269 research outputs found
Layer Analysis of the Structure of Water Confined in Vycor Glass
A Molecular Dynamics simulation of the microscopic structure of water
confined in a silica pore is presented. A single cavity in the silica glass has
been modeled as to reproduce the main features of the pores of real Vycor
glass. A layer analysis of the site-site radial distribution functions evidence
the presence in the pore of two subsets of water molecules with different
microscopic structure. Molecules which reside in the inner layer, close to the
center of the pore, have the same structure as bulk water but at a temperature
of 30 K higher. On the contrary the structure of the water molecules in the
outer layer, close to the substrate, is strongly influenced by the
water-substrate hydrophilic interaction and sensible distortions of the H-bond
network and of the orientational correlations between neighboring molecules
show up. Lowering the hydration has little effect on the structure of water in
the outer layer. The consequences on experimental determinations of the
structural properties of water in confinement are discussed.Comment: 6 pages, 8 figures included in the text, one figure added, changes in
the tex
Influence of the Environment Fluctuations on Incoherent Neutron Scattering Functions
In extending the conventional dynamic models, we consider a simple model to
account for the environment fluctuations of particle atoms in a protein system
and derive the elastic incoherent structure factor (EISF) and the incoherent
scattering correlation function C(Q,t) for both the jump dynamics between sites
with fluctuating site interspacing and for the diffusion inside a fluctuating
sphere. We find that the EISF of the system (or the normalized elastic
intensity) is equal to that in the absence of fluctuations averaged over the
distribution of site interspacing or sphere radius a. The scattering
correlation function is ,
where the average is taken over the Q-dependent effective distribution of
relaxation rates \lambda_n(a) and \psi(t) is the correlation function of the
length a. When \psi(t)=1, the relaxation of C(Q,t) is exponential for the jump
dynamics between sites (since \lambda_n(a) is independent of a) while it is
nonexponential for diffusion inside a sphere.Comment: 7 pages, 7 eps figure
Crystal-like high frequency phonons in the amorphous phases of solid water
The high frequency dynamics of low- (LDA) and high-density amorphous-ice
(HDA) and of cubic ice (I_c) has been measured by inelastic X-ray Scattering
(IXS) in the 1-15 nm^{-1} momentum transfer (Q) range. Sharp phonon-like
excitations are observed, and the longitudinal acoustic branch is identified up
to Q = 8nm^{-1} in LDA and I_c and up to 5nm^{-1} in HDA. The narrow width of
these excitations is in sharp contrast with the broad features observed in all
amorphous systems studied so far. The "crystal-like" behavior of amorphous
ices, therefore, implies a considerable reduction in the number of decay
channels available to sound-like excitations which is assimilated to low local
disorder.Comment: 4 pages, 3 figure
Liquid-Liquid Phase Transition for an Attractive Isotropic Potential with Wide Repulsive Range
Recent experimental and theoretical results have shown the existence of a
liquid-liquid phase transition in isotropic systems, such as biological
solutions and colloids, whose interaction can be represented via an effective
potential with a repulsive soft-core and an attractive part. We investigate how
the phase diagram of a schematic general isotropic system, interacting via a
soft-core squared attractive potential, changes by varying the parameters of
the potential. It has been shown that this potential has a phase diagram with a
liquid-liquid phase transition in addition to the standard gas-liquid phase
transition and that, for a short-range soft-core, the phase diagram resulting
from molecular dynamics simulations can be interpreted through a modified van
der Waals equation. Here we consider the case of soft-core ranges comparable
with or larger than the hard-core diameter. Because an analysis using molecular
dynamics simulations of such systems or potentials is too time-demanding, we
adopt an integral equation approach in the hypernetted-chain approximation.
Thus we can estimate how the temperature and density of both critical points
depend on the potential's parameters for large soft-core ranges. The present
results confirm and extend our previous analysis, showing that this potential
has two fluid-fluid critical points that are well separated in temperature and
in density only if there is a balance between the attractive and repulsive part
of the potential. We find that for large soft-core ranges our results satisfy a
simple relation between the potential's parameters
Dynamically Slow Processes in Supercooled Water Confined Between Hydrophobic Plates
We study the dynamics of water confined between hydrophobic flat surfaces at
low temperature. At different pressures, we observe different behaviors that we
understand in terms of the hydrogen bonds dynamics. At high pressure, the
formation of the open structure of the hydrogen bond network is inhibited and
the surfaces can be rapidly dehydrated by decreasing the temperature. At lower
pressure the rapid ordering of the hydrogen bonds generates heterogeneities
that are responsible for strong non-exponential behavior of the correlation
function, but with no strong increase of the correlation time. At very low
pressures, the gradual formation of the hydrogen bond network is responsible
for the large increase of the correlation time and, eventually, the dynamical
arrest of the system and of the dehydration process.Comment: 14 pages, 3 figure
Double dynamical regime of confined water
The Van Hove self correlation function of water confined in a silica pore is
calculated from Molecular Dynamics trajectories upon supercooling. At long time
in the relaxation region we found that the behaviour of the real space
time dependent correlators can be decomposed in a very slow, almost frozen,
dynamics due to the bound water close to the substrate and a faster dynamics of
the free water which resides far from the confining surface. For free water we
confirm the evidences of an approach to a crossover mode coupling transition,
previously found in Q space. In the short time region we found that the two
dynamical regimes are overimposed and cannot be distinguished. This shows that
the interplay between the slower and the faster dynamics emerges in going from
early times to the relaxation region, where a layer analysis of the
dynamical properties can be performed.Comment: 6 pages with 9 figures. RevTeX. Accepted for pulbication in J. Phys.
Cond. Mat
Liquid-Liquid Phase Transitions for Soft-Core Attractive Potentials
Using event driven molecular dynamics simulations, we study a three
dimensional one-component system of spherical particles interacting via a
discontinuous potential combining a repulsive square soft core and an
attractive square well. In the case of a narrow attractive well, it has been
shown that this potential has two metastable gas-liquid critical points. Here
we systematically investigate how the changes of the parameters of this
potential affect the phase diagram of the system. We find a broad range of
potential parameters for which the system has both a gas-liquid critical point
and a liquid-liquid critical point. For the liquid-gas critical point we find
that the derivatives of the critical temperature and pressure, with respect to
the parameters of the potential, have the same signs: they are positive for
increasing width of the attractive well and negative for increasing width and
repulsive energy of the soft core. This result resembles the behavior of the
liquid-gas critical point for standard liquids. In contrast, for the
liquid-liquid critical point the critical pressure decreases as the critical
temperature increases. As a consequence, the liquid-liquid critical point
exists at positive pressures only in a finite range of parameters. We present a
modified van der Waals equation which qualitatively reproduces the behavior of
both critical points within some range of parameters, and give us insight on
the mechanisms ruling the dependence of the two critical points on the
potential's parameters. The soft core potential studied here resembles model
potentials used for colloids, proteins, and potentials that have been related
to liquid metals, raising an interesting possibility that a liquid-liquid phase
transition may be present in some systems where it has not yet been observed.Comment: 29 pages, 15 figure
Ice XII in its second regime of metastability
We present neutron powder diffraction results which give unambiguous evidence
for the formation of the recently identified new crystalline ice phase[Lobban
et al.,Nature, 391, 268, (1998)], labeled ice XII, at completely different
conditions. Ice XII is produced here by compressing hexagonal ice I_h at T =
77, 100, 140 and 160 K up to 1.8 GPa. It can be maintained at ambient pressure
in the temperature range 1.5 < T < 135 K. High resolution diffraction is
carried out at T = 1.5 K and ambient pressure on ice XII and accurate
structural properties are obtained from Rietveld refinement. At T = 140 and 160
K additionally ice III/IX is formed. The increasing amount of ice III/IX with
increasing temperature gives an upper limit of T ~ 150 K for the successful
formation of ice XII with the presented procedure.Comment: 3 Pages of RevTeX, 3 tables, 3 figures (submitted to Physical Review
Letters
Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys
We report the results of ab initio molecular dynamics simulations of liquid
Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600
K, just above the melting point of GaAs. The liquid is predicted to be metallic
at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity
maximum near x = 0.5, consistent with the Faber-Ziman expression. The
electronic density of states is finite at the Fermi energy for all
concentrations; there is, however, a significant pseudogap especially in the
As-rich samples. The Ga-rich density of states more closely resembles that of a
free-electron metal. The partial structure factors show only a weak indication
of chemical short-range order. There is also some residue of the covalent
bonding found in the solid, which shows up in the bond-angle distribution
functions of the liquid state. Finally, the atomic diffusion coefficients at
1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in
Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in
Ga_{0.2}As_{0.8}.Comment: 29 pages, 10 eps figures, accepted for publication in Phys. Rev.
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