Large Surface Charge Density Research Articles

Molecular Dynamics Study on Swelling and Exfoliation Properties of Montmorillonite Nanosheets for Application as Proton Exchange Membranes

Existing proton exchange membranes have the disadvantages of low proton conductivity at high temperature, high cost, and poor barrier properties. Montmorillonite (MMT) materials have excellent thermal and chemical stability, and two-dimensional nanostructures formed by exfoliated MMT show excellent proton conductivity. Therefore, they are expected to become new proton exchange membrane materials. However, the existing exfoliation methods still lack a unified theoretical guidance. In this work, we systematically investigate the detailed swelling and exfoliation properties of MMT with different intercalation cations via extensive molecular dynamics simulations. We find that the swelling properties are determined by the hydration capacity of interlayer cations, and the exfoliation mainly occurs via a lateral sliding process between adjacent layers. The exfoliation energy barrier strongly depends on the interaction strength between interlayer cations and MMT nanosheets, swelling degree, and surface charge density. Lower interaction strength and a higher swelling degree together with larger surface charge density can result in an easy exfoliation process. Our work unveiled the detailed mechanism of the exfoliation process of MMT nanosheets and may contribute to the search for efficient MMT exfoliation methods.

Essence of the Enhanced Osmotic Energy Conversion in a Covalent Organic Framework Monolayer.

Low membrane conductivity originated from a high membrane thickness has long been the "Achilles heel" of the conventional polymeric membrane, greatly hampering the improvement of the output power density in osmotic power generation. Herein, we demonstrate a molecularly-thin two-dimensional (2D) covalent organic framework (COF) monolayer membrane, featured with ultimate thickness, high pore density, and tight pore size distribution, which performs as a highly efficient osmotic power generator. Despite the large pore size up to 3.8 nm and relatively low surface charge density of 2.2 mC m-2, the monolayer COF membrane exhibits a high osmotic current density of 16.7 kA m-2 and an output power density of 102 W m-2 under 50 times the NaCl salinity gradient (0.5 M/0.01 M). This superior power density could be further improved to 170 W m-2 in the real seawater/river water gradient system. When the large pore size and low surface charge density are considered, this superior performance is not expected. Computational studies further reveal that the ultimate membrane permeability originated from the high membrane porosity, rather than ion selectivity, plays a dominant role in the production of high current density, especially under high salinity. This work provides an alternative strategy to realize improved output power density in ultrapermeable membranes.

Effective Charged Exterior Surfaces for Enhanced Ionic Diffusion through Nanopores under Salt Gradients.

High-performance osmotic energy conversion requires both large ionic throughput and high ionic selectivity, which can be significantly promoted by exterior surface charges simultaneously, especially for short nanopores. Here, we investigate the enhancement of ionic diffusion by charged exterior surfaces under various conditions and explore corresponding effective charged areas. From simulations, ionic diffusion is promoted more significantly by exterior surface charges through nanopores with a shorter length, wider diameter, and larger surface charge density or under higher salt gradients. Effective widths of the charged ring regions near nanopores are reversely proportional to the pore length and linearly dependent on the pore diameter, salt gradient, and surface charge density. Due to the important role of effective charged areas in the propagation of ionic diffusion through single nanopores to cases with porous membranes, our results may provide useful guidance to the design and fabrication of porous membranes for practical high-performance osmotic energy harvesting.

Reversal of Electroosmotic Flow in Charged Nanopores with Multivalent Electrolyte.

We study the reversal of electroosmotic flow in charged cylindrical nanopores containing multivalent electrolyte. Dissipative particle dynamics is used to simulate the hydrodynamics of the electroosmotic flow. The electrostatic interactions are treated using 3D Ewald summation, corrected for a pseudo-one-dimensional geometry of the pore. We observe that, for sufficiently large surface charge density, condensation of multivalent counterions leads to the reversal of the pore's surface charge. This results in the reversal of electroosmotic flow. Our simulations show that the Smoluchowski equation is able to quantitatively account for the electroosmotic flow through the nanopore, if the shear plane is shifted from the position of the Stern contact surface.

Mine tailings in a redox-active environment: Iron geochemistry and potential environmental consequences

Iron (Fe) oxyhydroxides provide many functions in soils, mainly owing to their large surface area and high surface charge density. The reactivity of Fe oxyhydroxides is function of their mineralogical characteristics (e.g., crystallinity degree and crystal size). Detailed studies of these features are essential for predicting the stability and reactivity of these minerals within soil and sediments. The present study aimed to evaluate geochemical changes in Fe-rich tailings after the world's largest mining disaster in SE Brazil (in 2015) and to predict the potential environmental implications for the estuary. The mineralogical characteristics of the tailings were studied at three different times (2015, 2107, and 2019) to assess how an active redox environment affects Fe oxyhydroxides and to estimate the time frame within which significant changes occur. The study findings indicate a large decrease in the Fe oxyhydroxides crystallinity, which were initially composed (93%) of highly crystalline Fe oxyhydroxides (i.e., goethite and hematite) and 6.7% of poorly crystalline Fe oxyhydroxides (i.e., lepidocrocite and ferrihydrite). Within 4 years the mineralogical features of Fe oxyhydroxides had shifted, and in 2019 poorly crystalline Fe oxyhydroxides represented 47% of the Fe forms. Scanning electron microscope micrographs and the mean crystal size evidenced a decrease in particle size from 109 nm to 49 nm for goethite in the d111 direction. The changes in mean crystal size increased the reactivity of Fe oxyhydroxides, resulting in a greater number of interactions with cationic and anionic species. The decreased crystallinity and increased reactivity led to the compounds being more susceptible to reductive dissolution. Overall, the findings show that the decrease in crystallinity along with higher susceptibility to reductive dissolution of Fe oxyhydroxides can affect the fate of environmentally detrimental elements (e.g., phosphorus and trace metals) thereby increasing the concentration of these pollutants in estuarine soils and waters.

Intriguing triboelectrification behavior of identical P(VDF-TrFE) polymers

We report an intriguing triboelectrification behavior of thermally treated P(VDF-TrFE) polymers. Cooled (C-) and annealed (A-) P(VDF-TrFE) films were prepared by the rapid cooling of the melt solution and the subsequent annealing, respectively. The C–P(VDF-TrFE) had sparse grains and small elastic modulus and surface charge density, while, the A-P(VDF-TrFE) had dense grains and large elastic modulus and surface charge density. When the polymers contacted ITO electrodes, the triboelectric outputs of C-(PVDF-TrFE) were larger than those of A-P(VDF-TrFE). When the polymers contacted each other, the triboelectric outputs of different P(VDF-TrFE) films were significantly larger than those of similar P(VDF-TrFE) films. Based on detailed X-ray photoemission spectroscopy results, we suggested that roughness-induced frictional heat and elastic modulus differences may play an important role in minute material and/or ion transfer during the triboelectrification of identical P(VDF-TrFE) polymers.

An Inconspicuous Six-Coordinate Neutral DyIII Single-Ion Magnet with Remarkable Magnetic Anisotropy and Stability.

It is a crucial challenge to address both magnetic anisotropy and stability for single-molecule magnets (SMMs) used in next-generation nanodevices. Highly axial lanthanide SMMs with neutral charge and moderate coordination numbers represent promising magnetic materials. Here, using iodide ions with large volume and low surface charge density as weak donors, we report a six-coordinate neutral dysprosium SMM [Dy(Cy3PO)2I3(CH3CN)] with a certain degree of stability exhibiting a huge thermal barrier of 1062 K and hysteresis loops open up to 9 K. Through the elaborate reduction of ligand field strength, an apparent strongly axial crystal field is provided which elicits prominent crystal-field splitting and high axiality with the thermally activated relaxation via the third-excited Kramers' doublet. Moreover, the profound influence of strong equatorial ligand substitution on the electronic structure and relaxation pathway is clearly explored in DyIII analogues. The result suggests the great potential of the reducing the transverse ligand field in the improvement of SMMs performance.

Slip length and structure of liquid water flowing past atomistic smooth charged walls

In this work, the slip behavior and structure of liquid water flowing between two charged solid planar walls were investigated using non-equilibrium molecular dynamics simulations. The upper and lower walls are positive and negative charged, respectively. It was shown that the slip length increases at smaller water-solid interaction energy and become smaller with increasing the surface charge density. At the largest surface charge density, the slip length nearly independent of the water-solid interaction energy. The relationship between the slip length and surface charge density and water-solid interaction energy was rationalized by considering the static structure factor of liquid water. Interestingly, the positive charged surface induces less ordering structure and larger slip at the small surface charge density than that by the negative charged surface. While, at large surface charge density, the opposite correlation is observed. Furthermore, we find that the relationship between the slip length and the normalized main peak of static structure factor collapses onto a single curve for different water-solid interaction energies and surface charge densities. The results of the present work open perspectives for modeling complex systems with combined effects of surface charge and wettability.

The mechanism of Cu (II) adsorption onto 2,3-dialdehyde nano-fibrillated celluloses

Nano-fibrillated celluloses (NFCs) were separated from bleached softwood kraft pulp. Through the periodate oxidation method, 2,3-dialdehyde nano-fibrillated celluloses (DNFCs) with varied aldehyde content were prepared. The high aldehyde content, large specific surface area and high surface charge density of DNFCs benefited the adsorbing of Cu (II). The adsorption kinetics and isotherms presented good correlations with the Pseudo-second-order model and Freundlich model, respectively. Both physical and chemical adsorptions existed in the Cu (II) adsorption by DNFCs, while chemisorption was the rate-limiting step. The adsorption thermodynamic parameters Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were also investigated. This study provides theoretical support for applying DNFCs to remove metal ions from aqueous solutions.

Fern Dicranopteris linearis-derived biochars: Adjusting surface properties by direct processing of the silica phase

This work specifically emphasizes the effect of the silica phase contained in the fern Dicranopteris linearis (D. linearis), a common shrub occurring widely throughout tropical and subtropical regions, on biochar surface properties. Slow pyrolysis was performed in the temperature range from 400 to 900 °C under various ambient reaction conditions, i.e., non-biochar-oriented conditions (open pyrolysis) and biochar-oriented conditions (closed and N2-supported pyrolysis). The resulting changes in micromorphology and different surface properties, specific surface area and surface charge, were elucidated. Open pyrolysis resulted in excessive decomposition of biomass and condensation of silica, while the closed and N2-supported pyrolysis methods showed notable enhancement of biochar yield. The presence of silica as an inter-embedded part of the fern D. linearis and the derived biochars likely supported a carbon-silica structural model in which these two components might be integrated or decomposed during pyrolysis. In general, joint processing of organic carbon and silica greatly altered the surface properties. At lower temperatures and during N2-supported pyrolysis, condensation of organic compounds limited the development of high surface charge densities. At higher temperatures and during open pyrolysis, intensification of the silica phase was accompanied with an increasing number of charged surface sites, thereby increasing the surface charge density. Based on the porous structure, large surface area (up to 701 m2 g−1) and high surface charge density (up to 0.5 μmol(−) m−2), D. linearis-derived biochars can be highlighted as potential agro-environmental materials.

Asymmetric-Fluidic-Reservoirs Induced High Rectification Nanofluidic Diode

We demonstrate a novel nanofluidic diode that produces rectification factors in excess of 1000. The nanofluidic diode consists of ion permselective nanopores that connect two reservoirs of different diameters- a micropore reservoir and a macropore reservoir. On the application of +100 V to the micropore, a low OFF state current is observed. The OFF state is caused by formation of the ion depleted zone in the micropore because the anions are prevented from entering the nanopores from the micropore and the cations are depleted in this region to maintain charge neutrality. On the application of −100 V, we observe a high ON state current. The ON state is caused by formation of the ion enriched zone in the microchannel because the anions cannot pass through the nanopores and accumulate in the microchannel. To maintain charge neutrality the cations also become enriched in the microchannel. The ratio of ON state current to the OFF state current gives the rectification of current. Here, plasma oxidation is used to achieve a nanopore with a large wall surface charge density of σn = −55 mC/m2 which yields a rectification of current on the order of 3500 that is nearly two orders of magnitude higher than those reported thus far. In contrast to the other nanofluidic diodes, this nanofluidic diode does not introduce asymmetry to the nanopore, but asymmetry is produced by having the nanopores join a micropore and a macropore. Introduction of asymmetry into the fluidic reservoirs which the nanopores connect is quite simple. Hence, the nanofluidic diode is easy to scale up to industrial level.

Reverse electrodialysis in bilayer nanochannels: salinity gradient-driven power generation.

To evaluate the possibility of nano-fluidic reverse electrodialysis (RED) for salinity gradient energy harvesting, we consider the behavior of ion transportation in a bilayer cylindrical nanochannel consisting of different sized nanopores connecting two large reservoirs at different NaCl concentrations. Numerical simulations to illustrate the electrokinetic behavior at asymmetric sub-pore length and surface charge density are conducted, the impacts of which on transference number, osmotic current, diffusive voltage, maximum power and maximum power efficiency are systematically investigated. The results reveal that the transference number in Config. I (where high NaCl concentration is applied at the larger nanopore) is always larger than that in the opposite configuration (Config. II). At low concentration ratios, the osmotic current and maximum power have maximum values, while the maximum power efficiency decreases consistently. For Config. II, the ion transportation is impacted by the surface charge density at both sub-nanopores, while for Config. I, it is determined by the surface charge density at the downstream small nanopore. When large surface charge density is applied at the downstream small nanopore in contact with a very low concentration reservoir, there exists an interesting phenomenon: the larger surface charge density at the large nanopore induces a slight performance drop due to the impact of upstream EDL overlap.

Hydrodynamic model of a dielectric-barrier discharge in pure chlorine

A one-dimensional hydrodynamic model of a dielectric-barrier discharge (DBD) in pure chlorine is developed, and the properties of the discharge are modeled. The discharge is excited in an 8-mm-long discharge gap between 2-mm-thick dielectric quartz layers covering metal electrodes. The DBD spatiotemporal characteristics at gas pressures of 15–100 Torr are modeled for the case in which a 100-kHz harmonic voltage with an amplitude of 8 kV is applied to the electrodes. The average power density deposited in the discharge over one voltage period is 2.5–5.8 W/cm3. It is shown that ions and electrons absorb about 95 and 5% of the discharge power, respectively. In this case, from 67 to 97% of the power absorbed by electrons is spent on the dissociation and ionization of Cl2 molecules. Two phases can be distinguished in the discharge dynamics: the active (multispike) phase, which follows the breakdown of the discharge gap, and the passive phase. The active phase is characterized by the presence of multiple current spikes, a relatively high current, small surface charge density on the dielectrics, and large voltage drop across the discharge gap. The passive phase (with no current spikes) is characterized by a low current, large surface charge density on the dielectrics, and small voltage drop across the discharge gap. The peak current density in the spikes at all pressures is about 4 mA/cm2. In the multispike phase, there are distinct space charge sheaths with thicknesses of 1.5–1.8 mm and a mean electron energy of 4.3–7 eV and the central region of quasineutral plasma with a weak electric field and a mean electron energy of 0.8–3 eV. The degree of ionization of chlorine molecules in the discharge is ~0.02% at a pressure of 15 Torr and ~0.01% at 100 Torr. The DBD plasma is electronegative due to the fast attachment of electrons to chlorine atoms: e + Cl2 → Cl + Cl–. The most abundant charged particles are Cl 2 + and Cl− ions, and the degree of ionization during current spikes in the active phase is (4.1–5.5) × 10–7. The mechanism of discharge sustainment is analyzed. The appearance of a series of current spikes in the active phase of the discharge is explained.

Critical adsorption of polyelectrolytes onto planar and convex highly charged surfaces: the nonlinear Poisson–Boltzmann approach

We study the adsorption–desorption transition of polyelectrolyte chains onto planar, cylindrical and spherical surfaces with arbitrarily high surface charge densities by massive Monte Carlo computer simulations. We examine in detail how the well known scaling relations for the threshold transition—demarcating the adsorbed and desorbed domains of a polyelectrolyte near weakly charged surfaces—are altered for highly charged interfaces. In virtue of high surface potentials and large surface charge densities, the Debye–Hückel approximation is often not feasible and the nonlinear Poisson–Boltzmann approach should be implemented. At low salt conditions, for instance, the electrostatic potential from the nonlinear Poisson–Boltzmann equation is smaller than the Debye–Hückel result, such that the required critical surface charge density for polyelectrolyte adsorption increases. The nonlinear relation between the surface charge density and electrostatic potential leads to a sharply increasing critical surface charge density with growing ionic strength, imposing an additional limit to the critical salt concentration above which no polyelectrolyte adsorption occurs at all. We contrast our simulations findings with the known scaling results for weak critical polyelectrolyte adsorption onto oppositely charged surfaces for the three standard geometries. Finally, we discuss some applications of our results for some physical–chemical and biophysical systems.

Carrier mobility and scattering lifetime in electric double-layer gated few-layer graphene

We fabricate electric double-layer field-effect transistor (EDL-FET) devices on mechanically exfoliated few-layer graphene. We exploit the large capacitance of a polymeric electrolyte to study the transport properties of three, four and five-layer samples under a large induced surface charge density both above and below the glass transition temperature of the polymer. We find that the carrier mobility shows a strong asymmetry between the hole and electron doping regime. We then employ ab initio density functional theory (DFT) calculations to determine the average scattering lifetime from the experimental data. We explain its peculiar dependence on the carrier density in terms of the specific properties of the electrolyte we used in our experiments.

Poisson-Boltzmann thermodynamics of counterions confined by curved hard walls.

We consider a set of identical mobile pointlike charges (counterions) confined to a domain with curved hard walls carrying a uniform fixed surface charge density, the system as a whole being electroneutral. Three domain geometries are considered: a pair of parallel plates, the cylinder, and the sphere. The particle system in thermal equilibrium is assumed to be described by the nonlinear Poisson-Boltzmann theory. While the effectively one-dimensional plates and the two-dimensional cylinder have already been solved, the three-dimensional sphere problem is not integrable. It is shown that the contact density of particles at the charged surface is determined by a first-order Abel differential equation of the second kind which is a counterpart of Enig's equation in the critical theory of gravitation and combustion or explosion. This equation enables us to construct the exact series solutions of the contact density in the regions of small and large surface charge densities. The formalism provides, within the mean-field Poisson-Boltzmann framework, the complete thermodynamics of counterions inside a charged sphere (salt-free system).

The use of clay nanoplatelets to improve hydrogels for tissue engineering applications

Event Abstract Back to Event The use of clay nanoplatelets to improve hydrogels for tissue engineering applications Lyndon Charles1, Sara E. Strecker1, Shimon A. Unterman1, Adar Oren1, 2, Michael Philbrook3, Gloria L. Matthews3, Elazer R. Edelman1, 4 and Natalie Artzi1, 5 1 MIT, Institute for Medical Engineering and Science, United States 2 ORT Braude College, Department of Biotechnology Engineering, Israel 3 Genzyme, a Sanofi Company, Orthopaedic Research, United States 4 Brigham and Women's Hospital, Cardiovascular Division, United States 5 Brigham and Women's Hospital, Department of Anesthesiology, United States Introduction: Hydrogels are widely used in tissue regeneration including bone and cartilage, but their tunability is challenged by their hydrophilicity which presents limits in degradation control, mechanical stability, and drug release which strongly influence regenerative outcomes. Hydrogels of polyamidoamine (PAMAM) dendrimer and dextran-aldehyde offer excellent biocompatibility, tunability, and in situ gelation[1]. Here we show the ability of clay nanoplatelets (cNPs) to improve hydrogel degradation and drug release to add further tunability. cNPs also modify the rheological properties[2] of hydrogel precursor solutions and has been optimized by our lab to create easily injectable hydrogels to improve use in minimally invasive procedures. Different sized clays, montmorillonite and laponite, were selected for improved mechanical and rheological properties. The ability to tune these properties using cNPs to create an injectable, biocompatible, drug delivery hydrogel represents a significant improvement over current hydrogels. Tunable clay hydrogels are a suitable candidate for use in tissue regeneration to address common challenges. Materials and Methods: PAMAM dendrimer was purchased from Dedritech, Inc and 10 kDa Dextran (Sigma Aldrich) was oxidized by reacting with sodium periodate. Clays were obtained from Southern Clay Products, Inc. Degradation studies were done in phosphate buffered saline and mass recorded daily. Mouse mesenchymal cells (MSC) were used for in vitro cell studies. Toxicity studies were performed by placing hydrogels in direct contact with C3H10T1/2 cells and imaged using live/dead cell staining kit (Life Technologies). Hydrogels were introduced to cell environment by transwell assay in cell growth and alkaline phosphatase assays (ALP) assays. For ALP, 16 mg simvastatin was loaded into each hydrogel and W20-17 cell line was used. PrestoBlue (Life Technologies) reagent was used to assay cell growth and ALP kit (Sigma Aldrich) was used for ALP expression. Results and Discussion: Degradation behavior of hydrogels was measured by mass loss. Results (Fig1A) show the ability of cNPs to retard hydrogel degradation. While hydrogels show a 20% loss in mass, clay hydrogels lose only 5% of mass over 21 days. Due to their large surface area and charge density, cNPs may add stability by forming networks of clay-clay and clay-polymer interactions, in addition to polymer-polymer interactions. Clay hydrogels also improve biocompatibility and acute toxicity. Early effect on cell growth is reversed with the addition of cNPs (Fig1B) and clay hydrogels mitigate the local, acute toxicity of MSCs (Fig1C) seen adjacent to the hydrogel (Fig1C). Clay addition may enhance polymer interactions and prevent the early release of unbound polymer into the media. Use of hydrogels as drug delivery vehicles is important in tissue regeneration. When added to hydrogels loaded with the osteogenic drug simvastatin, cNPs lead to increased ALP expression in MSCs (Fig1D). This is likely due to increase in simvastatin release from the hydrogel driven by the interruption of the polymer network by cNPs. Conclusion: Here we show that cNPs can modify hydrogel properties to control drug release and improve cell compatibility. This ability to tune hydrogel behavior by addition of cNPs, as well as improving injectability, can enhance their flexibility for use in variable environments and tissue engineering applications.

Is mixed oxide of SnxTi1−xO2 more effective for H2O decomposition? A first principles study

The interaction of water with wide band gap metal oxides has wide ranging applications. Using first principles approach we demonstrate that a mixed oxide of SnxTi1−xO2 (x=0.5) significantly reduces the dissociation barrier of water in comparison to SnO2 and TiO2 surfaces. From the charge distribution at the top layer it is seen that SnxTi1−xO2 is more polar, where the charge separation between metal and oxygen atoms is more than SnO2 and TiO2 surfaces. The reason behind higher activity of the mixed surface has been explained by the surface morphology and larger surface charge density at the top layer.

Temperature dependence of electric transport in few-layer graphene under large charge doping induced by electrochemical gating.

The temperature dependence of electric transport properties of single-layer and few-layer graphene at large charge doping is of great interest both for the study of the scattering processes dominating the conductivity at different temperatures and in view of the theoretically predicted possibility to reach the superconducting state in such extreme conditions. Here we present the results obtained in 3-, 4- and 5-layer graphene devices down to 3.5 K, where a large surface charge density up to about 6.8·1014 cm−2 has been reached by employing a novel polymer electrolyte solution for the electrochemical gating. In contrast with recent results obtained in single-layer graphene, the temperature dependence of the sheet resistance between 20 K and 280 K shows a low-temperature dominance of a T2 component – that can be associated with electron-electron scattering – and, at about 100 K, a crossover to the classic electron-phonon regime. Unexpectedly, this crossover does not show any dependence on the induced charge density, i.e. on the large tuning of the Fermi energy.

Numerical modeling of electrostatic precipitation: Effect of Gas temperature

A Computational Fluid Dynamics model is presented to calculate the particle collection efficiency in electrostatic precipitators in terms of the interactions among gas-particle flow, dust resistivity and electric field. Various material properties and operation parameters are included to extend the model capability for process optimization. The current paper focuses on the effect of gas temperature for a cold-side precipitator. It is found that, as temperature increases in the range 90–120°C considered, the operation voltage drops significantly, due to increased dust resistivity. Behaviors of fine particles in the range from 0.05 to 25μm in diameter are simulated in a circular wire-plate configuration. A minimum efficiency is predicted for particle size around 0.5μm. Below this critical size, the increased collection efficiency results from relatively large surface charge density due to diffusion charging, and reduced inter-phase drag. For moist gas, temperature has significant non-linear effect on the fly ash resistivity. It is demonstrated that a same change in temperature from 120°C to 90°C is more effective than from 150°C to 120°C, as far as the collection efficiency is concerned.