The physisorption of Kr on graphitic amorphous carbon (g-C) has been investigated using a statistical approach. The interaction energy calculation process (i) established a structural model of g-C and (ii) determined the adsorbate-adsorbate and the adsorbate-substrate interaction potentials on g-C. The structural model of g-C was divided into three regions. For the interaction potential between a Kr atom and a carbon atom the short and medium range order of g-C was described with a discrete medium model based on three ring clusters using ring statistics from Beeman's continuous random network C1120 model of g-C. For the intermediate distance region, Beeman's radial distribution function was used to model g-C. A homogenous and isotropic continuous medium model was used at large distances. The Kr-Kr and Kr-g-C interaction potentials used for Kr on g-C, which are pair-wise Lennard-Jones 6-12 potentials, are similar to Kr on graphite potentials. the validity of the model for g-C and the potentials were verified though calculations for Kr on graphite. Results compared favorably with recent literature values. The interaction energy calculation results for Kr on a g-C substrate assert that (i) Kr adlayers will form on g-C, (ii) the structure of the Kr adlayer is governed by the substrate corrugation at low coverage and by the Kr-Kr interaction at high coverage, and (iii) there is no direct relation between the structure of Kr adlayers on g-C and those on graphite. The average binding energy of Kr on g-C is comparable with that on graphite, but the corrugation of g-C is perhaps six times that of a graphite substrate. The wrinkling of the g-C surface, due to the presence of a distribution of 5-, 6-, and 7- membered rings, is responsible for this large corrugation of the g-C substrate.

The selectivity mechanism of the UV-light activated metal oxide semiconductor (MOS) gas sensors was studied. A reaction model based on two competing redox processes was presented to solve the selectivity problem. A concept named dynamic equilibrium of adsorbed oxygen concentration was brought about in this model and two reaction responses were discussed: (1) when most of the MOS surface is adsorbed with oxygen, the resistance of the MOS gas sensor will decrease upon the injection of reducing agents (RAs); (2) when most of the MOS surface is not adsorbed with oxygen, the resistance of the MOS gas sensor will increase upon the injection of RAs. Finally, experiments were conducted on ZnO MOS gas sensors to prove the proposed hypothesis of the reaction mechanism. / Thesis / Master of Applied Science (MASc)

No description available.

The production of second generation biofuels -- cellulosic ethanol from renewable lignocellulosic biomass has the potential to lead the bioindustrial revolution necessary to the transition from a fossil fuel-based economy to a sustainable carbohydrate economy. Effective release of fermentable sugars through biomass pretreatment followed by enzymatic hydrolysis is among the most costly steps for emerging cellulosic ethanol biorefineries. In this project, two pretreatment methods (dilute acid, DA, and cellulose solvent- and organic solvent-lignocellulose fractionation, COSLIF) for corn stover were compared. It was found that glucan digestibility of the corn stover pretreated by COSLIF was much higher, along with faster hydrolysis rate, than that by DA- pretreated. This difference was more significant at a low enzyme loading. Quantitative measurements of total substrate accessibility to cellulase (TSAC), cellulose accessibility to cellulase (CAC), and non-cellulose accessibility to cellulase (NCAC) based on adsorption of a non-hydrolytic recombinant protein TGC were established to find out the cause. The COSLIF-pretreated corn stover had a CAC nearly twice that of the DA-pretreated biomass. Further supported by qualitative scanning electron microscopy images, these results suggested that COSLIF treatment disrupted microfibrillar structures within biomass while DA treatment mainly removed hemicelluloses, resulting in a much less substrate accessibility of the latter than of the former. It also concluded that enhancing substrate accessibility was the key to an efficient bioconversion of lignocellulose. A simple method for determining the adsorbed cellulase on cellulosic materials or pretreated lignocellulose was established for better understanding of cellulase adsorption and desorption. This method involved hydrolysis of adsorbed cellulase in the presence of 10 M of NaOH at 121oC for 20 min, followed by the ninhydrin assay for the amino acids released from the hydrolyzed cellulase. The major lignocellulosic components (i.e. cellulose, hemicellulose, and lignin) did not interfere with the ninhydrin assay. A number of cellulase desorption methods were investigated, including pH adjustment, detergents, high salt solution, and polyhydric alcohols. The pH adjustment to 13.0 and the elution by 72% ethylene glycol at a neutral pH were among the most efficient approaches for desorbing the adsorbed cellulase. For the recycling of active cellulase, a modest pH adjustment to 10.0 may be a low-cost method to desorb active cellulase. More than 90% of cellulase for hydrolysis of the pretreated corn stover could be recycled by washing at pH 10.0. This study provided an in-depth understanding of biomass saccharification for the production of cellulosic ethanol for cellulose hydrolysis and cellulase adsorption and desorption. It will be of great importance for developing better lignocellulose pretreatment technologies and improving cellulose hydrolysis by engineered cellulases. / Master of Science

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The aim of this thesis was to study structure-property relationships in porous materials using various adapted analytical techniques and in-house instruments. The thesis is divided into two sections, and the first section of work constitutes the majority of the thesis. The first section of work deals with the theoretical versus experimental classification of sorption selectivity in porous compounds. A transiently porous metallocycle that can adsorb acetylene and carbon dioxide served as a model host for this experiment. A volumetric sorption instrument had to be constructed to carry out sorption with acetylene. Even though the metallocycle should theoretically be selective for acetylene over carbon dioxide based on single-gas sorption isotherms, this was not the case during the sorption of a mixture of the two gases. Furthermore, high-pressure single-crystal diffraction was carried out utilising an in-house environmental gas cell, and structural elucidation indicated that both acetylene and carbon dioxide coexist in a single cavity of the host. Additional complementary techniques are discussed that were used to confirm that both gases are present in a single host cavity. The techniques included infrared spectroscopy as well as high-pressure florescence and Raman spectroscopy, which had to be conducted with a specially designed pressure vessel and with adapted instrumentation. Finally, density functional theory calculations were employed to explain how host-guest and guest-guest interactions lead to the change in adsorption selectivity. It is concluded that researchers need to show experimentally that a compound is selective for the adsorption of a specific gas, because theoretical models are not always accurate. The second part of this work focuses on a fundamental study of the structure-property relationships in a porous hydrogen-bonded organic framework. The section starts off by exploring the activation conditions and thermal stability of the framework. This is followed by an exploration of a possible phase transformation or thermal expansion in the framework, but neither of these occurred. Thereafter, an extended study of the framework’s sorption behaviour with various gases is discussed. Then, a structural study of its solvated phase is used to explain the framework’s stability. Finally, a novel analytical method is introduced, and two examples are used to demonstrate why the instrument is useful in the field of supramolecular chemistry. The chapter is concluded by stating the importance these fundamental studies, as well the development of new analytical techniques. / AFRIKAANSE OPSOMMING: Die doel van hierdie tesis was om die struktuur-afhanklike eienskappe van poreuse materiale te ondersoek. Die studie het gebruik gemaak van verskeie aangepasde analitiese metodes asook instrumente wat spesifiek vir die studie gebou was. Die werk word in twee dele verdeel, en die meerderheid van die tesis word in die eerste deel bevat. In die eerste deel van die tesis word die validiteit van teoretiese- teen eksperimentele adsorpsie selektiwiteit opgeweeg. ʼn Gasheer wat bestaan uit ringvormige koördinasie-verbindings en wat asetileen asook koolstof dioksied kan adsorbeer, dien as ʼn model gasheer vir die studie. ʼn Volumetriese sorpsie instrument was spesiaal vir die studie gebou sodat asetileen sorpsie gedoen kon word. Volgens asetileen en koolstof dioksied se enkel-gas adsorpsie isoterme moet asetileen teoreties met voorkeur geadsorbeer word gedurende ʼn adsorpsie eksperiment waarin beide gasse teenwoordig is, maar eksperimenteel was dit bepaal dat dit nie so is nie, dus is daar ʼn verandering in die gasheer se adsorpsie selektiwiteit. Hierna word strukturele data van die gasheer, onder ʼn hoë druk van die gas mengsel, versamel deur gebruik te maak van enkel-kristal diffraksie en ʼn spesiaal-gemaakde gas sel. Die strukturele data toon dat beide asetileen en koolstof dioksied teenwoordig is binne elke porie van die gasheer. Daar word dan van addisionele analitiese metodes gebruik te maak om die observasie te bevestig. Die analitiese metodes sluit in infrarooi spektroskopie asook hoë-druk fluoressensie en Raman spektroskopie wat geëis het dat ʼn spesiale druk-bestande monster houer gebou moes word en dat analitiese instrumente gemodifiseer moet word. Ten slotte was daar van “density functional theory” gebruik gemaak om te verduidelik dat die interaksie tussen die gasheer en gas sowel as die interaksie tussen twee gasse lei tot die verandering in adsorpsie selektiwiteit. Uit hierdie bevinding word die gevolgtrekking gemaak dat navorsers met meer eksperimentele data vorendag sal moet kom voordat ʼn gevolgtrekking gemaak kan word dat ʼn raamwerk selektief een gas adsorbeer. Die tweede afdeling van die werk fokus op ʼn fundamentele studie van die struktuur-afhanklike eienskappe van ʼn poreuse waterstof-verbinde organies raamwerk. Die afdeling begin deur ʼn ondersoek van die aktivering kondisies sowel as die temperatuur-afhanklike stabiliteit van die raamwerk. Dit word gevolg deur te soek na moontlike fase veranderings of temperatuur-afhanklike uitsetting van die raamwerk, maar nie een van die twee eienskappe word waargeneem nie. Daarna word die deeglike ondersoek van die raamwerk se adsorpsie vermoë met verskeie gasse bespreek. Dit word gevolg deur ʼn strukturele studie van die solvaat van die raamwerk, wat dan gebruik word om die stabiliteit van die raamwerk te verduidelik. Ten slotte word ʼn analitiese metode bekend gestel, en twee voorbeelde word gebruik om te wys hoe nuttig die metode is om ʼn kombinasie van resultate te bekom. Die hoofstuk word saamgevat deur te verduidelik hoekom dit belangrik is om hierdie tipe fundamentele studies te doen asook waarom nuwe analitiese metodes ontwerp moet word.

Electrostatics plays a crucial role in the membrane biology. Negatively charged lipids (such as PS, PA and PIP2) are subject to redistribution under the action of electrostatic forces during various signalling events. Membrane recruitment of multiple signalling proteins (such as MARCKS or Src kinase) is often maintained by positively charged polybasic domains (PD). Even though adsorption of these proteins to the cellular membrane has been extensively investigated, very little is known about how electrostatic interactions contribute to their membrane lateral dynamics. This thesis presents an investigation of the contribution of electrostatic interactions to the membrane lateral dynamics by means of novel computational tools. First, I developed a dynamic Monte-Carlo automaton that faithfully simulates lateral diffusion of the adsorbed positively charged PD of a peripheral membrane protein, as well as the dynamics of mono- (PS, PA) and polyvalent (PIP2) anionic lipids within the bilayer. This model allowed to investigate the major characteristics of protein-membrane diffusion on the uniform membrane. In agreement with earlier results, the simulations revealed the following microscopic phenomena: 1) Electrostatic lipid demixing in the vicinity of the PD; 2) PD interacts with PIP2 stronger than with monovalent lipids. On the spatially heterogeneous membrane the automaton predicted a directional drift of the PD, which was validated by a simple mean-field analytical model. The predicted phenomenon could potentially play a major role in membrane domain formation. To test this hypothesis and to investigate the membrane dynamics on larger scales I developed a continuous model, which was based on the results of the automaton simulations. The results of the continuous model and the Monte-Carlo simulations were shown to be in quantitative agreement. The continuous model allows one to simulate the electrostatic membrane dynamics on micrometer scales and can be used to describe various biologically important processes, such as endocytic cup initiation.

Phenolic compounds are the chemicals which are used by many different industries and as a result of this spread to the environment. These compounds can be absorbed easily through the human and animal skin and through the mucosal membrane, mix in to the blood circulation and thus create a toxic effect on several tissue and organs including, liver, lung and kidneys. For this reason, determination of phenolic compounds emitted to environment is a very important issue. In fact, there are standard methods for the determination of these compounds like HPLC, Spectrophotometric and calorimetric methods however, these are time consuming methods and requires to be expertise. On the other hand, there are also different types of biosensors developed for the phenolic compound detection. In this study, a new, disposable, cheap and convenient tyrosinase biosensor was developed for the phenolic compound detection. By means of absorption method, the enzyme tyrosinase and the chromophore MBTH were immobilized on the support material and as a model substrate L- dopa was used. As a result of optimization studies 1mg/ml tyrosinase concentration and 1.5mM MBTH concentration were determined for using in biosensor construction. Detection limit of l-dopa, model substrate, found as 0,064 mM and for other phenolic compounds, 4-chlorophenol, catechol, m-cresol and p-cresol, detection limit was obtained 0.032 mM, 0.032 mM, 0.128 mM, 0.128 mM, respectively. In addition, we found that the biosensor response was not affected by pH changes ranging from 3 to 11. The stability of biosensor which is one of the important parameter for commercialization was not change through 70 days at room temperature and 4&deg / C when compared to at the beginning response.

The ongoing energy and environmental crises have pushed the transportation sector, a major greenhouse gas emitter, to seek sustainable fuel and technology alternatives. Natural gas and bio-methane are potential alternatives with numerous advantages over conventional fuels. Adsorbed natural gas (ANG) technology uses porous adsorbent material to store methane efficiently at lower pressures. An issue limiting this technology is the lack of compact tanks with efficient adsorbent packing that increase storage capacity. This study addresses the need for more compact ANG tanks by creating novel binder-less monolithic activated carbon monolith adsorbents with targeted porosity. A template is produced using 3D printing and a commercially available phenolic resin as a filling material. Upon thermal treatment, the 3D-printed template combusts with molecular oxygen in its structure, and the resin is transformed into activated carbon by pyrolysis. Longer activation times led to higher BET surface areas. However, after activation periods beyond 15 minutes, the surface area increase is obtained at the expense of a higher burn-off, which affects the material density. Adsorption of 0.04g/g of methane was measured at 30 bar and 298 K on the activated carbon monolith with the highest BET surface area (516 m2/g). Results in the same conditions on a super high surface area Maxsorb activated carbon were 0.13g/g. Although the methane capacity obtained is lower than in a commercial sample, it was demonstrated that producing an activated carbon monolith with tailored porosity is possible. New techniques for activation should be studied to enhance their gravimetric capacity to make ANG competitive.

No description available.

Des cultures de Chlorella vulgaris ont été réalisées en triplicata sur de longues périodes (15-30 jours) en autotrophie et en mixotrophie afin d’étudier l’effet de Mg2+ sur les microalgues et déterminer sa concentration dans la biomasse au cours de la croissance. Différentes concentrations ont été testées (de 8.9 à 465 mg L-1). Ceci a montré que le Mg2+ n’est pas toxique pour les algues à des concentrations élevées provoquant habituellement l’inhibition de la croissance dans le cas d’autres ions divalents (métaux lourds). Des bilans matière ont été établis et vérifiés avec une précision satisfaisante (écart moyen de 90%). La quantité de Mg2+ bioaccumulée augmente avec la concentration initiale de Mg2+ du milieu de culture. Une saturation en Mg2+ intracellulaire a été observée à partir d’environ 45 mg de Mg2+ L-1. Typiquement, la répartition entre le Mg2+ extracellulaire et intracellulaire est de 18% contre 51% dans le bioréacteur (5L) pour une concentration initiale de 16 mg L-1. En mixotrophie (addition de 10 g L-1 de glucose), une production de biomasse significativement plus importante et plus rapide que celle en autotrophie a été observée. En revanche, la quantité de Mg2+ accumulée dans les cellules est plus importante en autotrophie qu’en mixotrophie. Des mesures de chlorophylle a, de nitrate et de glucose ont été également réalisées. Afin de comprendre la cinétique relativement complexe de bioaccumulation du Mg2+, un modèle cinétique original a été élaboré couplant transfert (externe, membrane, interne) et réactions (ad/absorption) en milieu hétérogène. Le modèle a été validé expérimentalement. Il permet de rendre compte au mieux de l’ensemble des résultats obtenus. / Chlorella vulgaris cultures were grown in triplicate over 15-30 days under autotrophic or mixotrophic conditions in order to study Mg2+ uptake and accumulation into the biomass. The concentrations of Mg2+ tested were 8.9 to 465 mg L-1. The results showed that Mg2+ is not toxic to C. vulgaris even at 465 mg L-1 which is not the case for other divalent ions (especially heavy metals). The mass balances for Mg2+ accumulation were determined for the cultures and were confirmed to an average of 90%. Beyond 46.1 mg of Mg2+ L-1 in the culture medium, the cells became saturated at 4 mg of Mg2+ absorbed per g of dry biomass. In the bioreactor (5L), at an initial Mg2+ concentration of 16 mg L-1, the cells accumulated 69% of the initial concentration, in which 18% was adsorbed and 51% absorbed. The chlorophyll a, nitrate and glucose concentrations were measured during the experiments. Under mixotrophic conditions (glucose 10 g L-1), a greater and faster biomass production was obtained than under autotrophic conditions. The Mg2+ bioaccumulation was higher in the autotrophic rather than the mixotrophic phase. In order to understand the kinetics of Mg2+ uptake, an original kinetic model was developed coupling the transfer and reaction phenomena in heterogeneous media. This model was confirmed experimentally.