1,721,083 research outputs found
Highly recyclable surfactant-based supramolecular eutectogels for iodine removal
Full text linkIn this work, we screened several surfactants for their ability to harden Eutectic Mixtures. We found cetyldiethanolamine-N-oxide as the best candidate, which was able to form eutectogel in four eutectic mixtures. Characterization of the gels obtained in terms of thermal stability, rheology and gelation kinetics, showed a prominent influence of the nature of the hydrogen bonding accepting and donating components for the first two parameter and the other one, respectively. Then, we assessed the eutectogels for their ability to adsorb I2, as a model for its radioactive nuclides, from apolar solvent solutions. The best performing gel allowed fast and thorough removal of I2, over a broad range of initial concentrations. This gel displayed a maximum adsorption capacity of 280 mg/g, competitive with most gel-based sorbent reported in the literature for the adsorption of iodine. Interestingly, the eutectogel proved to be highly recyclable, it could be regenerated and reused without loss in performance
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
Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate as kinetic probe for piperazinium-based cationic micelles
No full textA new class of cationic surfactants containing the heterocyclic piperazinium ring in their covalent structure was prepared; cetyldialkylpiperazinium
halides, CRPX, with alkyl = Me (CMPX), Et (CEPX), n-Pr (CPPX), and with halide = Br and Cl. They were characterized by measures of critical micellar concentration, cmc, and ionization degree, α, and also by use of the decarboxylation of 6-nitrobenzisoxazole-3-
carboxylate as a kinetic probe to investigate the properties of the microinterface they provide in aqueous solutions. The pseudophase kinetic treatment fails to fit the data at high [surfactant], which show anomalies with abrupt increase in kobs, especially for CEPX and CPPX. Data can be fitted up to [surfactant] ca. 0.1 M, and 0.2 M in some cases. Compared with cetyltrialkylammonium halides, values of k'M indicate a water-richer microenvironment and less important interface property changes with increasing head group bulk. The reaction could be studied both in the presence and in the absence of NaOH; comparison shows that NaOH affects only the shape of the kinetic profile at low [surfactant], without affecting the microenvironment provided by mature micelles
Alfa-Chymotrypsin Superactivity in Cetyltrialkylammonium Bromide Rich Media
No full texta-Chymotrypsin (a-CT) activity was tested with N-glutaryl-L-phenylalanine p-nitroanilide in buffered media with added cationic surfactants. The effect of the commercial cetyltrimethylammonium bromide (CTABr) was compared with that of three other surfactants with ethyl (CTEABr), propyl (CTPABr), and butyl (CTBABr) head groups. These were synthesized and purified in this laboratory. Surfactant head groups provided distinct environments that largely modulated the catalytic performance. Larger alkyl head group hydrophobicity led to a marked enhancement of ?-CT activity. CTBABr-rich media induced the highest superactivity. Kinetic measurements were performed in Tris-HCl buffer at a surfactant concentration either below or above CMC, and ?-CT superactivity occurred in both media. Positive interactions between the enzyme and surfactants happened independently of the supramolecular organization of the medium. The reaction followed the Michaelis-Menten kinetics. The substrate to micelle aggregates binding constant was used to calculate the substrate concentration available for catalysis. The k(cat) to K(m) ratio was in CTBABr-rich media always higher than in pure buffer and depended on the surfactant concentration. ?-CT superactivity depended on the pH value of buffer solution. Enzyme inactivation followed a single-step mechanism in pure buffer and a series mechanism in the presence of a surfactant. The rate of activity decay obeyed a first-order kinetics
Effect of conditioners upon the thermodynamics and kinetics of methane hydrate formation. A preliminary structure-properties relationship study
No full textThe synthesis and stability of gas hydrates was found to be heavily affected by the presence of small quantities of additives, or conditioners, particularly surfactants. In a recent work, we showed that the enhancement of hydrate formation, both from previously described and newly synthesized surfactants, is probably due to surfactant monomers, rather than micelles, and that the features of hydrate induction time should not be used as a measure of critical micelle concentration. In the present paper, we discuss the results of a structure-properties relationship study in which a preliminary attempt to relate the structural features of several amphiphilic additives to some kinetic and thermodynamic parameters of methane hydrate formation - e.g., induction times, rate of formation, occupancy, etc. - is conducted. According to the present study, it is found that, for a particular conditioner, a reduction of induction time does not correlate to an increase of the formation rate and occupancy, and vice versa. This may be related to the nature of chemical moieties forming a particular amphiphile (e.g., the hydrophobic tail, head group, counterion, etc.). The understanding of the mechanisms by which those moieties play their differential role may be the key tool to the design and synthesis of tailored conditioners
SN2 displacement by bromide ions in dichloromethane. The role of reverse micelles
No full textReverse micellar systems are of interest as reaction media
because they are powerful models for biological compartmentalization,
enzymatic catalysis and separation of biomolecules.
Solutions of ionic surfactants in apolar solvents may contain reverse micelles, but they may also contain ion pairs, or small clusters, with waters of hydration. We studied the
bimolecular reaction in CH2Cl2 solutions of cationic tetraalkylammonium
bromide salts (onium salts), such as cetyltrimethylammonium
bromide (CTABr), cetyltripropylammonium
bromide (CTPABr) and tetra-n-butylammonium bromide
(TBABr). Methylnaphthalene-2-sulfonate (β-MeONs),
its 6-sulfonate derivative (β-MeONsS–) as the 2,6-lutidinium salt and methyl-5-N,N,N,trimethylammonium naphthalene-1-sulfonate (α-MeONsNT+) as the trifluoromethanesulfonate
salt react with Br– in CH2Cl2. First-order rate constants, kobs, increase linearly and similarly for the three substrates with
increasing concentrations of the onium salts. Reactions are
faster with TBABr than they are with CTPABr and CTABr,
and the reactivity of the three substrates is in the order: α-
MeONsNT+ >> β-MeONsS– > β-MeONs. The reactions are
inhibited by the addition of H2O, but CTABr tolerates H2O
in large excess. At [H2O]/[CTABr] = w0 = 6, “water-pool” reverse micelles form, and kobs for all three substrates is then independent of w0
Method for the production of binary clathrate hydrates of hydrogen
No full textA method for the effective production of binary clathrate hydrates of hydrogen and a co-former is provided, said method comprising the formation of water-in-oil nanoemulsions, such as, e.g., surfactant reverse micelles in an organic solvent, and the formation of clathrate hydrate nanocrystals from the water droplets thus obtained
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