Single Ion Conductivities Research Articles

Gramicidin Pores Report the Activity of Membrane-Active Enzymes

Phospholipases constitute a ubiquitous class of membrane-active enzymes that play a key role in cellular signaling, proliferation, and membrane trafficking. Aberrant phospholipase activity is implicated in a range of diseases including cancer, inflammation, and myocardial disease. Characterization of these enzymes is therefore important, both for improving the understanding of phospholipase catalysis and for accelerating pharmaceutical and biotechnological applications. This paper describes a novel approach to monitor, in situ and in real-time, the activity of phospholipase D (PLD) and phospholipase C (PLC) on planar lipid bilayers. This method is based on lipase-induced changes in the electrical charge of lipid bilayers and on the concomitant change in ion concentration near lipid membranes. The approach reports these changes in local ion concentration by a measurable change in the single channel ion conductance through pores of the ion channel-forming peptide gramicidin A. This enzyme assay takes advant...

Gramicidin Pores Report the Activity of Membrane-Active Enzymes

Phospholipases constitute a ubiquitous class of membrane-active enzymes that play a key role in cellular signaling, proliferation, and membrane trafficking. Aberrant phospholipase activity is implicated in a range of diseases including cancer, inflammation, and myocardial disease. Characterization of these enzymes is therefore important, both for improving the understanding of phospholipase catalysis and for accelerating pharmaceutical and biotechnological applications. This paper describes a novel approach to monitor, in situ and in real-time, the activity of phospholipase D (PLD) and phospholipase C (PLC) on planar lipid bilayers. This method is based on lipase-induced changes in the electrical charge of lipid bilayers and on the concomitant change in ion concentration near lipid membranes. The approach reports these changes in local ion concentration by a measurable change in the single channel ion conductance through pores of the ion channel-forming peptide gramicidin A. This enzyme assay takes advantage of the amplification characteristics of gramicidin pores to sense the activity of picomolar to nanomolar concentrations of membrane-active enzymes without requiring labeled substrates or products. The resulting method proceeds on lipid bilayers without the need for detergents, quantifies enzyme activity on native lipid substrates within minutes, and provides unique access to both leaflets of well-defined lipid bilayers; this method also makes it possible to generate planar lipid bilayers with transverse lipid asymmetry.

Electrical Conductances of Tetrabutylammonium Bromide, Sodium Tetraphenylborate, and Sodium Bromide in Ethylene Glycol (1) + Water (2) Mixtures at (298.15, 303.15, 308.15, and 318.15) K

The electrical conductances of solutions of tetrabutylammonium bromide (Bu4NBr), sodium tetraphenylborate (NaBPh4), and sodium bromide (NaBr) in ethylene glycol (EG) (1) + water (2) mixed solvent media containing 0.10, 0.20, and 0.30 mass fractions of EG (w1) have been reported at (298.15, 303.15, 308.15, and 318.15) K. The conductance data have been analyzed by the 1978 Fuoss conductance-concentration equation in terms of the limiting molar conductance (Λ0), the association constant (KA), and the association diameter (R). The ionic contributions to the limiting molar conductances have been estimated using tetrabutylammonium tetraphenylborate (Bu4NBPh4) as the “reference electrolyte”. All of these three electrolytes are found to exist as free ions in the present solvent mixtures within the temperature range from (298.15 to 318.15) K. Although the association constants of the electrolytes do not vary significantly as a function of temperature, the limiting molar conductances of the electrolytes as well as the single-ion conductivity values increase appreciably with temperature.

On a novel rate theory for transport in narrow ion channels and its application to the study of flux optimization via geometric effects

We present a novel rate theory based on the notions of splitting probability and mean first passage time to describe single-ion conduction in narrow, effectively one-dimensional membrane channels. In contrast to traditional approaches such as transition state theory or Kramers theory, transitions between different conduction states in our model are governed by rates which depend on the full geometry of the potential of mean force (PMF) resulting from the superposition of an equilibrium free energy profile and a transmembrane potential induced by a nonequilibrium constraint. If a detailed theoretical PMF is available (e.g., from atomistic molecular dynamics simulations), it can be used to compute characteristic conductance curves in the framework of our model, thereby bridging the gap between the atomistic and the mesoscopic level of description. Explicit analytic solutions for the rates, the ion flux, and the associated electric current can be obtained by approximating the actual PMF by a piecewise linear potential. As illustrative examples, we consider both a theoretical and an experimental application of the model. The theoretical example is based on a hypothetical channel with a fully symmetric sawtooth equilibrium PMF. For this system, we explore how changes in the spatial extent of the binding sites affect the rate of transport when a linear voltage ramp is applied. Already for the case of a single binding site, we find that there is an optimum size of the site which maximizes the current through the channel provided that the applied voltage exceeds a threshold value given by the binding energy of the site. The above optimization effect is shown to arise from the complex interplay between the channel structure and the applied electric field, expressed by a nonlinear dependence of the rates with respect to the linear size of the binding site. In studying the properties of current-voltage curves, we find a double crossover between sublinear and superlinear behaviors as the size of the binding site is varied. The ratio of unidirectional fluxes clearly deviates from the Ussing limit and can be characterized by a flux ratio exponent which decreases below unity as the binding site becomes wider. We also explore effects arising from changes in the ion bulk concentration under symmetric ionic conditions and the presence of additional binding sites in the hypothetical channel. As for the experimental application, we show that our rate theory is able to provide good fits to conductance data for sodium permeation through the gramicidin A channel. Possible extensions of the theory to treat the case of an asymmetric equilibrium PMF, fluctuations in the mean number of translocating ions, the case of fluctuating energy barriers, and multi-ion conductance are briefly discussed.

Molecular mobility and Li+ conduction in polyester copolymer ionomers based on poly(ethylene oxide)

We investigate the segmental and local dynamics as well as the transport of Li(+) cations in a series of model poly(ethylene oxide)-based single-ion conductors with varying ion content, using dielectric relaxation spectroscopy. We observe a slowing down of segmental dynamics and an increase in glass transition temperature above a critical ion content, as well as the appearance of an additional relaxation process associated with rotation of ion pairs. Conductivity is strongly coupled to segmental relaxation. For a fixed segmental relaxation frequency, molar conductivity increases with increasing ion content. A physical model of electrode polarization is used to separate ionic conductivity into the contributions of mobile ion concentration and ion mobility, and a model for the conduction mechanism involving transient triple ions is proposed to rationalize the behavior of these quantities as a function of ion content and the measured dielectric constant.

Synthesis and characterization of solid single ion conductors based on poly[lithium tetrakis(ethyleneboryl)borate

Main chain anionic polymers with lithium cations, which have tetrakis(ethyleneboryl)borate as repeating units, were prepared and their ionic conductivities were determined. The polyelectrolyte is characterized both as-synthesized and after annealing at 275 °C by means of impedance spectroscopy, direct current measurements, 11B, 13C, 1H, static 7Li solid state NMR and IR spectroscopies, and DT/TG/MS techniques. The important feature of this novel solid polyelectrolyte is that the anion is immobilized on the polymer chain. The single ionic conduction was confirmed by the observation of constant current under dc electric field with non-blocking electrodes. The material exhibits a high Li+ transference number. The annealed polymer has an ionic conductivity of 1.6 × 10−6 S cm−1 at 275 °C with an activation energy of 70 kJ mol−1.

Molecular Mobility, Ion Mobility, and Mobile Ion Concentration in Poly(ethylene oxide)-Based Polyurethane Ionomers

The segmental and local chain dynamics as well as the transport of Na+ and Li+ cations in a series of model poly(ethylene oxide)-based polyurethane ionomers is investigated using dielectric relaxation spectroscopy. A physical model of electrode polarization is employed to separately determine mobile ion concentration and ion mobility in these single-ion conductors. A model including unpaired ions, separated ion pairs, and contact ion pairs is used to reconcile the very small fraction of free ions obtained using the electrode polarization model with those of previous studies of ion association in polyether-based single-ion conducting and salt-containing systems.

Single-ion conductors for lithium batteries via silica surface modification

Single-ion conductors (SICs) have been prepared by free-radical polymerization of sulfonic acid-containing monomer on high-purity silica surface that was first tailored with unsaturated functionality using a silanation reaction. It was found that steric effects limited polyelectrolyte surface loading even when large amount of silane molecules were grafted by forming a cross-linked structure. The results indicate that large surface area is an important factor to achieve high-surface loading of ionic moieties. Composite electrolytes were prepared by dispersing these SICs in aprotic solvents. The effects of filler content and solvent on ionic conductivity were investigated.

A novel gel polymer electrolyte based on poly ionic liquid 1-ethyl 3-(2-methacryloyloxy ethyl) imidazolium iodide

Poly ionic liquid 1-ethyl 3-(2-methacryloyloxy ethyl) imidazolium iodide (PEMEImI) as a single-ion conductor was designed and synthesized. When appropriate amount of suitable plasticizers, I 2 and polyacrylonitrile (PAN) were incorporated into it, the complex formed gel polymer electrolyte. Chemical structure, thermal behavior and ionic conductive properties of the gel polymer electrolyte were investigated by Raman spectra, UV–Vis spectra, differential scanning calorimetry (DSC), and complex impedance analysis, respectively. For the new gel polymer electrolyte, the ionic conductivity of about 1 × 10 −3 S cm −1 at room temperature was achieved.

Synthesis and Characterization of Poly(Ethylene Glycol)-Based Single-Ion Conductors

A series of ionomers was synthesized by melt polycondensation of poly(ethylene glycol) (PEG) oligomers and dimethyl 5-sulfoisophthalate sodium salt. The molar mass of the PEG spacer was either 400, 600, or 900, and the cation was exchanged from sodium to lithium or cesium by dialysis. Since the anions are covalently attached to the ionomer chains and are essentially immobile relative to the cations, these ionomers are excellent models for polymeric single-ion electrolytes. The experimental evidence to date suggests that the cations do not aggregate to form the usual ion clusters seen in other ionomers. No relaxation time associated with ion clusters was observed in rheological measurements, nor was an “ionomer peak” observed in small-angle X-ray scattering measurements. The ionic conductivity increases significantly with increasing PEG spacer molecular weight, although the total cation content decreases at the same time. At room temperature, the highest conductivity (10-6 S/cm) was achieved for the sodium...

Ion Transport in Single Ion Conductors

not Available.

Electrical Conductances of Tetrabutylammonium Bromide, Sodium Tetraphenylborate, and Sodium Bromide in Methanol (1) + Water (2) Mixtures at (298.15, 308.15, and 318.15) K

The electrical conductances of the solutions of tetrabutylammonium bromide (Bu4NBr), sodium tetraphenylborate (NaBPh4), and sodium bromide (NaBr) in methanol (1) + water (2) mixed solvent media containing 0.10, 0.20, and 0.30 volume fractions of methanol have been reported at (298.15, 308.15, and 318.15) K. The conductance data have been analyzed by the 1978 Fuoss conductance−concentration equation in terms of the limiting molar conductance (Λ0), the association constant (KA), and the association diameter (R). The ionic contributions to the limiting molar conductance have been estimated using tetrabutylammonium tetraphenylborate (Bu4NBPh4) as the “reference electrolyte”. All of these three electrolytes are found to exist as free ions in the present solvent mixtures within the temperature range (298.15 to 318.15) K. Besides the relative permittivity and the viscosity of the media, specific interactions of the ions with the solvent media have been found to play a profound influence on their mobilities. Although the association constants of the electrolytes do not vary significantly as a function of temperature, the limiting molar conductances of the electrolytes as well as the single-ion conductivity values increase appreciably with temperature.

Polarization in Cells Containing Single-Ion Graft Copolymer Electrolytes

The single-ion conductor, -incorporated poly[ methacrylate-ran-lithium methacrylate]-graft-poly(dimethyl siloxane), P(OEM--LiMA)--PDMS, was used as an electrolyte in cells containing a lithium anode and a thin-film vanadium oxide cathode. Cycle testing revealed an unanticipated high polarization which resulted in a significant drop in capacity compared to that of cells constructed with conventional salt-doped electrolytes produced by the addition of a lithium salt to an uncharged electrolyte poly( methacrylate-graft-polydimethyl siloxane. Impedance spectra obtained from a vanadium oxide symmetric cell fitted with a single-ion electrolyte showed a large resistance associated with the cathode/electrolyte interface. Further battery testing illustrated that the polarization remained even when lithium triflate was added to the single-ion electrolyte. In contrast, cells consisting of a lithium anode, single-ion electrolyte, and an alloying cathode showed no rise in polarization over what is found in similar cells constructed with a salt-doped electrolyte. These observations are consistent with the hypothesis that diffusion of lithium ions into the bulk vanadium oxide may be coulombically hindered by single-ion electrolytes.

Single ion conductors—polyphosphazenes with sulfonimide functional groups

A novel single ion conductive polymer electrolyte was developed by covalently linking an arylsulfonimide substituent to the polyphosphazene backbone. Polymeric single-ion conductors incorporate the anion of a salt either into the polymer backbone or as a pendent group linked to the polymer backbone. Immobilization of the anion could provide access to electrochemical devices that would be less vulnerable to increased resistance associated with salt concentration gradients at the interfaces during charging and discharging. In this work, an immobilized sulfonimide lithium salt is the source of lithium cations, while a cation-solvating cosubstituent, 2-(2-methoxyethoxy)ethoxy, was used to increase free volume and assist cation transport. The ionic conductivities showed a dependence on the percentage of lithiated sulfonimide substituent present. Increasing amounts of the lithium sulfonimide component increased the charge carrier concentration but decreased the ionic conductivity due to decreased macromolecular motion and possible increased shielding of the nitrogen atoms in the polyphosphazene backbone. Maximum ionic conductivity values of 2.45 × 10 − 6 S/cm at ambient temperature and 4.99 × 10 − 5 S/cm at 80 °C were obtained. Gel polymer electrolytes containing N-methyl-2-pyrrolidone gave ionic conductivities in the 10 − 3 S/cm range. The ion conduction process was investigated through model polymers that contained the non-immobilized sulfonimide — systems that had higher conductivities than their single ion counterparts.

Single Ion Conductor Polymer Electrolytes for Lithium Batteries and Fuel Cells

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Solid Polymer Single-Ion Conductors: Synthesis and Properties

Optimal battery electrolyte materials provide for high transport of a single ionic species. Here we report solid single-ion conductors (SICs) based on composite polymer electrolytes consisting of poly(ethylene glycol) (PEG) and an organic−inorganic component (OIC) formed in situ. The resulting solid films showed transport properties with conductivity as high as 10-4 S/cm at room temperature and a cation transference number of 0.9. The materials were characterized using solid-state NMR, differential scanning calorimetry, X-ray diffraction, transmission electron microscopy (TEM), and impedance spectroscopy. The OIC was formed by sol−gel reaction of the three precursors: the sodium salt of 3-trihydroxysilylpropylmethylphosphonate (SPMP), (3-glycidylopropyl)trimethoxysilane (GLYMO), and tetramethoxysilane, resulting in a high degree of three-dimensional network character of OIC within SIC solid polymer electrolytes as indicated by solid-state NMR. TEM examination of calcined OIC shows that the silicate parti...

Synthesis and Characterization of Network Single Ion Conductors Based on Comb-Branched Polyepoxide Ethers and Lithium Bis(allylmalonato)borate

Network single ion conductors (NSICs) based on comb-branch polyepoxide ethers and lithium bis(allylmalonato) borate have been synthesized and thoroughly characterized by means of ionic conductivity measurements, electrochemical impedance, and cycling in symmetrical Li/Li half cells, Li/V6O13 full cells in which a NSIC was used as both binder and electrolyte in the cathode electrode and as the electrolyte separator membrane, and by dynamic mechanical analysis (DMA). The substitution of the trimethylene oxide (TMO) unit into the side chains in place of ethylene oxide (EO) units increased the polymer−ion mobility (lower glass transition temperature). However, the ionic conductivity was nearly one and half orders of magnitude lower than the corresponding pure EO-based single ion conductor at the same salt concentration, which may be ascribed to the lower dielectric constant of the TMO side chains that result in a lower concentration of free conducting lithium cations. For a highly cross-linked system (EO/Li = 20), only 47 wt % plasticizing solvent (ethylene carbonate (EC)/ethyl methyl carbonate (EMC), 1/1 by wt) could be taken up, and the ionic conductivity was only increased by 1 order of magnitude over the dry polyelectrolyte, while for a less densely cross-linked system (EO/Li = 80), up to 75 wt % plasticizer could be taken up and the ionic conductivity was increased by nearly 2 orders of magnitude. A Li/Li symmetric cell that was cycled at 85 °C at a current density of 25μA cm-2 showed no concentration polarization or diffusional relaxation, which was consistent with a lithium ion transference number of 1. However, both the bulk and interfacial impedance increased after 20 cycles, which was apparently due to continued cross-linking reactions within the membrane and on the surface of the lithium electrodes. A Li/V6O13 full cell constructed using a single ion conductor gel (propylene carbonate (PC)/EMC, 1/1 in wt) was cycled at 25 °C at a current density of 25μA cm-2 and showed an initial capacity of 268 mAh g-1 of V6O13, which stabilized at around 200 mAh g-1 after the first 20 cycles. During the DMA measurements on the NSICs, it was found that besides the main glass transition (α transition) there was a distinct secondary glass transition (β transition) for NSICs having five EO units in the side chains, while this (the secondary transition) was not clearly visible in the network single ion conductors (NSICs) with shorter side chains (two, three, and four EO units). The main glass transition (α transition) was attributed to the whole network structure of the single ion conductors and secondary glass transition (β transition) appeared to be due to the complexation of lithium by the side-chain chains. Both the main glass transition and the secondary transition were found to shift to higher temperature with increasing salt concentration.

Synthesis and Characterization of Single-Ion Graft Copolymer Electrolytes

A microphase-separating single-ion conductor, poly[ methacrylate-ran-lithium methacrylate]-graft-poly(dimethyl siloxane), P(OEM--LiMA)--PDMS, was prepared by lithiating a precursor polymer synthesized by free radical methods using commercially available macromonomers. This material possessed a low conductivity, stemming from high ion-pairing interactions that severely restricted the number of charge carriers available for conduction. Subsequent conversion of the LiMA units via the addition of , a Lewis acid, resulted in a 2 orders-of-magnitude rise in conductivity, a gain that could be attributed to a large increase in the number of mobile cations. By blending this material with uncharged POEM--PDMS, the room-temperature conductivity was optimized to . With a lithium transference number of unity, these materials exhibit higher dc-measured conductivities at elevated currents than their salt-doped counterparts and are electrochemically stable to .

Electrochemical characteristics of multi-functional polymer electrolyte based on dual-layer membrane for lithium metal polymer battery

A novel dual-layer type polymer electrolyte was prepared by impregnating the interconnected pores with an ethylene carbonate(EC)/dimethyl carbonate(DMC)/lithium hexafluorophosphate(LiPF 6) solution. An incompatible layer is based on a micro-porous polyethylene(PE) and a compatible layer, based on a poly(vinylidenefluoride-co-hexafluoropropylene)(P(VdF-co-HFP)), is submicro-porous and compatible with an electrolyte solution. To enhance the ionic conductivity in the compatible layer, inorganic type Li + single ion conductor particle based on hydrophilic silica has been synthesized. The maximum apparent ionic conductivity of the dual-layer polymer electrolyte is 7.7 × 10 −3 S/cm at the ambient temperature. High rate capability of the Li/the dual-layer polymer electrolytes/Li[Ni 0.15Li 0.23M n0.62]O 2 cell was significantly improved as the incompatible layer thickness decreases.

Fundamental studies of novel inorganic–organic zwitterionic hybrids. 1. Preparation and characterizations of hybrid zwitterionic polymers

Four types of novel inorganic–organic hybrid zwitterionic polymers were successfully synthesized through sol–gel process. The polymer precursors were obtained by a reaction of N-[3-(trimethoxysilyl) propyl] ethylene diamine (TMSPEDA), under non-aqueous conditions, with 3-glycidoxypropyltrimethoxysilane (GPTMS), and followed by a reaction with γ-butyrolactone (γ-BL), and then exposure to open air to generate the hybrid zwitterionic polymers. The final polymers and products were characterized by FTIR, 13C NMR, TGA and conductometric titration curves. Both FTIR and 13C NMR spectra of polymers demonstrate the generation of ion pairs in these hybrid zwitterionic polymers. The TGA analysis reveals that the thermal stability of these types of hybrid zwitterionic polymers could go up to 350 °C. The relationships between conductivity and pH values show that the conductivities and the amphiphilic behaviors of hybrid zwitterionic polymers can be influenced by pH values; and the isoelectric point (IEP) of these kinds of hybrid zwitterionic polymers was pH 6.68–8.20, while the apparent contents of both anionic and cationic groups in the polymers are in the range of (1.1–2.1) × 10 −4 and (2–8.0) × 10 −5 mol/g, respectively. These kinds of polymers are expected to have applications to create anti-fouling membranes and single ionic conductors as well as to separate multi-valent ionic salts, etc.