128 resultados para POSITIVE-IONS


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Differences in shell morphology in the intertidal prosobranch mollusc Calliostoma zizyphinum were studied from a number of sites within four geographical reo ions of the British Isles with varying exposures to wave action and crab predation. Mean values of damage scarring were highest in shells sampled from Strangford Lough, Northern Ireland, and lowe;t in individuals from the Atlantic Coast. Shells collected front the Isle of Man were smaller than those from either the Atlantic coast of Ireland or Strangford Lough. Shells front Strangford Lough had taller shells (higher aspect ratio) than shells from the County Down and Atlantic Coast and shells from the County Down coast were more squat (lower aspect ratio) than those from all other areas. Shells from Strangford Lough were significantly thinner than shells from all other geographical areas. The relationships between shell damage scarring and shell size and tallness were not consistent among areas. Shells from Strangford Lough are notable in showing a steep, positive relationship between shell size and scarring and a steep, negative relationship between scarring and tallness. The unusual shell phenotypes observed in shells from Strangford 1,Lough may be explained by rapid shell growth, which would not only allow Calliostoma to attain a size refuge from crab predators but also to recover successfully from repeated crab attacks on the shell lip. Such a strategy would result in larger, thinner shells with a high number of damage scars.

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The effects of linear scaling of the atomic charges of a reference potential on the structure, dynamics, and energetics of the ionic liquid 1,3-dimethylimidazolium chloride are investigated. Diffusion coefficients that span over four orders of magnitude are observed between the original model and a scaled model in which the ionic charges are +/- 0.5 e. While the three-dimensional structure of the liquid is less affected, the partial radial distribution functions change markedly-with the positive result that for ionic charges of +/- 0.7 e, an excellent agreement is observed with ab initio molecular dynamics data. Cohesive energy densities calculated from these partial-charge models are also in better agreement with those calculated from the ab initio data. We postulate that ionic-liquid models in which the ionic charges are assumed to be +/- 1 e overestimate the intermolecular attractions between ions, which results in overstructuring, slow dynamics, and increased cohesive energy densities. The use of scaled-charge sets may be of benefit in the simulation of these systems-especially when looking at properties beyond liquid structure-thus providing on alternative to computationally expensive polarisable force fields.

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The electrochemical oxidation of potassium nitrite has been studied in the room temperature ionic liquid (RTIL) [C(2)mim][NTf2] by cyclic voltammetry at platinum electrodes. A chemically irreversible oxidation peak was observed, and a solubility of 7.5(+/- 0.5) mM and diffusion coefficient of 2.0(+/- 0.2) x 10(-11) m(2) s(-1) were calculated from potential step chronoamperometry on the microdisk electrode. A second, and sometimes third, oxidation peak was also observed when the anodic limit was extended, and these were provisionally assigned to the oxidation of nitrogen dioxide (NO2) and nitrate (NO3-), respectively. The electrochemical oxidation of nitrogen dioxide gas (NO2) was also studied by cyclic voltammetry in [C(2)mim][NTf2] on Pt electrodes of various size, giving a solubility of ca. 51(+/- 0.2) mM and diffusion coefficient of 1.6(+/- 0.05) x 10(-10) m(2) s(-1) (at 25 degrees C). It is likely that NO2 exists predominantly as its dimer, N2O4, at room temperature. The oxidation mechanism follows a CE process, which involves the initial dissociation of the dimer to the monomer, followed by a one-electron oxidation. A second, larger oxidation peak was observed at more positive potentials and is thought to be the direct oxidation of N2O4. In addition to understanding the mechanisms of NO2- and NO2 oxidations, this work has implications in the electrochemical detection of nitrite ions and of NO2 gas in RTIL media, the latter which may be of particular use in gas sensing.

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The electrochemical oxidation of 1-butyl-3-methylimidazolium nitrate [C(4)mim][NO3] was studied by cyclic voltammetry in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [C(2)mim][NTf2]. A sharp peak was observed on a Pt microelectrode (d = 10 mu m), and a diffusion coefficient at infinite dilution of ca. 2.0 x 10(-11) m(2) s(-1) was obtained. Next, the cyclic voltammetry of sodium nitrate (NaNO3) and potassium nitrate (KNO3) was studied, by dissolving small amounts of solid into the RTIL [ C2mim][ NTf2]. Similar oxidation peaks were observed, revealing diffusion coefficients of ca. 8.8 and 9.0 x 10(-12) m(2) s(-1) and solubilities of 11.9 and 10.8 mM for NaNO3 and KNO3, respectively. The smaller diffusion coefficients for NaNO3 and KNO3 (compared to [C(4)mim][NO3]) may indicate that NO3- is ion-paired with Na+ or K+. This work may have applications in the electroanalytical determination of nitrate in RTIL solutions. Furthermore, a reduction feature was observed for both NaNO3 and KNO3, with additional anodic peaks indicating the formation of oxides, peroxides, superoxides and nitrites. This behaviour is surprisingly similar to that obtained from melts of NaNO3 and KNO3 at high temperatures ( ca. 350 - 500 degrees C), and this observation could significantly simplify experimental conditions required to investigate these compounds. We then used X-ray photoelectron spectroscopy (XPS) to suggest that disodium( I) oxide (Na2O), which has found use as a storage compound for hydrogen, was deposited on a Pt electrode surface following the reduction of NaNO3.