Understanding Multi-Ion Transport Mechanisms in Bipolar Membranes

Bipolar membranes (BPMs) have the potential to become critical components in electrochemical devices for a variety of electrolysis and electrosynthesis applications. Because they can operate under large pH gradients, BPMs enable favorable environments for electrocatalysis at the individual electrodes. Critical to the implementation of BPMs in these devices is understanding the kinetics of water dissociation that occurs within the BPM as well as the co- and counter-ion crossover through the BPM, which both present significant obstacles to developing efficient and stable BPM-electrolyzers. In this study, a continuum model of multi-ion transport in a BPM is developed and fit to experimental data. Specifically, concentration profiles are determined for all ionic species, and the importance of a water-dissociation catalyst is demonstrated. The model describes internal concentration polarization and co- and counter-ion crossover in BPMs, determining the mode of transport for ions within the BPM and revealing the significance of salt-ion crossover when operated with pH gradients relevant to electrolysis and electrosynthesis. Finally, a sensitivity analysis reveals that the performance and lifetime of BPMs can be improved substantially by using of thinner dissociation catalysts, managing water transport, modulating the thickness of the individual layers in the BPM to control salt-ion crossover, and increasing the ion-exchange capacity of the ion-exchange layers in order to amplify the water-dissociation kinetics at the interface

Probing ion transport mechanisms with synthetic channel-forming molecules

Maintaining asymmetric balances of intra- and extra-cellular ion concentrations is essential for the healthy and regular functioning of a cell as the presence of specific ionic gradients are responsible for a number of cellular and physiological processes. In order to establish and maintain these ionic gradients, ion channels provide one mechanism of controlled transport of physiological ions such as sodium (Na+), potassium (K+), calcium (Ca2+) and chloride (Cl-) into and out of the cell. Natural ion channels are large and very complex proteins that are able to rapidly transport ions across the cell‟s bilayer membrane with high selectivity. In order to understand which structural characteristics are required for effective and selective ion transport, a range of macrocyclic compounds based on calix[4]arenes, oxacalix[3]arenes, pillar[5]arenes and diaza-18-crown-6 was synthesised as models for channel-forming biomolecules. Through synthetic modifications and comparisons with their monomeric equivalents, it was possible to determine relationships between their structures and their activities. Polyether-based substituents, including an extended polyether, which incorporated a trans-but-2-ene linker, were attached in order to produce membrane-spanning molecules. The key elements investigated were the effect of the macrocycles compared to their monomeric equivalents, the effect of macrocyclic cavity size on ion selectivity, the effect on ion conductance observed between a rigid macrocycle compared with a flexible macrocycle and the impact of altering polyether chain length and functionality on ion conductance and selectivity. The compounds‟ ion transport abilities were assessed on artificial planar lipid bilayers and their antimicrobial activities determined by a variation of the Kirby-Bauer disc diffusion test on the common pathogens: E. coli, S. aureus, P. aeruginosa and S. pyogenes. Planar lipid bilayer results demonstrated that a predetermined structure, the length of the polyether substituent and the fit between the size of the macrocycle to the cation were important for transmembrane ion conduction; whereas the monomeric analogues formed unregulated sized pores leading to irregular to no activity and general non-selectivity. The rigid macrocycle compared to the flexible macrocycle demonstrated key differences in conduction where it is postulated that the flexible macrocycle conducted ionophorically. Antimicrobial tests revealed that the monomeric derivatives were significantly more potent towards bacteria than their macrocyclic equivalents, presumably due to the production of surfactant-like activity whilst the macrocyclic analogues displayed limited aqueous solubility

Effects of the toxins of Lophopodella carteri (ectoprocta), on blood lactate and electrolytes of Lepomis macrochirus (bluegill)

Specimens of bluegill (Lepomis macrochirus) were exposed to homogenates of Lophopodella carter, an ectoproct that contains substances toxic to some gilled vertebrates. The homogenates caused significant decreases in blood Na+ and Cl - and a significant increase in lactate. Potassium levels were unaffected. It is proposed that these changes in blood. properties were caused by damage to ion transport mechanisms of the gill epithelium

Quantum Tunneling of Thermal Protons Through Pristine Graphene

Atomically thin two-dimensional materials such as graphene and hexagonal boron nitride have recently been found to exhibit appreciable permeability to thermal protons, making these materials emerging candidates for separation technologies [S. Hu et al., Nature 516, 227 (2014); M. Lozada-Hidalgo et al., Science 351, 68 (2016).]. These remarkable findings remain unexplained by density-functional electronic structure calculations, which instead yield barriers that exceed by 1.0 eV those found in experiments. Here we resolve this puzzle by demonstrating that the proton transfer through pristine graphene is driven by quantum nuclear effects, which substantially reduce the transport barrier by up to 1.4 eV compared to the results of classical molecular dynamics (MD). Our Feynman-Kac path-integral MD simulations unambiguously reveal the quantum tunneling mechanism of strongly interacting hydrogen ions through two-dimensional materials. In addition, we predict a strong isotope effect of 1 eV on the transport barrier for graphene in vacuum and at room temperature. These findings not only shed light on the graphene permeability to protons and deuterons, but also offer new insights for controlling the underlying quantum ion transport mechanisms in nanostructured separation membranes

Ion Transport Mechanisms via Time-Dependent Local Structure and Dynamics in Highly Concentrated Electrolytes

Highly concentrated electrolytes (HCEs) are attracting interest as safer and more stable alternatives to current lithium-ion battery electrolytes, but their structure, solvation dynamics and ion transport mechanisms are arguably more complex. We here present a novel general method for analyzing both the structure and the dynamics, and ultimately the ion transport mechanism(s), of electrolytes including HCEs. This is based on automated detection of bonds, both covalent and coordination bonds, including how they dynamically change, in molecular dynamics (MD) simulation trajectories. We thereafter classify distinct local structures by their bond topology and characterize their physicochemical properties by statistical mechanics, giving both a qualitative and quantitative description of the structure, solvation and coordination dynamics, and ion transport mechanism(s). We demonstrate the method by in detail analyzing an ab initio MD simulation trajectory of an HCE consisting of the LiTFSI salt dissolved in acetonitrile at a 1:2 molar ratio. We find this electrolyte to form a flexible percolating network which limits vehicular ion transport but enables the Li+\ua0ions to move between different TFSI coordination sites along with their first solvation shells. In contrast, the TFSI anions are immobilized in the network, but often free to rotate which further facilitates the Li+\ua0hopping mechanism.Highly concentrated electrolytes (HCEs) are attracting interest as safer and more stable alternatives to current lithium-ion battery electrolytes, but their structure, solvation dynamics and ion transport mechanisms are arguably more complex. We here present a novel general method for analyzing both the structure and the dynamics, and ultimately the ion transport mechanism(s), of electrolytes including HCEs. This is based on automated detection of bonds, both covalent and coordination bonds, including how they dynamically change, in molecular dynamics (MD) simulation trajectories. We thereafter classify distinct local structures by their bond topology and characterize their physicochemical properties by statistical mechanics, giving both a qualitative and quantitative description of the structure, solvation and coordination dynamics, and ion transport mechanism(s). We demonstrate the method by in detail analyzing an ab initio MD simulation trajectory of an HCE consisting of the LiTFSI salt dissolved in acetonitrile at a 1:2 molar ratio. We find this electrolyte to form a flexible percolating network which limits vehicular ion transport but enables the Li+\ua0ions to move between different TFSI coordination sites along with their first solvation shells. In contrast, the TFSI anions are immobilized in the network, but often free to rotate which further facilitates the Li+\ua0hopping mechanism.Highly concentrated electrolytes (HCEs) are attracting interest as safer and more stable alternatives to current lithium-ion battery electrolytes, but their structure, solvation dynamics and ion transport mechanisms are arguably more complex. We here present a novel general method for analyzing both the structure and the dynamics, and ultimately the ion transport mechanism(s), of electrolytes including HCEs. This is based on automated detection of bonds, both covalent and coordination bonds, including how they dynamically change, in molecular dynamics (MD) simulation trajectories. We thereafter classify distinct local structures by their bond topology and characterize their physicochemical properties by statistical mechanics, giving both a qualitative and quantitative description of the structure, solvation and coordination dynamics, and ion transport mechanism(s). We demonstrate the method by in detail analyzing an ab initio MD simulation trajectory of an HCE consisting of the LiTFSI salt dissolved in acetonitrile at a 1:2 molar ratio. We find this electrolyte to form a flexible percolating network which limits vehicular ion transport but enables the Li+\ua0ions to move between different TFSI coordination sites along with their first solvation shells. In contrast, the TFSI anions are immobilized in the network, but often free to rotate which further facilitates the Li+\ua0hopping mechanism

Systematic Computational and Experimental Investigation of Lithium-Ion Transport Mechanisms in Polyester-Based Polymer Electrolytes

Understanding the mechanisms of lithium-ion transport in polymers is crucial for the design of polymer electrolytes. We combine modular synthesis, electrochemical characterization, and molecular simulation to investigate lithium-ion transport in a new family of polyester-based polymers and in poly(ethylene oxide) (PEO). Theoretical predictions of glass-transition temperatures and ionic conductivities in the polymers agree well with experimental measurements. Interestingly, both the experiments and simulations indicate that the ionic conductivity of PEO, relative to the polyesters, is far higher than would be expected from its relative glass-transition temperature. The simulations reveal that diffusion of the lithium cations in the polyesters proceeds via a different mechanism than in PEO, and analysis of the distribution of available cation solvation sites in the various polymers provides a novel and intuitive way to explain the experimentally observed ionic conductivities. This work provides a platform for the evaluation and prediction of ionic conductivities in polymer electrolyte materials

Gallstones: Bad Company for the Steatotic Liver

Gallstones are very frequent worldwide with a prevalence ranging from 10% to 15% in Western countries to <5% in Africa, with the geographic variations being associated with genetic and environmental factors.1 Although asymptomatic in more than 80% of patients, gallstone disease incurs one of the highest health care costs among digestive diseases and hospitalization is frequent as a consequence of its complications

Inverse coarse–graining methodologies to understand ion transport in block copolymer electrolytes

This research is focused on two fronts (i) developing multiscale simulation strategies for multicomponent polymers which can generate self assembled morphologies at both mesoscopic and atomistic length scales (ii) understanding the conformational attributes and dynamics of polymers in structured morphologies to understand the ion–transport mechanisms in block copolymer electrolytes. First part of the work is devoted in developing strategies to create equilibrated block copolymer morphologies below ODT with hard repulsive potentials. To this end, ordered morphologies with the help soft repulsive potentials are generated which possess equilibrated long range order within very short computational time. A rigorous mapping between the interaction parameters of the hard and soft potentials is then utilized to obtain the intermolecular interaction parameter of the soft potential corresponding to the target hard potential repulsion parameter. Subsequent to establishing the long range structure, short repulsive potential (within a coarse-grained framework) is reintroduced and equilibrated to generate ordered morphologies using hard repulsive potentials. Further to this, both topological and dynamic properties in ordered lamellar phases were characterized. The topological constraints are seen to increase with increasing degree of segregation. On characterizing the local dynamics of polymeric segments, we found that inhomogeneities exist in the spatially local dynamics and the length scale of perturbation of such inhomogeneities is controlled by the interfacial width of the block copolymer. The last part of the work involved the generation of ion–doped block copolymer melts at the atomistic level and to compare the results obtained therein with those for pure homopolymeric melts. To this end, we employed a multiscale simulation method to generate PS–PEO block copolymer doped with LiPF₆ ions. Our results demonstrate that the cation-anion radial distribution functions (RDF) display stronger coordination in the block copolymer melts compared to pure PEO homopolymer melts. Radial distribution functions isolated in the PEO and PS domains demonstrate that the stronger coordination seen in BCPs arise from the influence of both the higher fraction of ions segregated in the PS phase and the influence of interactions in the PS domain. Further, the cation-anion RDFs display spatial heterogeneity, with a stronger cation-anion binding in the interfacial region compared to bulk of the PEO domain. Investigations into the ion transport mechanisms in PS-PEO block copolymer melt reveal that ions exhibit slower dynamics in both the block copolymer (overall) and in the PEO phase of the BCP melt. Such results are shown to arise from the effects of slower polymer segmental dynamics in the BCP melt and the coordination characteristics of the ions. Polymer backbone-ion residence times analyzed as a function of distance from the interface indicate that ions have a larger residence time near the interface compared to that near the bulk of lamella, and demonstrates the influence of the glassy PS blocks and microphase segregation on the ion transport properties. Ion transport mechanisms in BCP melts reveal that there exist five distinct mechanisms for ion transport along the backbone of the chain and exhibit qualitative differences from the behavior in homopolymer melts.Chemical Engineerin

Intracellular pH regulation in mantle epithelial cells of the Pacific oyster, Crassostrea gigas

Shell formation and repair occurs under the control of mantle epithelial cells in bivalve molluscs. However, limited information is available on the precise acid–base regulatory machinery present within these cells, which are fundamental to calcification. Here, we isolate mantle epithelial cells from the Pacific oyster, Crassostrea gigas and utilise live cell imaging in combination with the fluorescent dye, BCECF-AM to study intracellular pH (pHi) regulation. To elucidate the involvement of various ion transport mechanisms, modified seawater solutions (low sodium, low bicarbonate) and specific inhibitors for acid–base proteins were used. Diminished pH recovery in the absence of Na+ and under inhibition of sodium/hydrogen exchangers (NHEs) implicate the involvement of a sodium dependent cellular proton extrusion mechanism. In addition, pH recovery was reduced under inhibition of carbonic anhydrases. These data provide the foundation for a better understanding of acid–base regulation underlying the physiology of calcification in bivalves