Model Of Ion Transport Research Articles

An assessment of electroneutrality implementations for accurate electrochemical ion transport models

During the diffusion and migration of ions in electrolytes, the electrodynamic ion-ion interactions prevent charge separation despite different ionic mobilities, ultimately enforcing electroneutrality in the bulk electrolyte. To model ion transport accurately, a method to enforce electroneutrality must be implemented. In this study, four strategies to implement electroneutrality are discussed and evaluated. The ion distributions that result from a transport model with the different electroneutrality implementations are calculated, considering various electrolytes and sets of electrochemical parameters. The meaningfulness and applicability of each implementation are assessed through spatial charge accumulations, transference numbers, and experimental data from the literature. Combining the electrochemical ion transport models with the electroneutrality constraint for all ions is shown to result in an overdetermined system of equations if the driving forces are calculated under neglection of diffusion potentials. The often-reported model simplification of using the electroneutrality constraint to resolve the transport of one specific species explicitly results in non-physically correct mass transport. A practical approach to precisely describe the measured physicochemical ion movements is obtained by equilibrating spatial charges with the ion conduction for every time step in the ion transport model, which is reasonably applicable to multi-ion systems in three-dimensional frameworks. This comprehensive assessment aims to guide readers in selecting an appropriate electroneutrality implementation framework for ion transport models.

Alternating access of a bacterial homolog of neurotransmitter: sodium symporters determined from AlphaFold2 ensembles and DEER spectroscopy

Neurotransmitter:sodium symporters (NSSs) play critical roles in neural signaling by regulating neurotransmitter uptake into cells powered by sodium electrochemical gradients. Bacterial NSSs orthologs, including MhsT from Bacillus halodurans, have emerged as model systems to understand the structural motifs of alternating access in NSSs and the extent of conservation of these motifs across the family. Here, we apply a computational/experimental methodology to illuminate the conformational landscape of MhsT alternating access. Capitalizing on our recently developed method, Sampling Protein Ensembles and Conformational Heterogeneity with AlphaFold2 (SPEACH_AF), we derived clusters of MhsT models spanning the transition from inward-facing to outward-facing conformations. Systematic application of double electron-electron resonance (DEER) spectroscopy revealed ligand-dependent movements of multiple structural motifs that underpin MhsT's conformational cycle. Remarkably, comparative DEER analysis in detergent micelles and lipid nanodiscs highlights the profound effect of the environment on the energetics of conformational changes. Through experimentally derived selection of collective variables, we present a model of ion and substrate-powered transport by MhsT consistent with the conformational cycle derived from DEER. Our findings not only advance the understanding of MhsT's function but also uncover motifs of conformational dynamics conserved within the broader context of the NSS family and within the LeuT-fold class of transporters. Importantly, our methodological blueprint introduces an approach that can be applied across a diverse spectrum of transporters to describe their conformational landscapes.

A new fractal model for flow-solid-chemical multi-field coupling in underground engineering

The permeation diffusion behavior of chloride ions with seawater is one of the key factors causing damage to reinforced concrete structures due to seawater erosion of steel bars. This study establishes a new mathematical model for chloride ion transport in reinforced concrete structures exposed to seawater. The model captures the effect of chloride ions on underwater tunnel structures effectively and provides a valuable theoretical basis for the maintenance and safety of these structures. The proposed model uniquely incorporates the microscopic pore structure characteristics of concrete and the effects of static water pressure and porosity on chloride ion diffusion. The results show that: (1) the diffusion coefficient of chloride ions first increases rapidly, then remains basically unchanged and gradually decreases; (2) there is a non-linear positive correlation between porosity, static water pressure, and maximum corrosion thickness; and (3) the fractal dimension of pore curvature is opposite to the fractal dimension of pore size and crack length.

Cryo-EM structures of the human band 3 transporter indicate a transport mechanism involving the coupled movement of chloride and bicarbonate ions.

The band 3 transporter is a critical integral membrane protein of the red blood cell (RBC), as it is responsible for catalyzing the exchange of bicarbonate and chloride anions across the plasma membrane. To elucidate the structural mechanism of the band 3 transporter, detergent solubilized human ghost membrane reconstituted in nanodiscs was applied to a cryo-EM holey carbon grid to define its composition. With this approach, we identified and determined structural information of the human band 3 transporter. Here, we present 5 different cryo-EM structures of the transmembrane domain of dimeric band 3, either alone or bound with chloride or bicarbonate. Interestingly, we observed that human band 3 can form both symmetric and asymmetric dimers with a different combination of outward-facing (OF) and inward-facing (IF) states. These structures also allow us to obtain the first model of a human band 3 molecule at the IF conformation. Based on the structural data of these dimers, we propose a model of ion transport that is in favor of the elevator-type mechanism.

Damage-based ion transport in recycled aggregate concrete under external sulfate attack

The utilization of recycled coarse aggregate (RCA) in replacement of natural coarse aggregate in the production of recycled aggregate concrete (RAC) may potentially lead to a reduction in the resistance of RAC to sulfate attack. In this study, the dynamic elastic modulus and sulfate ion transport of RAC was investigated by fully immersing RAC specimens produced with different water-to-cement ratios (W/C) and RCA volume fractions (VRCA) into sulfate (Na2SO4) solutions with different concentrations. The effects of W/C and VRCA on the initial damage velocity, damage acceleration, and service life of the RAC were analyzed by the defined damage evolution equations. The variation patterns of the ion diffusion coefficient and the surface ion concentration were discussed in detail, and a general equation was developed to describe the variation of surface ion concentration over time. A damage-based ion transport model was developed based on the damage evolution of the dynamic elastic modulus. The results demonstrated that the damage-based ion transport model exhibited excellent accuracy in predicting the ion transport behavior in the RAC. Furthermore, the mechanism of the effect of RCA on ion transport is more complex and requires a comprehensive study. This study provides theoretical support for the application of RAC in engineering construction and the calibration of important physical parameters in related numerical studies.

Water-mediated super-correlated proton-assisted transport mode for solid-state K−O2 batteries

A comprehensive understanding of the behavior nature of fast ions at the atomic level is essential for the development of advanced solid-state ionic conductors. The inadequate inter-ion correlation effects of current ion transport models lead to a conductivity bottleneck in designing conductors. Herein, based on water-mediated proton-assisted ion transport, a novel transport mode with simultaneous anion-cation and inter-cation coupling is designed, enabling the K-ions of the modeled solid-state ionic conductor K2Fe4O7 crystals to achieve an ultra-high conductivity of 7.6 × 10–2 S cm–1, an enhancement of two orders of magnitude. The principle of the accelerated K-ion diffusion through the rotation and vibration of water molecules around the framework oxygen atom is elaborated, and the coupling correlation between proton and K-ion transport is confirmed using in-situ impedance spectroscopy under labeled isotopes. The application of this mechanism enabled the fabricated K−O2 solid-state battery exhibit an ultra-low overpotential (0.1 V) and excellent rate performance. Further, the mechanism is also applicable for Li and Na-ion conductors, providing significant theoretical guidance for breaking existing universal design rules and for the development for faster ionic conductors.

(Invited) Energy-Efficient and Impurity-Tolerant Electrochemical Co-Generation of Acid and Base Solutions

The sequential application of acid and base solutions can be used to perform chemical transformations that are of interest for carbon management and sustainable resource utilization, including CO2 capture, CO2 removal, production of slaked lime, and recovery of critical minerals. Electrochemical co-generation of acid and base from salt solutions enables their use in closed-loop processes that could be powered by renewable electricity, wherein acid and base are regenerated from the salt byproduct of their application. To be viable at scale, acid-base co-generating systems need to be energy-efficient and tolerant to impurities that are inevitably released into recycle loops. Conventional electrochemical methods for co-generating acid and base, such as bipolar membrane electrodialysis (BPMED), rely on the use of ion exchange membranes (IEMs) to inhibit H+ and OH– recombination, but IEMs cause large resistive losses that severely limit the energy efficiency and current density of these systems. In addition, cation exchange membranes (CEMs) do not tolerate polyvalent cations (e.g., Ca2+, Mg2+) in the presence of base, yet polyvalent cations are intrinsic to all high-impact applications. In this talk, I will describe our development of acid-base co-generating systems in which H+ and OH– recombination is inhibited by competitive transport of supporting electrolyte and H+ masking, which enables the use of a simple porous diaphragm separator instead of IEMs. We developed an ion transport model to predict the current efficiency for acid-base co-generation as a function of the electrolyte composition and operating parameters. Instead of a bipolar membrane, our systems use the H2 oxidation reaction (HOR) and H2 evolution reaction for H+ and OH– generation, respectively. We demonstrate IEM-free production of acid and base at useful concentrations in the presence of polyvalent cations with much higher energy efficiency and current density than state of the art BPMED systems. Individual cells can be stacked by combining HOR and HER electrodes into bipolar gas diffusion electrodes that recirculate H2 with near-unity efficiency. To demonstrate the utility of the acid and base solutions generated by this system, I will describe their use to extract Mg(OH)2 from ultramafic rocks for application in CO2 removal.

Modeling of Concrete Deterioration under External Sulfate Attack and Drying-Wetting Cycles: A Review.

This paper comprehensively summarizes moisture transport, ion transport, and mechanical damage models applied to concrete under sulfate attack and drying-wetting cycles. It highlights the essential aspects and principles of each model, emphasizing their significance in understanding the movement of moisture and ions, as well as the resulting mechanical damage within the concrete during these degradation processes. The paper critically analyzes the assumptions made in each model, shedding light on their limitations and implications for prediction accuracy. Two primary challenges faced by current models under sulfate attack and drying-wetting cycles are identified: the limited consideration of the coupled effects of chemical and physical attacks from sulfate, and the unclear mechanism of the sulfate attacks. Future research directions are proposed, focusing on exploring the transport mechanism of sulfate ions under various driving forces and further clarifying the crystallization process and expansion damage mechanism in concrete pores. Addressing these research directions will advance our understanding of sulfate attack under drying-wetting cycles, leading to improved models and mitigation strategies for enhancing the durability and performance of concrete structures.

Coupled model for electro‐osmosis consolidation and ion transport considering chemical osmosis in saturated clay soils

AbstractThe electro‐osmosis approach efficiently facilitates the rapid dewatering of soil with high water content and contributes to reducing contaminant levels within the clay soil. However, the changes of chemical field caused by ion transport in the clay soil during electro‐osmosis process will also influence the clay soil consolidation effect. Existing theories predominantly tend to disregard this crucial physical process and its resultant effects, thereby restraining a comprehensive analysis of electro‐osmosis consolidation (EOC) behavior under intricate chemical conditions. This study introduces a concise model of EOC and ion transport considering chemical osmosis. The model considers the nonlinear variation of clay soil parameters such as compressibility, permeability, and effective diffusion coefficients, along with the interaction between EOC and ion transport. Meanwhile, the correctness of the model is verified from different aspects such as theoretical derivation and model comparison. Based on the proposed model, the impacts of the variation in electrical field intensity and chemical concentration on the coupled behaviors between EOC and ion transport are systematically investigated, with and without incorporating nonlinear consolidation characteristics. The results show that diffusion and electro‐migration exhibit a more pronounced effect on ion transport during EOC. Simultaneously, with the increase of ion concentration in clay soil pore solution, the effects of chemical osmosis become increasingly apparent, thereby enhancing clay soil settlement.

Synergy effects of pH and thermal localization on membrane-based salinity gradient energy harvesting

Salinity gradient power generation receives increasing attention due to its availability in nature. However, membrane surface charge is not fixed value in isolation but strongly dependent of various conditions. Here, a coupled ion transport model is proposed to study the synergy effects of pH and thermal localization on membrane-based salinity gradient energy conversion. The research illustrates that the surface charge is regulated by solution environment, membrane properties and thermal conditions. Specially, the surface charge sign can be even reversed at certain case that induces distinct ion transports behaviors. Consequently, Al2O3 membrane achieves the better energy conversion performance at low pH with negative temperature difference. While SiO2 membrane obtains the best output performance at high pH with negative temperature difference. Such is ascribed to pH-regulated surface charge and thermal up-diffusion enhancement. The maximum output power of 10 pW with efficiency of 16 % is achieved for individual membrane channel. As a proof of concept, it shows significant deviations between the single and coupling model on power generation. The findings of this study can help for better understanding the coupling mechanisms of solution, membrane and thermal conditions during the energy conversions, thereby providing a reasonable design route for membrane-based energy devices.

Novel modelling of metal atoms diffusion and ion transport in ECR plasma relevant to ion sources and in-plasma nuclear physics studies

Metals can be injected into electron cyclotron resonance ion sources (ECRIS) via different techniques, among which resistive ovens are used to vaporize neutral materials, later captured by the energetic plasma that will step-wise ionize them, hence giving multiply charged ion beams for accelerators. Recently, PANDORA, a novel ECR plasma trap, has been conceived to perform interdisciplinary research spanning from nuclear physics to astrophysics, where in-plasma high charge states of metallic species are demanded. However, a full knowledge on the vaporization method and on the coupling of neutral atoms with plasma and its overall dynamics is still not available. Simulations, hence, are of fundamental relevance to improve the overall efficiency, reduce consumption of rare expensive isotopes, and to improve the ion source performance. We present a numerical study about metallic species suitable for oven injection in ECRIS, focusing on metals diffusion, transport, and wall deposition under molecular flow regime. We studied the metal dynamics with and without plasma. Results underline the plasma role on a space-dependent conversion yield, reflecting the strongly inhomogeneous ECR plasma. The plasma and its parameters have been modelled using an established self-consistent particle-in-cell model. The numerical tool is conceived for the PANDORA plasma trap but can be extended to other ECR plasmas and traps. As test cases we studied the 134Cs and 48Ca radioisotopes, as metals of interest for the modern nuclear physics. A focus is given on the β-decaying 134Cs, as an application case for PANDORA, providing quantitative estimates of the γ-detection signal-poisoning effect by neutral metals deposition at the chamber wall.

A coupled physical–chemical model for mass transfer considering damage evolution: Sulfate ions transport in a cement-based material as an example

Damage induced in mass transfer process is a ubiquitous phenomenon in nature and living systems. A coupled physical–chemical modeling framework is proposed to clarify the mass transfer in damaged media. Such a theoretical framework shows that the mass transfer problem is no longer linear but non-linear, due to the mass transfer behavior is influenced not only by transport driving mechanisms but also by physical consumption mechanisms, chemical consumption mechanisms, and damage evolution. Based on the theoretical framework presented in this paper, one can choose specific mechanisms in each category, different mass transfer theories can be constructed to meet specific practical applications requirements. To verify the correctness and validity of this theoretical framework, we choose the theoretical model of sulfate ion transport in cement-based materials. This is because sulfate ions are the main cause to induce the corrosion cracks in concrete in the marine environment. The following mechanisms are considered: concentration gradient driven transfer, chemical reaction, continued hydration, filling effect, and expansion damage. 3D nonlinear partial differential equations with variable coefficients are derived for the system. The numerical results show that the chemical reaction has the dual characteristics of self-stagnation and acceleration of sulfate ion transport. The chemical reaction products consume the free ions and fill the pore structure, which reflects the self-stagnation of mass transfer, while the expansion force generated by the chemical products leads to the initiation of corrosion crack, which reflects the acceleration of mass transfer. The competitive outcome of the two mechanisms is acceleration feature as the dominant one. Finally, the proposed model efficacy is validated by comparing the numerical results with previously reported experimental data.

Modeling of ion transport from ionization region to entrance of mass spectrometer in HiPIMS argon/Cr target

Plasma global kinetic model coupled with the Monte Carlo method is used to study the ion transport in HiPIMS Ar/Cr target. The plasma kinetic global model is developed to study the time evolution of neutral, ion, and electron species created in the ionization region. To analyze the ion temporal spectra at the entrance of the mass spectrometer, a simple model based on the Monte Carlo technique is developed to track the ion trajectories from the ionization region to the mass spectrometer. The ion temporal spectra obtained by the global kinetic model in the ionization region are introduced in the Monte Carlo model as input data. The simulation results reveal a temporal shift of the ion spectra as well as their spreading in comparison with those obtained in the ionization region. Such temporal shapes of the ion spectra are more sensitive to the ion temperatures in the ionization region, and the position of the mass spectrometer is connected to the reactor. A satisfactory agreement between simulated ion temporal spectra and those measured by the mass spectrometer is obtained when we have represented the ion population energies by two Maxwellian distributions, where the first one corresponds to the low temperature and the second to the high temperature.

On the discrepancy between nominal and effective charge density of polyamide reverse osmosis membranes

Quantitative and reliable information on the ionization of functional groups in polyamide (PA) active layers of thin film composite membranes is of vital importance for understanding and modeling of ion transport. Independent measurements using Ag+ ion-probing with RBS, XPS, or ICP-MS, indicate a nominal charge density in the order of 0.5 M. Conversely, model fitting to salt rejection experiments suggests an effective membrane charge at least three orders of magnitude lower for brackish water (BWRO) and seawater (SWRO) membranes. In this study we show that the original interpretation of the ion-probe experiments is inconsistent with the principles of ion partitioning used in ion transport models. We provide an alternative interpretation of ion-probe experiments in the literature, explaining the results through binding of silver ions to the carboxylic groups. By using this interpretation, the discrepancy between nominal and effective charge can be resolved: the Ag measured in the ion-probe experiments is mostly bound to the membrane (-COOAg, not –COO−/Ag+), while the membrane is virtually uncharged. Consequently, the ion-probe experiments can quantify the concentration of functional groups, but not theirpKa. We propose that confinement affecting membrane ionization, should be explicitly included in ion-transport modeling.

Physics-informed deep learning for multi-species membrane separations

Conventional continuum models for ion transport across polyamide membranes require solving partial differential equations (PDEs). These models typically introduce a host of assumptions and simplifications to improve the computational tractability of existing solvers. As a consequence of these constraints, conventional models struggle to generalize predictive performance to new unseen conditions. Deep learning has recently shown promise in alleviating many of these concerns, making it a promising avenue for surrogate models that can replace conventional PDE-based approaches. In this work, we develop a physics-informed deep learning model to predict ion transport across diverse membrane types. The proposed architecture leverages neural differential equations in conjunction with classical closure models as inductive biases directly encoded into the neural framework. The neural methods are pre-trained on simulated data from continuum models and fine-tuned on independent experiments to learn multi-ionic rejection behaviour. We also harness the attention mechanism, commonly observed in language modelling, to learn and infer key paired transport relationships. Gaussian noise augmentations from experimental uncertainty estimates are also introduced into the measured data to improve robustness and generalization. We study the neural framework’s performance relative to conventional PDE-based methods, and also compare the use of hard/soft inductive bias constraints on prediction accuracy. Lastly, we compare our approach to other competitive deep learning architectures and illustrate strong agreement with experimental measurements across all studied datasets.

Modeling of Ion Transport in a Three-Layer System with an Ion-Exchange Membrane Based on the Nernst–Planck and Displacement Current Equations

Modeling of Ion Transport in a Three-Layer System with an Ion-Exchange Membrane Based on the Nernst–Planck and Displacement Current Equations

A charged existence: A century of transmembrane ion transport in plants.

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

Maximizing blue energy: the role of ion partitioning in nanochannel systems.

This study describes a numerical analysis on blue energy generation using a charged nanochannel with an integrated pH-sensitive polyelectrolyte layer (PEL), considering ion partitioning effects due to permittivity differences. The mathematical model for ionic and fluidic transport is solved using the finite element method, and the model validation is performed against existing theoretical and experimental results. The study investigates the influence of electrolyte concentration, permittivity ratio, and salt types (KCl, BeCl2, AlCl3) on the energy conversion process. The findings illustrate the substantial role of ion partitioning in modulating ionic concentration and potential fields, thereby affecting current profiles and energy conversion efficiencies. Remarkably, overlooking ion partitioning leads to significant overestimations of power density, highlighting the necessity of this consideration for accurate device performance predictions. This work introduces a promising configuration that achieves higher power densities, paving the way for the next generation of efficient energy-harvesting devices. The findings offer valuable insights into the development of state-of-the-art blue energy harvesting nanofluidic devices, advancing sustainable energy production.

Ionic regulatory strategies of crabs: the transition from water to land.

Terrestrial crabs (brachyurans and anomurans) have invaded land following a variety of pathways from marine and/or via freshwater environments. This transition from water to land requires physiological, ecological, and behavioral adaptations to allow the exploitation of these new environmental conditions. Arguably, the management of salt and water balance (e.g., osmoregulation) is integral for their survival and success in an environment where predominantly low-salinity aquatic (e.g., freshwater) water sources are found, sometimes in only minimal amounts. This requires a suite of morphological and biochemical modifications, especially at the branchial chamber of semi-terrestrial and terrestrial crabs to allow reprocessing of urine to maximize ion uptake. Using knowledge gained from electrophysiology, biochemistry, and more recent molecular biology techniques, we present summarized updated models for ion transport for all major taxonomic groups of terrestrial crabs. This is an exciting and fast-moving field of research, and we hope that this review will stimulate further study. Terrestrial crabs retain their crown as the ideal model group for studying the evolutionary pathways that facilitated terrestrial invasion.

(Invited) Microwave Dielectric Relaxation Spectroscopy: A Technique to Inform Ion Transport in Hydrated Polymer Membranes

Providing sustainable supplies of purified water and energy is a critical global challenge for the future, and polymer membranes will play a key role in addressing these clear and pressing global needs for water and energy. Polymer membrane-based processes dominate the desalination market because they are more energy efficient than thermal desalination processes, and polymer membranes are crucial components in several rapidly developing power generation and energy storage applications that rely on membranes to control, selectively, rates of ion transport. Much remains unknown about the influence of polymer structure on basic intrinsic transport properties, and these relationships are important for designing next generation polymer membrane materials.Key to controlling ion transport in membranes for water purification and energy applications is engineering interactions between ions and the solvated polymer matrix. Microwave dielectric relaxation spectroscopy is a tool that enables study of hydrated polymer membranes because water molecule dipoles can be probed in the microwave frequency region. This technique can provide unique insight into hydrated polymer dielectric permittivity (or dielectric constant) properties, which are critical in emerging thermodynamic modeling of ion transport in hydrated polymers. This presentation discusses the use of microwave dielectric relaxation spectroscopy to uncover structure property relationships in hydrated polymers and, more broadly, presents an overview of research aimed at further understanding fundamental structure/property relationships that govern small molecule transport in polymeric materials considered for desalination and electric potential field-driven membrane applications that can help address global needs for clean water and energy.