1,720,980 research outputs found
Speciation of dimethyltin(IV) – and trimethyltin(IV) –carbocysteinate and – glutamate systems in aqueous media
No full textThe formation of complex species in the dimethyltin(IV) and trimethyltin(IV)-carboxymethyl-L-cysteinate (carbocysteinate) systems in NaClaq, at different ionic strengths, and in a multicomponent Na+, K+,
Ca2+, Mg2+, Cl- and SO42- medium representative of the seawater major composition, is discussed. Experimental results give evidence for the formation of the following species (L1⁄4carbocysteinate):
[(CH3)2Sn(L)]0, [(CH3)2Sn(HL)]+, [(CH3)2Sn(OH)(L)]-, [(CH3)2Sn(OH)2(L)]2- in the DMT–CCYS system, and [(CH3)3Sn(HL)]0, [(CH3)3Sn(L)]- and [(CH3)3Sn(OH)(L)]2- in the TMT-CCYS system. The ionic strength dependence of formation constants was taken into account by an extended Debye Huckel type equation and by the SIT (Specific ion Interaction Theory). Measurements were carried out also on the dimethyltin(IV)-glutamate and trimethyltin(IV)-glutamate systems in NaClaq, owing the strict similarity of glutamate and carbocysteinate. Results obtained show the formation of complex species having the same stoichiometry as those formed in the DMT- and TMT-carbocysteinate systems, with very
similar stability, confirming that carbocysteinate behaves as a dicarboxylic amino acid without involving the sulfur-bridge potential binding site in metal coordination
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
THERMODYNAMIC STUDY FOR THE PROTONATION OF HALLOYSITE
Full text linkThe Halloysite (Al2Si2O5(OH)4·2H2O) is an abundant and cheap clay and is
considered one of the most promising nano structured and naturally
occurring clay mineral. Large deposits of this material are present in France,
Belgium, China, New Zealand and USA [1,2]. Among the spheroidal,
tubular or platy morphologies, the tubular is the most common and abundant
one. Typically, halloysite nanotubes (HNTs) are formed by 15 – 20
aluminosilicate layers, having a length of 1 ± 0.5 μm, and inner and outer
diameters of 10 – 15 nm and 50 -70 nm, respectively [1]. In each layer, the
SiOH and the AlOH groups are disposed on the external and the internal
surfaces, respectively. As consequence, in each nanotube and in a large pH
range, there is a charge separation between the inner and the outer surfaces.
In particular, the inner surface is positively charged because of protonation
of AlOH groups and the outer surface is negatively charged because of the
gradual deprotonation of SiOH groups at pH higher than 2.
This charge separation is one of the most important features for the HNTs
versatility and is strictly dependent on the acid – base properties of their
functional groups. In this work, the protonation / deprotonation of SiOH and
AlOH groups of HNTs is evaluated by ISE-H+ potentiometric titrations of
HNTs aqueous suspensions in different ionic media and ionic strengths
(0.025 ≤ I / mol L-1 ≤ 0.750) and in a wide pH range (2 – 11), at T =
25°C.ISE-H+ potentiometric titration data have been processed by using the
same models successfully used in a previous work for carboxylic and phenol
groups of humic and fulvic substances [3]. Moreover, the change of
superficial charge with the changing of pH was also evaluated by ζ potential
measurements.Correlations between the ζ potential trend and the
distribution of protonated and deprotonated species of HNTs were found
MODELLING OF PROTONATION CONSTANTS OF HALLOYSITE CLAY NANOTUBES IN VARIOUS AQUEOUS MEDIA, AT DIFFERENT IONIC STRENGTHS
No full textIn the last decade nanoparticles have assumed more and more importance because of their particular properties mainly due to the nanometer-scale dimensions that confer them a large surface/volume ratio. Among nanomaterials one of the most studied is the halloysite that, as well as the other natural clay minerals is safe for human and environmental friendly. Halloysite is abundant and cheap and is present in large deposits worldwide like those in New Zealand, France, Belgium and China [1,2]. It is similar to kaolin but has a hollow tubular structure that can be attributable to particular crystallization conditions. Typically, halloysite nanotubes (HNTs) are formed by 15 – 20 aluminosilicate layers, has a length of 1 ± 0.5 μm and inner and outer diameters of ~15 and 50 -70 nm, respectively [1,2]. In each layer the SiOH and the AlOH groups are disposed on the external and the internal surfaces, respectively. As consequence, the chemistry of the lumen and of the outer surface of HNTs is completely different. In particular, in each nanotube the inner surface is positively charged and the outer surface has an excess of negative charges in a wide pH range. The particular structure of HNTs makes this kind of clay mineral very useful for different purposes and several papers and
reviews have been published on their different applications [1-3]. The behavior of HNTs in all these applications is strictly related to their acid-base properties that here have been studied by ISE-H+ potentiometric titrations in several interacting and non interacting ionic media, in the range 0.025 ≤ I mol L-1 ≤ 1.000, at T = 25°C. Two functional groups indicated with HNT1 (SiOH) and HNT2 (AlOH) were considered in the analysis of the experimental data. Potentiometric data related to the HNT1 groups were processed with four different models previously used for the study of the acid-base properties of natural and synthetic polyelectrolytes: Högfeldt, Linear, modified Henderson-Hasselbalch and diprotic like models [4]. The acid – base behavior of HNT2 groups were well defined by one protonation constant calculated by processing the potentiometric titration data with the computer programs STACO and BSTAC
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
Polycarboxylic acids in sea water: acid–base properties, solubilities, activity coefficients, and complex formation constants at different salinities
No full textThis paper reports the results of the investigations carried out in synthetic sea water at different salinities for different classes of polycarboxylic acids. The investigations can be summarized as follows: (a) Determination of the protonation constants in such multicomponent solution in a salinity range 15 ≤ S ≤ 45, at t = 25 °C, for the linear dicarboxylic acids HOOC-(CH2)n–COOH (0 ≤ n ≤ 8), and aromatic polycarboxylic acids (o-phthalic and 1,2,4-benzenetricarboxylic acids). For malonic, succinic, 1,2,3-benzenetricarboxylic, and 1,2,3,4-benzenetetracarboxylic acids, investigations were also carried out at t = 10 and 37 °C; (b) Determination of the total and intrinsic solubility (ST and S0, respectively) of the linear dicarboxylic acids HOOC-(CH2)n-COOH (0 ≤ n ≤ 8), o-phthalic, 1,2,4-benzenetricarboxylic acids at t = 25 °C and 15 ≤ S ≤ 45, and calculation of the corresponding Setschenow parameters and activity coefficients; (c) Modeling the dependence of the experimental and literature protonation constants of the polycarboxylic acids on salinity, acid concentration, temperature, and number of the methylene groups in the molecules by means of new empirical equations; (d) Determination of the specific interaction parameters in synthetic sea water of the ionic species of the acids by means of the specific ion interaction theory and Pitzer models; (e) Determination of the protonation constant of the anion A1.117− of the single salt BA at different salinities and temperatures; (f) Determination and modeling in dependence of the salinity of the ΔH/kJ mol−1 of protonation of the linear dicarboxylic acids and of the A1.117− anion, by means of a Debye-Hückel type equation; (g) Determination of the complex formation constants (log βBpLHi) between the cation B1.117+ and the different deprotonated species of the carboxylic acids at different salinities and temperatures. Independent of the thermodynamic aqueous properties determined, a significant dependence of these parameters (log βiH, log βBpLHi, ΔH/kJ mol−1 of protonation, ST and S0) on the ionic medium, salinity, and temperature was observed. Moreover, the huge number of data collected allowed us to propose some empirical equations to model/predict the behavior of these classes of O-donor ligands in a multicomponent solution such as synthetic sea water. Graphical abstract: [Figure not available: see fulltext.
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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