Surface Of Multi-walled Carbon Nanotubes Research Articles

Synthesis and characterization of Schiff base ligand-mutliwalled carbon nanotubes as mercury-free electrochemical sensor for detecting toxic metals in aquatic treatment

The aspire of the present investigation is to develop a novel mercury-free electrochemical sensor for the sensitive detection of toxic pollutant in environmental sources. The synthesis of N,N′-bis(salicylaldehyde)-1,2-diaminobenzene (BSD) as a pre designed Schiff-base ligand (SBL) coated on surface of multi-walled carbon nanotubes (MWCNTs) modified working electrode for the detection of Pb2+ and Hg2+ in various water sources. The role of BSD contains hydroxyl and imine ions acting as active sites to detect the metal ions in 0.1 M of acetate buffer solution. A star like crystal structure for BSD/MWCNTs modified surface of electrode was observed using SEM analysis. The conductance (σ) for PIG electrode (unmodified), MWCNTs modified and BSD/MWCNTs modified electrodes are 4.7 × 10−5 S cm−1, 408.1 × 10−5 S cm−1 and 553.2 × 10−5 S cm−1 were estimated. BSD/MWCNTs modified electrode shows the best conductivity compared with other electrodes. For the selective detection of Pb2+ and Hg2+, the square wave anodic stripping voltammetry (SWASV) was utilised. The linear range of Pb2+ and Hg2+ in the concentration from 2.0 to 155 nM and limits of detection for Pb2+ and Hg2+ were observed at 0.3 nM and 0.6 nM by SWASV. This sensor is often used for sensitive detection of analytes like Pb2+ and Hg2+ in different water samples. The results of SWASV recovered value are comparing with atomic absorption spectroscopy (AAS).

Synthesis, characterization, and electrochemical evaluation of SnFe2O4@MWCNTS nanocomposite as a potential hydrogen storage material

The widespread use of hydrogen as a vehicle fuel has prompted us to develop a new nanocomposite by immobilizing of tin ferrite nanoparticles (SnFe2O4) on the surface of multi-walled carbon nanotubes (abbreviated as MWCNTS) for the first time. The prepared nanocomposite powder (SnFe2O4@MWCNTS) was investigated utilizing various microscopy and spectroscopy methods, such as FT-IR, XRD, SEM, EDX, and BET techniques. Moreover, the electrochemical property of SnFe2O4@MWCNTS nanocomposite was investigated by cyclic voltammogram (CV) and charge–discharge chronopotentiometry (CHP) techniques. A variety of factors on the hydrogen storage capacity, such as current density, surface area of the copper foam, and the influence of repeated hydrogen adsorption-desorption cycles were assessed. The electrochemical results indicated that the SnFe2O4@MWCNTS has high capability and excellent reversibility compared to SnFe2O4 nanoparticles (NPs) for hydrogen storage. The highest hydrogen discharge capability of SnFe2O4@MWCNTs was achieved ∼ 365 mAh/g during the 1st cycle, and the storage capacity enhanced to ∼ 2350 mAh/g at the end of 20 cycles using a current of 2 mA. Consequently, the SnFe2O4@MWCNTS illustrated great capacity as a prospective active material for hydrogen storage systems.

Coaxial structured Bi2S3–SnS2-MWCNT hybrid nanocomposite with its improved thermoelectric properties

Bismuth(III) sulfide (Bi2S3), tin(IV) sulfide (SnS2), and a multi-walled carbon nanotube (MWCNT) are prepared as a hybrid composites (Bi2S3–SnS2-MWCNT) using a simple hydrothermal method. The as-prepared hybrid composite exhibit n-type thermoelectric (TE) characteristics and an enhanced power factor (PF) value. Bi2S3–SnS2 composites are anchored to the surface of MWCNT and form a coaxial structure. The nanostructure formation through bonding between these different materials is confirmed via scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS). The TE properties of the Bi2S3–SnS2-MWCNT composite is improved by changing the Bi2S3, SnS2 and MWCNT content. The maximum PF (∼150.8 μ·W/m·K2) was obtained for Bi2S3–SnS2-MWCNT composite with Bi:Sn ratio of 7:3 and 2 wt% of CNT. The highest PF values were ∼34 and ∼58 times higher than the PF of Bi2S3 and SnS2 at room temperature. The synthesized Bi2S3–SnS2-MWCNT hybrid nanocomposite can provide important components for fabrication CNT-based TE composites with high conversion efficiency, further advancing TE device.

Composite of DNA-Stabilized Multiwalled Carbon Nanotube and Nematic Liquid Crystal for Display Performance Optimization

We have prepared composites of a nematic liquid crystal (NLC) mixture to improve display performance by doping multiwalled carbon nanotubes (MWCNTs) and single-stranded-DNA-wrapped MWCNTs. Polarized optical microscopy images indicated the uniformity of the composite. Infrared absorption spectra showed there existed an interaction between MWCNTs and 4-pentyl-4′-cyanobiphenyl (5CB, a practical solvent-type monomer NLC) due to π–π stacking and charge transfer, indicating the anchoring of 5CB molecules on the MWCNT surface. The electro-optical property proved the incorporation of MWCNTs and DNA-wrapped MWCNTs reduced the threshold voltage and response time, thus following the changed splay elastic constant and rotational viscosity coefficient of LC, reflecting that DNA can improve the stability of MWCNTs in 5CB. Moreover, dielectric measurement showed the enhanced value to dielectric anisotropy and shift in relaxation frequency, including the changes of dielectric permittivity and loss, revealing the electrically “inertness” MWCNTs at high frequencies and the DNA-induced broadened frequency range of low dielectric loss. Then electrical conductivity tests further proved the cooperative orientation of MWCNTs with 5CB under the applied voltage and the contribution of MWCNTs’ high conducting along the long axis. This study revealed that MWCNT doping could improve the physical properties of NLCs, and DNA wrapping could pave a promising way toward stabilized MWCNT/NLC composites for display performance optimization.

Vapor-phase derived ultra-fine Bismuth nanoparticles embedded in carbon nanotube networks as anodes for sodium and potassium ion batteries

Bismuth (Bi) is a promising material as the anode for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) due to its characteristics such as reasonable price and high theoretical volumetric capacity (3800 mAh cm−3). Nevertheless, considerable drawbacks have hindered the practical applications of Bi, including its relatively low electrical conductivity and inevitable volumetric change during the alloying/dealloying processes. To solve these problems, we proposed a novel design:Bi nanoparticles were synthesized via a single-step low-pressure vapor-phase reaction and embedded onto the surfaces of multi-walled carbon nanotubes (MWCNTs). After being vaporized at 650℃ and 10-5 Pa, Bi nanoparticles less than 10 nm were uniformly distributed in the three-dimensional (3D) MWCNT networks to form a Bi/MWNTs composite. In this unique design, the nanostructured Bi can reduce the risk of structural rupture during cycling, and the structure of the MWCMT network is beneficial in shortening the electron/ion transport path. In addition, MWCNTs can improve the overall conductivity of the Bi/MWCNTs composite and prevent particle aggregation, thus improving the cycling stability and rate performance. As an anode material for SIB, the Bi/MWCNTs composite has demonstrated excellent fast charging performance with a reversible capacity of 254 mAh/g at 20 A/g. A capacity of 221mAhg−1 after cycling at 10 A/g for 8000 cycles has also been achieved for SIB. As an anode material for PIB, the Bi/MWCNTs composite has delivered excellent rate performances with a reversible capacity of 251 mAh/g at 20 A/g. A specific capacity of 270mAhg−1 after cycling at 1Ag−1 for 5000 cycles has also been achieved for PIB.

Resorc[4]arene Modifiers for Supramolecular Site-Directed Immobilization of Antibodies on Multi-Walled Carbon Nanotubes.

One of the main problems in developing immunosensors featuring carbon nanotubes (CNTs) is immobilizing antibodies (Abs) onto the CNT surface to afford selective binding to target antigens (Ags). In this work, we developed a practical supramolecular Ab conjugation strategy based on resorc[4]arene modifiers. To improve the Ab orientation on the CNTs surface and optimizing the Ab/Ag interaction, we exploited the host-guest approach by synthesizing two newly resorc[4]arene linkers R1 and R2 via well-established procedures. The upper rim was decorated with eight methoxyl groups to promote selective recognition of the fragment crystallizable (Fc ) region of the Ab. Moreover, the lower rim was functionalized with 3-bromopropyloxy or 3-azidopropiloxy substituents to bind the macrocycles on the multi-walled carbon nanotubes (MWCNTs) surface. Accordingly, several chemical modifications of MWCNTs were evaluated. After the morphological and electrochemical characterization of nanomaterials, the resorc[4]arene-modified MWCNTs were deposited onto a glassy carbon electrode surface to evaluate their potential applicability for label-free immunosensor development. The most promising system showed an improved electrode active area (AEL ) of almost 20 % and a site-oriented immobilization of the SARS-CoV-2 spike protein S1 antibody (Ab-SPS1). The developed immunosensor revealed a good sensitivity (23.64 μA mL ng-1 cm-2 ) towards the SPS1 antigen and a limit of detection (LOD) of 1.01 ng mL-1 .

Smart wearable triboelectric nanogenerator for self-powered bioelectronics and therapeutics

With the development of Triboelectric Nanogenerators (TENGs), the field of energy harvesting has experienced a new era of revolution following the trend to configure innovative materials, design, integrated sensing and high triboelectric output. Here, we investigated the electricity-generation process of TENG by conducting its finite-element analysis using COMSOL Multiphysics software. Further, the effect of different weight concentrations (0%, 2%, 4%, 6%) of multi-walled carbon nanotubes (MWCNT) into the Polydimethylsiloxane (PDMS) matrix was investigated. It has enabled us to find the optimal weight ratio of MWCNT and increase surface charge density, thereby enhancing triboelectric performance for realising smart wearable triboelectric nanogenerators (SW-TENG). On the other hand, a novel dynamic test platform was developed for automated tapping on TENG for quantified performance analysis of SW-TENG. The results indicate that 4 wt% is the optimal weight ratio for fabrication of MWCNT-PDMS based TENG as it recorded the highest open circuit voltage of 195.4 V revealing ∼79% enhancement in triboelectric output as compared to pure PDMS based TENG. Furthermore, SW-TENG was integrated with a bridge rectifier and 10 μF capacitor in order to power bioelectric devices such as a digital thermometer and a pulse sensor. Besides serving as a power source, the SW-TENG shows innovative sensing capabilities for delivering self-powered rehabilitation therapy augmenting the next generation healthcare systems.

CO Oxidation Reaction by Platinum Clusters on the Surface of Multiwalled Carbon Nanotubes: Experimental and Theoretical Study of Kinetics in a Wide Range of O2/CO Ratios

This work presents a systematic study of the kinetic aspects of CO oxidation reaction catalyzed by platinum nanoparticles (NPs) supported on the surface of multiwalled carbon nanotubes (MWCNTs). The investigation presented is closely related to the actual practical task of air purification in enclosed spaces. Therefore, the catalytic reaction was carried out in the presence of an excess of oxygen (5 vol.%) and over a wide range of CO concentrations from 50 ppm to 1600 ppm. For the catalyst characterization, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were applied. Kinetic modelling based on the Langmuir–Hinshelwood and Mars-van Krevelen mechanisms was taken as a basis, using the results obtained on Pt foil. Simulation of CO oxidation reaction on platinum NPs at temperatures above 90 °C was carried out using a kinetic model describing the reaction mechanism on bulk platinum. The description of the kinetics of CO oxidation reaction on Pt NPs over the entire temperature range, including the low temperatures down to −40 °C, required the introduction of the steps characterizing an additional concerted mechanism related to CO-assisted O2 dissociation. Using the presented model, some predictions of the kinetic behaviour of the system were made.

Block Copolymer Adsorption on the Surface of Multi-Walled Carbon Nanotubes for Dispersion in N,N Dimethyl Formamide

This research aims to characterize the adsorption morphology of block copolymer dispersants of the styrene-block-4-vinylpyridine family (S4VP) on the surface of multi-walled carbon nanotubes (MWCNT) in a polar organic solvent, N,N-dimethyl formamide (DMF). Good, unagglomerated dispersion is important in several applications such as fabricating CNT nanocomposites in a polymer film for electronic or optical devices. Small-angle neutron scattering (SANS) measurements, using the contrast variation (CV) method, are used to evaluate the density and extension of the polymer chains adsorbed on the nanotube surface, which can yield insight into the means of successful dispersion. The results show that the block copolymers adsorb onto the MWCNT surface as a continuous coverage of low polymer concentration. Poly(styrene) (PS) blocks adsorb more tightly, forming a 20 Å layer containing about 6 wt.% PS, whereas poly(4-vinylpyridine) (P4VP) blocks emanate into the solvent, forming a thicker shell (totaling 110 Å in radius) but of very dilute (<1 wt.%) polymer concentration. This indicates strong chain extension. Increasing the PS molecular weight increases the thickness of the adsorbed layer but decreases the overall polymer concentration within it. These results are relevant for the ability of dispersed CNTs to form a strong interface with matrix polymers in composites, due to the extension of the 4VP chains allowing for entanglement with matrix chains. The sparse polymer coverage of the CNT surface may provide sufficient space to form CNT-CNT contacts in processed films and composites, which are important for electrical or thermal conductivity.

Characterization of adhesion strength between carbon nanotubes and cementitious materials

Multi-walled carbon nanotube (MWCNT) and carbon nanofiber (CNF) additions increase the elastic modulus, flexural strength, and toughness of Portland cement concrete. However, the interaction mechanism between cement constituents and these nanomaterials is not fully understood. A modified MWCNT-coated atomic force microscopy (AFM) probe is developed by coating a silica particle with oxidized MWCNT through layer-by-layer assembly and adhering it to a tipless AFM cantilever. The probe allows measurement of adhesion between MWCNT and the substrate with a force control procedure. SEM-EDS is acquired in the same region as AFM measurements through a benchmarking scheme to correlate chemistry with the measured adhesion. Statistical deconvolution shows C–S–H regions have lower adhesion to MWCNT than intermixed regions (C–S–H/Clinker). Furthermore, in C–S–H regions, the normalized adhesion strength increases with calcium concentration. This result is due to the higher interaction between the oxygen functional groups in the MWCNT surface and the calcium in the substrate.

Chiral Recognition of Phenylglycinamide Enantiomer Based on Electrode Modified by Silver-Ammonia Ion-Functionalized Carbon Nanotubes Complex

Polyacrylic acid (PAA) chains were used to decorate the surface of multi-walled carbon nanotubes (MWCNTs) via in situ free radical polymerization, and sulfonated chitosan (SCS) was synthesized via a simple and environmental method. Silver-ammonia ions were introduced as the fixative with PAA-MWCNTs as the basic framework, and SCS was used to decorate the surface, thereby obtaining PAA-MWCNTs-Ag-SCS. The modified electrode exhibited excellent cyclic voltammogram (CV) stability after 100 cycles of scanning. According to differential pulse voltammetry (DPV), the peak current value was approximately 250 μA, exhibiting outstanding sensitivity to phenylglycinamide (Pen) enantiomers. The peak current ratio of D-Pen to L-Pen reached 2.16, showing excellent selectivity. The detection limit (DL) was calculated as 0.015 mM and 0.036 mM for L-Pen and D-Pen, respectively, using the signal-to-noise ratio (S/N = 3). This study provides a new idea for the construction of a chiral-sensing platform with outstanding sensitivity, superior stability, and excellent recognition efficiency.

The promotional effects of carbon nanotube on Fe5C2-ZnO catalysts for CO2 hydrogenation to heavy olefins

The multi-walled carbon nanotubes (MWNTs) supported Fe-based catalysts, including FeZnNa/MWNTs, FeZnK/MWNTs, and FeZnRb/MWNTs were synthesized through co-precipitation, and incipient wetness impregnation approaches. The physicochemical results demonstrated that the addition of MWNTs had significantly influenced the morphology, reduction, phase structure, adsorption behavior of Fe-based catalysts, and consequently, the reactivity of CO2 hydrogenation to olefins. Furthermore, adding MWNTs to the alkali metal-promoted FeZn catalysts inhibits the synthesis of CH4 and CO and favors an improved selectivity for olefins. With the incorporation of MWNTs, the heavy olefins (C5-C8=) selectivity over FeZnNa, FeZnK, and FeZnRb catalysts increased from 27.1 to 32.2, 27.0–31.1, and 24.2–28.6 %, respectively. The results proved that the superior selectivity of heavy olefin over the MWNTs-supported catalysts might be accomplished by the electron-donating ability of the MWNTs surface, the presence of extra Hägg carbide, and higher adsorption strength with light olefins. In addition, the catalyst supported by MWNTs demonstrated exceptional stability throughout the 100-hour test at 593 K.

Improvement in the corrosion resistance of electrodeposited Ni-W alloy by MWCNT co-deposition and prevention of metal-carbon interfacial corrosion by carbide formation

Composite plating is an effective method which can enhance the properties of conventional metal plating by co-deposition of composite materials. In this study, multi-walled carbon nanotubes (MWCNTs) were used as a composite material to improve the corrosion resistance of Ni-W alloys, and the effect of the MWCNTs addition on the anti-corrosion properties was discussed. The corrosion current density of the Ni-W/MWCNTs composite decreased approximately 4 times from 16.6 to 4.6 μA·cm-2 in 3 wt% NaCl solution compared to the pristine Ni-W; this showed that the co-deposition of MWCNTs was effective for improvement in the corrosion resistance. However, the interfacial corrosion between the metal matrix and MWCNTs occurred in the high potential region and degraded the passivation properties of plating. This interfacial corrosion is caused by the high O content due to the hydroxyl groups on the acid-treated MWCNTs surface and the difference in W concentration between the interface of MWCNTs and the Ni-W matrix. Heat treatment in Ar atmosphere removes these hydroxyl groups and diffuses Ni and W into MWCNTs to form metal carbide (Ni6W6C), effectively alleviating interfacial corrosion. This study presents interfacial corrosion can be effectively mitigated by forming metal carbides at the interface of carbon materials in metal/carbon composites.

INFLUENCE OF DOMAIN STRUCTURE ON THERMAL PROPERTIES OF CARBON NANOTUBES

In this work, we studied the structure of carbon nanotubes (CNTs) and its effect on the heat capacity of CNTs. We show that single-wall CNTs (SWCNTs) contain zigzag structures that are &amp;#126; 200 nm in size and the surface of multi-walled CNTs (MWCNTs) is formed by zigzag-armchair crystallites that are &amp;#126; 40 nm in size. A decrease in the size of the crystallites in MWCNTs leads to a higher density of domain interfaces that scatter phonons, resulting in an increase in the lower cutoff frequency of these phonons and 10&amp;#37; decrease in the heat capacity of the MWCNTs compared to SWCNTs. The revealed dependence of the heat capacity on the structural disorder allows one to control of the thermal properties of nanotubes and may be useful in the development of thermoelectric and thermal interface materials.

Влияние термической обработки на структуру и электрохимические характеристики композита на основе многостенных углеродных нанотрубок и оксида марганца

In this paper, the effect of heat treatment in an inert medium on the structure and electrochemical characteristics of a composite based on multi-walled carbon nanotubes (MWCNTs) and manganese oxide formed by deposition from a KMnO4 solution is investigated. As a result of heat treatment composite (500°C) in argon on the surface of multi-walled carbon nanotubes are formed nanoparticles of non-stoichiometric manganese oxide. X-ray photoelectron spectroscopy data indicate a high degree of functionalization of the surface of carbon nanotubes by oxygen-containing groups, as well as the possible formation chemical bonds at the interfaces of the composite. Electrochemical measurements showed that the specific capacity of the composite after heat treatment (88-50 F/g) exceeds the capacity of the original MWCNTs (18-15 F/g) in the current range discharge 0.1-1A/g.

A multi-walled carbon nanotube-supported acidic ionic liquid catalyst for the conversion of biomass-derived saccharides to 5-hydroxymethylfurfural

Herein, the surface of multi-walled carbon nanotubes (MWCNTs) was functionalized with 3-mercaptopropyltrimethoxysilane followed by anchoring of 1-vinylimidazole on the surface through the thiol–ene click reaction.

Modification of the surface of multi-walled carbon nanotubes to impart technological properties

Modification of the surface of multi-walled carbon nanotubes to impart technological properties

HRMAS-NMR and simulation study of the self-assembly of surfactants on carbon nanotubes.

Polyethoxylated surfactants, such as those of the Tween and Pluronic series, are commonly used to disperse carbon nanotubes (CNTs) and other nanoparticles. However, the current understanding of the nature of interactions between these surfactants and CNTs is limited. The nature of the interactions between surfactants (Tween-80 [T80] and Pluronic F68 [PF68]) and CNTs was investigated using high-resolution magic angle spinning nuclear magnetic resonance (HRMAS-NMR) and coarse-grained molecular dynamics (MD) simulations. HRMAS-NMR revealed that T80 molecules interact with single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) via the oleyl chain, whereas PF68 molecules interact with the surface of SWCNTs and MWCNTs via the polypropylene oxide residues. The polyethylene oxide chains were oriented towards the external aqueous environment. The HRMAS-NMR results were supported by MD simulations, and the latter provided further insights into the nature of the interactions.

Poly(N-vinyl triazole-b-N-vinyl imidazole) Grafted on MWCNTs as Nanofillers to Improve Proton Conducting Membranes

Carbon nanotubes (CNTs) are of particular interest because of their ability to enhance the mechanical strength in the material; however, processing difficulties of CNTs often restrict utilization up to its full potential. To resolve this, in the current study, we have developed a simple and efficient method to functionalize the surface of a multiwalled carbon nanotube (MWCNT) with precise functional polymer chains, which were covalently grafted on the surface of the MWCNT, and delved into an application to demonstrate this material as an efficient nanofiller in developing a proton conducting membrane (PEM) from polybenzimidazole (PBI). At first, the MWCNT surface was converted to a polymerizable surface by attaching a trithiocarbonate based chain transfer agent (CTA). Then, a N-heterocyclic block copolymer, namely poly-N-vinyl-1,2,4-triazole-b-poly-N-vinyl imidazole (pNVT-b-pNVI), was grown from the CTA anchored surface with a one-pot surface initiated reversible addition–fragmentation chain transfer (SI-RAFT) technique. Grafting of block copolymer chain was confirmed by GPC, NMR, TGA, TEM, FESEM, and EDX studies. To the best of our knowledge, this will be the first report of a growing block copolymer structure grafted covalently on the surface of the MWCNT. The novelty of the work was further enhanced by incorporating pNVT-b-pNVI-g-MWCNT as a nanofiller into the oxypolybenzimidazole (OPBI) membrane to obtain homogeneous nanocomposite membranes with excellent thermomechanical and tensile properties, thermal stability, superior proton conduction when doped with phosphoric acid (PA), and PA holding capacity. The nanocomposite membrane with 2.5 wt % nanofiller loading displayed a tensile stress of 1.8 MPa and a strain of 176% at break. The basic N-heterocyclic rings dangling from the block copolymer chains grafted on the MWCNT surface allowed formation of strong H-bonding, acid–base interaction with PA, which is responsible for high acid uptake and superior PA retention, and also exhibited proton conductivity as high as 0.164 S cm–1 at 180 °C, which is a 2.6-fold increment when compared with a pristine OPBI membrane. This significant increase in conductivity is attributed to the proton conducting nanochannel pathway generated along the polymer-g-MWCNT surface.

Composite Based on Multi-Walled Carbon Nanotubes and Manganese Oxide with Rhenium Additive for Supercapacitors: Structural and Electrochemical Studies

The structure and electrochemical characteristics of composites based on multi-walled carbon nanotubes (MWCNTs) and manganese oxide with the addition of rhenium oxide has been studied. It has shown that the decorating of the MWCNT surface with layers or nanoparticles of manganese oxide (Mn(III) + Mn(IV)) provides more than a twofold increase in the value of the specific capacitance at low potential scan rates. However, composites based only on manganese oxide exhibit poor electrochemical behavior and the value of the specific capacitance decreases rapidly with increasing potential scan rate due to the limitation of diffusion processes. The addition of rhenium oxide to composites significantly increases their electrochemical properties due to changes in the chemical composition and morphology of composites. Studies of the structure and chemical state have shown that an improvement in the specific capacitance is provided by increasing in the proportion of Mn(IV) oxide in such composites, which has the ability to rapidly and completely reverse redox reactions and has lower electrical resistance values, compared to Mn(III) oxide. A detailed analysis of the voltammetric data showed that an increase in the rate capability in composites with the addition of rhenium oxide can also be provided by increasing the availability of the electrode surface for electrolyte ions and increasing the amount of charge stored due to the formation of a double electric layer.