A comprehensive Car-Parrinello molecular dynamics (CP-MD) study of aqueous solutions of carbonic acid (H(2)CO(3)), bicarbonate (HCO(3)(-)), carbonate (CO(3)(2-)), and carbon dioxide (CO(2)) provides new quantitative insight into the structural and dynamic aspects of the hydrogen-bonding environments for these important aqueous species and their effects on the structure, H-bonding, and dynamical behavior of the surrounding water molecules. The hydration structures of the different carbonate species depend on their ability to accept and donate H-bonds with H(2)O. The H-bonds donated by the C-O-H sites of the carbonate species to water molecules are generally stronger and longer-lived than those accepted by these sites from water molecules. The structural relaxation among the water molecules is dominated by diffusional (translational) motion of H(2)O, whereas the H-bond reorganization is dominated by the librational motion of the water molecules and the carbonate species. The rates of structural relaxation of the H(2)O molecules and the rates of H-bond reorganization among them are slower in systems containing carbonate species, consistent with previous studies of simple salt solutions. The strengths and lifetimes of H-bonds involving the carbonate species positively correlate with the total negative charge on the species. H-bond donation from H(2)O to CO(2) is weak, but the presence of CO(2) noticeably affects the structure and structural relaxation of the surrounding H-bonding network leading to generally stronger H-bonds and slower relaxation rates, the behavior typical of a hydrophobic solute.

Solid state ionic conductors are important from an industrial viewpoint. A variety of such conductors have been found. In order to understand the reasons for high ionic conductivity in these solids, there have been a number of experimental, theoretical and computational studies in the literature. We provide here a survey of these investigations with focus on what is known and elaborate on issues that still remain unresolved. Conductivity depends on a number of factors such as presence of interstitial sites, ion size, temperature, crystal structure etc. We discuss the recent results from atomistic computer simulations on the dependence of conductivity in NASICONs as a function of composition, temperature, phase change and cation among others. A new potential for modelling of NASICON structure that has been proposed is also discussed.

The influence of Si/P ordering on Na(+) transport in the high conducting NASICON composition, Na3Zr2Si2PO12, is investigated using molecular dynamics simulation. The study demonstrates that the Na(+) conductivity in the system changes by more than an order of magnitude depending on the Si/P order in the structure. This rather surprising observation is attributed to the modulations in the electrostatic potential energy landscape along the conduction channel originating out of the increased coulombic repulsion of Na(+) with P(5+) ions compared to Si(4+). The study reveals the significance of the order of framework cations in fast ion transport in solids with aliovalent substitutions.

Sodium-ion battery (NIB) cathode performance based on ammonium vanadate is demonstrated here as having high capacity, long cycle life and good rate capability. The simple preparation process and morphology study enable us to explore this electrode as suitable NIB cathode. Furthermore, density functional theory (DFT) calculation is envisioned for the NH4V4O10 cathode, and three possible sodium arrangements in the structure are depicted for the first time. Relevant NIB-related properties such as average voltage, lattice constants, and atomic coordinates have been derived, and the estimated values are in good agreement with the current experimental values. A screening study shows ammonium vanadate electrodes prepared on carbon coat onto Al-current collector exhibits a better electrochemical performance toward sodium, with a sustained reversible capacity and outstanding rate capability. With the current cathode with nanobelt morphology, a reversible capacity of 190 mAh g(-1) is attained at a charging rate of 200 mA g(-1), and a stable capacity of above 120 mAh g(-1) is retained for an extended 50 cycles tested at 1000 mA g(-1) without the addition of any expensive electrolyte additive.

Constant-pressure, constant-temperature variable-shape simulation cell Monte Carlo and microcanonical ensemble molecular dynamics simulation of superionic conducting rhombohedral phase of Nasicon, Na 1+x Zr 2 Si x P 3-x O 12 , 0 e x e 3, at a temperature of 600 K is reported. Changes in structure, conductivity, hop path, site occupancies, bond lengths of framework atoms with composition are discussed. Average Na(1)-O distance shows a peak at x ) 2, while Na(2)-O distance shows a monotonic increase. Sum of the sodium occupancies at Na(1) and mid-Na sites adds up to a constant value of one which supports the conclusion of Boilot et al. 1 based on X-ray diffraction. Occupancy of Na(1) site attains a minimum at x ) 2. The predominant conduction channel (which carries more than 90% of the sodium ions) is found to be the one connecting Na(1)-mid-Na-Na(2). Density contours for sodium, depicting this conduction channel, are reported. Free energy profile along the conduction channel suggests that entropy contribution cannot be neglected. The mid-Na site is not associated with a free energy minimum.

A molecular dynamics (MD) investigation of LiCl in water, methanol, and ethylene glycol (EG) at 298 K is reported. Several structural and dynamical properties of the ions as well as the solvent such as self-diffusivity, radial distribution functions, void and neck distributions, velocity autocorrelation functions, and mean residence times of solvent in the first solvation shell have been computed. The results show that the reciprocal relationship between the self-diffusivity of the ions and the viscosity is valid in almost all solvents with the exception of water. From an analysis of radial distribution functions and coordination numbers the nature of hydrogen bonding within the solvent and its influence on the void and neck distribution becomes evident. It is seen that the solvent−solvent interaction is important in EG while solute−solvent interactions dominate in water and methanol. From Voronoi tessellation, it is seen that the voids and necks within methanol are larger as compared to those within water or EG. On the basis of the void and neck distributions obtained from MD simulations and literature experimental data of limiting ion conductivity for various ions of different sizes, we show that there is a relation between the void and neck radius on the one hand and dependence of conductivity on the ionic radius on the other. It is shown that the presence of large diameter voids and necks in methanol is responsible for maximum in limiting ion conductivity (λ0) of TMA+, while in water and EG, the maximum is seen for Rb+. In the case of monovalent anions, maximum in λ0 as a function ionic radius is seen for Br− in water and EG but for the larger ClO4 − ion in methanol. The relation between the void and neck distribution and the variation in λ0 with ionic radius arises via the Levitation effect which is discussed. These studies show the importance of the solvent structure and the associated void structure.

Arsenic (As) contamination in groundwater has become a geo-environmental as well as a toxicological problem across the globe affecting more than 100-million people in nearly 21 countries with its associated disease "arsenicosis." Arsenic poisoning may lead to fatal skin and internal cancers. In present review, an attempt has been made to generate awareness among the readers about various sources of occurrence of arsenic, its geochemistry and speciation, mobilization, metabolism, genotoxicity, and toxicological exposure on humans. The article also emphasizes the possible remedies for combating the problem. The knowledge of these facts may help to work on some workable remedial measure.

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