Search results for "THERMODYNAMICS"
showing 10 items of 2774 documents
Coarse-graining dipolar interactions in simple fluids and polymer solutions: Monte Carlo studies of the phase behavior
2009
In this paper we investigate the phase diagram of pure dipolar substances and their mixtures with short alkanes, using grand canonical Monte Carlo simulations of simplified coarse-grained models. Recently, an efficient coarse-grained model for simple quadrupolar molecules, based on a Lennard-Jones (LJ) interaction plus a spherically averaged quadrupolar potential, has been shown to be successful in predicting single-component and mixture phase diagrams. Motivated by these results, we investigate the phase diagrams of simple dipolar molecules (and their mixtures with alkanes) using a spherically averaged potential. First, we test the model on pure components. A generalized (state-dependent) …
The influence of external uniaxial stretching on the electromigration and diffusion of hydrogen in iron
2009
Electromigration and diffusion of hydrogen in iron, depending on unit elongation, were investigated. The resistivity method was applied in the investigations. It was found that the obtained experimental results cannot be described by means of a single flux of hydrogen, which diffuses in the electric field. In order to explain the obtained results, we assumed the existence of the following two fluxes of hydrogen in the stretched metal: migration, according to the interstitial mechanism, and migration according to the interstitial-defective mechanism. The dependence of effective valences and diffusion coefficients for both mechanisms were determined.
Lennard-Jones Parameters for B3LYP/CHARMM27 QM/MM Modeling of Nucleic Acid Bases.
2015
Combined quantum mechanics/molecular mechanics (QM/MM) methods allow computations on chemical events in large molecular systems. Here, we have tested the suitability of the standard CHARMM27 forcefield Lennard-Jones van der Waals (vdW) parameters for the treatment of nucleic acid bases in QM/MM calculations at the B3LYP/6-311+G(d,p)-CHARMM27 level. Alternative parameters were also tested by comparing the QM/MM hydrogen bond lengths and interaction energies with full QM [B3LYP/6-311+G(d,p)] results. The optimization of vdW parameters for nucleic acid bases is challenging because of the likelihood of multiple hydrogen bonds between the nucleic acid base and a water molecule. Two sets of optim…
Tautomerism in pyridazin-3(2H)-one: a theoretical study using implicit/explicit solvation models.
2013
Abstract The tautomeric conversion of pyridazin-3(2H)-one 1 into pyridazin-3-ol 2 has been theoretically studied using density functional theory (DFT) methods at the B3LYP/6-311++G** level. Two mechanisms have been considered for this process: (i) one in which the hydrogen is directly transferred through TS12; and (ii) another one in which a double hydrogen transfer takes place via TS1122 upon formation of the corresponding dimer. The former requires a very high activation energy of 42.64 kcal/mol as a consequence of the strain associated with the formation of the four-membered TS12, while the latter requires a much lower activation energy, 14.66 kcal/mol. Implicit, explicit, and a combinat…
From determination of the fugacity coefficients to estimation of hydrogen storage capacity: A convenient theoretical method
2015
Abstract The equation of state (EOS) from virial expansion (VE) is used in this work to pave the way for determining the fugacity coefficients of the hydrogen fluid at arbitrary temperature and pressure. The fugacity coefficients from our VE method have more physical meanings than the empirical values. In this way, the hydrogen storage capacity of a novel material model can be estimated by using few density functional theory (DFT) calculations with the aid of a continuum model. The efficient continuum model can provide a more accurate estimation of the hydrogen storage capacity than the pure DFT calculations. Furthermore, the expensive grand canonical ensemble (μNT) simulations combining wi…
Accumulation of low oxygen water in deep waters of ice-covered lakes cooled below 4 °C
2013
We studied vertical distribution of oxygen under the ice of 5 medium-sized, morphologically variable lakes that cooled well below 4 °C before freezing. In the upper part of the water column, dissolved oxygen and dissolved inorganic carbon concentrations generally remained vertically almost uniform, but in the deepest water, concentrations changed rapidly near the bottom. The coincidence of the changes with an increase in deep water temperature shows that they were due to advection of water made heavier by the heat flux from the sediment. Consequently, water with low concentrations of dissolved oxygen and high concentrations of dissolved inorganic carbon accumulated in the deepest part of th…
Calibrating storage tanks for soil erosion measurement from plots
1998
Many plots for soil loss measurements are equipped with a sequence of tanks for measuring runoff volume and sediment concentration. The stored water volume is easily determined by a water depth measurement while the sediment concentration is often measured by collecting samples of the mixed suspension. In this paper, using the Π-theorem of dimensional analysis, the functional relationship describing the mixing of the suspension in the tank is expressed in a dimensionless form. The recognized dimensionless groups allow the establishment, for given soil and water depth in the tank, of the relationship (calibration curve) between the actual and the measured concentration. The calibration curve…
Mountain dairy wastewater treatment with the use of a 'irregularly shaped' constructed wetland (Aosta Valley, Italy)
2014
Abstract In mountain areas, economical activities related to milk processing represent both a key source of income and job opportunities. One of the main characteristics of cheese production is the seasonal variability in the volume of milk processed and wastewater production that tend to limit the capacity of ecosystems to absorb their inputs. In alpine environment, the scarcity of plain surfaces and the climatic conditions results in the need for high CW performances of variable nutrient inputs in different seasons. By evaluating a CW seasonal efficiency for dairy wastewaters in a mountain region (Aosta Valley-NW Italy), this research was aimed to understand how performances of nutrient r…
Bypass flow, salinization and sodication in a cracking clay soil
2004
Abstract In Sicily, the increasing scarcity of good-quality waters is spreading irrigation with saline–sodic waters, thus enhancing the risk of soil secondary salinization and sodication. Sustainable management strategies are urgently needed in Sicily to prevent extent of salinization and sodication, thus preserving soil quality. Since irrigation is performed in cracking soils using irrigation systems involving high application rates, bypass flow of water and solutes occurs during irrigation. The objectives of this paper were (i) to investigate the process of Na–Ca exchange and subsequent salinization/sodication during bypass flow and (ii) to explore possibilities of using cyclic strategies…
Combining a data-driven approach with seasonal forecasts data to predicting reservoir water volume in the Mediterranean area.
2021
<p>Artificial reservoirs are one of the main water supply resources in the Mediterranean areas; their management can be strongly affected by the problems of drought and water scarcity. The reservoir water level is the result of the hydrological processes occurring in the upstream catchment, which, in turn, depend on meteorological variables, such as rainfall and temperature. It follows that a reliable forecast model of the meteorological forcing, along with a reliable water balance model, could enhance the correct management of a reservoir. With regard to the rainfall/temperature forecast model, the use of forecast climate data in the mid-term may provide further support for t…