7,378 research outputs found
Molecular dynamics study of the glass transition in confined water
A molecular dynamics simulation of SPC/E water confined in a Silica pore is
presented. The pore has been constructed to reproduce the average properties of
a pore of Vycor glass. Due to the confinement and to the presence of a strong
hydrophilic surface, the dynamic behaviour of the liquid appears to be strongly
dependent on the hydration level. The approach to the glass transition of
confined water is investigated on lowering hydration and on supercooling in the
framework of Mode Coupling Theories. At higher hydrations two quite distinct
subsets of water molecules are detectable. Those belonging to the first layer
close to the substrate suffer a severe slowing down, while the remaining ones
display a scenario typical of supercooled liquids approaching the kinetic glass
transition.Comment: Talk presented at "International Workshop on Dynamics in
Confinement", Grenoble January 2000, to be appear in J. Phys. Franc
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)
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
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
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.
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
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 -mers, up to in two dimensions with particular attention
to the case . We focus on the behavior of the coverage and of vacancy
dynamics as a function of time. We observe that for 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
. The long time behavior of the coverage is not mean field and nonanalytic,
with 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
Jasmonic acid methyl ester induces xylogenesis and modulates auxin-induced xylary cell identity with NO Involvement
In Arabidopsis basal hypocotyls of dark-grown seedlings, xylary cells may form from the pericycle as an alternative to adventitious roots. Several hormones may induce xylogenesis, as Jasmonic acid (JA), as well as indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) auxins, which also affect xylary identity. Studies with the ethylene (ET)-perception mutant ein3eil1 and the ET-precursor 1-aminocyclopropane-1-carboxylic acid (ACC), also demonstrate ET involvement in IBA-induced ectopic metaxylem. Moreover, nitric oxide (NO), produced after IBA/IAA-treatments, may affect JA signalling and interact positively/negatively with ET. To date, NO-involvement in ET/JA-mediated xylogenesis has never been investigated. To study this, and unravel JA-effects on xylary identity, xylogenesis was investigated in hypocotyls of seedlings treated with JA methyl-ester (JAMe) with/without ACC, IBA, IAA. Wild-type (wt) and ein3eil1 responses to hormonal treatments were compared, and the NO signal was quantified and its role evaluated by using NO-donors/scavengers. Ectopic-protoxylem increased in the wt only after treatment with JAMe(10 μM), whereas in ein3eil1 with any JAMe concentration. NO was detected in cells leading to either xylogenesis or adventitious rooting, and increased after treatment with JAMe(10 μM) combined or not with IBA(10 μM). Xylary identity changed when JAMe was applied with each auxin. Altogether, the results show that xylogenesis is induced by JA and NO positively regulates this process. In addition, NO also negatively interacts with ET-signalling and modulates auxin-induced xylary identity
Modifications of the hydrogen bond network of liquid water in a cylindrical SiO_2 pore
We present results of molecular dynamics simulations of water confined in a
silica pore. A cylindrical cavity is created inside a vitreous silica cell with
geometry and size similar to the pores of real Vycor glass. The simulations are
performed at different hydration levels. At all hydration levels water adsorbs
strongly on the Vycor surface; a double layer structure is evident at higher
hydrations. At almost full hydration the modifications of the
confinement-induced site-site pair distribution functions are in qualitative
agreement with neutron diffraction experiment. A decrease in the number of
hydrogen bonds between water molecules is observed along the pore radius, due
to the tendency of the molecules close to the substrate to form hydrogen-bonds
with the hydrophilic pore surface. As a consequence we observe a substrate
induced distortion of the H-bond tetrahedral network of water molecules in the
regions close to the surface.Comment: Talk presented at "Physics of Liquids: Foundations, Highlights,
Challenge", Murau Sept. 1998. To appear in J. Mol. Li
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