1,587 research outputs found
Phase Behavior of the Patchy Colloids Confined in the Patchy Porous Media
A simple model for functionalized disordered porous media is proposed and the
effects of confinement on self-association, percolation and phase behavior of a
fluid of patchy particles are studied. The media is formed by a randomly
distributed hard-sphere obstacles fixed in space and decorated by a certain
number of off-center square-well sites. The properties of the fluid of patchy
particles, represented by the fluid of hard spheres each bearing a set of the
off-center square-well sites, are studied using an appropriate combination of
the scaled particle theory for the porous media, Wertheim's thermodynamic
perturbation theory, and the Flory-Stockmayer theory. To assess the accuracy of
the theory a set of computer simulations have been performed. In general,
predictions of the theory appear to be in a good agreement with computer
simulation results. Confinement and competition between the formation of bonds
connecting the fluid particles, and connecting fluid particles and obstacles of
the matrix, give rise to a re-entrant phase behavior with three critical points
and two separate regions of the liquid-gas phase coexistence
The oscillatory damped behaviour of incommensurate double-walled carbon nanotubes
Abstract The mechanical properties of sliding carbon nanotubes have been investigated by classical molecular dynamics simulations in the canonical ensemble. In particular we have studied damped oscillations in the separation between the centres of mass of the inner and outer tubes of double-walled carbon nanotubes (DWCN). Incommensurate DWCNs forming (7, 0)@(9, 9) structures were simulated for systems at 298.15 K with axial lengths from 12.21 to 98.24 nm. The oscillations exhibited frequencies in the range of gigahertz with the frequency decreasing as the length of the system increases. The time until oscillations become negligible exhibited a nearly linear dependence on the length of the system. Two macroscopic models were developed in order to understand the forces involved in terms of macroscopic properties like friction and shear. The first model considered constant restoring forces during the whole event, while in the second the value of these constant restoring forces depended on the initial conditions of each oscillation. Both models reproduced the oscillations quite well, while the second model allows us to predict the dynamic shear strength in terms of the axial length of the system for tubes with the same diameters. The calculated dynamic shear strength exhibited monotonic behaviour with an inverse dependence on the length of the system. For systems with unequal axial lengths, the restoring force, which drives the oscillation, is reduced compared to the system with equal lengths, regardless of whether the outer nanotube is longer or shorter. M This article contains online multimedia enhancement
Swift Observations of GRB 050603: An afterglow with a steep late time decay slope
We report the results of Swift observations of the Gamma Ray Burst GRB
050603. With a V magnitude V=18.2 about 10 hours after the burst the optical
afterglow was the brightest so far detected by Swift and one of the brightest
optical afterglows ever seen. The Burst Alert Telescope (BAT) light curves show
three fast-rise-exponential-decay spikes with =12s and a fluence of
7.6 ergs cm in the 15-150 keV band. With an ergs it was also one of the most energetic
bursts of all times. The Swift spacecraft began observing of the afterglow with
the narrow-field instruments about 10 hours after the detection of the burst.
The burst was bright enough to be detected by the Swift UV/Optical telescope
(UVOT) for almost 3 days and by the X-ray Telescope (XRT) for a week after the
burst. The X-ray light curve shows a rapidly fading afterglow with a decay
index =1.76. The X-ray energy spectral index was
=0.71\plm0.10 with the column density in agreement with the
Galactic value. The spectral analysis does not show an obvious change in the
X-ray spectral slope over time. The optical UVOT light curve decays with a
slope of =1.8\plm0.2.
The steepness and the similarity of the optical and X-ray decay rates suggest
that the afterglow was observed after the jet break. We estimate a jet opening
angle of about 1-2Comment: 14 pages, accepted for publication in Ap
Experimental and theoretical justifications for the observed discriminations between enantiomers of prochiral alcohols by chirally blind EI-MS
To all appearances, electron impact mass spectrometer (EI-MS) is considered a
"chirally blind" instrument. Yet, numerous non-identical R (right) and S (left)
configurations of prochiral alcohols' mass spectra alcohols have appeared in
the literature with almost no justification. Such observations are often
attributed to impurities, experimental circumstances, inaccurate measurements,
etc. In an experimental attempt to explain this phenomenon, here we have
avoided the above mentioned pitfalls by conducting control experiments using
different pure enantiomers under the same circumstances. Hence, we report the
mass spectra of R- and S-enantiomers of 2-octanol (1R, 1S) and 1-octyn-3-ol
(2R, 2S) collected by running 20 independent experiments for each R- and
S-enantiomer. Statistical analyses confirmed that the peak intensities were
significant to an acceptable level of confidence. The 1R and 1S enantiomers
were separated reasonably in the PC space, implying that the chirally blind
EI-MS is able to discriminate between R and S prochiral alcohols.
Theoretically, self-complexation through H-bonding for S (or R) appears to give
a new chiral center at the H-bonded oxygen atom, producing a new dimeric pair
of diastereomers SRS and SSS (or RRR and RSR) before ionization, and SRS.+ and
SSS.+ (or RRR.+ and RSR.+) after ionization. The results of our calculations
have explicitly shown that these hydrogen bonds formed. Interestingly, the
latter four ionized diastereomers appear with different structural and
thermodynamic parameters at the M06-2X/6-311++g (d,p) level of theory
Formalizing Atom-typing and the Dissemination of Force Fields with Foyer
A key component to enhancing reproducibility in the molecular simulation
community is reducing ambiguity in the parameterization of molecular models.
Ambiguity in molecular models often stems from the dissemination of molecular
force fields in a format that is not directly usable or is ambiguously
documented via a non-machine readable mechanism. Specifically, the lack of a
general tool for performing automated atom-typing under the rules of a
particular force field facilitates errors in model parameterization that may go
unnoticed if other researchers are unable reproduce this process. Here, we
present Foyer, a Python tool that enables users to define force field
atom-typing rules in a format that is both machine- and human-readable thus
eliminating ambiguity in atom-typing and additionally providing a framework for
force field dissemination. Foyer defines force fields in an XML format, where
SMARTS strings are used to define the chemical context of a particular atom
type. Herein we describe the underlying methodology of the Foyer package,
highlighting its advantages over typical atom-typing approaches and demonstrate
is application in several use-cases.Comment: 39 Page, 4 Figures, 8 Listing
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