OpenUCT :: Browsing by Author "Linder, Peter W"

Browsing by Author "Linder, Peter W"

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Open Access
The adsorption of Ni, Cu, Zn, Cd and Pb by δ-MnO₂ and its inclusion in an equilibrium model of metal partitioning in soils
(1997) Pretorius, Patrius Julius; Linder, Peter W
Chemical equilibrium modelling provides a mechanistic tool for the prediction of metal partitioning in soils, which is important in predicting the fate and effects of metals in soil systems. In order to set up an equilibrium model of metal partitioning in soil systems, thermodynamic data for all processes influencing metal fate in soil systems are needed. In this work, the adsorption of the metals nickel, copper, zinc, cadmium and lead by δ-MnO₂ is investigated. This was done in order to provide thermodynamic data for an important adsorption phase in soils systems. Before this work, consistent adsorption data for only one adsorption phase, hydrous ferric oxide, were available. This precluded the inclusion of manganese dioxide in equilibrium models of metal fate in soil systems. Surface complexation properties of a synthetic manganese dioxide were investigated using glass electrode potentiometry. Experimental data were interpreted according to the surface complexation model in conjunction with the diffuse double layer model of the solid/solution interface. Adsorption constants were derived using the non-linear optimization program FITEQL. The surface complexation parameters determined in this fashion were validated against results obtained from the open literature. Following this, the surface complexation parameters were included in a chemical equilibrium model of soil systems. This model was used to predict the partitioning of nickel, copper, zinc, lead and cadmium in a number of soil samples collected in The Netherlands. Contrary to results presented by other workers, it was found that a non-homogeneous surface site model was needed to explain the potentiometric data obtained for proton and metal adsorption by manganese dioxide. Best fits of alkalimetric titration data were obtained with a two-site, three surface-species model of the δ-MnO₂ surface. Site concentrations of 2.231x10⁻³ mol.g⁻¹ and 7.656x10⁻⁴ mol.g⁻¹ were obtained. Corresponding equilibrium constants for the formation of the postulated surface species are -1.27 (=XO⁻), -5.99 (=YO⁻) and 3.52 (= YOH2₂⁺). This model was successful in a qualitative manner in describing adsorption results obtained from the open literature. The prediction of metal partitioning in soil systems showed that although the inclusion of manganese dioxide in the modelled to some improvement in the agreement between observed and predicted results, other factors are present which influence metal partitioning in soils. The discrepancy between observed and predicted results furthermore showed that the processes accounted for in the equilibrium model are incomplete.
Open Access
The adsorption of nitrogen and argon by evaporated films of sodium chloride
(1969) Rodgers, Allen L; Linder, Peter W
An ultra-high vacuum apparatus has been designed and constructed. Pressures of the order of 1 x 10⁻⁸ torr have been attained routinely. The adsorption of nitrogen by Pyrex has been investigated and the results obtained have shown good agreement with those in the literature. Rigorously cleaned evaporated films of sodium chloride have been prepared and the adsorption of nitrogen and argon by them has been investigated. Isosteric heats and entropies of adsorption have been determined and theoretical entropies calculated. The appearance of a group of sites of uniform activity, hitherto unavailable owing to contamination of the surface, is postulated. The adsorption of nitrogen and argon has been found to be reasonably well represented by the immobile, localized model with indications of the adsorbed argon molecules occupying a different set of sites from the adsorbed nitrogen. Two novel methods for the preparation of evaporated films of alkali halides are suggested.
Open Access
The adsorption of vinyl chloride and vinylidene chloride monomers on their polymers
(1970) Torrington, Ralph George; Linder, Peter W
Following semi-quantitative investigations of the kinetics and equilibria of:- (a) Vinyl chloride monomer vapour adsorbed on polyvinyl chloride; (b) Vinylidene chloride monomer vapour adsorbed on polyvinyl chloride; (c) Vinylidene chloride monomer vapour adsorbed on polyvinylidene chloride; it became evident that, for a number of reasons detailed in the text, it would be worthwhile to study system (a) in detail. Thorough investigations were therefore made at the two temperatures, 25⁰ C and 42.6⁰ C, and in the pressure range 20 torr to 600 torr, with the following results: (i) Attainment of equilibrium, especially at the higher pressures, was very slow and as a result only approximate isotherms could be obtained. These isotherms were shown to exhibit hysteresis. Comparison of the amount of monomer adsorbed at the higher pressures within the above limits, with the amount of krypton required to cover the surface with a monolayer, indicated that multimolecular adsorption occurred. The ascending branches of the isotherms appeared to obey the Freundlich isotherm and the approximate isosteric heats of adsorption calculated from the equilibrium data fell off linearly as the logarithm of the amount of monomer adsorbed increased; as would be expected if the Freundlich isotherm were obeyed. The values of the heats of adsorption found were such that comparison between them and the latent heat of vapourisation of the monomer made it difficult to decide whether the first layer on the surface was chemisorbed or physically adsorbed. (ii) The kinetic data showed that there was always an initial instantaneous adsorption which was reversible towards pressure, followed by a very much slower uptake of monomer. The kinetics of the slow uptake of monomer was studied, at the two temperatures, under both constant pressure and constant volume conditions and was shown to consist of two processes.
Open Access
Complexation of divalent copper, zinc and calcium ions by phosphate esters in aqueous solution
(1988) Kramer, Ulrike; Linder, Peter W
The role of metal ions as catalysts for numerous biochemical reactions has been the subject of many investigations. One of the most important classes of ligands are phosphate esters. In this thesis I describe the investigation of some phosphate ester-metal ion equilibria. Formation constants for the complexation of p-nitrophenyl phosphate, phenyl phosphate, 1-naphthyl phosphate, α-D-glucose-1'-phosphate, glycerol-2-phosphate, methyl phosphate, 8-quinolyl phosphate, 8-quinolyl methyl phosphate, triphosphate and fluorotriphosphate with protons, copper, zinc and calcium ions were determined by potentiometry. In addition, the complexation of 1-naphthyl phosphate, 8-quinolyl phosphate and 8-quinolyl methyl phosphate with nickel and cobalt ions was also studied. Protonation enthalpies and copper complexation enthalpies of p-nitrophenyl phosphate, phenyl phosphate, 1-naphthyl phosphate, α-D-glucose-1'-phosphate, glycerol-2-phosphate and methyl phosphate were determined by calorimetry. A correlation between the nucleophilicity of the ester group and the magnitude of the stability constants of the proton, copper and zinc complexes of p-nitrophenyl phosphate, phenyl phosphate, 1-naphthyl phosphate, α-D-glucose-1'-phosphate, glycerol-2-phosphate and methyl phosphate is found and explained in terms of electronic induction effects, i.e. by polarisation of the phosphate oxygens by the ester group. The calorimetric results show that the desolvation of ligand and metal ion during the complexation plays an important role. The possibility of similar correlations for complexes of triphosphates is also discussed.
Open Access
Computer modelling of the chemical speciation in seawater
(1990) Van der Meulen, Karen; Linder, Peter W; Jackson, Graham Ellis
The primary aim of this thesis is to establish a computer model of the chemical elements in seawater and to use this model to gain insight into the chemical processes controlling dissolution, precipitation, redox levels and coordinative interactions in surface seawater. In order to accomplish this task a relatively extensive database consisting of about 580 complexation equilibria arising from 32 inorganic and 10 organic components has been set up. Constants have been selected from critical compilations. Included in these equilibria are solubility products for the formation of potential solids, redox reactions and interactions with the atmospheric gases carbon dioxide and oxygen.
Open Access
A computer simulation of the protonation and metal complexation properties of fulvic acids
(1990) Pretorius, P J; Linder, Peter W
Electrostatic interactions between binding sites on a fulvic acid molecule are incorporated into a computer model for the simulation of fulvic acid cation equilibria. A preliminary investigation into the influence of nitrogen containing constituents on simulated metal complexation results is also reported. The metal investigated is Cu²⁺. The influence of electrostatic interactions on the complexation properties of macromolecules is evaluated by focussing on the Gibbs free energy of the complexation process. The change in Gibbs free energy, characteristic of a specific complexation reaction, is modified by the electrostatic interactions between sites. ΔG is modified by a factor dependent on the work of charging a system previously uncharged. This work is, under conditions of constant temperature and pressure, equivalent to the electrostatic contribution to ΔG. In order to evaluate the electrostatic free energy, it is necessary to evaluate the electrostatic potential on the macromolecule due to the charges present. This is accomplished using dielectric models of the macromolecules. The main points to emerge from this investigation are that the electrostatic models range from very sophisticated to very crude. Typically, the sophisticated models need detailed structural information on the macromolecule, such as atomic and charge coordinates. It is found that no such structural data are available for humic substances. This limits the choice of models to those which do not require detailed structural information. These models are however fraught with approximations, introducing uncertainty of unknown extent into the calculated results. The chosen electrostatic models are incorporated into protonation and metal complexation models of fulvic acids. Parameters which may influence simulated results to a large extent are identified. This investigation indicates that input parameters for the various modelling steps should be chosen with a great degree of circumspection. The cation complexation models are used to simulate experimental results published in the open literature. The results indicate that the incorporation of electrostatic interactions leads to some improvement between experimental and simulated results. The results further suggest that there are secondary refinements to the models which may improve the agreement between simulated and experimental results. The degree of fit between experimental and simulated results proves to be rather independent of the choice of electrostatic model. The influence of nitrogen containing ligands on the Cu²⁺ complexation capacity of fulvic acid does not turn out to be very pronounced. The assumption of neglecting these constituents as possible binding sites does not seem to be a limiting assumption. The results suggest that the predictive models developed in the course of this investigation may have some potential to simulate the cation complexation characteristics of fulvic acids. Further refinement of the models is, however, necessary.
Open Access
Diamino diamide copper complexes as anti-arthritic agents
(1993) Voyé, Alexander; Linder, Peter W; Jackson, Graham Ellis
This thesis describes the investigation of a series of diamino diamide copper(II) complexes with respect to their potential as anti-inflammatory agents in the treatment of rheumatoid arthritis. Several physico-chemical techniques were employed in the investigation including glass electrode potentiometry, UV/VIS spectroscopy, molecular mechanics calculations, speciation modelling as well as X-ray crystallography. Animal experiments were also carried out.
Open Access
Endemism and richness in the Cape floristic region: phytogeographic patterns and environmental correlates in a global biodiversity hotspot
(2009) Bradshaw, Peter L; Linder, Peter W
This study reports on an investigation of the phytogeographical patterns retrieved within the exceptionally species rich Cape Floristic Region (CFR), a global biodiversity hotspot in South Africa. Modern Analytical techniques, including a novel approach developed within this study, were used to identify Phytogeographical Centres. Moreover, the efficiency and optimality of these techniques were tested against each other using several different datasets. Endemism and species richness in the core CFR were assessed against contemporary environmental conditions, using a spatially sensitive regression technique. A combined dataset of 4414 taxa was analysed, of which 4000 were recorded in the general CFR area. This represented 44.4% of the 9087 total taxa recorded in the CFR, and the largest dataset examined to date on floristic patterns in the CFR. The combined data (Combined Dataset), a relatively representative sample of the CFR flora, was used to establish overall phytogeographic patterns of endemism. Further, derivative subsets, based predominantly on phylogeny/taxonomy including taxa from eleven plant families, and two ecological guilds, Geophytes and Red Data Listed (RDL) Taxa, were also analysed. Differences between the derivative datasets revealed insights into taxonomically distinct floristic patterns, determined by the dominance of particular floristic/biotic elements within each dataset. This helped explain phytogeographical differences between previous CFR phytogeographical studies, which focused on different floristic/biotic elements. In the Combined Dataset, nearly all Quarter Degree Square (QDS) cells were assigned to PCs in the core CFR, indicating endemism is common throughout the CFR. However, endemic taxa were concentrated in the high winter rainfall west, and southwest areas. The large size of the dataset, and detailed analyses revealed additional finer phytogeographical sub-division, not previously recorded, including six Phytogeographical Provinces, 16 Centres, and 36 Sub-Centres; compared to five equivalent Phytogeographical Provinces and nine equivalent Centres of Weimarck. Hierarchical analyses of the Combined Dataset displayed congruent patterns to the previous two comprehensive phytogeographical studies of Goldblatt and Manning, (2000) and Weimarck (1941). Within the core CFR phytogeographical provinces, common patterns across studies included choria being strongly associated with TMS mountain ranges. This highlighted the importance of substrate and topography explaining floristic patterns, consequent PC formation, and the numeric dominance of montane TMS 'fynbos' taxa. The latter was confirmed through habitat analysis of endemic taxa. However, PC development was also noted on the lower lying areas, notably the Agulhas Plains, and the lowland areas neighbouring the mountains of the Southwest Phytogeographical Province, and to a lesser extent, the Northwest Province. The five analytical methods used to determine candidate PCs were evaluated for performance optimality. Bell Shaped Curve Weighting using UPGMA proved marginally more optimal than the other four methods, but differences between various weighting and clustering algorithms were less than anticipated. However, weighting was seen to be substantially better than not weighting. The novel technique of using a multiple clustering analysis approach was found to highlight areas of conflict, where floristic/biotic elements overlapped, and to supplement occasional poorly resolved trees. In addition, the approach of using GIS interrogation of candidate centres, substantially enhanced the endemic composition and size of PCs, and is strongly recommended. Moreover, post clustering GIS analyses may correct any marginal disadvantages of any single approach. Significant correlations between endemism and PC size, and richness of non-endemics and PC size were found. However, certain PCs still contained more taxa than predicted by area alone, while others contained fewer. In these PCs with over- or under-represented richness, alternative explanations (either historical and/or environmental) were required to explain why these PCs differ from other PCs in the CFR, which was subsequently investigated. Analyses of patterns of endemism and taxon richness against contemporary climatic and environmental variables using a regression technique which accounted for spatial variation in parameter estimates highlighted the importance of both the energy-water hypothesis, and the habitat heterogeneity hypothesis in the CFR. Potential evapo-transpiration and various measures of topographic heterogeneity provided much explanatory power. In addition, length of growing/rain season was particularly important in the western winter rainfall portion of the CFR, and is here retrieved for the first time as a strong predictor variable. Different input units and different floristic components of the dataset required different models for optimisation. The models were adequately able to account for much of the variability in richness (~ = 78.3- 91.2%). The Cape Floristic Region (CFR) -a global hotspot- proved an excellent area to study patterns of endemism, taxon richness, floristic patterns, and how environmental conditions affect richness because of its exceptionally high concentration of both richness because of its exceptionally high concentration of both richness and endemism, and high levels of beta and gamma diversity over relatively short geographic distances.
Open Access
The formation of cyanoborate liquid clathrates with water or cresol as promoters
(1996) Davis, Bridgit Helen; Hunter, Roger; Linder, Peter W
If host and guest components form a liquid clathrate only when a third component is present, the third component is a promoter of liquid clathrate formation. The importance of promoters was verified by promoting liquid clathrate formation in guest-host combinations which do not form liquid clathrates without a promoter. The validity of the conventional definition of liquid clathrates was probed with respect to the absence of host in the excess guest layer and with respect to the fixed maximum guest to host ratio. A model which allows for the effect of promoters on liquid clathrates was proposed. The II electron rich nature of the guest is important in the model, so the guests, furan, thiophene and benzene, were used to probe the dependence of cyanoborate liquid clathrate formation on the IT electron rich nature of the guest. Differences in the electronic nature of the promoters, water and cresol, caused differences in the ease of promoting cyanoborate liquid clathrate formation, in the stability and composition of the liquid clathrates formed, and in the interactions between the promoter and the components of the liquid clathrate (host anion, host cation and guest). In promoting liquid clathrate formation, the promoter separated the host cation-anion pairs so that the guest interacted favourably with the host cation. The guest to host ratio was dependent on the amount of promoter present in the liquid clathrate, and favourable interactions of the promoter with liquid clathrate components caused the liquid clathrate to expand as it accommodated increased amounts of promoter and guest. Favourable interaction of the promoter with the components of the liquid clathrate compensated for the energy required to separate the host cation-anion pairs.
Open Access
An investigation of character variation in Chaetobromus Nees (Danthonieae: Poaceae) in relation to taxonomic and ecological pattern
(1995) Verboom, George Anthony; Linder, Peter W
Character variation in Chaetobromus, a genus of palatable grasses endemic to the arid western areas of southern Africa, was used to derive a classification reflecting taxonomic and ecological pattern. The present study differs from earlier biosystematic investigations by its much more intensive approach to sampling, with 75 anatomical, morphological and cytological characters and 169 individual samples being used. The use of larger population samples permitted quantification of variation within populations, in addition to that among populations and groups. Phenetic methods revealed the existence of three groups, approximating three formerly described taxa and reflecting divergent ecological strategies in Chaetobromus. A lack of diagnostic field characters argues against their recognition at species level, and Chaetobromus Nees is here described as monotypic, the type species, C. involucratus (Schrad.) Nees, comprising three subspecies C. involucratus subsp. involucratus, C. involucratus subsp. villosus Verboom and C. involucratus subsp. dregeanus (Nees) Verboom. There is overlap among subspecies in most characters although many showed significant mean differences. Within subspecies, character variation appears to be homogeneously distributed with respect to population boundaries suggesting that these are likely to have little impact on sampling. An investigation testing the effect of sample strategy on variation capture and taxonomic group detection suggests that a sample of 10-15 specimens is likely to account for most variation present. Phylogenetically, Chaetobromus is included in the tribe Danthonieae, and, on morphological evidence, is probably basal to a clade containing Pentaschistis, Pentameris and Pseudopentameris. The genus appears to occupy a niche unique among the African danthonioids, favouring lime-rich, basic soils and a strongly-seasonal winter-rainfall regime with arid summers. Ecological differences among the subspecies are reflected in differences in growth form and vegetative and reproductive phenologies. The niche requirements of Chaetobromus may be adequately specific to explain the patchy distribution of the genus. Bibliography: pages 119-131.
Open Access
Phenology and branching in eight selected species of the genus Cliffortia
(1999) Fellingham, Anna Catherina; Linder, Peter W
To date the peculiarities of the inflorescence structure in the wind-pollinated genus Cliffortia (Rosaceae), has gone unnoticed. In this study stereo microscope examination and sketching of fresh and dried specimens, combined with extensive field observations, were performed on eight species of the genus. In its simplest form the inflorescence is a reduced short shoot, bearing a lateral e-bracteate flower and a potentially viable apical bud. Variations in the basic structure can be in the number of flowers, the mix of the sexes of the flowers and the number and type of short shoots as primary, secondary and tertiary axes. A high incidence of structural plasticity of the inflorescence exists. Structural changes can take place throughout the development of the inflorescence or only at the onset of the vegetative stage. These changes occur in the short shoot(s) constituting the axes of the inflorescence, causing either an increase in the length of the internodes, apical proliferation of the axes or a combination of these two effects. A specific combination of changes is linked to a specific inflorescence type. The vegetative elements of the inflorescence thus modified, are retained as an integral part of the vegetative branching system, instead of being discarded after the shedding of the fruits. Thus the inflorescence disappears without being discarded, while the integration of the inflorescence matrix into the vegetative elements of the plant, has a marked effect on the vegetative branching pattern. In some species the structural changes in the inflorescence are linked to changes in the predominance of one sex over the other over time, so that an individual, initially of the one sex, can become one of the opposite sex by the end of the season. In the past the erroneous interpretation of a single point in the process of a sex change as if it is a permanent state of sexuality, led to the prevalent acceptance of dioecy as the norm for the genus, while in truth, it is monoecy with dichogamy in this genus, as in many other wind-pollinated taxa.
Open Access
Some properties of silica-supported nickel catalysts
(1970) Firer, Colin; Linder, Peter W
A literature survey has been made of earlier work on factors which have been found to influence the catalytic activity of supported metal catalysts. In particular an interaction has been shown to exist between the metal and its support in these catalysts. Two of the important factors which were shown to affect activity are the average metal crystallite size and the fraction of the metal present in a non-metallic form (for example as metal ions) in the catalyst. The earlier work consists almost exclusively of kinetic studies of heterogeneously catalyzed reactions. It is suggested that the conclusions are of a limited value owing to the widespread existence of the compensation effect, the fact that different test reactions may give contrasting results as to the relative activities of a series of catalysts and the belief that only a small fraction of the surface sites of a catalyst is generally involved in a given heterogeneous reaction. The work described in this thesis consists of an investigation of the effects of various factors on the isosteric heat of adsorption of an adsorbate per unit metal area on supported metal catalysts. The heat of adsorption per unit metal area provides a measure of the strength of the bond between the adsorbate and the surface atoms of the adsorbent. It was felt that this approach would give further insight into the factors affecting catalytic activity because the disadvantages, mentioned above, of kinetic studies would be eliminated.
Open Access
Synthesis and metal binding properties of selected organophosphorus plant growth regulators
(1992) Dhansay, Mohamed Afzal; Linder, Peter W
Open Access
Systematic implications of leaf anatomy and palynology in the Disinae and Coryciinae (Orchidaceae)
(1989) Chesselet, Pascale Claude Marcelle Henriette; Linder, Peter W
Pollen morphology of 8 species (TEM), 86 species (SEM), and leaf anatomy of 62 species (LM), were surveyed in the Oisinae, Coryclinae and, as outgroup taxa, the Orchideae and Satyriinae. Characters extracted from observations made of leaf anatomy and pollen were analysed using cladistic methods, and assessed in relation to the present phylogeny of the group. Leaf anatomy data gave little phylogenetic information. Sclerification associated with vascular bundles was systematically useful. Pollen data served to resolve taxa at the subtribal level. Both data sets provided evidence of relationship for taxonomically problematic taxa. The Coryciinae are palynologically defined by a suite of synapomorphies, including a secondarily tectate exine structure, fasciculate massulae, and elongated tetrads with linear microspore configuration. Bibliography: pages 107-123.

Browsing by Author "Linder, Peter W"

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