1,720,965 research outputs found
How increasing pressure affects the ion hydration structure and shell properties at ambient temperature
Full text linkThis work deals with the effect of increasing pressure at 298.15 K on the structure and hydration shell properties of ions in infinitely diluted solutions. Results were obtained from NPT Monte Carlo simulations at various pressures, from 1 atm up to 8000 atm, for some alkali metal, alkaline earth and halide ions in TIP4P water. As pressure increases, the ion-O and the ion-H radial distribution functions (rdfs) are subjected to changes to differing extents depending on both the charge and the size of the ion. The first peak of the ion-O rdf is shifted to a shorter distance from the ion for Cs+, Br− and I−, while in the other cases there is no evidence of shortening. In contrast, for the alkaline earth ions, the most prominent effect is the decrease in the height of the first peak. Minima positions of the ion-O rdfs were used to define the first and the second hydration shells at a given pressure. Water dipole orientation with respect to the radial direction was examined, showing that at higher pressures the ion-dipole interaction becomes less attractive than at 1 atm. A much less favorable orientation was found for waters in the first shell of halide ions. Shell properties were computed from definite integrals of the ion-O rdfs, such as the coordination number and the shell contribution to the excess volume. For the first hydration shell, apart from Ca2+, there is a significant increase in the coordination number upon increasing pressure. This effect becomes more important the larger the ion size is and this is very significant for alkali metal and halide ions. In the case of I− the gap observed between 4 katm and 5 katm reflects the striking effect of increasing pressure on the shell definition. The coordination number remains almost constant when using an alternative boundary for the shell. This was suggested by the radial distribution of water-dipole orientations. Shell excess volume contributions are discussed by examining their dependence on pressure. Electrostriction is shown for the first shell, while the second shell's contribution to the excess volume is positive. At a higher pressure, the shell electrostrictive volume per water molecule is always less than at 1 atm. The greatest effect is shown for the first shell of alkaline earth. The effect of a different shell boundary is examined on the shell quantities of halide ions
Ionic hydration at ambient and higher pressures: Computed chemical potentials from simulations and finite-size effects
No full textFor the chemical potential of a hydrated mono-atomic ion,
finite- size effects on simulation results obtained using molecular potential truncation are investigated. Free energy perturbation (FEP) calculations
were carried out by scaling in two processes the Lennard-Jones (LJ) ion-water parameters and the ion-charge (q) for Br-, K+ and Ca2+ interacting with TIP4P water. Corrections which scale with q2 enable us to reduce finite-size effects.
However, at ambient conditions, discrepancies which depend on q are shown by the corrected values when comparison is made with the experimental data
of the Marcus compilation. Similar behavior was observed
by extrapolating the original FEP results to an infinitely large system.
Hence, these errors were assumed to depend on water density and corrected at high pressures. Consistency, within statistical uncertainties, is shown when comparing with results derived from computed volumetric quantities.
Results are also compared with those derived from experimental values of excess volumes at ambient conditions
DSCC Converter as Energy Router for Rural Energy Communities: a Case Study Under Vertical Imbalance
No full textThe sustainable electrification of rural communities relies on the effective utilization and integration of renewable energy sources, which are generally connected to the grid using multiple independent converters. In contrast, the adoption of a single centralized multiport converter acting as energy router can help to reduce operation costs, to increase power density, and to simplify energy management and maintenance, offering a more efficient solution compared to multiple converters. This paper investigates the use of modular multilevel converters acting as a multiport energy router to connect various energy sources to the grid. In this scenario, a vertical imbalance in the arm voltages of the converter, due to the different operation patterns of the connected sources, can harm the stability of the converter. The system's internal dynamics can be leveraged as a control mechanism to ensure system functionality and to prevent unequal stress on semiconductor devices within the individual cells. A case study involving vertical imbalance is analyzed in this paper to assess the converter's potential to improve the reliability and efficiency of renewable energy integration in rural electrification projects
Hydrophilic Versus Hydrophobic Coupling in the Pressure Dependence of the Chemical Potential of Alkali Metal and Halide Ions in Water
No full text[Image: see text] We computed the chemical potential for some alkali metal ions (K(+), Rb(+), and Cs(+)) and two halide ions (Br(–) and I(–)) in aqueous solution at ambient T and various pressures in the range 1–8000 atm. Results were obtained from classic Monte Carlo simulations in the NPT ensemble by means of the free energy perturbation method. Here, the chemical potential is computed as the sum of a term relative to a Lennard-Jones solute and a term relative to the process in which this solute is transformed into the ion. Hydrophobic and hydrophilic features of these two components of the chemical potential show opposite behaviors under isothermal compression. The increase in pressure determines an increase in the hydrophobic component, which becomes more positive with a stronger effect for larger ions. Correspondingly, the values of the hydrophilic component become more negative for alkali ions, whereas they are only slightly affected by compression for halide ions. Hydrophobic–hydrophilic quasi-compensation in the slopes is observed for Rb(+). For a smaller ion, such as K(+), the dependence on pressure of the hydrophilic component is slightly dominant. For a larger ion, as observed in the cases of Cs(+), Br(–), and I(–), the hydrophobic component assumes the determinant role. Pressure dependence of the chemical potential is little affected by corrections introduced for molecular potential truncation. This view can change for possible boundary artifacts that could have affected the static electrostatic potential. Some inference is obtained from comparison with experimental data at 1 atm on the free energy of hydration. Discrepancies show the characteristic asymmetry between cations and anions. The further addition of a correction based on the static potential significantly reduces these discrepancies with important error cancellation on the sum of chemical potentials of ions of opposite charge. The correction is applied also at higher pressures, and results are compared with those obtained by adding an alternative correction that is based on the water number density. Regardless of the ion, changes of the chemical potential induced by an increase in pressure appear to be dominated by the hydrophobic component, in particular when using the alternative correction. For bromide and iodide electrolytes, the two corrections give chemical potentials in good agreement
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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