Low Concentration Regime Research Articles

Describing ion exchange membrane-electrolyte interactions for high electrolyte concentrations used in electrochemical reactors

In this work we describe the application of thermodynamic approaches to evaluate the uptake of acids into ion exchange membranes under conditions of equilibration of the membranes with concentrated solutions. This situation is encountered in practical electrochemical reactor systems such as redox flow batteries (RFBs). First, we collect data describing that uptake for a series of acids in contact with perfluorosulfonic acid membranes. We then develop a treatment of the activity coefficients in such systems with a standard thermodynamic analysis of equilibrium between the solution and membrane phases. This is augmented by the Freeman-Manning counterion condensation approach. The latter fails to capture the behavior. As an alternative, based on the first part of the Freeman-Manning analysis we estimate the Donnan potential difference at the membrane surface. This parameter reliably captures the membrane behavior, with the potential tending toward the same constant value for the membrane exposed to different acids. The counterion condensation theory may provide insight into the evolution of the Donnan potential as a function of concentration in the low concentration regime. Finally, we comment on the possible development of this approach as a useful predictive parameter for membrane behavior in RFBs.

The shear mechanics of enzymatically degraded articular cartilage are predicted by a rigidity percolation model

Purpose: An accurate model for the mechanics of cartilage would allow for the prediction of future damage to diseased tissue and would direct the necessary advancements for artificial construct development. Recently it has been shown that orders of magnitude of variation of the shear modulus of articular cartilage are captured by describing the disordered collagen network with a rigidity percolation model. This model suggests that low concentrations of collagen, found near the surface of cartilage, are not sufficiently connected to support a shear load, leading to the mechanical properties in this low concentration regime to be dictated by the background aggrecan gel. This result contrasts with the deeper regions of the tissue where there is a higher concentration of collagen. In this region, the collagen rigidity percolates and the shear modulus of the tissue rapidly increases by several orders of magnitude. This model provides a framework for understanding the effect compositional changes cause to the mechanics of the tissue. The model predicts that the removal of the reinforcing background modulus shifts the rigidity percolation threshold of the collagen concentration, and results in a large decrease in shear modulus for the same collagen concentration. In the surface region of the tissue, as the mechanics are dominated by the aggrecan gel, then the shear modulus of the tissue will greatly decrease when the gel is removed - unlike deep into the tissue where the removal of the aggrecan gel will result in only a small decrease of shear modulus as the collagen network is still sufficiently connected. To test these predictions, we degrade the proteoglycan network of cartilage using the enzyme trypsin. Methods: Cartilage explants (N=24) were harvested from femoral condyles of neonatal calves (Fig 1.A.). The plugs (N=12) were coated by an epoxy surface which protected everywhere except the surface. The samples were then submerged in trypsin-EDTA (0.25%) at 37°C for 300 minutes, which removed proteoglycans as a front from the surface towards the deep zone. The samples were then rinsed with protease inhibitors. These plugs were bisected, and one half was used for measurement of the depth dependent shear modulus and biochemical analysis. The other half was fixed and used for histology and FTIR measurement, then biochemical essays were used to measure proteoglycan and collagen content. We were able to stain the tissue to validate proteoglycan loss, and the FTIR measurement provided a relative concentration of the aggrecan and collagen concentration throughout the tissue. We performed our simulations in Fortran, treating collagen as a random Kagome network with a reinforcing uniform background. We were able to calculate a shear modulus of this network, and then add in the reinforcing background afterwards. Results: The histology staining shows that we have a consistent front progressing with depth (Fig 1.B.). This verifies our FTIR measurements which shows a region of significantly reduced aggrecan concentration, with an unchanged collagen content (Fig 1.D.1.). Our shear measurements showed that the loss of proteoglycans caused an uneven decrease of shear modulus by up to a factor of 5 (Fig 1.D.2.). We then fitted the measurements of both the healthy articular cartilage and the degraded tissue (Fig 1.D.3.), and by scaling the reinforcing background modulus in the mechanical simulations, we can make predictions about the mechanical properties of other tissues for a range of proteoglycan concentrations. Conclusions: The trypsin degradation was able to mostly remove the aggrecan gel but deeper in the tissue there was still a non-zero concentration of aggrecan. Despite the removal of most of the proteoglycan background, the shear modulus of the deep zone was still an order of magnitude larger than that of the healthy surface tissue. This backs the prediction of the model which suggests that at high concentrations of collagen, it is the collagen network which provides most of the tissue’s structural support. This rigidity percolation model captures the mechanics of both healthy and deteriorated articular cartilage. Using this model, full mechanical descriptions of articular cartilage can be described using simple constituent scans of the collagen and proteoglycan content of the tissue. The model can also be used for the diagnosis of early proteoglycan loss through mechanical measurement. Finally, this model provides the understanding to direct the necessary advancements in artificial construct development.

Super-resolution approaches in photoacoustic imaging

The resolution of photoacoustic imaging (PAI) is limited at depths by the diffraction limit. Several ways have been introduced to achieve super-resolution. In the context of imaging the vasculature, the presence of flow can be exploited in two regimes, distinct by the concentration of flowing absorbing particles. In the high concentration regime, we proposed to exploit the absorption fluctuation caused by flowing absorbers by analyzing nth-order statistics of temporal signal fluctuations. In the low concentration regime, when absorbers appear one-by-one in each acoustic resolution spots, the localization microscopy technique can be adapted to our problem. While these two methods improve the resolution greatly, their cost is to reduce temporal resolution, because of the need to record thousands of images. Supposing the knowledge of the PSF (point spread function) of the imaging system, it is possible to the regain temporal resolution. After the simulation of the forward model, the imaged object can be recovered by solving a minimization problem. We will show that adding a sparsity constraint to this problem can enhance the resolution. These techniques have been investigated in both simulations and experiments in microfluidic channels. Such super-resolution approaches bring the optical contrast at depth closer to the cellular level.

Threshold Voltage Control in Organic Field-Effect Transistors by Surface Doping with a Fluorinated Alkylsilane.

Doping is a powerful tool to control the majority charge carrier density in organic field-effect transistors and the threshold voltage of these devices. Here, a surface doping approach is shown, where the dopant is deposited on the prefabricated polycrystalline semiconducting layer. In this study, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTCS), a fluorinated alkylsilane is used as a dopant, which is solution processable and much cheaper than conventional p-type dopants, such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). In this work, the depositions from the gas phase and from solution are compared. Both deposition approaches led to an increased conductivity and to a shift in the threshold voltage to more positive values, both of which indicate a p-type doping effect. The magnitude of the threshold voltage shift could be controlled by the FTCS deposition time (from vapor) or FTCS concentration (from solution); for short deposition times and low concentrations, the off current stayed constant and the mobility decreased only slightly. In the low doping concentration regime, both approaches resulted in similar transistor characteristics, i.e., similar values of shift in the threshold and turn-on voltage as well as mobility, ION/ IOFF ratio and amount of introduced free charge carriers. In comparison with vapor deposition, the solution-based approach can be conducted with less material and in a shorter time, which is critical for industrial applications.

The link between the kinetics of gas hydrate formation and surface ion distribution in the low salt concentration regime

Inorganic salts can thermodynamically inhibit gas hydrate formation. However, some inorganic salts at low concentration can act as a kinetic hydrate promoter. The mechanism of kinetic hydrate promotion in the presence of low concentration of inorganic salts is still unknown. This paper presents an experimental study into methane hydrate formation in an impeller-agitated vessel in the presence of sodium halides and alkali metal chlorides at low concentrations. It is shown that alkali metal chlorides and sodium halides at low concentration can reduce the induction time and kinetically promote gas hydrate formation. It has been proposed that bubbles form inside the agitated vessel as a result of the gas pocket break-up. Simulated gas pocket break-up studies show a smaller gas bubble formation in the salts solution with low concentrations in comparison with in the pure water. The small bubbles formation leads to an increase in the gas-water interface area and gas hold-up of the vessel. Consequently, there will be an increase in the mass transfer for gas hydration formation. In addition, the strength of hydrogen bonds at the gas/water interface affect the gas dissolution rate into the aqueous phase. Ions that have more affinity for the interface order water molecules weakly and improve the gas hydrate formation. Bubbles zeta potential measurements also confirm the ion-specific effect of the applied salts at the gas-water interface. Ultimately, gas-water interfacial area and ion-specific effect play critical roles in the gas hydrate formation.

Intermolecular Singlet Fission in Unsymmetrical Derivatives of Pentacene in Solution

AbstractIntermolecular singlet fission (SF) is an electronically coupled process between two chromophores, where distance dependences are decisive in terms of rates and yields. In the current work, a family of pentacene derivatives featuring different functional groups have been designed, synthesized, and probed with respect to intermolecular SF in the low, medium, and high concentration regimes rather than in the solid state. By means of advanced photophysical techniques, global analysis modeling, and ab initio calculations, a model for intermolecular SF is postulated. The model is based on an early key intermediate, which involves the diffusional encounter between one pentacene in its singlet excited‐state with another one in its ground state and which features excimer characteristics. This is followed by a transformation into a coupled triplet excited‐state. The role of the functional group appended to pentacene is analyzed with respect to steric shielding of the pentacene core as a means to prevent photophysical degradation, as well as control diffusional encounter and, subsequently, SF. The findings demonstrate the potential of new molecular materials for SF, especially in solution studies, as well as the challenges of implementing them in energy conversion schemes due to the appearance of photodegradation processes that compete with SF.

Interaction of a cationic amphiphile with monomeric and polymeric electrolytes: From morphological transition to associative phase separation.

The discrete effects of a series of structurally divergent monomeric viz. Sodium Chloride (NaCl), Tetra-butyl Ammonium Chloride (TBAC) and Sodium Benzoate (NaBz) and polymeric viz. Sodium Polystyrene Sulfonate (NaPSS) electrolytes towards the morphological and/or aggregation properties of Octadecyl-trimethyl Ammonium Bromide (OTAB) micelles have been quantified spectroscopically by means of the modulations of the absorption and emission spectral properties of an extrinsic anthracene-based probe 9-methyl anthroate (9-MA) within the concerned media. Further corroboration of the spectroscopic results was acquired from the non-invasive dynamic light scattering technique. The qualitatively similar mode of action of all the monomeric salts has been explained on the basis of the archetypal Israelachvili model whereas the corresponding extent of the morphological transition of the micelles, which is found to follow the order NaBz > NaCl > TBAC, has been explained invoking the co-sphere overlap model. Conversely, to explain the aggregation behaviour of the micelles in the presence of the polymeric electrolyte, a two-step model has been formulated. According to this model, at the low concentration regime, the polymeric salt is found to only neutralize the surface charge of the micelles inducing micellar growth; whereas further increment in the concentration of the polymer assists the hydrophobic association between the micelles leading to the formation of larger aggregates, eventually causing a phase separation.

Non-equilibrium solvation dynamics in water-DMSO binary mixture: Composition dependence of non-linear relaxation.

Because of a larger number of intermolecular interactions and configurations available to them, aqueous binary mixtures exhibit a variety of dynamics that are not seen in pure liquids, often hard to understand or predict, and have attracted considerable interest recently. Among all such solutions, mixtures of water and dimethyl sulfoxide (DMSO) stand out for their unique role in chemistry and biology. The low DMSO concentration regime of the water-dimethyl sulfoxide (DMSO) mixture is relevant in wide ranging chemical and biological processes. Interestingly, this low concentration regime is known to display a string of yet unexplained anomalies. We probe these anomalies in atomistic simulations by studying (i) equilibrium solvation dynamics both in the ground and the excited states of the probe separately and (ii) the non-equilibrium solvation dynamics subsequent to excitation at time t = 0 and then following the solvation process. The latter needed a large number of simulations to obtain a reliable average. We carried out such studies across a large number of compositions of the water-DMSO mixture. We find that the usually employed linear response approximation breaks down at those concentrations where binary mixtures display other anomalies. The non-linearity is reflected in substantially different solvent responses in the ground and in the excited states of the solute probe indole and also in non-equilibrium solvation. The difference is maximum near 20%-35% of the DMSO concentration regime.

Paramagnetic Surface Active Ionic Liquids: Interaction with DNA and MRI Application

Paramagnetic surface active ionic liquids (PMSAILs) have been synthesized wherein long alkyl chain bearing imidazoilium, pyridinium or isoquinolinium are cationic and bromotrichloroferrate (III) is paramagnetic counterion constituents. PMSAILs have been investigated for their aggregation properties using surface tension, conductivity, and dynamic light scattering (DLS). Interaction of PMSAILs with DNA in aqueous solutions has been examined through CD, fluorescence, ITC, DLS, zeta potential, and agarose gel electrophoresis. DNA compaction has been observed upon interaction with PMSAILs in dilute solutions. PMSAIL with isoquinolinium cation was found more efficient towards the DNA compaction. Decompaction of DNA could be achieved simply with the addition of NaCl. Further, the application of PMSAILS as MRI contrast materials has been explored. All the investigated PMSAILS exhibited excellent dual mode T1 & T2 contrast property in low concentration regime. To improve the real-time practical importance, MRI results have been compared with clinically available Gd-BOPTA based MRI contrast agent.

Nuclear Magnetic Resonance-Based Structural Characterization and Backbone Dynamics of Recombinant Bee Venom Melittin.

In recent years, there has been a resurgence of interest in melittin and its variants as their therapeutic potential has become increasingly evident. Melittin is a 26-residue peptide and a toxic component of honey bee venom. The versatility of melittin in interacting with various biological substrates, such as membranes, glycosaminoglycans, and a variety of proteins, has inspired a slew of studies that aim to improve our understanding of the structural basis of such interactions. However, these studies have largely focused on melittin solutions at high concentrations (>1 mM), even though melittin is generally effective at lower (micromolar) concentrations. Here we present high-resolution nuclear magnetic resonance studies in the lower-concentration regime using a novel method to produce isotope-labeled (15N and 13C) recombinant melittin. We provide residue-specific structural characterization of melittin in dilute aqueous solution and in 2,2,2-trifluoroethanol/water mixtures, which mimic melittin structure-function and interactions in aqueous and membrane-like environments, respectively. We find that the cis-trans isomerization of Pro14 is key to changes in the secondary structure of melittin. Thus, this study provides residue-specific structural information about melittin in the free state and in a model of the substrate-bound state. These results, taken together with published work from other laboratories, reveal the peptide's structural versatility that resembles that of intrinsically disordered proteins and peptides.

Investigation of PdCuSi metallic glass film for hysteresis-free and fast response capacitive MEMS hydrogen sensors

In this study, we investigated PdCuSi metallic glass (MG) as a sensing material for capacitive MEMS hydrogen sensors. We first confirmed by film analysis that the fabricated PdCuSi film was MG and that it had a trigonal prism cluster. The measured pressure-composition-temperature curve of PdCuSi MG exhibited no hysteresis during hydrogen absorption and desorption. The response time was found to become faster by two orders of magnitudes compared with that of Pd polycrystal. These properties were attributed to the trigonal prism clusters. Strain was evaluated in the low hydrogen concentration regime of 0.05 vol% to 4.0 vol%, and the strain of PdCuSi MG was found to follow Sieverts' law well, indicating that hydrogen is present in the MG in a diffuse state. The hydrogen-concentration dependence of a capacitive MEMS hydrogen sensor was measured and hysteresis-free characteristics were obtained, implying advantages in hydrogen leak detection applications.

Coassembly of Linear Diblock Copolymer Chains and Homopolymer Brushes on Silica Particles: A Combined Computer Simulation and Experimental Study

A combined computer simulation and experimental study on coassembly of poly(2-(dimethylamino)ethyl methacrylate)-block-polystyrene (PDMAEMA-b-PS) block copolymers and PS brushes on silica particles was performed. PS brushes on silica particles at two different grafting densities were prepared by the “grafting to” approach, and PDMAEMA-b-PS block copolymers with different molecular weights and compositions were synthesized by reversible addition–fragmentation chain transfer polymerization. In THF/methanol mixtures, block copolymer chains and PS brushes coassemble into surface micelles (s-micelles), with collapsed PS cores and PDMAEMA coronae. Meanwhile, block copolymer chains are able to self-assemble into block copolymer micelles (b-micelles). Computer simulation results and experimental results indicate that block copolymer concentration, PS and PDMAEMA block lengths, and PS grafting density exert significant influences on the coassembly process. In low BCP concentration regime, the average size of s-mic...

Consistent biases in Antarctic sea ice concentration simulated by climate models

Abstract. The simulation of Antarctic sea ice in global climate models often does not agree with observations. In this study, we examine the compactness of sea ice, as well as the regional distribution of sea ice concentration, in climate models from the latest Coupled Model Intercomparison Project (CMIP5) and in satellite observations. We find substantial differences in concentration values between different sets of satellite observations, particularly at high concentrations, requiring careful treatment when comparing to models. As a fraction of total sea ice extent, models simulate too much loose, low-concentration sea ice cover throughout the year, and too little compact, high-concentration cover in the summer. In spite of the differences in physics between models, these tendencies are broadly consistent across the population of 40 CMIP5 simulations, a result not previously highlighted. Separating models with and without an explicit lateral melt term, we find that inclusion of lateral melt may account for overestimation of low-concentration cover. Targeted model experiments with a coupled ocean–sea ice model show that choice of constant floe diameter in the lateral melt scheme can also impact representation of loose ice. This suggests that current sea ice thermodynamics contribute to the inadequate simulation of the low-concentration regime in many models.

Method for Low Nanomolar Concentration Analyte Sensing Using Electrochemical Enzymatic Biosensors.

We introduce a new electrochemical measurement method compatible with an enzymatic biosensor that is capable of analyte sensing down to the low nanomolar concentration regime. This method is termed accumulation mode sensing and utilizes an immobilized redox polymer mediator wired to an oxidoreductase enzyme to store charge during a premeasurement charge concentration step, followed by a measurement step in which this accumulated charge is quantified. We demonstrate this new method using a model glucose sensor and show how the sensitivity of a sensor can be modified simply by adjusting the time duration of the charge concentration step. We achieve a limit of detection of 4.7 ± 1.4 nM using accumulation mode sensing, which represents a 25-fold improvement over traditional amperometry.

Temperature-dependent transport properties of graphene decorated by alkali metal adatoms (Li, K)

We report the electrical transport properties of graphene for dilute alkali metal decoration (n ∼ 2 × 1012 cm−2) at cryogenic temperatures. Upon deposition of K and Li atoms at T = 20 K, graphene devices are doped with electrons, and the charge carrier mobility is decreased. As temperature is increased, the number of electrons donated to the graphene and the number of charged scatterers are reduced, and the mobility of the metal decorated graphene is increased. This differs from the typical temperature-dependent transport in undecorated graphene, where the mobility decreases with increasing temperature. To investigate the kinetic behavior of adatoms on graphene, we estimate the hopping time of the Li and K adatoms on graphene based on the migration barrier in the low concentration regime of the metal adatoms by Density Functional Theory calculations. The calculations reveal that these adatoms are mobile even at cryogenic temperatures and become more mobile with increasing temperature, allowing for cluster formation of adatoms. This indicates that the dominant factor in the electron transport on warming is a cluster formation.

Operational Implementation of Sea Ice Concentration Estimates From the AMSR2 Sensor

An operation implementation of a passive microwave sea ice concentration algorithm to support NOAA's operational mission is presented. The NASA team 2 algorithm, previously developed for the NASA advanced microwave scanning radiometer for the Earth observing system (AMSR-E) product suite, is adapted for operational use with the JAXA AMSR2 sensor through several enhancements. First, the algorithm is modified to process individual swaths and provide concentration from the most recent swaths instead of a 24-hour average. A latency (time since observation) field and a 24-hour concentration range (maximum–minimum) are included to provide indications of data timeliness and variability. Concentration from the Bootstrap algorithm is a secondary field to provide complementary sea ice information. A quality flag is implemented to provide information on interpolation, filtering, and other quality control steps. The AMSR2 concentration fields are compared with a different AMSR2 passive microwave product, and then validated via comparison with sea ice concentration from the Suomi visible and infrared imaging radiometer suite. This validation indicates the AMSR2 concentrations have a bias of 3.9% and an RMSE of 11.0% in the Arctic, and a bias of 4.45% and RMSE of 8.8% in the Antarctic. In most cases, the NOAA operational requirements for accuracy are met. However, in low-concentration regimes, such as during melt and near the ice edge, errors are higher because of the limitations of passive microwave sensors and the algorithm retrieval.

1.54 μm photoluminescence from Er:Ox centers at extremely low concentration in silicon at 300K.

The demand for single photon emitters at λ=1.54 μm, which follows from the consistent development of quantum networks based on optical fiber technologies, makes Er:Ox centers in Si a viable resource, thanks to the I13/24→I415/2 optical transition of Er3+. While its implementation in high-power applications is hindered by the extremely low emission rate, the study of such systems in the low concentration regime remains relevant for quantum technologies. In this Letter, we explore the room-temperature photoluminescence at the telecomm wavelength from very low implantation doses of Er:Ox in Si. The lower-bound number of optically active Er atoms detected is of the order of 102, corresponding to a higher-bound value for the emission rate per individual ion of about 104 s-1.

Critical Properties of the Two-Dimensional Villain Model with Ferromagnetic Impurities

This work investigates the square lattice in the presence of an-ti-ferromagnetic exchange considering the two-dimensional Ising Villain model. A computational scheme is constructed to evaluate the degeneracy of ground states at zero temperature employing the Pfaffian method through perturbation theory. The entropy and canonical spin glass of the considered model are precisely obtained. The critical point between the Villain phase and canonical spin glass phase with the line fit of entropy at the thermodynamic limit is investigated. It is also shown that the transition point of the Villain model lies in the low concentration regime.

Combining random walk and regression models to understand solvation in multi-component solvent systems.

Polysaccharides, such as cellulose, are often processed by dissolution in solvent mixtures, e.g. an ionic liquid (IL) combined with a dipolar aprotic co-solvent (CS) that the polymer does not dissolve in. A multi-walker, discrete-time, discrete-space 1-dimensional random walk can be applied to model solvation of a polymer in a multi-component solvent mixture. The number of IL pairs in a solvent mixture and the number of solvent shells formable, x, is associated with n, the model time-step, and N, the number of random walkers. The mean number of distinct sites visited is proportional to the amount of polymer soluble in a solution. By also fitting a polynomial regression model to the data, we can associate the random walk terms with chemical interactions between components and probe where the system deviates from a 1-D random walk. The 'frustration' between solvents shells is given as ln x in the random walk model and as a negative IL:IL interaction term in the regression model. This frustration appears in regime II of the random walk model (high volume fractions of IL) where walkers interfere with each other, and the system tends to its limiting behaviour. In the low concentration regime, (regime I) the solvent shells do not interact, and the system depends only on IL and CS terms. In both models (and both regimes), the system is almost entirely controlled by the volume available to solvation shells, and thus is a counting/space-filling problem, where the molar volume of the CS is important. Small deviations are observed when there is an IL-CS interaction. The use of two models, built on separate approaches, confirm these findings, demonstrating that this is a real effect and offering a route to identifying such systems. Specifically, the majority of CSs - such as dimethylformide - follow the random walk model, whilst 1-methylimidazole, dimethyl sulfoxide, 1,3-dimethyl-2-imidazolidinone and tetramethylurea offer a CS-mediated improvement and propylene carbonate results in a CS-mediated hindrance. It is shown here that systems, which are very complex at a molecular level, may, nonetheless, be effectively modelled as a simple random walk in phase-space. The 1-D random walk model allows prediction of the ability of solvent mixtures to dissolve cellulose based on only two dissolution measurements (one in neat IL) and molar volume.

Continuous monitoring of the bulk oxidation states in LixNi1/3Mn1/3Co1/3O2 during charging and discharging

Operando magnetic susceptibility measurements on the LixNi1/3Mn1/3Co1/3O2 cathode material during repetitive electrochemical cycling were performed, enabling a continuous and bulk sensitive monitoring of the charge compensation process. Upon charging and Li extraction down to the Li contents of x = 1/3, exclusively Ni undergoes oxidation in two consecutive steps, namely, Ni2+→Ni3+ for x > 2/3 and Ni3+→Ni4+ for 2/3 > x > 1/3 with a continuous transition in between. In the regime of low Li concentrations x < 1∕3, both Co- and O-ions contribute to the charge compensation. While the oxidation of Ni and O during charging is reversible, the Co oxidation is found to be irreversible.