• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 110
  • 86
  • 22
  • 8
  • 7
  • 7
  • 4
  • 4
  • 2
  • 1
  • Tagged with
  • 272
  • 45
  • 36
  • 33
  • 32
  • 28
  • 27
  • 26
  • 25
  • 23
  • 23
  • 20
  • 20
  • 19
  • 19
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Mechanism of the dissolution of acrylonitrile based polymers in aqueous inorganic salt solutions

Hoskins, Alan Richard January 1992 (has links)
No description available.
2

Neutral and Cationic Main Group Lewis Acids - Synthesis, Anion Complexation and Redox Properties

Dorsey, Christopher L. 2009 May 1900 (has links)
The primary goal of this research concerns the synthesis and characterization of hybrid main group Lewis acids. Initially, the focus of this work was on the synthesis of derivatives possessing unusual bonding interactions enforced by a rigid 1,8- naphthalenediyl scaffold. After discovering a route to a new dilithio reagent, silicon based derivatives featuring R3Si-F->CR3 + and R3C-H->SiFR3 interactions of 2.703(2) and 2.32(2) Angstrom respectively were successfully synthesized and fully characterized. Another hybrid Lewis acid based on the 1,8-naphthalenediyl scaffold that was studied was a trinuclear B2/Hg Lewis acid. This molecule has been shown to bind two fluoride anions sequentially, and the binding events can be followed by differential pulsed votammetry. The final part of this work concerns the reactivity and redox behavior of main group systems. It has been shown that the p-phenylene linker in 4-dimesitylboryl-1- diarylmethylium benzenes effectively reduces electrochemical communication between the carbocation and borane moieties when compared to systems without the linker. Reduction of these species produces a derivative whose EPR signal is only slightly influenced by the ^11 B center. These findings have been further substantiated by theoretical calculations. Finally, the redox properties of alpha-phosphonio- and alpha- phosphonyl-carbocations have been studied. Chemical reduction of both species leads to a predominately carbon centered radical with coupling to the ^31P center of 18 and 19.7 G respectively. The alpha-phosphonio carbocations, however, also undergo ligand exchange reactions with pyridine derivatives suggesting that these species can also be referred to as ligand stabilized carbodications.
3

Synthèse de polymères porteurs de sites complexants associés, au sein du squelette, à des segments de propriétés différentes

Doan, Thi Hong Nhung Soutif, Jean-Claude January 2006 (has links) (PDF)
Reproduction de : Thèse de doctorat : Chimie et physico-chimie des polymères : Le Mans : 2006. Reproduction de : Thèse de doctorat : Chimie et physico-chimie des polymères : eHô-Chi-Minh-Ville : 2006. / Thèse soutenue en co-tutelle. Titre provenant de l'écran-titre. Bibliogr. p. 142-149.
4

Synthèse de polymères dendronisés pour la reconnaissance ionique et à propriétés thermosensibles

Roeser, Jérôme Arnaud, Françoise. Méry, Stéphane. January 2009 (has links) (PDF)
Thèse de doctorat : Chimie : Strasbourg 1 : 2008. / Titre provenant de l'écran-titre.
5

Cyclic ligands based on bis-ureidopyridines

Yan, Zegui January 1997 (has links)
No description available.
6

Aspects thermodynamiques et cinétiques de la complexation de cations métalliques (Cu²+ et Ni²+) par la 5-phénylazo-8-hydroxyquinoline (5Ph8HQ) et le cyclame greffés sur des nanoparticules de silice en suspension colloïdale / Thermodynamic and kinetic aspects of metal cations (Cu2+ and Ni2+) complexation by 5-phenylazo-8-hydroxyquinoline (5Ph8HQ) and cyclame grafted onto silica nanoparticles in colloidal suspension

Rose-Hélène, Maureen 11 October 2011 (has links)
Nous avons considéré autant les aspects thermodynamiques que cinétiques de la complexation d'ions métalliques (Cu2+ et Ni2+) par la 5-phénylazo-8-hydroxyquinoline (5Ph8HQ) et le cyclame greffés sur des nanoparticules de silice en dispersion colloïdale. Les silices pyrogénées, caractérisées par des surfaces spécifiques respectives d'environ 200 et 390 m2/g, ont été sélectionnées avec l'objectif d'obtenir des suspensions colloïdales stables. Nous avons démontré l'aptitude des colloïdes obtenus à extraire des cations divalents à l'état de traces (de l'ordre du micromolaire). Nous avons eu recours à l'ultrafiltration pour séparer la phase silicique dispersée de la phase aqueuse. Nous avons également montré l'intérêt de remplacer une pseudophase micellaire solubilisant l'extractant par nos phases solides. Nous avons décrit les cinétiques de complexation en utilisant la technique de la spectrophotométrie à écoulement bloqué pour les réactions les plus rapides. La dépendance de la constante de vitesse observée vis-à-vis de la concentration en cation métallique, du contre-ion considéré (acétate ou chlorure), de la force ionique et du pH a été évaluée. Pour décrire la cinétique de réaction du Ni2+ avec la 5Ph8HQ greffé, il nous a fallu prendre en compte les propriétés spectrophotométriques du complexe sur nanoparticules de silice. C'est un comportement original de la 5Ph8HQ sur silice puisque les densités optiques de solutions contenant la 5Ph8HQ en milieu micellaire ou le cyclame greffé sur silice suivent la loi de Beer-Lambert. L'étude du cyclame greffé qu'il s'agisse des cinétiques ou à l'équilibre est compliquée en raison du relargage de ce dernier en solution. / We considered both the thermodynamic and kinetic aspects of metal ions (Cu2+ and Ni2+) complexation by 5-phenylazo-8-hydroxyquinoline (5Ph8HQ) and cyclam grafted onto silica nanoparticles in colloidal dispersion. Fumed silicas with specific areas of respectively 200 and 390 m2/g were selected to get stable colloidal suspensions. We demonstrated the ability of these colloids to extract trace elements (at micromolar level). We used the ultrafiltration process to separate the dispersed silica phase from the aqueous phase. We also showed interest of our solid dispersed phases instead of a pseudo micellar one. We described complexation kinetics by stopped flow technique for the fastest reactions. The influence of metal cation concentration, counter-ion nature (acetate or chloride), ionic strength and pH on observed rate constant has been investigated. To describe the Ni2+ kinetics with grafted 5Ph8HQ, we took into account the spectrophotometric properties of the complex on silica nanoparticles. This is an original behaviour of 5Ph8HQ on silica since the optical densities of solutions containing 5Ph8HQ solubilised in micelles or cyclam grafted onto silica follow the Beer-Lambert's law. The study of grafted cyclam whether the kinetics or equilibrium is complicated due to the release of the latter in solution.
7

Complexation d'actinides et d'analogues par des ligands hydroxamates / Complexation of actinides and analogues with hydroxamate ligands

He, Mingjian 14 November 2019 (has links)
L’augmentation des activités humaines dans le domaine nucléaire civil comme militaire est à l’origine d’une dissémination effective ou potentielle de radionucléides dans l’environnement. Leur mobilité dépend de plusieurs facteurs tels que le pH, la force ionique, le degré d’oxydation et la présence de ligands organiques. Afin de garantir la sûreté de sites d’entreposage ou de stockage, de développer des procédés de remédiation de sols contaminés, il est nécessaire de disposer de données fondamentales sur les interactions entre actinides et ligands organiques susceptibles d’être présents dans l’environnement, vecteurs de migration ou de piégeage de ces radioéléments. Dans ce contexte, l’étude s’est axée sur les interactions entre Th(IV), U(IV,VI), Cf(III) and Eu(III) et des dérivés hydroxamates, en particulier, la desferrioxamine B, un sidérophore bactérien comportant trois fonctions hydroxamiques. Les constantes de formation des complexes sont déterminées en fonction de l’acidité et de la concentration de ligand à force ionique et température fixées en mettant en jeu plusieurs techniques expérimentales et des concentrations en élément comprises entre 10⁻¹⁰ et 10⁻³ M (extraction liquide-liquide combinée à une détection par spectrométrie γ, électrophorèse capillaire avec détection UV, spectrophotométrie d’absorption UV-visible). L’approche thermodynamique de cette étude est complétée par une étude structurale à l’aide de techniques spectroscopiques telles que la spectroscopie infrarouge à transformée de Fourier et de la spectroscopie d’absorption des rayons X. Ces mesures expérimentales sont alors confrontées à des calculs théoriques (DFT), afin de déterminer l’arrangement structural du métal et les distances interatomiques. / Due to the increasing human activities in the civilian nuclear fields, the actual and potential release of radionuclides into the environment is a matter of concern. The mobility of radio-nuclides depends on several factors such as pH, ionic strength, oxidation state and the presence of organic ligands. In order to guarantee the safety of radioactive waste storage sites and to develop contaminated soil remediation processes, it is necessary to have fundamental data on actinides and natural organic ligands interactions. This study focuses on the interaction between Th(IV), U(IV,VI), Cf(III) and Eu(III) and hydroxamates derivatives, desferrioxamine B, a bacterial siderophore with three hydroxamic functions. The stability constants of complexes are determined as function of acidity and ligand concentration, at fixed ionic strength and temperature, using several techniques and metal concentrations ranging from 10⁻¹⁰ to 10⁻³ M (liquid-liquid extraction coupled with γ-spectrometry, capillary electrophoresis with UV detection, UV-Vis absorption spectrophotometry). The thermodynamic study is supplemented by a structural one using spectroscopic techniques such as Fourier transform infrared spectroscopy and X-ray absorption spectroscopy. Experimental measurements are compared with quantum chemistry calculations (DFT) in order to determine the coordination geometry of the metal ion and the interatomic distances.
8

Removal of nitrogen compounds from bitumen-derived gas oil and its impact on hydrotreating

ParraGalvis, Lina R Unknown Date
No description available.
9

Dissolved organic carbon in aquitard environments : properties, complexation, and transport

Reszat, Thorsten 22 May 2007
Clay-rich glacial till aquitards are widespread throughout the northern hemisphere. Due to their low hydraulic conductivity, these geologic units are commonly used to contain wastes. Dissolved Organic Carbon (DOC) in natural environments influences the speciation and mobility of contaminants, such as heavy metals and radionuclides, and is present in high concentrations in clay-rich tills (5 to 150 mg l-1). Detailed knowledge of the influence of DOC on the long-term stability, speciation, and mobility of elements is lacking. Studies in this thesis characterize the properties and function of DOC with respect to element speciation and transport at the King research site, an archetypal clay-till aquitard in Saskatchewan, Canada. Characterization of DOC using Asymmetrical Flow-Field Flow Fractionation (AsFlFFF) with on-line UV and DOC detection demonstrated the molecular weight (Mw) of DOC within the aquitard environment is low, ranging from 1160 to 1286 daltons (Da), and the relative amount of aromatic carbon in aquitard DOC is lower than in surface water DOC. These findings imply the complexation ability of DOC in aquitards is lower than surface water DOC. DOC aromaticity decreased with depth in the aquitard, while Mw remained constant. DOC Mw in other aquitards investigated was comparably low (1470-1630 Da). Coupling AsFlFFF with on-line ICP-MS analysis allowed the identification of Fe, U and Zn associated with the DOC, and demonstrated that <4% of total aqueous elements tested at the King site were complexed with DOC. Experimental and numerical modeling results demonstrate the low masses of metals (Cu, Mn, Mo, Ni, Sr, U and Zn) complexed with the DOC can be attributed to competitive complexation with carbonate and sulphate ligands naturally present in the pore water. The in-situ association constant, Kd, for U and Zn as determined by complexation experiments decreased with depth in the aquitard and was attributed to the corresponding decrease in DOC aromaticity. The maximum mass of complexation of U and Zn to DOC takes place at pH 3-6, and decreases above and below this pH range (range tested: pH 1.3-10). These results were supported by geochemical modeling and suggest the complexation of aqueous metals by DOC should be limited in ground water environments of similar chemistries. Laboratory transport studies using double reservoir diffusion cells showed DOC and other similarly sized colloidal material can diffuse through the matrix at the King site aquitard. The effective pore throat diameter in the matrix media was determined to be 2 - 2.2 nm and travel times of colloids increased with increasing colloidal diameter. Colloids >2 nm were prevented from movement in the till by sieving mechanisms, which suggests bacteria and viruses as well as larger colloids should not migrate through till aquitards. Due to preferential sieving of larger Mw DOC, DOC in aquitard environments is typified by a small diameter compared with surface water DOC, and a small range in hydrodynamic diameter and Mw.
10

Dissolved organic carbon in aquitard environments : properties, complexation, and transport

Reszat, Thorsten 22 May 2007 (has links)
Clay-rich glacial till aquitards are widespread throughout the northern hemisphere. Due to their low hydraulic conductivity, these geologic units are commonly used to contain wastes. Dissolved Organic Carbon (DOC) in natural environments influences the speciation and mobility of contaminants, such as heavy metals and radionuclides, and is present in high concentrations in clay-rich tills (5 to 150 mg l-1). Detailed knowledge of the influence of DOC on the long-term stability, speciation, and mobility of elements is lacking. Studies in this thesis characterize the properties and function of DOC with respect to element speciation and transport at the King research site, an archetypal clay-till aquitard in Saskatchewan, Canada. Characterization of DOC using Asymmetrical Flow-Field Flow Fractionation (AsFlFFF) with on-line UV and DOC detection demonstrated the molecular weight (Mw) of DOC within the aquitard environment is low, ranging from 1160 to 1286 daltons (Da), and the relative amount of aromatic carbon in aquitard DOC is lower than in surface water DOC. These findings imply the complexation ability of DOC in aquitards is lower than surface water DOC. DOC aromaticity decreased with depth in the aquitard, while Mw remained constant. DOC Mw in other aquitards investigated was comparably low (1470-1630 Da). Coupling AsFlFFF with on-line ICP-MS analysis allowed the identification of Fe, U and Zn associated with the DOC, and demonstrated that <4% of total aqueous elements tested at the King site were complexed with DOC. Experimental and numerical modeling results demonstrate the low masses of metals (Cu, Mn, Mo, Ni, Sr, U and Zn) complexed with the DOC can be attributed to competitive complexation with carbonate and sulphate ligands naturally present in the pore water. The in-situ association constant, Kd, for U and Zn as determined by complexation experiments decreased with depth in the aquitard and was attributed to the corresponding decrease in DOC aromaticity. The maximum mass of complexation of U and Zn to DOC takes place at pH 3-6, and decreases above and below this pH range (range tested: pH 1.3-10). These results were supported by geochemical modeling and suggest the complexation of aqueous metals by DOC should be limited in ground water environments of similar chemistries. Laboratory transport studies using double reservoir diffusion cells showed DOC and other similarly sized colloidal material can diffuse through the matrix at the King site aquitard. The effective pore throat diameter in the matrix media was determined to be 2 - 2.2 nm and travel times of colloids increased with increasing colloidal diameter. Colloids >2 nm were prevented from movement in the till by sieving mechanisms, which suggests bacteria and viruses as well as larger colloids should not migrate through till aquitards. Due to preferential sieving of larger Mw DOC, DOC in aquitard environments is typified by a small diameter compared with surface water DOC, and a small range in hydrodynamic diameter and Mw.

Page generated in 0.1026 seconds