquid interface orderliness, and enhancement of water molecule migration capability. The implications of this research extend to providing theoretical insights for addressing foam control challenges in DDA flotation processes.

Biomass-tar-derived oxygen-enriched porous carbon as a high-performance carbon electrode material for double electric layer and zinc-ion hybrid supercapacitor

Carbon-based capacitor electrodes have the advantages of high capacitance value, superior recyclability, and inexpensive raw materials, which fuel the application of capacitors in large-scale energy storage. Biomass tar is a hazardous waste that can be recycled as a low-cost carbon electrode precursor for capacitors, due to its retaining biomass-based heteroatom. In this study, hierarchically porous carbon with cross-linked structure was fabricated by using a nano-Al2O3 template combined with the KOH activation technique, in which the γ-Al2O3 has a size of only 10 nm with high specific surface area, thus optimizing the pore structure of the carbon material. The resulting optimal material, BTC-0.2-2-800, contains abundant pore structure and extremely high oxygen content, delivers low resistance and excellent rate performance with high specific capacitance of 292.1 F g−1 and 221.7 F g−1 at 1 A g−1 and 20 A g−1, respectively, in double electric layer supercapacitors (EDLCs). Besides, no significant capacity degradation after 10,000 charging and discharging cycles at 10 A g−1 was found. The two-electrode device prepared using Na2SO4 electrolyte achieves a high energy density of 43.18 Wh kg−1 at the power density of 1000 W kg−1, which can light up a 2 V LED lamp. Moreover, Zinc-ion hybrid supercapacitor (ZIHSC) fabricated with BTC-0.2-2-800 carbon electrode accounts for the energy densities of 69.82 Wh kg−1 and 49.39 Wh kg−1 at the power densities of 40 W kg−1 and 400 W kg−1, respectively. This study reveals the great potential of biomass-tar-derived carbon as a high-performance electrode material, which may provide an opportunities for the resource utilization of hazardous wastes.

Single-Particle Imaging Reveals the Electrical Double-Layer Modulated Ion Dynamics at Crowded Interface.

The dynamics of ion transport at the interface is the critical factor for determining the performance of an electrochemical energy storage device. While practical applications are realized in concentrated electrolytes and nanopores, there is a limited understanding of their ion dynamic features. Herein, we studied the interfacial ion dynamics in room-temperature ionic liquids by transient single-particle imaging with microsecond-scale resolution. We observed slowed-down dynamics at lower potential while acceleration was observed at higher potential. Combined with simulation, we found that the microstructure evolution of the electric double layer (EDL) results in potential-dependent kinetics. Then, we established a correspondence between the ion dynamics and interfacial ion composition. Besides, the ordered ion orientation within EDL is also an essential factor for accelerating interfacial ion transport. These results inspire us with a new possibility to optimize electrochemical energy storage through the good control of the rational design of the interfacial ion structures.

Electrochemical on-surface synthesis of a strong electron-donating graphene nanoribbon catalyst

On-surface synthesis of edge-functionalized graphene nanoribbons (GNRs) has attracted much attention. However, producing such GNRs on a large scale through on-surface synthesis under ultra-high vacuum on thermally activated metal surfaces has been challenging. This is mainly due to the decomposition of functional groups at temperatures of 300 to 500 °C and limited monolayer GNR growth based on the metal catalysis. To overcome these obstacles, we developed an on-surface electrochemical technique that utilizes redox reactions of asymmetric precursors at an electric double layer where a strong electric field is confined to the liquid-solid interface. We successfully demonstrate layer-by-layer growth of strong electron-donating GNRs on electrodes at temperatures <80 °C without decomposing functional groups. We show that high-voltage facilitates previously unknown heterochiral di-cationic polymerization. Electrochemically produced GNRs exhibiting one of the strongest electron-donating properties known, enable extraordinary silicon-etching catalytic activity, exceeding those of noble metals, with superior photoconductive properties. Our technique advances the possibility of producing various edge-functional GNRs.

Investigation of heat transfer phenomenon in a complex wavy divergent channel under the effects of thermal radiation, joule heating and electroosmosis

The heat transfer analysis in electro osmotic flow of nanofluid is studied through the complex wavy channel under the consideration of joule heat and thermal radiation effects. The Ellis fluid is considered a base fluid and gold particles are suspended in them. The complex system of equations is converted to dimensionless form with the help of dimensionless transformation and parameters. The concepts of longer wavelength and lower Reynolds number are utilized to simplify the problem and obtain the exact solution of velocity and temperature distribution by using the built-in command DSolve in the mathematical software MATHEMATICA 13.3. The graphs are plotted with the same software to check the effects of various parameters on velocity, temperature, heat transfer rate, and streamlines with the following range of parameters − 5 ≤ U H S ≤ 5 , 0 ≤ β ≤ 5 , 0 ≤ α ≤ 3 , 0 < k ≤ 5 , 0 ≤ ϕ ≤ 0.04 , 20 ≤ P r ≤ 28 , 0 ≤ R n ≤ 5 . The computational results reported that the electroosmotic parameter, Helmholtz-Smoluchowski velocity, and material parameter reduce the velocity in the initial region and enhance the final region of the channel. It is also noted that the velocity profile exhibits increasing and decreasing behavior in the initial and final region respectively with upgrading the values of volumetric concentration of nanoparticles. The heat transfer rate is strongly affected by material parameters α and β , thermal radiation parameter, Prandtl number, and joule heating and these parameters promote the heat transfer rate of the system. The novel research of the current study shows the examination of the heat transfer analysis of blood-gold nanofluid in the presence of an electrical double layer and thermal radiation with the help of the Ellis nanofluid flow model through a complex symmetric wavy channel. The results of the present investigation are useful for the early detection of renal diseases and monitoring of disease progression because of the unique properties of gold nanoparticles. The role of gold nanoparticles as imaging agents, drug delivery antioxidant properties, biosensing, targeted therapy, and renal clearance makes them the perfect choice for renal diseases.

Low-voltage solution-processed Sn-doped CuI neuromorphic transistors with synaptic plasticity and pain mimicked

In this article, SnxCu1−xI thin-film transistors were fabricated on a glass substrate, with CuI doped with varying concentrations of SnI2 serving as the channel and chitosan as the dielectric. When x = 0.06, the device exhibited optimal performance: a current on/off ratio of 2.56 × 105, a subthreshold slope of 31.67 mV/dec, a threshold voltage of 1.33 V, and a saturated field-effect mobility of 21.75 cm2 V−1 s−1. Due to the electric double layer effect of chitosan, the operating voltage of the devices was reduced to below 2 V. Simulations were also conducted on the behavior and functionality of artificial synapses, such as short-term plasticity, long-term plasticity, and paired-pulse facilitation. Building upon the functionalities of artificial synapses, the Sn0.06Cu0.94I neuromorphic transistors simulated the fundamental pain perception function of biological nociceptors. Finally, the effects of bias stress and laser irradiation on the devices were investigated, indicating the excellent stability of the Sn0.06Cu0.94I neuromorphic transistors. Fabricated via the solution process, this low-voltage neuromorphic transistors hold significant implications for applications in bionic sensing systems and neuromorphic chip technology.

Electrokinetic properties of NaCl solution via molecular dynamics simulations with scaled-charge electrolytes.

Scaling ionic charges has become an alternative to polarizable force fields for representing indirect charge transfer effects in molecular simulations. In our work, we apply molecular dynamics simulations to investigate the properties of NaCl aqueous solutions in homogeneous and confined media. We compare classical integer- and scaled-charge force fields for the ions. In the bulk, we validate the force fields by computing equilibrium and transport properties and comparing them with experimental data. Integer-charge ions overestimate dielectric saturation and ionic association. Both force fields present an excess in ion-ion correlation, which leads to a deviation in the ionic conductivity at higher ionic strengths. Negatively charged quartz is used to simulate the confinement effect. Electrostatic interactions dominate counter-ion adsorption. Full-charge ions have stronger and more defined adsorption planes. We obtain the electroosmotic mobility of the solution by combining the shear plane location from non-equilibrium simulations with the ionic distribution from equilibrium simulations. From the Helmholtz-Smoluchowski equation, the zeta potential and the streaming potential coupling coefficient are computed. From an atomic-scale perspective, our molecular dynamics simulations corroborate the hypothesis of maximum packing of the Stern layer, which results in a stable and non-zero zeta potential at high salinity. The scaled-charge model representation of both properties is in excellent qualitative and quantitative agreement with experimental data. With our work, we demonstrate how useful and precise simple scaled-charge models for electrolytes can be to represent complex systems, such as the electrical double layer.

Selection of suitable organic electrolyte for CO2 electro-reduction to CO in three-compartment electrolysis cell with NaOH and Cl2 yielded as byproducts

Developing high performance electrolyte is essential in improving the performance of CO2 electro-reduction. In this work, we have constructed a three-compartment electrolysis cell for CO2 reduction to CO in organic electrolyte, with NaOH and Cl2 produced as byproducts. The generated CO and Cl2 can be served as feedstock for the production of phosgene, which is a widely used acylating agent for producing valuable downstream products. In order to improve the performance of the electrolysis cell, various commonly used organic solvents and supporting electrolytes were investigated. Consequently, 0.72 M Bu4NCF3SO3 in propylene carbonate (PC) was selected due to its high CO2 solubility. This electrolyte enabled the achievement of a current density of 27 mA/cm2 at a potential of −2.0 V (vs. SHE), with a stable Faradaic efficiency of 93 % for CO production. Compared with an aqueous catholytes, the altered double electric layer in the organic electrolyte prevents cathode poisoning, which is crucial for practical applications.

New insights into the role of nitrogen doping in microporous carbon on the capacitive charge storage mechanism: From ab initio to machine learning accelerated molecular dynamics

Fundamental understandings of the relationship between ion-electrode interaction and structural feature in porous carbon electrodes at a molecular level provides guidelines for the design of high-performance electric double layer supercapacitors. It is certified by experiments that porous carbon structures doped with nitrogen show enhanced capacitive performance. However, in the theoretical simulations, the fundamental charge storage mechanism is still elusive. In particular, the recent experimental result shows that the generally ignored nitrolic nitrogen (N5) in porous carbon exhibits a positive effect on capacitance, while graphitic nitrogen (N3) does the opposite, which is against with the simulation results based the 2D-modeled porous graphene structure. Here, we perform ab initio molecular dynamics simulations on the N3 and N5-doped carbon/electrolyte interfaces, including both 2D planar and 3D microporous carbon electrodes. Our calculation indicates that N3 in the 3D pore hinders the electrolyte transport, while N5-doped micropore still serves as an electrolyte transport channel through the formation of H-bond. The charge storage mechanism is further elucidated by the analysis of the well equilibrated interfaces obtained from the machine learning force field accelerated molecular dynamics. Our work provides a new insight into the effect of nitrogen doping in 3D porous carbon, which is exactly opposite to the 2D planar graphene. Therefore, we emphasize that differences in the electrochemical conditions of 2D planar and 3D microporous carbon electrodes should be fully considered when analyzing the effects of surface chemistry on charge storage mechanisms.

Investigation of induced electroosmotic flow in small-scale capillary electrophoresis devices: Strategies for control and reversal.

Electroosmotic flow (EOF) is the bulk flow of solution in a capillary or microchannel induced by an applied electric potential. For capillary and microchip electrophoresis, the EOF enables analysis of both cations and anions in one separation and can be varied to modify separation speed and resolution. The EOF arises from an electrical double layer at the capillary wall and is normally controlled through the pH and ionic strength of the background buffer or with the use of additives. Understanding and controlling the electrical double layer is therefore critical for maintaining acceptable repeatability during method development. Surprisingly, in fused silica capillaries at low pH, studies observe an EOF even though the capillary surface should be neutralized. Previous work has suggested the presence of an "induced electroosmotic flow" from radial electric fields generated across the capillary wall due to the separation voltage and grounded components external to the capillary. Using thin-wall (15µm) fused silica separation capillaries to facilitate the study of radial fields, we show that the EOF mobility depends on both the separation voltage and the location of external grounds. This is consistent with the induced EOF model, in which radial electric fields embed positive charges at the capillary walls to create an electrical double layer. The magnitude of the effect is characterized and shown to have long-range influences that are difficult to completely null by moving grounded components away from the separation capillary. Instead, active EOF control using externally applied potentials or a passive approach using a negative separation voltage are discussed as two possible methods for controlling the induced EOF. Both methods can reverse the EOF and improve the resolution and peak efficiency in amino acid separations.

Cation valency in water-in-salt electrolytes alters the short- and long-range structure of the electrical double layer

Highly concentrated aqueous electrolytes (termed water-in-salt electrolytes, WiSEs) at solid-liquid interfaces are ubiquitous in myriad applications including biological signaling, electrosynthesis, and energy storage. This interface, known as the electrical double layer (EDL), has a different structure in WiSEs than in dilute electrolytes. Here, we investigate how divalent salts [zinc bis(trifluoromethylsulfonyl)imide, Zn(TFSI)2], as well as mixtures of mono- and divalent salts [lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) mixed with Zn(TFSI)2], affect the short- and long-range structure of the EDL under confinement using a multimodal combination of scattering, spectroscopy, and surface forces measurements. Raman spectroscopy of bulk electrolytes suggests that the cation is closely associated with the anion regardless of valency. Wide-angle X-ray scattering reveals that all bulk electrolytes form ion clusters; however, the clusters are suppressed with increasing concentration of the divalent ion. To probe the EDL under confinement, we use a Surface Forces Apparatus and demonstrate that the thickness of the adsorbed layer of ions at the interface grows with increasing divalent ion concentration. Multiple interfacial layers form following this adlayer; their thicknesses appear dependent on anion size, rather than cation. Importantly, all electrolytes exhibit very long electrostatic decay lengths that are insensitive to valency. It is likely that in the WiSE regime, electrostatic screening is mediated by the formation of ion clusters rather than individual well-solvated ions. This work contributes to understanding the structure and charge-neutralization mechanism in this class of electrolytes and the interfacial behavior of mixed-electrolyte systems encountered in electrochemistry and biology.

Predicting the Electrophoretic Mobility of Charged Particles in an Aqueous Medium.

Electrophoresis of charged particles has important applications in biochemical separation processes. The mobility of these particles depends on the surrounding electric double layer (EDL), which is impacted by solvent restructuring because of hydration interactions. Nevertheless, most theoretical estimates ignore such interactions during computation of the electrophoretic mobility. Here, we employ a complementary blend of mean-field analysis and molecular dynamics simulations performed for a peptide-G-quadruplex complex to assess how hydration interactions alter the mobility of a charged particle in an aqueous medium. These interactions are seen to stabilize the EDL, resulting in more significant localized counterion concentrations while strengthening the ensuing electrokinetic flow. The ordering of ions near the particle surface is obtained only upon including hydration interaction, revealing that the hydration water molecules act as a glue for forming a stable EDL, a key finding of this work. Conversely, the observed microstructure of ions near the charged surface as obtained from our theory establishes a bridge link between the micro and continuum model. The presence of larger counter ions enhances the drag on the particle, thus restricting its mobility. The mobility also becomes dependent on size, which may be useful for isolating a wide array of biomolecules. The impact of hydration interactions intensifies with increases in particle size, surface charge density, and bulk ion concentration.

Droplet Electrophoresis with Internal Free Ions: Effect of Permittivity Changes in the Electric Double Layer.

Droplet electrophoresis (EP) is of interest in biological systems, microfluidics, and separation techniques. We investigate EP of an oil droplet that contains free ions and is stabilized in an electrolyte solution through an amphoteric surfactant. The presence of mobile ions within the droplet leads to the creation of a distinct nonzero space charge density inside the droplet and consequently, formation of an inner EDL inside the droplet in addition to the traditionally considered outside EDL. While we assume the permittivity inside the inner EDL to remain constant, we consider both the case of constant and variable permittivity in the outer EDL. Our findings demonstrate a change in the droplet direction of motion in the electric field when transitioning from acidic to alkaline pH, regardless of permittivity and ionic strength in both oil and electrolyte. We further find a significant reduction in the magnitude of droplet velocity in the case of a variable permittivity due to reduction of the local space charge density within the EDL surrounding the droplet. When decreasing the viscosity ratio of the oil to the electrolyte, in all cases we find a reduction in droplet velocity. This decline is attributed mostly to the formation and strength of a vortex around the droplet. We finally demonstrate that with constant permittivity in the outer EDL, the variation in κaouter has a more significant effect on the droplet's EP velocity than altering κainner. However, in cases where the body forces inside of the droplet dominate, minor changes in the outer electrolyte concentration have no influence on the droplet motion, which is relevant for biological colloids that can contain significant free internal charges. Our results are important for the manipulation of biological colloids, water and waste treatment such as lubricant removal from processing streams.