Steven Greenbaum

Pyrite-induced hydroxyl radical formation and its effect on nucleic acids

BackgroundPyrite, the most abundant metal sulphide on Earth, is known to spontaneously form hydrogen peroxide when exposed to water. In this study the hypothesis that pyrite-induced hydrogen peroxide is transformed to hydroxyl radicals is tested.ResultsUsing a combination of electron spin resonance (ESR) spin-trapping techniques and scavenging reactions involving nucleic acids, the formation of hydroxyl radicals in pyrite/aqueous suspensions is demonstrated. The addition of EDTA to pyrite slurries inhibits the hydrogen peroxide-to-hydroxyl radical conversion, but does not inhibit the formation of hydrogen peroxide. Given the stability of EDTA chelation with both ferrous and ferric iron, this suggests that the addition of the EDTA prevents the transformation by chelation of dissolved iron species.ConclusionWhile the exact mechanism or mechanisms of the hydrogen peroxide-to-hydroxyl radical conversion cannot be resolved on the basis of the experiments reported in this study, it is clear that the pyrite surface promotes the reaction. The formation of hydroxyl radicals is significant because they react nearly instantaneously with most organic molecules. This suggests that the presence of pyrite in natural, engineered, or physiological aqueous systems may induce the transformation of a wide range of organic molecules. This finding has implications for the role pyrite may play in aquatic environments and raises the question whether inhalation of pyrite dust contributes to the development of lung diseases.

Characterization of lithiated natural graphite before and after mild oxidation

Abstract Partial oxidation of natural graphite utilized in lithium ion batteries was found to increase its reversible capacity, while decreasing the irreversible capacity. Several chemically distinct Li sites in lithiated graphite were identified by solid state 7 Li nuclear magnetic resonance (NMR): intercalated Li, and Li chemically bonded within the surface passivation layer or solid electrolyte interface (SEI). The partially oxidized graphite exhibited a third site, attributed to Li bonded to armchair, zigzag, or other edge sites in the carbon. In addition, the NMR signal from the SEI in the partially oxidized graphite is consistent with earlier work suggesting that oxidation lays the foundation for a chemically bonded SEI that is implicated in improved electrochemical performance. Electron paramagnetic resonance (EPR) signals observed in lithiated graphite are attributed to conduction electrons, as noted by other authors. EPR in unlithiated graphite, however, failed to detect a correlation between possible radical sites to which Li could bond and excess Li capacity in the partially oxidized graphite.

NMR, DSC, DMA, and High Pressure Electrical Conductivity Studies in PPO Complexed with Sodium Perchlorate

Abstract : Audio frequency electrical conductivity, DSC, DMA, and 23 Na nMR measurements have been carried out on Parel 58 elastomer complexed with sodium perchlorate. (As Parel 58 is primarily poly(propylene oxide), it will be referred to as PPO.) The DSC and DMA measurements yield similar values for Tg which are about 72 C higher than the central Tg for uncomplexed PPO. In addition, the DSC studies show that the sodium perchlorate is insoluble above about 140 C. The conductivity measurements have been carried out in vacuum over the temperature range 290-370K. From a VTF analysis Ea is found to be about 0.09 eV and T0 is found to be about 45 C below the central glass transition temperature which is the same behavior observed previously for PPP complexed with lithium salts and for the alpha relaxation in uncomplexed material. In addition, it is found that the vacuum activation volumes for the electrical conductivity and the alpha relaxation are approximately the same when compared relative To. The 23Na NMR measurements reveal the presence of both bound and mobile sodium species, the relative concentrations of which change by about a factor of ten over the temperature range -90 to +90 C. In addition the mobile 23Na resonance becomes motionally narrowed above Tg. The NMR results combined with the conductivity data imply that ion motion is controlled by large scale segmental motions of the polymer chains.

High field multinuclear NMR investigation of the SEI layer in lithium rechargeable batteries

7Li and 1 9 F magic angle spinning nuclear magnetic resonance (NMR) allows for the identification and quantification of LiF in the solid electrolyte interphase (SEI) at both electrodes in lithium-ion rechargeable batteries. An estimate of the percent of lithium not reintercalated into the cathode after numerous cycles is also obtained. In the cells presented, more LiF formed on the surface of the anode compared to its respective cathode. Positive correlations have been established for the amount of LiF formed on the cathode with both the number of cycles and the % Li loss from the cathode.

NMR, DSC and high pressure electrical conductivity studies of liquid and hybrid electrolytes

Abstract Electrical conductivity, differential scanning calorimetry (DSC) and 7 Li nuclear magnetic resonance (NMR) studies have been carried out on liquid electrolytes such as ethylene carbonate:propylene carbonate (EC:PC) and EC:dimethyl carbonate (DMC) containing LiPF 6 (and LiCF 3 SO 3 for NMR) and films plasticized using the same liquid electrolytes. The films are based on poly(vinylidene fluoride) (PVdF) copolymerized with hexafluoropropylene and contain fumed silica. All measurements were carried out at atmospheric pressure from room temperature to about −150°C and the electrical conductivity studies were performed at room temperature at pressures up to 0.3 GPa. The liquids and hybrid electrolytes are similar in that the electrical conductivity of the EC:PC-based substances exhibits Vogel–Tammann–Fulcher (VTF) behaviour while that for the EC:DMC-based substances does not. Part of the deviation from VTF behaviour for the EC:DMC-based materials is attributed to crystallization. Further, the glass transition temperatures as determined from NMR, DSC and electrical conductivity measurements are about the same for the liquids and hybrid electrolytes. However, substantial differences are found. The electrical conductivity of the hybrid electrolytes at room temperature is lower than expected and, more importantly, the relative change of conductivity with pressure is larger than for the liquids. In addition, above the glass transition temperature, the NMR T 1 values are smaller and the NMR linewidths are larger for the hybrid electrolytes than for the liquids while at both low and high temperature the NMR linewidths are larger. Consequently, it is concluded that significant solid matrix–lithium ion interactions take place.

Electrical conductivity and 6,7Li NMR studies of Li1 + yCoO2

Abstract The battery cathode material LiCoO2 was synthesized with a deliberate excess of Li, according to Li1 + yCoO2, where y = 0.08 and 0.35 (nominally). The effect of divalent doping with Mg2+ was also explored for some samples, with y values of 0.0 (stoichiometric) and 0.08. Electrical conductivity measurements of the stoichiometric material, without Mg, as functions of oxygen partial pressure and temperature exhibit p-type semiconducting behaviour and suggest that the defects primarily responsible for the generation of holes are cobalt ion vacancies. Excess Li increases the electrical conductivity, while the incorporation of Mg leads to a more dramatic enhancement in conductivity, the latter interpreted as a transition to metallic behaviour. NMR spectroscopic measurements of both 6,7Li isotopes suggest that only a small fraction (

Complex impedance studies of S-SEBS block polymer proton-conducting membranes

Water uptake, swelling, 1 H pulsed gradient spin-echo nuclear magnetic resonance (NMR) and variable temperature and pressure complex impedance electrical conductivity studies have been carried out on sulfonated styrene/ethylenebutylene/styrene (S-SEBS) triblock polymer proton conducting membranes. At the highest water contents, the activation volume calculated from the effect of pressure on the electrical conductivity is negative. Previously reported results for Nafion 117 show the same behavior. In addition, above about 10 wt% water, the diffusion coefficients, D from NMR and Do calculated from conductivity data, are similar for S-SEBS. The same result is obtained for Nafion 117. The conclusion is that proton transport at high water content is by molecular diffusion for both materials. For low water contents, however, the materials are significantly different. For low water content S-SEBS, D and D a are different while they are the same for Nafion 117. In addition, the variation of the conductivity with temperature for S-SEBS is Arrhenius while that for Nafion 117 is not. Finally, the variation of the electrical conductivity with pressure gives rise to activation volumes on the order of 14 cm 3 /mol for S-SEBS while those for Nation 117 are about four times larger. These results indicate that proton transport in low water content S-SEBS occurs via a thermally activated process (ion motion via energy barriers) that is consistent with the more rigid side chains in that material.

Charge Transport and Water Molecular Motion in Variable Molecular Weight NAFION Membranes. High Pressure Electrical Conductivity and NMR.

Abstract Measurements of the electrical conductivity and deuteron NMR spin-lattice relaxation times ( T 1 ) in three different molecular weights of acid form NAFION conditioned at various levels of relative humidity have been carried out. Complex impedance studies were made along the plane of the film at frequencies from 10-10 8 Hz at room temperature and pressures up to 0.3 GPa. The high pressure electrical data were only obtained for water contents less than 8 wt%. The NMR measurements were also made at room temperature and pressures up to 0.25 GPa. The NMR data are primarily for water contents greater than 6 wt%. The calculated activation volume exhibits a large decrease (from 16 to 3 cm 3 /mol) as the water content is increased from 2.4–8 wt%. In addition, the activation volumes are larger and the electrical conductivity is smaller for the higher molecular weight material. These results represent further evidence that the transport mechanism in low water content materials is dominated by segmental motions of the polymer chain and that proton transport and water molecular rotation are correlated. The activation volumes extracted from the NMR data show only a small further decrease as the water content is increased from 6–22 wt%. Possible explanations for the high water content NMR pressure results are given.

Studies of Water in Nafion Membranes Using Deuteron and Oxygen‐17 Nuclear Magnetic Resonance, and Dielectric Relaxation Techniques

Deuteron and oxygen-17 nuclear magnetic resonance measurements and dielectric relaxation studies of Nafion-117 membranes with variable water content (approximately 5-18% by weight) have been carried out. Glassy behavior of the water domains at low temperature, below ca. 200 K, is evidenced by the specific nature of the 1 H NMR line shapes. Activation energies extracted from 1 H spin-lattice relaxation data on the high temperature side of the T 1 minimum exhibit a steady increase with increasing water content. In spite of a high degree of molecular mobility, angular-dependent spectra in both as-received and stretched samples reflect considerable anisotropy of the host polymer

Electrical Conductivity and NMR Studies of Methanol/Water Mixtures in Nafion Membranes.

Abstract Complex impedance studies have been carried out in acid form Nafion 117 treated with various amounts of methanol and methanol–water mixtures. At room temperature and atmospheric pressure the conductivity for Nafion treated with “pure” methanol is about a factor of ten less than for Nafion which contains the same wt.% of water. In samples treated with the water–methanol mixtures, the conductivity is lower than for samples having the same total wt.% of water. However, for low mixed fluid wt.% the conductivity is significantly higher than for samples with the same amount of water, only, as was in the mix. This enhancement of conductivity over that for the corresponding water uptake is attributed to a plasticizing effect of the methanol facilitating the segmental motion of the polymer. At higher water concentrations, the conductivity is generally lower in the mixed solution-treated samples than in samples treated with the corresponding amount of water. This is to be expected since in this regime, proton conduction occurs in fluid-rich regions, which in the solution case includes a large fraction of methanol. For a 40 wt.% 1.4:1 molar ratio film, the studies were carried out at pressures up to 0.3 GPa. It is found that the electrical conductivity decreases with increasing pressure. Both the electrical conductivity and the activation volume are similar to the result for Nafion containing the same amount of water only. Deuteron NMR spin-lattice relaxation measurements of isotopically enriched methanol/water mixtures in Nafion 117 at elevated pressure demonstrate greater molecular-level interactions between methanol and Nafion than between water and Nafion. This is consistent with the plasticizing effect observed in the conductivity results.