• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 226
  • 68
  • 34
  • 34
  • 10
  • 9
  • 9
  • 9
  • 9
  • 9
  • 9
  • 4
  • 3
  • 3
  • 3
  • Tagged with
  • 507
  • 87
  • 80
  • 67
  • 40
  • 37
  • 35
  • 33
  • 30
  • 27
  • 27
  • 25
  • 25
  • 25
  • 25
  • 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.

The synthesis of novel oligosaccharides using the glycosidase #alpha#-mannosidase

Smith, Nigel K. January 1997 (has links)
No description available.

The bulk and surface properties of HFC replacement refrigerants

Neilson, Martin M. January 2000 (has links)
No description available.

Time resolved infrared spectroscopy of transient species in conventional and supercritical fluid solutions

Yang, Jixin January 2003 (has links)
No description available.

Design, Synthesis and Characterization of D-glucosamine Low Molecular Weight Gelators

Parikh, Bhargiv 14 May 2010 (has links)
Low molecular weight gelators (LMWGs) have gained much attention over the last few decades, because of their ability to form supramolecular architectures as well as their many potential applications in biomedical research and as advanced materials. Most of the gelators were discovered through serendipity, and their structural requirements are somewhat ambiguous. This is due, in part, to the fact that the supramolecular gelation phenomenon is not yet fully understood, though many structural classes have been found to be excellent organogelators. Carbohydrates are abundant natural resources that are useful in preparing advanced materials. We have previously showed that monosaccharide derivatives can form effective low molecular weight gelators for both organic solvents and aqueous mixtures. In this research, we have studied the gelation capability of several glucosamine derivatives. Several series of 4,6-O-acetal protected glucosamine derivatives were synthesized and screened for their gelation properties in several solvents.

Carbothermic reduction of alumina into a metallic solvent phase

Caizergues, Derek January 1998 (has links)
A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 1998. / Experiments have been conducted at around 17000C to determine the whether carbothermic reduction of alumina is possible at these temperatures. Total pressure of the system was reduced to around 30 kPa and various metallic solvents such as copper, nickel, iron and tin were used to dissolve the metallic aluminium produced. The use of a solvent (and hence decreasing the activity of metallic aluminium) and a lower pressure are thermodynamic requirements to increase the extent of reduction under a given set of conditions. This enables the use of lower temperatures than are required under atmospheric conditions. The highest recovery of aluminium was achieved with the nickel solvent decreasing in order from iron, copper and tin. This ranking was also in accord with the extent of deviation from ideality in the respective binary solutions of these solvents with aluminiur, The nickel-aluminium system displays the largest negative deviation from ideality whereas the till, aluminium system showed a positive deviation. The rate and extent of the reduction was found to be highly dependent on temperature and pressure. The pseudo first order reaction rate was found to be the primary order for the reduction of aluminium in all the solvents used. It is also suggested that the reduction rate was controlled primarily by chemical reaction rate father than by transport processes. This is due to the extreme sensitivity of the rate and extent of the reaction to temperature. / AC2017

Supramolecular chemistry at the polymer-oil interface

Truscott, Christopher Leslie January 2018 (has links)
The movement away from metals towards polymers for automobile bearing coatings opens a new area for the possible modification of these coatings. This project, done in collaboration with Castrol, was to understand adsorption of small molecules at the surface of these polymers and work towards engineering host-guest interactions at the surface to provide binding of specific molecules. Initially, the commercial polyamide-imide was characterised via Nuclear Magnetic Resonance (NMR), Infrared (IR) and Ultra-Violet/Visible (UV-Vis) spectroscopy to understand the nature of the functional groups and structures present. This revealed that the imidisation reaction was incomplete so the functionality within the polymer could change and thus alter how molecules interact with it. This reaction was then investigated via IR and NMR spectroscopy and it was shown that the degree of imidisation varied with temperature. Then the characterised polyamide-imide was used to study the adsorption of alkylphenols, a class of molecules that have structures and functionality like common additives used in engine oils. Additionally, the adsorption of water was studied as it is a common contaminant. These two systems were studied via solution depletion isotherms and neutron reflectivity measurements. The isotherms confirmed the adsorption of the molecules whilst neutron reflection was used to characterise the layers. For alkylphenol, a rather sparsely packed layer of the molecules and solvent existed at the surface with their alkyl tails extending into the solvent. For water; the molecules diffuse into the polyamide-imide. It can be partially removed by washing the surface with dry dodecane; however, some water remains in the polymer layer. A viologen-cucurbit[8]uril binding site was chosen as the supramolecular surface interaction. The binding unit was incorporated into the polyamide-imide. The synthesis of the binding site polymer was achieved in two stages; the reproduction of the commercial polymer and the synthesis of a polymer containing 100% viologen with and without threaded cucurbit[8]uril. Whilst the synthesis appeared successful, neutron reflectivity measurements showed that, when in contact with a solution containing a second guest, no adsorption was seen on the polymer containing cucurbit[8]uril. In order to study the supramolecular interaction in non-polar solvent, a series of rotaxanes were synthesised with viologen-cucurbit[8]uril cores and bulky stopper groups to prevent unthreading of the cucurbituril as well as enhancing the solubility of the system. The two components were linked via an amidisation reaction between an acid chloride and an amine. Due to the low solubility of the products confirmation of synthesis was only possible in one case. As well as using a viologen-cucurbit[8]uril binding site, binding in cucurbiturils via halogen bonding was investigated as common halogen bonding species show good solubility in non-polar solvents. Initially co-crystals of 2,5-diiodo-1,3,4,6-tetrafluorobenzene with tertiary amides were studied. With N,N-dimethylformamide, a 1:2 co-crystal was seen but the structure with N-methyl-2-pyrrolidone had a stoichiometry of 1:1. In both structures the oxygen of the amide is involved in a halogen bonding; however, for N-methyl-2-pyrrolidone it is bifurcated leading to the formation of chains. A 50-50 mixture of pyridine and formic acid was found to solubilize both halogen bond donors and cucurbit[n]uril. Over the course of these experiments the structure of the co-crystals of two halogen bond donors, 2,5-diiodo-1,3,4,6-tetrafluorobenzene and 2,4,6-triiodo-1,3,5-trifluorobenzene, with pyridine were determined. Crystals of the cucurbituril-adducts weren’t of sufficient quality to determine the structure. The presence of binding solution was confirmed by 1H and 19F NMR experiments on the methyliodide-cucurbit[6]uril and Trans-diiodooctrafluoroazobenzene-cucurbit[8]uril systems.

Solvent and conformational effects on molecular volumes

Shahidi, Fereidoon, 1951- January 1976 (has links)
No description available.

Transport and Phase-Transfer Catalysis in Gas-Expanded Liquids

Maxey, Natalie Brimer 11 April 2006 (has links)
Gas-expanded liquids (GXL) are a new and benign class of liquid solvents that are intermediate in physical properties between normal liquids and supercritical fluids and therefore may offer advantages in separations, reactions, and advanced materials. Phase-transfer catalysis (PTC) is a powerful tool in chemistry that facilitates interaction and reaction between two or more species present in immiscible phases and offers the ability to eliminate the use of frequently expensive, environmentally undesirable, and difficult to remove polar, aprotic solvents. The work presented here seeks to further characterize the transport properties of GXLs and apply these new solvents to PTC systems, which could result in both greener chemistry and improved process economics. The transport properties of GXL are characterized by the measurement of diffusivities by the Taylor-Aris dispersion method and calculation of solvent viscosity based on those measurements. The measurement of these bulk properties is part of a larger effort to probe the effect of changes in the local structure surrounding a solute on the solution behavior. The two technologies of PTC and GXL are combined when the distribution of a phase-transfer catalyst between GXL and aqueous phases is measured and compared to changes in the kinetics of a reaction performed in the same system. The results show that increased reaction rates and more efficient catalyst recovery are possible with GXL solvents.

Organic solvents for catalysis and organic reactions

Blasucci, Vittoria Madonna 15 October 2009 (has links)
We develop, characterize, and apply novel solvent systems for enhanced separations. The field of separations has long been explored by chemical engineers. One way to optimize separations is through solvent manipulation. Through molecular design, smart solvents can be created which accomplish this task. Smart solvents undergo step or gradual changes in properties when activated by a stimulus. These property changes enable unique chemistry and separations. This thesis explores the application of two different types of smart solvents: switchable and tunable solvents. First we show that a neutral liquid can react with carbon dioxide and be switched into an ionic liquid which can then be thermally reversed back to its molecular form. Each form that the solvent takes has unique properties that can be structurally tuned to span a large range. We also look at a tunable solvent system based on polyethylene glycol/dioxane that is initially homogeneous, but induced to a heterogeneous system through carbon dioxide pressurization. Finally, we look at the advantage of using carbon dioxide as a co-solvent that is easily removed post-reaction for the grafting of silanes onto polyolefin backbones.

Cometabolic degradation of MTBE at low concentration

Liu, Catherine Yuen Yiu. January 2001 (has links)
Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Page generated in 0.0664 seconds