A route to explain water anomalies from results on an aqueous solution of salt

In this paper we investigate the possibility to detect the hypothesized liquid-liquid critical point of water in supercooled aqueous solutions of salts. Molecular dynamics computer simulations are conducted on bulk TIP4P water and on an aqueous solution of sodium chloride in TIP4P water, with concentration c = 0.67 mol/kg. The liquid-liquid critical point is found both in the bulk and in the solution. Its position in the thermodynamic plane shifts to higher temperature and lower pressure for the solution. Comparison with available experimental data allowed us to produce the phase diagrams of both bulk water and the aqueous solution as measurable in experiments. Given the position of the liquid-liquid critical point in the solution as obtained from our simulations, the experimental determination of the hypothesized liquid-liquid critical point of water in aqueous solutions of salts appears possible.Comment: 5 pages, 6 figures. Accepted for publication on the Journal of Chemical Physics (2010)

A molecular dynamics simulation of water confined in a cylindrical SiO2 pore

A molecular dynamics simulation of water confined in a silica pore is performed in order to compare it with recent experimental results on water confined in porous Vycor glass at room temperature. A cylindrical pore of 40 A is created inside a vitreous SiO2 cell, obtained by computer simulation. The resulting cavity offers to water a rough hydrophilic surface and its geometry and size are similar to those of a typical pore in porous Vycor glass. The site-site distribution functions of water inside the pore are evaluated and compared with bulk water results. We find that the modifications of the site-site distribution functions, induced by confinement, are in qualitative agreement with the recent neutron diffraction experiment, confirming that the disturbance to the microscopic structure of water mainly concerns orientational arrangement of neighbouring molecules. A layer analysis of MD results indicates that, while the geometrical constraint gives an almost constant density profile up to the layers closest to the interface, with an uniform average number of hydrogen bonds (HB), the hydrophilic interaction produces the wetting of the pore surface at the expenses of the adjacent water layers. Moreover the orientational disorder togheter with a reduction of the average number of HB persists in the layers close to the interface, while water molecules cluster in the middle of the pore at a density and with a coordination similar to bulk water.Comment: RevTex, 11 pages, 12 figures; to appear in June 15 issue of J. Chem. Phy

Glass transition and layering effects in confined water: a computer simulation study

Single particle dynamics of water confined in a nanopore is studied through Computer Molecular Dynamics. The pore is modeled to represent the average properties of a pore of Vycor glass. Dynamics is analyzed at different hydration levels and upon supercooling. At all hydration levels and all temperatures investigated a layering effect is observed due to the strong hydrophilicity of the substrate. The time density correlators show, already at ambient temperature, strong deviations from the Debye and the stretched exponential behavior. Both on decreasing hydration level and upon supercooling we find features that can be related to the cage effect typical of a supercooled liquid undergoing a kinetic glass transition. Nonetheless the behavior predicted by Mode Coupling Theory can be observed only by carrying out a proper shell analysis of the density correlators. Water molecules within the first two layers from the substrate are in a glassy state already at ambient temperature (bound water). The remaining subset of molecules (free water) undergoes a kinetic glass transition; the relaxation of the density correlators agree with the main predictions of the theory. From our data we can predict the temperature of structural arrest of free water.Comment: 14 pages, 15 figures inserted in the text, to be published in J. Chem. Phys. (2000

Structural Properties of High and Low Density Water in a Supercooled Aqueous Solution of Salt

We consider and compare the structural properties of bulk TIP4P water and of a sodium chloride aqueous solution in TIP4P water with concentration c = 0.67 mol/kg, in the metastable supercooled region. In a previous paper [D. Corradini, M. Rovere and P. Gallo, J. Chem. Phys. 132, 134508 (2010)] we found in both systems the presence of a liquid-liquid critical point (LLCP). The LLCP is believed to be the end point of the coexistence line between a high density liquid (HDL) and a low density liquid (LDL) phase of water. In the present paper we study the different features of water-water structure in HDL and LDL both in bulk water and in the solution. We find that the ions are able to modify the bulk LDL structure, rendering water-water structure more similar to the bulk HDL case. By the study of the hydration structure in HDL and LDL, a possible mechanism for the modification of the bulk LDL structure in the solution is identified in the substitution of the oxygen by the chloride ion in oxygen coordination shells.Comment: 10 pages, 10 figures, 2 tables. Accepted for publication on J. Phys. Chem

Nitric oxide alleviates cadmium- but not arsenic-induced damages in rice roots

Nitric oxide (NO) has signalling roles in plant stress responses. Cadmium (Cd) and arsenic (As) soil pollutants alter plant development, mainly the root-system, by increasing NO-content, triggering reactive oxygen species (ROS), and forming peroxynitrite by NO-reaction with the superoxide anion. Interactions of NO with ROS and peroxynitrite seem important for plant tolerance to heavy metal(oid)s, but the mechanisms underlying this process remain unclear. Our goal was to investigate NO-involvement in rice (Oryza sativa L.) root-system after exposure to Cd or As, to highlight possible differences in NO-behaviour between the two pollutants. To the aim, morpho-histological, chemical and epifluorescence analyses were carried out on roots of different origin in the root-system, under exposure to Cd or As, combined or not with sodium nitroprusside (SNP), a NO-donor compound. Results show that increased intracellular NO levels alleviate the root-system alterations induced by Cd, i.e., inhibition of adventitious root elongation and lateral root formation, increment in lignin deposition in the sclerenchyma/endodermis cell-walls, but, even if reducing As-induced endodermis lignification, do not recover the majority of the As-damages, i.e., enhancement of AR-elongation, reduction of LR-formation, anomalous tissue-proliferation. However, NO decreases both Cd and As uptake, without affecting the pollutants translocation-capability from roots to shoots. Moreover, NO reduces the Cd-induced, but not the As-induced, ROS levels by triggering peroxynitrite production. Altogether, results highlight a different behaviour of NO in modulating rice root-system response to the toxicity of the heavy metal Cd and the metalloid As, which depends by the NO-interaction with the specific pollutant

Cadmium and arsenic affect root development in Oryza sativa L. negatively interacting with auxin

Cadmium (Cd) and arsenic (As), non essential, but toxic, elements for animals and plants are frequently present in paddy fields. Oryza sativa L., a staple food for at least the half of world population, easily absorbs As and Cd by the root, and in this organ the pollutants evoke consistent damages, reducing/modifying the root system. Auxins are key hormones in regulating all developmental processes, including root organogenesis. Moreover, plants respond to environmental stresses, such as those caused by Cd and As, by changing levels and distribution of endogenous phytohormones. Even though the effects of Cd and As on the roots have been investigated in some species, it remains necessary to deepen the knowledge about the cross-talk between these toxic elements and auxin during root formation and development, in particular in agronomically important plants, such as rice. Hence, the research goal was to investigate the interactions between Cd and As, alone or combined, and auxin during the development of rice roots. To reach the aim, morphological, histological and histochemical analyses were carried out on seedlings, exposed or not to Cd and/or As, belonging to the wild type and transgenic lines useful for monitoring indole-3-acetic acid (IAA) localization, i.e., OsDR5:GUS, and IAA cellular influx and efflux, i.e., OsAUX1:GUS and OsPIN5b:GUS. Moreover, the transcript levels of the YUCCA2 and ASA2, IAA biosynthetic genes were also monitored in Cd and/or As exposed wild type seedlings. The results highlight that As and Cd affect cyto-histology and morphology of the roots. In particular, they alter the lateral root primordia organization and development with negative consequences on root system architecture. This is due to a disturbance of IAA biosynthesis and transport, as indicated by the altered expression of both ASA2 and YUCCA2 biosynthetic genes, and AUX1 and PIN5b transporter genes

Computer simulation of the phase diagram for a fluid confined in a fractal and disordered porous material

We present a grand canonical Monte Carlo simulation study of the phase diagram of a Lennard-Jones fluid adsorbed in a fractal and highly porous aerogel. The gel environment is generated from an off-lattice diffusion limited cluster-cluster aggregation process. Simulations have been performed with the multicanonical ensemble sampling technique. The biased sampling function has been obtained by histogram reweighting calculations. Comparing the confined and the bulk system liquid-vapor coexistence curves we observe a decrease of both the critical temperature and density in qualitative agreement with experiments and other Monte Carlo studies on Lennard-Jones fluids confined in random matrices of spheres. At variance with these numerical studies we do not observe upon confinement a peak on the liquid side of the coexistence curve associated with a liquid-liquid phase coexistence. In our case only a shouldering of the coexistence curve appears upon confinement. This shoulder can be associated with high density fluctuations in the liquid phase. The coexisting vapor and liquid phases in our system show a high degree of spatial disorder and inhomogeneity.Comment: 8 pages, 8 figures, to be published in Phys. Rev.

Confined water in the low hydration regime

Molecular dynamics results on water confined in a silica pore in the low hydration regime are presented. Strong layering effects are found due to the hydrophilic character of the substrate. The local properties of water are studied as function of both temperature and hydration level. The interaction of the thin films of water with the silica atoms induces a strong distortion of the hydrogen bond network. The residence time of the water molecules is dependent on the distance from the surface. Its behavior shows a transition from a brownian to a non-brownian regime approaching the substrate in agreement with results found in studies of water at contact with globular proteins.Comment: 7 pages with 12 figures (RevTeX4). To be published on J. Chem. Phy

Random sequential adsorption and diffusion of dimers and k-mers on a square lattice

We have performed extensive simulations of random sequential adsorption and diffusion of $k$-mers, up to $k=5$ in two dimensions with particular attention to the case $k=2$. We focus on the behavior of the coverage and of vacancy dynamics as a function of time. We observe that for $k=2,3$ a complete coverage of the lattice is never reached, because of the existence of frozen configurations that prevent isolated vacancies in the lattice to join. From this result we argue that complete coverage is never attained for any value of $k$. The long time behavior of the coverage is not mean field and nonanalytic, with $t^{-1/2}$ as leading term. Long time coverage regimes are independent of the initial conditions while strongly depend on the diffusion probability and deposition rate and, in particular, different values of these parameters lead to different final values of the coverage. The geometrical complexity of these systems is also highlighted through an investigation of the vacancy population dynamics.Comment: 9 pages, 9 figures, to be published in the Journal of Chemical Physic

Spinodal of supercooled polarizable water

We develop a series of molecular dynamics computer simulations of liquid water, performed with a polarizable potential model, to calculate the spinodal line and the curve of maximum density inside the metastable supercooled region. After analysing the structural properties,the liquid spinodal line is followed down to T=210 K. A monotonic decrease is found in the explored region. The curve of maximum density bends on approaching the spinodal line. These results, in agreement with similar studies on non polarizable models of water, are consistent with the existence of a second critical point for water.Comment: 8 pages, 5 figures, 2 tables. To be published in Phys. Re