16,805 research outputs found
On the composition dependence of the microscopic structure, thermodynamic, dynamic and dielectric properties of water-dimethyl formamide model mixtures. Molecular dynamics simulation results
Isothermal-isobaric molecular dynamics simulations have been performed to
examine an ample set of properties of the model water-N,N-dimethylformamide
(DMF) mixture as a function of composition. The SPC-E and TIP4P-Ew water models
together with two united atom models for DMF [Chalaris M., Samios J., J. Chem.
Phys., 2000, 112, 8581; Cordeiro J., Int. J. Quantum Chem., 1997, 65, 709] were
used. Our principal analyses concern the behaviour of structural properties in
terms of radial distribution functions, and the number of hydrogen bonds
between molecules of different species as well as thermodynamic properties.
Namely, we explore the density, excess mixing molar volume and enthalpy, the
heat capacity and excess mixing heat capacity. Finally, the self-diffusion
coefficients of species and the dielectric constant of the system are
discussed. In addition, surface tension of water-DMF mixtures has been
calculated and analyzed.Comment: 15 pages, 11 figure
Quark masses in QCD: a progress report
Recent progress on QCD sum rule determinations of the light and heavy quark
masses is reported. In the light quark sector a major breakthrough has been
made recently in connection with the historical systematic uncertainties due to
a lack of experimental information on the pseudoscalar resonance spectral
functions. It is now possible to suppress this contribution to the 1% level by
using suitable integration kernels in Finite Energy QCD sum rules. This allows
to determine the up-, down-, and strange-quark masses with an unprecedented
precision of some 8-10%. Further reduction of this uncertainty will be possible
with improved accuracy in the strong coupling, now the main source of error. In
the heavy quark sector, the availability of experimental data in the vector
channel, and the use of suitable multipurpose integration kernels allows to
increase the accuracy of the charm- and bottom-quarks masses to the 1% level.Comment: Invited review paper to be published in Modern Physics Letters
Computational studies on the behaviour of anionic and nonionic surfactants at the SiO (silicon dioxide)/water interface
Molecular dynamics simulations to study the behaviour of anionic (Sodium
Dodecylsulfate, SDS) and nonionic (Monooleate of Sorbitan, SPAN80) surfactants
close to a SiO (silicon dioxide) surface were carried out. Simulations
showed that a water layer was first adsorbed on the surface and then the
surfactants were attached on that layer. Moreover, it was observed that water
behaviour close to the surface influenced the surfactant adsorption since a
semi-spherical micelle was formed on the SiO surface with SDS molecules
whereas a cylindrical micelle was formed with SPAN80 molecules. Adsorption of
the micelles was conducted in terms of structural properties (density profiles
and angular distributions) and dynamical behaviour (diffusion coefficients) of
the systems. Finally, it was also shown that some water molecules moved inside
the solid surface and located at specific sites of the solid surface.Comment: 8 pages, 6 fiigure
Temperature dependence of the microscopic structure and density anomaly of the SPC/E and TIP4P-Ew water models. Molecular dynamics simulation results
We have investigated temperature trends of the microscopic structure of the
SPC/E and TIP4P-Ew water models in terms of the pair distribution functions,
coordination numbers, the average number of hydrogen bonds, the distribution of
bonding states of a single molecule as well as the angular distribution of
molecules by using the constant pressure molecular dynamics simulations. The
evolution of the structure is put in correspondence with the dependence of
water density on high temperatures down to the region of temperatures where the
system becomes supercooled. It is shown that the fraction of molecules with
three and four bonds determine the maximum density for both models. Moreover,
the temperature dependence of the dielectric constant is obtained and analyzed.Comment: 11 pages, 5 figure
Electromagnetic nucleon form factors from QCD sum rules
The electromagnetic form factors of the nucleon, in the space-like region,
are determined from three-point function Finite Energy QCD Sum Rules. The QCD
calculation is performed to leading order in perturbation theory in the chiral
limit, and to leading order in the non-perturbative power corrections. The
results for the Dirac form factor, , are in very good agreement with
data for both the proton and the neutron, in the currently accessible
experimental region of momentum transfers. This is not the case, though, for
the Pauli form factor , which has a soft -dependence
proportional to the quark condensate .Comment: Replaced Version. An error has been corrected in the numerical
evaluation of the Pauli form factor. This changes the results for F_2(q^2),
as well as the conclusion
QCD determination of the leading order hadronic contribution to the muon g-2
The leading order hadronic contribution to the muon magnetic moment anomaly,
, is determined entirely in the framework of QCD. The result in
the light-quark sector, in units of , is , and in the heavy-quark sector , and
, resulting in . The
main uncertainty is due to the current lattice QCD value of the first and
second derivative of the electromagnetic current correlator at the origin.
Expected improvement in the precision of these derivatives may render this
approach the most accurate and trustworthy determination of the leading order
.Comment: Invited talk at "Les Rencontres de Physique de la Vallee d'Aosta",
March 2017. Speaker: C. A. Dominguez. To be published in Nuovo Cimento
Isobaric-isothermal molecular dynamics computer simulations of the properties of water-1,2-dimethoxyethane model mixtures
Isothermal-isobaric molecular dynamics simulations have been performed to
examine a broad set of properties of the model water-1,2-dimethoxyethane (DME)
mixture as a function of composition. The SPC-E and TIP4P-Ew water models and
the modified TraPPE model for DME were applied. Our principal focus was to
explore the trends of behaviour of the structural properties in terms of the
radial distribution functions, coordination numbers and number of hydrogen
bonds between molecules of different species, and of conformations of DME
molecules. Thermodynamic properties, such as density, molar volume, enthalpy of
mixing and heat capacity at constant pressure have been examined. Finally, the
self-diffusion coefficients of species and the dielectric constant of the
system were calculated and analyzed.Comment: 14 pages, 9 figure
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