Equilibrium and kinetic data for the solvent extraction of germanium by three impure commercial 7-alkylated-8hydroxyquinoline extractants which vary in structure at the 7alkyl group, are evaluated in order to elucidate an holistic kinetic extraction model which accounts for the various reactions and partition effects occurring during the metal-ion chelation process. It is proposed that for the extraction process, which is first order in germanium concentration, by the ligand reagents Lix 26, TN 01787 and TN 02181, the rate-determining step, on stereochemical grounds, is the attachment of either a neutral ligand or a protonated ligand species-to the biligand intermediate GeL2 2+ (L:ligand) at the interface. In high speed shaking/mixing assemblies the extraction process was observed to occur in two discrete reaction regimes : a fast initial rate for which the orders with respect to ligand reagent are 1,06, 2,10 and 1,77 for TN 02181, Lix 26 and TN 0178-7 respectively, and a slower subsequent rate for which the apparent reaction orders with respect to ligand concentration are 1,12, 2,70 and 3,08 for TN 02181, Lix 26 and TN 01787 respectively. For the slower reaction regime, orders between 1 and 3 are explicable if the steady state approximation is invoked for the intermediate germanium species GeL3+ and GeL2 2+ formed at the interface. In the fast reaction regime, it is proposed that the accelerated extraction rates are a function of (i) the speciation of germanium and (ii) participation in the rate-determining step by the protonated ligand moiety H2L+HS04- which is rapidly formed after phase contact. At low ligand concentration, the following order of ligand efficacy has been observed : TN 01787 < Lix 26 < TN 02181 whereas at high concentration ligand efficacies are similar because the interface is saturated with ligand. Orders with respect to [H+] for the reagents vary from -1 to - 3 during the course of reaction, indicating complex mixedorder behaviour. The effects upon extraction of ionic strength, temperature, the addition of organic modifiers and diluent nature are investigated as well as the kinetics of germanium stripping by aqueous hydroxide. The physical effects of interfacial tension, viscosity and relative dielectric constant are also reported and suggestions are made as to their effect upon the extraction characteristics. Computer modelling of the extractants has been used as an aid in describing size, structure and stereochemical considerations of the ligands and the chelate products. / Thesis (Ph.D.)-University of Natal, Durban, 1990.

The models used in the prediction of activity coefficients are important tools for designing major unit operations (distillation columns, liquid-liquid extractors etc). In the petrochemical and chemical industry, well established methods such as UNIFAC and ASOG are routinely employed for the prediction of the activity coefficient. These methods are, however, reliant on binary group interaction parameters which need to be fitted to reliable experimental data. It is for this reason that these methods are often not applicable to systems which involve complex molecules. In these systems, typically solid-liquid equilibria are of interest where the solid is some pharmaceutical product or intermediate or a molecule of similar complexity (the term complex here refers to situations where molecules contain several functional groups which are either polar, hydrogen bonding, or lead to mesomeric structures in equilibrium). In many applications, due to economic and environmental considerations, a list of no more than 20 solvents is usually considered. It is for this reason that the objective of this work is to develop a method for predicting the activity coefficient of complex multifunctional compounds in some common solvents. The segment activity coefficient approaches proposed by Hansen, MOSCED and the NRTL-SAC models show that it should be possible to “interpolate” between solvents if suitable reference solvents are available (e.g. non-polar, polar and hydrogen bonding). Therefore it is useful to classify the different solvents into suitable categories inside which analogous behaviour should be observed. To accomplish this, a significant amount of data needs to be collected for the common solvents. Data with water as a solvent was freely available and multiple sources were found with suitable data. Both infinite dilution activity coefficient (y∞) and SLE (Solid-Liquid Equilibrium) data were used for model development. The y∞ data were taken from the DDB (Dortmund Data Bank) and SLE data were taken from Beilstein, Chemspider and DDB. The limiting factor for the usage of SLE data was the availability of fusion data (heat of fusion and melting temperature) for the solute. Since y∞ in water is essentially a pure component property it was modelled as such, using the experience gained previously by this group. The overall RMD percentage (in ln y∞) for the training set was 7.3 % for 630 compounds. For the test set the RMD (in ln y∞) was 9.1 % for 25 fairly complex compounds. Typically the temperature dependence of y∞ data is ignored when considering model development such as this. Nevertheless, the temperature dependence was investigated and it was found that a very simple general correlation showed moderate accuracy when predicting the temperature dependence of compounds with low solubility. Data for solvents other than water were very scarce, with insufficient data to develop a model with reasonable accuracy. A novel method is proposed for the alkane solvents, which allows the values in any alkane solvent to be converted to a value in the solvent hexane. The method relies on a first principles application of the solution of groups concept. Quite unexpectedly throughout the course of developing the method, several shortfalls were uncovered in the combinatorial expressions used by UNIFAC and mod. UNIFAC. These shortfalls were empirically accounted for and a new expression for infinite dilution activity coefficient is proposed. This expression is however not readily applicable to mixtures and therefore requires some further attention. The method allows for the extension of the data available in hexane (chosen since it is a common solvent for complex compounds). In the same way as the y∞ data in water, the y∞ data in hexane were modelled as a pure component property. The overall RMD percentage (in ln y∞) for the training set was 21.4 % for 181 compounds. For the test set the RMD (in ln y∞) was 11.7 % for 14 fairly complex compounds. The great advantage of both these methods is that, since they are treated as pure component properties, the number of model parameters grows linearly with the number of groups, unlike with mixture models (UNIFAC, ASOG, etc.) where it grows quadratically. For both the water and the hexane method the predictions of the method developed in this work were compared to the predictions of UNIFAC, mod. UNIFAC, COSMO-RS(OL) and COSMO-SAC. Since water and hexane are not the only solvents of practical interest, a method was developed to interpolate the alcohol behaviour based on the water and hexane behaviour. The ability to predict the infinite dilution activity coefficient in various solvents allowed for the prediction of various other properties, viz. air-water partition coefficient, octanol-water partition coefficient, and water-alcohol cosolvent mixtures. In most cases the predictions of these properties were good, even for the fairly complex compounds tested. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2009.

No description available.

No description available.

A partially purified chlorophyllase, obtained from alga Phaeodactylum tricornutum, was assayed for its hydrolytic activity towards the pheophytin in ternary micellar systems of hexane/Tris-HCl/surfactant. A wide range of surfactants, sorbitans (Span 20, 40, 60, 80 and 85) and polysorbates (Tween 20, 40, 60, 80 and 85), was used. The use of either 50 $ mu$M of Span 85 or 1 $ mu$M of Tween 80 increased the hydrolytic activity of chlorophyllase by 110 and 23%, respectively. The optimum values of pH, enzyme content, incubation time and temperature for the hydrolytic activity of chlorophyllase were determined as 8.25, 8 $ mu$g protein/ml, 60 min and 27.5$ sp circ$C, respectively. The enzyme was assayed for its hydrolytic activity in the most appropriate ternary system containing Span 85 with purified pheophytin, as well as chlorophyll derivatives, as substrates. Moreover, the values of $V sb{ rm max}/K sb{ rm m}$ ratio for chlorophyllase, using the partially purified pheophytin as substrate, in ternary systems with Span 85 and Tween 80 as surfactants, were 0.15 and 0.08, respectively; however, the value of $V sb{ rm max}/K sb{ rm m}$ ratio for the enzyme, in the ternary system with Span 85, using purified pheophytin as substrate was 0.07. The addition of optimized amounts of individual membrane lipids, L-$ alpha$-phosphatidylcholine, L-$ alpha$-phosphatidyl-DL-glycerol and $ beta$-carotene increased the hydrolytic activity of chlorophyllase, using partially purified pheophytin as substrate, by 50, 36 and 10%, respectively, for Span 85, and 30, 48 and 15%, respectively, for Tween 80; in addition, these lipids increased the enzyme activity by 6, 23 and 31%, respectively, in the Span 85 media, using purified pheophytin as substrate. Phytol showed a competitive inhibitory effect on chlorophyllase activity in both Span and Tween systems containing partially purified pheophytin substrate; however, phytol had an uncompetitive inhibitory effect on the enzyme activity in the S

No description available.

In recent years, ionic liquids have emerged as one of the most promising alternatives to traditional volatile organic solvents when it comes to catalytic reactions. Stable metal nanoparticles suspended in ionic liquids, are catalytic systems that mimic aspects of nanoparticles on solid supports, as well as traditional metal-ligand complexes used in organometallic catalysis. While alkylimidazolium ionic liquids, with or without appended functionalities, have been earmarked as the media of choice for the dispersal of nanoparticles, the tetraalkylphosphonium family of ionic liquids has largely been overlooked, despite their facile synthesis, commercial availability, chemical resemblance to surfactants traditionally used for nanoparticle stabilization, stability under basic conditions, and wide thermal as well as electrochemical windows. It is only recently that a number of research groups have given this family of novel alternative solvents the recognition it deserves, and used metal NPs dispersed in these ILs as catalysts in reactions such as hydrogenations, oxidations, C-C cross-couplings, hydrodeoxygenations, aminations, etc. This thesis investigates the synthesis, characterization, and catalytic applications of transition metal nanoparticles in tetraalkylphosphonium ionic liquids. The ionic liquids described in this thesis functioned as the reaction media as well as intrinsic nanoparticle stabilizers during the course of the catalytic processes. Metallic nanoparticles synthesized in these ionic liquids proved to be stable, efficient and recyclable catalytic systems for reactions of industrial significance, such as hydrodeoxygenations, hydrogenations, and oxidations. It was demonstrated that stability and catalytic activity of these systems were profoundly dependent on the properties of the ionic liquids, such as the nature of the alkyl chains attached to the phosphonium cation, and the coordination ability of the anion. Since heat-induced nanoparticle sintering was a problem, a procedure was devised to redisperse the aggregated and/or sintered nanoparticles so as to restore their initial sizes and catalytic activities. The presence of halides as counter-ions in tetraalkylphosphonium ionic liquids was seen to facilitate the oxidative degradation of agglomerated metal nanoparticles, which was a key step in our redispersion protocol. It was demonstrated that this redispersion protocol, when applied to heat-sintered nanoparticles, produces nanostructures that resemble the freshly made nanoparticles not only in size but also in catalytic activities. The presence of by-products from the borohydride reduction step used to generate the nanoparticles in the ionic liquids actually facilitated multistep reactions such as hydrodeoxygenation of phenol, where a Lewis Acid was necessary for a dehydration step. Finally, an attempt was made to utilize nanoparticles of an earth-abundant metal (iron) as a hydrogenation catalyst in a variety of alternative solvents (including tetraalkylphosphonium ionic liquids) in order to enhance the “green”ness of the catalyst systems. X-ray absorption spectroscopy (XAS) of the iron- nanoparticles/ionic liquid systems at the Canadian Light Source revealed significant details about the chemical interaction between iron and the ionic liquid matrices, which added to our understanding of this neoteric family of catalysts.

No description available.

The aim of this study was to determine in an empirical manner whether the phenomenon of hearing loss due to the exposure to volatile organic solvents was present in the gasification plants of a petrochemical factory. The experimental groups included unexposed (N=20) workers, workers (N=20) exposed to noise and workers (N=19) exposed to noise and volatile organic solvents. The following assessments were made: diagnostic audiometry for the assessment of hearing thresholds; biological monitoring of ortho-cresol and hippuric acid for the biological markers of toluene; passive air sampling and noise measurements. Questionnaires were completed by the three groups to collect information about age, ethnicity, smoking habits, alcohol consumption and work history regarding the exposure to ototoxic substances as well as the period of employment. There were no statistically significant differences between the average ages of the three groups, but more of the workers in the noise group were from a higher age group. This age composition and the work history of the noise group were considered as possible influences on the results of this hearing study. The results of the passive air sampling show that the levels of volatile organic solvents were lower than the occupational exposure limits on the day of sampling. The biological monitoring shows that the markers, namely hippuric acid and o-cresol, were lower than the biological exposure indices. The noise group was exposed to sound pressure levels of 79-90 dB(A) and the noise + hydrocarbon group to levels of 85-88 dB(A). The average hearing thresholds of the noise group were at most of the frequencies (0.5-8 kHz) significantly higher (p≤0,05) than that of the noise + hydrocarbon group. The average hearing thresholds of the control group were overall lower than the noise group's hearing thresholds, but on average a little higher than the noise + hydrocarbon group's hearing thresholds. No relationship was found in this study between the exposure to volatile organic solvents (for example toluene that was used as marker in this study) and the elevation of hearing thresholds of workers at the gasification plants of the petrochemical factory. / Thesis (M.Sc.)--North-West University, Potchefstroom Campus, 2004.

Phosphate esters and their hydrolysis reactions underpin many of the most important reactions in biology and have therefore been the focus of continued research. In this thesis, a range of theoretical studies on two phosphate ester hydrolysis reactions is presented. Particular attention is paid to the role of solvent in these reactions. The first reaction studied is the hydrolysis of dimethyl phosphate. A study using density functional theory examines three possible mechanisms for this reaction in the gas phase and the presence of solvent is also considered through the use of reaction field methods. The base-catalysed mechanism for this reaction receives further attention and the reaction energy profile in solution is adiabatically mapped using two different hybrid QM/MM potentials. The use of a QM/MM potential facilitates an atomistic representation of the solvent. Another study determines the potential of mean force for this reaction by simulating the reaction using QM/MM molecular dynamics simulations in conjunction with umbrella sampling methods. Both QM/MM studies demonstrate that, in solution, the rate-determining step for the base-catalysed reaction is the cleavage of the bond between the phosphorous and the leaving group. The QM/MM studies also established that the reaction proceeds via the formation of a short-lived pentacoordinated intermediate species. The second reaction studied was the base-catalysed hydrolysis of methyl ribose phosphate, a realistic model of the nonenzymatic hydrolysis of RNA. Since the hydrolysis of thio-substituted analogues of RNA have been the study of many experimental studies probing the mechanism of RNA hydrolysis, theoretical studies of the hydrolysis of two thio analogues of methyl ribose phosphate are also presented. The results of these studies provide valuable insight into the mechanism of RNA hydrolysis and concludes that the cleavage of P-O5' bond corresponds to the reaction transition state.