506 research outputs found
Kinetic and ion pairing contributions in the dielectric spectra of electrolyte aqueous solutions
Understanding dielectric spectra can reveal important information about the
dynamics of solvents and solutes from the dipolar relaxation times down to
electronic ones. In the late 1970s, Hubbard and Onsager predicted that adding
salt ions to a polar solution would result in a reduced dielectric permittivity
that arises from the unexpected tendency of solvent dipoles to align opposite
to the applied field. So far, this effect has escaped an experimental
verification, mainly because of the concomitant appearance of dielectric
saturation from which the Hubbard-Onsager decrement cannot be easily separated.
Here we develop a novel non-equilibrium molecular dynamics simulation approach
to determine this decrement accurately for the first time. Using a
thermodynamic consistent all-atom force field we show that for an aqueous
solution containing sodium chloride around 4.8 Mol/l, this effect accounts for
12\% of the total dielectric permittivity. The dielectric decrement can be
strikingly different if a less accurate force field for the ions is used. Using
the widespread GROMOS parameters, we observe in fact an {\it increment} of the
dielectric permittivity rather than a decrement. We can show that this
increment is caused by ion pairing, introduced by a too low dispersion force,
and clarify the microscopic connection between long-living ion pairs and the
appearance of specific features in the dielectric spectrum of the solution
Puckering Free Energy of Pyranoses: an NMR and Metadynamics--Umbrella Sampling Investigation
We present the results of a combined metadynamics--umbrella sampling
investigation of the puckered conformers of pyranoses described using the
gromos 45a4 force field. The free energy landscape of Cremer--Pople puckering
coordinates has been calculated for the whole series of alpha and beta
aldohexoses, showing that the current force field parameters fail in
reproducing proper puckering free energy differences between chair conformers.
We suggest a modification to the gromos 45a4 parameter set which improves
considerably the agreement of simulation results with theoretical and
experimental estimates of puckering free energies. We also report on the
experimental measurement of altrose conformers populations by means of NMR
spectroscopy, which show good agreement with the predictions of current
theoretical models
The generalized identification of truly interfacial molecules (ITIM) algorithm for nonplanar interfaces
We present a generalized version of the ITIM algorithm for the identification of interfacial molecules, which is able to treat arbitrarily shaped interfaces. The algorithm exploits the similarities between the concept of probe sphere used in ITIM and the circumsphere criterion used in the α-shapes approach, and can be regarded either as a reference-frame independent version of the former, or as an extended version of the latter that includes the atomic excluded volume. The new algorithm is applied to compute the intrinsic orientational order parameters of water around a dodecylphosphocholine and a cholic acid micelle in aqueous environment, and to the identification of solvent-reachable sites in four model structures for soot. The additional algorithm introduced for the calculation of intrinsic density profiles in arbitrary geometries proved to be extremely useful also for planar interfaces, as it allows to solve the paradox of smeared intrinsic profiles far from the interface. © 2013 American Institute of Physics
Crossover to non-Fermi-liquid spin dynamics in cuprates
The antiferromagnetic spin correlation function , the staggered
spin susceptibility and the energy scale are studied numerically within the t-J model and the Hubbard
model, as relevant to cuprates. It is shown that , related to the
onset of the non-Fermi-liquid spin response at , is very low in
the regime below the 'optimum' hole doping , while it
shows a steep increase in the overdoped regime. A quantitative analysis of NMR
spin-spin relaxation-rate for various cuprates reveals a similar
behavior, indicating on a sharp, but continuous, crossover between a
Fermi-liquid and a non-Fermi-liquid behavior as a function of doping.Comment: 4 pages, 4 figures. Submitted to PR
Electron Spin Resonance of SrCu2(BO3)2 at High Magnetic Field
We calculate the electron spin resonance (ESR) spectra of the
quasi-two-dimensional dimer spin liquid SrCu2(BO3)2 as a function of magnetic
field B. Using the standard Lanczos method, we solve a Shastry-Sutherland
Hamiltonian with additional Dzyaloshinsky-Moriya (DM) terms which are crucial
to explain different qualitative aspects of the ESR spectra. In particular, a
nearest-neighbor DM interaction with a non-zero D_z component is required to
explain the low frequency ESR lines for B || c. This suggests that crystal
symmetry is lowered at low temperatures due to a structural phase transition.Comment: 4 pages, 4 b&w figure
Stability study and uncertainty evaluation of CO2 certified reference materials for greenhouse gases monitoring
The continuous rising in the concentration of carbon dioxide (CO2) in the atmosphere is one of the main causes of
the increase in the greenhouse effect and global warming. To monitor the alarming scenario and to provide
Governments and decision makers with reliable emission data, gaseous certified reference materials (CRMs) at
atmospheric CO2 amount fraction are needed. This paper describes two independent metrological traceability
paths established at INRiM for the preparation of this kind of CRMs. The aim of this publication is to show a
method for evaluating the uncertainty associated with CRM stability and to demonstrate that there is no significant
trend in the results over time. Such CRMs are produced as an intermediate step towards the development
of novel generation CRMs certified also for the isotopic composition
Traumatic brain injury and NADPH oxidase: A deep relationship
Traumatic brain injury (TBI) represents one of the major causes of mortality and disability in the world. TBI is characterized by primary damage resulting from the mechanical forces applied to the head as a direct result of the trauma and by the subsequent secondary injury due to a complex cascade of biochemical events that eventually lead to neuronal cell death. Oxidative stress plays a pivotal role in the genesis of the delayed harmful effects contributing to permanent damage. NADPH oxidases (Nox), ubiquitary membrane multisubunit enzymes whose unique function is the production of reactive oxygen species (ROS), have been shown to be a major source of ROS in the brain and to be involved in several neurological diseases. Emerging evidence demonstrates that Nox is upregulated after TBI, suggesting Nox critical role in the onset and development of this pathology. In this review, we summarize the current evidence about the role of Nox enzymes in the pathophysiology of TBI
The intrinsic structure of the interface of partially miscible fluids : an application to ionic liquids
We investigate by means of Molecular Dynamics simulations how the intrinsic sur- face structure of liquid/liquid interfaces involving ionic liquids depends on the opposite phase of varying polarity. We study 1-n-butyl-3-methylimidazolium hexa uorophos- phate (BMIM PF 6 ) and 1-n-butyl-3-methylimidazolium bis(tri uoromethylsulfonyl)imid (BMIM NTf 2 ). The opposite phase is either cyclohexane or water, but as a reference, IL { vacuum interfaces are also studied. We combine a distance-based cluster search algorithm with the ITIM intrinsic analyzing method to separate liquid phases showing non-negligible mutual miscibility and to identify atoms residing at the instantaneous surface. In contrast to the well structured surface of IL { vacuum systems, at liq- uid/liquid interfaces of ILs density correlations, ionic associations and orientational preferences are all weakened, this eect being much more pronounced when the other species is water. In such systems we observe a drastic reduction in the presence of the cation at the surface and an increase of appearance of polar moieties (of both the cations and anions) leading to decreased apolar character of the interface. Furthermore, cations are mostly found to turn with their butyl chains toward the bulk while having their methyl groups sticking towards water. Anion-cation associations are reduced and partially replaced by water-anion and rarely also water-cation associations
Traumatic brain injury and NADPH oxidase: A deep relationship
Traumatic brain injury (TBI) represents one of the major causes of mortality and disability in the world. TBI is characterized by primary damage resulting from the mechanical forces applied to the head as a direct result of the trauma and by the subsequent secondary injury due to a complex cascade of biochemical events that eventually lead to neuronal cell death. Oxidative stress plays a pivotal role in the genesis of the delayed harmful effects contributing to permanent damage. NADPH oxidases (Nox), ubiquitary membrane multisubunit enzymes whose unique function is the production of reactive oxygen species (ROS), have been shown to be a major source of ROS in the brain and to be involved in several neurological diseases. Emerging evidence demonstrates that Nox is upregulated after TBI, suggesting Nox critical role in the onset and development of this pathology. In this review, we summarize the current evidence about the role of Nox enzymes in the pathophysiology of TBI
- …