519 research outputs found
Structural properties and liquid spinodal of water confined in a hydrophobic environment
We present the results of a computer simulation study of thermodynamical
properties of TIP4P water confined in a hydrophobic disordered matrix of soft
spheres upon supercooling. The hydrogen bond network of water appears preserved
in this hydrophobic confinement. Nonetheless a reduction in the average number
of hydrogen bonds due to the geometrical constraints is observed. The liquid
branch of the spinodal line is calculated from 350 K down to 210 K. The same
thermodynamic scenario of the bulk is found: the spinodal curve is
monotonically decreasing. The line of maximum density bends avoiding a crossing
of the spinodal. There is however a shift both of the line of maximum density
and of the spinodal toward higher pressures and lower temperatures with respect
to bulk.Comment: 7 pages, 12 figure
A Family of Tunable Spherically-Symmetric Potentials that Span the Range from Hard Spheres to Water-like Behavior
We investigate the equation of state, diffusion coefficient, and structural
order of a family of spherically-symmetric potentials consisting of a hard core
and a linear repulsive ramp. This generic potential has two characteristic
length scales: the hard and soft core diameters. The family of potentials is
generated by varying their ratio, . We find negative thermal expansion
(thermodynamic anomaly) and an increase of the diffusion coefficient upon
isothermal compression (dynamic anomaly) for . As in water,
the regions where these anomalies occur are nested domes in the () or
() planes, with the thermodynamic anomaly dome contained entirely within
the dynamic anomaly dome. We calculate translational and orientational order
parameters ( and ), and project equilibrium state points onto the () plane, or order map. The order map evolves from water-like behavior to
hard-sphere-like behavior upon varying between 4/7 and 6/7. Thus, we
traverse the range of liquid behavior encompassed by hard spheres ()
and water-like () with a family of tunable
spherically-symmetric potentials by simply varying the ratio of hard to
soft-core diameters. Although dynamic and thermodynamic anomalies occur almost
across the entire range , water-like structural anomalies
(i.e., decrease in both and upon compression and strictly correlated
and in the anomalous region) occur only around .
Water-like anomalies in structure, dynamics and thermodynamics arise solely due
to the existence of two length scales, orientation-dependent interactions being
absent by design.Comment: total 21 pages, 6 figure
Potential Energy Landscape of the Apparent First-Order Phase Transition between Low-Density and High-Density Amorphous Ice
The potential energy landscape (PEL) formalism is a valuable approach within
statistical mechanics for describing supercooled liquids and glasses. Here we
use the PEL formalism and computer simulations to study the pressure-induced
transformations between low-density amorphous ice (LDA) and high-density
amorphous ice (HDA) at different temperatures. We employ the ST2 water model
for which the LDA-HDA transformations are remarkably sharp, similar to what is
observed in experiments, and reminiscent of a first-order phase transition. Our
results are consistent with the view that LDA and HDA configurations are
associated with two distinct regions (megabasins) of the PEL that are separated
by a potential energy barrier. At higher temperature, we find that low-density
liquid (LDL) configurations are located in the same megabasin as LDA, and that
high-density liquid (HDL) configurations are located in the same megabasin as
HDA. We show that the pressure-induced LDL-HDL and LDA-HDA transformations
occur along paths that interconnect these two megabasins, but that the path
followed by the liquid is different than the path followed by the amorphous
solid. At higher pressure, we also study the liquid-to-ice-VII first-order
phase transition, and find that the behavior of the PEL properties across this
transition are qualitatively similar to the changes found during the LDA-HDA
transformation. This similarity supports the interpretation that the LDA-HDA
transformation is a first-order-like phase transition between
out-of-equilibrium states.Comment: 29 pages, 8 figure
Glass Polymorphism in TIP4P/2005 Water: A Description Based on the Potential Energy Landscape Formalism
The potential energy landscape (PEL) formalism is a statistical mechanical
approach to describe supercooled liquids and glasses. Here we use the PEL
formalism to study the pressure-induced transformations between low-density
amorphous ice (LDA) and high-density amorphous ice (HDA) using computer
simulations of the TIP4P/2005 molecular model of water. We find that the
properties of the PEL sampled by the system during the LDA-HDA transformation
exhibit anomalous behavior. In particular, at conditions where the change in
density during the LDA-HDA transformation is approximately discontinuous,
reminiscent of a first-order phase transition, we find that (i) the inherent
structure (IS) energy, , is a concave function of the volume,
and (ii) the IS pressure, , exhibits a van der Waals-like loop.
In addition, the curvature of the PEL at the IS is anomalous, a non-monotonic
function of . In agreement with previous studies, our work suggests that
conditions (i) and (ii) are necessary (but not sufficient) signatures of the
PEL for the LDA-HDA transformation to be reminiscent of a first-order phase
transition. We also find that one can identify two different regions of the
PEL, one associated to LDA and another to HDA. Our computer simulations are
performed using a wide range of compression/decompression and cooling rates. In
particular, our slowest cooling rate (0.01 K/ns) is within the experimental
rates employed in hyperquenching experiments to produce LDA. Interestingly, the
LDA-HDA transformation pressure that we obtain at K and at different
rates extrapolates remarkably well to the corresponding experimental pressure.Comment: Manuscript and Supplementary Materia
Cooling rate, heating rate and aging effects in glassy water
We report a molecular dynamics simulation study of the properties of the
potential energy landscape sampled by a system of water molecules during the
process of generating a glass by cooling, and during the process of
regenerating the equilibrium liquid by heating the glass. We study the
dependence of these processes on the cooling/heating rates as well as on the
role of aging (the time elapsed in the glass state). We compare the properties
of the potential energy landscape sampled during these processes with the
corresponding properties sampled in the liquid equilibrium state to elucidate
under which conditions glass configurations can be associated with equilibrium
liquid configurations.Comment: to be published in Phys. Rev. E (rapid comunication
A Potential Energy Landscape Study of the Amorphous-Amorphous Transformation in HO
We study the potential energy landscape explored during a
compression-decompression cycle for the SPC/E (extended simple point charge)
model of water. During the cycle, the system changes from low density amorphous
ice (LDA) to high density amorphous ice (HDA). After the cycle, the system does
not return to the same region of the landscape, supporting the interesting
possibility that more than one significantly different configuration
corresponds to LDA. We find that the regions of the landscape explored during
this transition have properties remarkably different from those explored in
thermal equilibrium in the liquid phase
Thermodynamics, Structure, and Dynamics of Water Confined between Hydrophobic Plates
We perform molecular dynamics simulations of 512 water-like molecules that
interact via the TIP5P potential and are confined between two smooth
hydrophobic plates that are separated by 1.10 nm. We find that the anomalous
thermodynamic properties of water are shifted to lower temperatures relative to
the bulk by K. The dynamics and structure of the confined water
resemble bulk water at higher temperatures, consistent with the shift of
thermodynamic anomalies to lower temperature. Due to this shift, our
confined water simulations (down to K) do not reach sufficiently low
temperature to observe a liquid-liquid phase transition found for bulk water at
K using the TIP5P potential. We find that the different
crystalline structures that can form for two different separations of the
plates, 0.7 nm and 1.10 nm, have no counterparts in the bulk system, and
discuss the relevance to experiments on confined water.Comment: 31 pages, 14 figure
Testis structure and function in a nongenetic hyperadipose rat model at prepubertal and adult ages
There are few data for hormonal levels and testis structure and function during postnatal development in rats neonatally treated with monosodium L-glutamate (MSG). In our study, newborn male pups were ip injected with MSG (4 mg/g body weight) every 2 d up to 10 d of age and investigated at prepubertal and adult ages. Plasma levels of leptin, LH, FSH, prolactin, testosterone (T), corticosterone, and free T4 (FT4) were measured. MSG rats displayed elevated circulating levels of corticosterone and hyperadiposity/ hyperleptinemia, regardless of the age examined; conversely, circulating prolactin levels were not affected. Moreover, prepubertal MSG rats revealed a significant (P < 0.05) reduction in testis weight and the number of Sertoli (SC) and Leydig cells per testis. Leptin plasma levels were severalfold higher (2.41 vs. 8.07; P < 0.05) in prepubertal MSG rats, and these animals displayed plasma LH, FSH, T, and FT4 levels significantly decreased (P < 0.05). Taken together, these data indicate that testis development, as well as SC and Leydig cell proliferation, were disturbed in prepubertal MSG rats. Adult MSG rats also displayed significantly higher leptin plasma levels (7.26 vs. 27.04; P < 0.05) and lower (P < 0.05) LH and FSH plasma levels. However, T and FT4 plasma levels were normal, and no apparent alterations were observed in testis structure of MSG rats. Only the number of SCs per testis was significantly (P < 0.05) reduced in the adult MSG rats. In conclusion, although early installed hyperadipose/hyperleptinemia phenotype was probably responsible for the reproductive axis damages in MSG animals, it remains to be investigated whether this condition is the main factor for hypothalamus-pituitary-gonadal axis dysfunction in MSG rats.Instituto Multidisciplinario de BiologĂa Celula
Testis structure and function in a nongenetic hyperadipose rat model at prepubertal and adult ages
There are few data for hormonal levels and testis structure and function during postnatal development in rats neonatally treated with monosodium L-glutamate (MSG). In our study, newborn male pups were ip injected with MSG (4 mg/g body weight) every 2 d up to 10 d of age and investigated at prepubertal and adult ages. Plasma levels of leptin, LH, FSH, prolactin, testosterone (T), corticosterone, and free T4 (FT4) were measured. MSG rats displayed elevated circulating levels of corticosterone and hyperadiposity/ hyperleptinemia, regardless of the age examined; conversely, circulating prolactin levels were not affected. Moreover, prepubertal MSG rats revealed a significant (P < 0.05) reduction in testis weight and the number of Sertoli (SC) and Leydig cells per testis. Leptin plasma levels were severalfold higher (2.41 vs. 8.07; P < 0.05) in prepubertal MSG rats, and these animals displayed plasma LH, FSH, T, and FT4 levels significantly decreased (P < 0.05). Taken together, these data indicate that testis development, as well as SC and Leydig cell proliferation, were disturbed in prepubertal MSG rats. Adult MSG rats also displayed significantly higher leptin plasma levels (7.26 vs. 27.04; P < 0.05) and lower (P < 0.05) LH and FSH plasma levels. However, T and FT4 plasma levels were normal, and no apparent alterations were observed in testis structure of MSG rats. Only the number of SCs per testis was significantly (P < 0.05) reduced in the adult MSG rats. In conclusion, although early installed hyperadipose/hyperleptinemia phenotype was probably responsible for the reproductive axis damages in MSG animals, it remains to be investigated whether this condition is the main factor for hypothalamus-pituitary-gonadal axis dysfunction in MSG rats.Instituto Multidisciplinario de BiologĂa Celula
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