The objective of this research was to study the impact of ripening inhibitor level and type on the formation, stability, and activity of antimicrobial thyme oil nanoemulsions formed by spontaneous emulsification. Oil-in-water antimicrobial nanoemulsions (10 wt%) were formed by titrating a mixture of essential oil, ripening inhibitor, and surfactant (Tween 80) into 5mM sodium citrate buffer (pH 3.5). Stable nanoemulsions containing small droplets (d < 70 nm) were formed. The antimicrobial activity of the nanoemulsions decreased with increasing ripening inhibitor concentration, which was attributed to a reduction in the amount of hydrophobic antimicrobial constituents transferred to the separated hydrophobic domain, mimicking bacterial cell membranes, by using dialysis and chromatography. The antimicrobial activity of the nanoemulsions also depended on the nature of the ripening inhibitor used: palm ≈ corn > canola > coconut which also depended on their ability to transfer hydrophobic antimicrobial constituents to the separated hydrophobic domain.

This project deals with the mixed micellar and interfacial properties of mixtures of three surfactants [sodium dodecyl sulphate (SDS), cetyltrimethylammonium bromide (CTAB), and tetraethylene glycol monododecyl ether (C12E4)] with ABA symmetrical triblock copolymer (Pluronic F127), which has many industrial applications. Evidence of F127 micellisation and interaction with surfactants in the aqueous phase is inferred through interfacial tension measurements. The solution containing diluted monomeric F127 showed complex formation with surfactants before the latter self-aggregate as pure micelles. The simultaneous presence of ionic surfactants and micellar F127 in solutions displayed a decrease of interfacial activity and led to the conclusion of F127 micelles disruption. C12E4 was found to interact differently with micellar F127 in forming mixed micelles, and no loss of interfacial activity was recorded. This “association-dissociation” behaviour of F127 and surfactants was leveraged to understand the stability of mineral oil in water emulsions formulated with them in the presence of sodium phosphate (Na3PO4). The mechanisms of emulsions breakdown were found to rely on aggregation behaviour and complex structure of F127 and surfactants mixtures in solution. Laser diffraction showed that unlike SDS and CTAB, mixed-emulsifier systems containing C12E4 are stable to both flocculation, Ostwald ripening and coalescence. Due to electrostatic repulsion between its head group and F127 hydrophilic block, and also because of the combined effect of Ostwald ripening and coalescence, CTAB emulsifier containing systems displayed quicker instability than SDS. SDS containing systems showed a progressive shifting of droplets size distributions to bimodality as SDS concentration was increased and heat exposure pursued, revealing the activity of two distinct population of droplets in the emulsions. More insight on the mechanisms underlying the stability of the three mixed emulsifier systems was gained in performing optical microscopy and rheology measurements; the results were found to be consistent with particle size distribution. / Dissertation (MSc (Applied Science: Chemical Technology))--University of Pretoria, 2020. / Chemical Engineering / MSc (Applied Science: Chemical Technology) / Unrestricted

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Transdermal drug delivery systems (TDS) are pharmaceutical devices that are designed to deliver specific drugs to the human body by diffusion through skin. The TDS effectiveness suffers from crystallization in the patch when they are kept in storage for more than two years. It has been reported that there are two types of crystals in the patch: needle and aggregate, and growth of drug crystals in TDS generally occurs only in the middle third of the polymer layer. In our study, fluorescence microscopy, EDS (SEM) and Raman microspectroscopy were used to further characterize the crystals. The results show that the needle crystals most probably contain estradiol and acrylic resin conjugate. The FTIR spectrum of the model sample proved the occurrence of a reaction between estradiol and acrylic resin. Crystal growth in an unstressed matrix of a dissolved crystallizable drug component was simulated using a kinetic Monte Carlo model. Simulation using Potts model with proper boundary condition gives the crystals in the middle of matrix in the higher temperature. Bond fluctuation model is also being implemented to study representative dense TDS polymer matrix. This model can account for the size effect of polymer chain on the crystal growth. The drug release profile from TDS was also studied by simulating the diffusion of drug molecules using Monte Carlo techniques for different initial TDS microstructure. The release rate and profile of TDS depend on the dissolution process of the crystal. At low storage temperature, the grains are evenly distributed throughout the thickness of the TDS patch, thus the release rate and profile is similar to the randomly initiated system. Further work on stress induced crystallization is currently under development. Although the study was specifically done for drug in a polymer medium, the techniques developed in this investigation is in general applicable to any constrained crystallization in a polymer medium.

The phase separation of mixed Langmuir-Blodgett (LB) monolayers was investigated using a combination of atomic force microscopy (AFM), X-ray photoelectron emission microscopy (X-PEEM) and confocal fluorescent microscopy measurements. Shapes of phase-separated domains that formed on solid substrate surfaces depended on a competition between line tension and dipole-dipole interactions. In the mixed LB film of arachidic acid (C19H39COOH) (C20) and perfluorotetradecanoic acid (C13F27COOH) (F14), the components phase separated into elevated hexagonal domains of C20 surrounded by a continuous domain primarily consisting of F14. The underlying molecular arrangement of C20 was found to be an oblique packing. The domains in this system grew via Ostwald ripening and the kinetics of their growth was modeled by twodimensional LifshitzSlyozov equation. In the stearic acid (C17H35COOH) (C18) and F14 mixed films, the C18 domains formed a linear pattern where the F14 molecules filled the areas in between the lines occupied by C18. For the mixed film of palmitic acid (C15H31COOH) (C16) and perfluorooctadecanoic acid (C17F35COOH) (F18), the surfactants phaseseparated into elevated hexagonal domains with hairy extensions radiating from them. These domains were composed of F18 and surrounded by C16. Ostwald ripening was found to be the mechanism of domain growth. Phase separation was controlled by different forces such as line tension and dipole interactions, as well as the diffusion of the molecules, solubility of the surfactant in the sub-phase, temperature and surface pressure. Simple mechanisms regarding phase separation and pattern formation were discussed in these mixed systems. It was observed that all fatty acid / F14 systems in this study were immiscible at all molar fractions examined. The fatty acid / F18 systems were immiscible at short chains of fatty acids (myristic acid (C13H27COOH) C14, C16, C18), whereas at longer fatty acid chains (C20, C22 behenic acid (C21H43COOH)) the components of the mixed system became miscible. When perfluorocarboxylic acid chain combined with fatty acids, the domains changed from large hexagonal domains into narrow lines as the fatty acid chain decreased in length.

The phase separation of mixed Langmuir-Blodgett (LB) monolayers was investigated using a combination of atomic force microscopy (AFM), X-ray photoelectron emission microscopy (X-PEEM) and confocal fluorescent microscopy measurements. Shapes of phase-separated domains that formed on solid substrate surfaces depended on a competition between line tension and dipole-dipole interactions. In the mixed LB film of arachidic acid (C19H39COOH) (C20) and perfluorotetradecanoic acid (C13F27COOH) (F14), the components phase separated into elevated hexagonal domains of C20 surrounded by a continuous domain primarily consisting of F14. The underlying molecular arrangement of C20 was found to be an oblique packing. The domains in this system grew via Ostwald ripening and the kinetics of their growth was modeled by twodimensional LifshitzSlyozov equation. In the stearic acid (C17H35COOH) (C18) and F14 mixed films, the C18 domains formed a linear pattern where the F14 molecules filled the areas in between the lines occupied by C18. For the mixed film of palmitic acid (C15H31COOH) (C16) and perfluorooctadecanoic acid (C17F35COOH) (F18), the surfactants phaseseparated into elevated hexagonal domains with hairy extensions radiating from them. These domains were composed of F18 and surrounded by C16. Ostwald ripening was found to be the mechanism of domain growth. Phase separation was controlled by different forces such as line tension and dipole interactions, as well as the diffusion of the molecules, solubility of the surfactant in the sub-phase, temperature and surface pressure. Simple mechanisms regarding phase separation and pattern formation were discussed in these mixed systems. It was observed that all fatty acid / F14 systems in this study were immiscible at all molar fractions examined. The fatty acid / F18 systems were immiscible at short chains of fatty acids (myristic acid (C13H27COOH) C14, C16, C18), whereas at longer fatty acid chains (C20, C22 behenic acid (C21H43COOH)) the components of the mixed system became miscible. When perfluorocarboxylic acid chain combined with fatty acids, the domains changed from large hexagonal domains into narrow lines as the fatty acid chain decreased in length.

La règle des phases d'Ostwald classiquement utilisée pour justifier la cristallisation d'un système polymorphique, stipule que la phase métastable apparait en premier puis subit une transition polymorphique vers la phase stable. Les modèles classiques, qui ne considèrent que la nucléation et la croissance, ne permettent pas de refléter l'avantage cinétique de la phase métastable formulé par la règle d'Ostwald. Cette étude propose d'étudier et de mieux comprendre la cristallisation d'un système polymorphe en prenant en compte le mécanisme de mûrissement d'Ostwald, habituellement négligé. Un produit modèle, l'acide L-Glutamique, est choisi pour l'étude expérimentale menée en milieu agité et stagnant. Deux modèles, l'un basé sur les bilans de population, l'autre basé sur les équations cinétiques, sont développés et qualitativement comparés pour simuler le comportement expérimental des phases polymorphes. Alors que le modèle de bilan de population s'avère limité, le modèle des équations cinétiques a permis de souligner l'effet du mécanisme de mûrissement sur la compétition entre les phases polymorphes et de valider, ainsi, une nouvelle explication pour la règle des phases d'Ostwald / The Ostwald rule of stages is conventionally used to explain the crystallization behavior of a polymorphic system. It states that the metastable phase first appears and undergoes a polymorphic transition toward the stable phase, in a second step. The Classical models, which only consider nucleation and growth, fail to reflect the kinetic advantage of the metastable phase formulated by Ostwald’s rule. Hence, this work intends to study and better understand the crystallization of a polymorphic system, taking into account the Ostwald ripening mechanism, usually neglected. A model compound, L-Glutamic acid, is chosen for the experimental study in agitated and stagnant conditions. Two numerical models, one based on the population balance equation and the other based on the kinetic equations, are developed to simulate the behavior of that polymorphic system, observed experimentally. A qualitative comparison between these two models is proposed. The model that relates the population balance equation does not permit correct implementation of all the mechanisms. Conversely, the model based on the kinetic equations highlights the effect of the ripening mechanism on the competition between the two polymorphic phases and allows us to propose a new explanation of the Ostwald rule of stages

<div>Metal-zeolites are promising materials for passive adsorber technologies for the abatement of nitrogen oxides (NOx, x = 1,2) and aldehydes during low-temperature operation in automotive exhaust aftertreatment systems. The aqueous-phase exchange processes used commonly to prepare metal-zeolites typically require mononuclear, transition metal complexes to diffuse within intrazeolite pore networks with their solvation shells and replace extra framework cations of higher chemical potential. When metal complexes are larger than the zeolite pore-limiting diameter, this imposes intracrystalline transport restrictions; thus, complexes and agglomerates tend to preferentially deposit near the surfaces of crystallites, requiring post-synthetic treatments to disperse metal species more uniformly throughout zeolite crystallites via solid-state ion-exchange processes. Here, we address the influence of post-synthetic gas treatments and zeolite material properties on the structural interconversion and exchange of extra framework Pd in CHA zeolites with a focus on the thermodynamic, kinetic, and mechanistic factors that dictate the Pd site structures and spatial distributions that form.<br></div><div><br></div><div>Pd-amine complexes introduced via incipient wetness impregnation on CHA zeolites were found to preferentially site near crystallite surfaces. Post-synthetic treatments in flowing air results in Pd-amine decomposition and agglomeration to metallic Pd0and supersequent oxidation to PdO, before converting to mononuclear Pd²⁺cations through an Ostwald ripening mechanism at high temperatures (>550 K). Progressively higher air treatment temperatures (up to 1023 K) were found to (1) thermodynamically favor the formation of mononu-clear Pd²⁺cations relative to agglomerated PdO particles, (2) increase the apparent rate of structural interconversion to mononuclear Pd²⁺, and (3) facilitate longer-range mobility of molecular intermediates involved in Ostwald ripening processes that allow Pd cations to form deeper within zeolite crystallites to form more uniformly dispersed Pd-zeolite materials. Additionally, the controlled synthetic variation of the atomic arrangement of 1 or 2Al sites in the 6-membered ring of CHA was used to show a thermodynamic preference to form mononuclear Pd2+cations charge-compensated by 2 Al sites over [PdOH]⁺complexes at 1 Al site. Colloidal Pd nanoparticle syntheses and deposition methods were used to prepare monodisperse Pd-CHA materials to isolate the effects of Pd particle size on structuralinterconversion to mononuclear Pd²⁺under a range of external environments. Consistent with computational thermodynamic predictions, smaller Pd particle sizes favor structural interconversion to mononuclear Pd²⁺under high-temperature air treatments (598–973 K),while adding H2O to the air stream inhibits the thermodynamics but not the kinetics of mononuclear Pd²⁺formation, demonstrating that water vapor in exhaust streams may be deleterious to the long-term stability of Pd-zeolite materials for passive NOx adsorption.<br></div><div><br></div><div>The influence of metal-zeolite material properties on the adsorption, desorption, and conversion of formaldehyde, a government-regulated automotive pollutant, under realistic conditions was investigated to identify beneficial material properties for this emerging application in mobile engine pollution abatement. A suite of Beta zeolite materials was synthesized with varied adsorption site identity (Brønsted acid, Lewis acid, silanol groups, and extra framework metal oxide) and bulk site densities. All materials stored formaldehyde and converted a large fraction of formaldehyde to more environmentally benign CO and CO₂, demonstrating the efficacy of silanol defects and zeolitic supports for the storage of formaldehyde. Sn-containing zeotypes, containing either Lewis acidic framework Sn sites or extra framework SnO_x particles, resulted in the greatest selectivity to CO and CO₂ formed during formaldehyde desorption, suggests that Sn species are a beneficial component in metal-zeolite formulations for the abatement of formaldehyde in automotive exhaust streams.<br></div><div><br></div><div>This work demonstrates how combining precise synthesis of metal-zeolites of varied bulk and atomic properties with site-specific characterization and titration methods enables systematically disentangling the influence of separate material properties (e.g., Pd particle size, zeolite framework Al arrangement, silanol density, heteroatom identify) and external environment on changes to metal structure, speciation, and oxidation state. This approach provides thermodynamic, kinetic, and mechanistic insights into the factors that influence metal re-structuring under the practical conditions encountered in automotive exhaust after treatment applications and guidance for materials design and treatment strategies to form desired metal structures during synthesis and after regeneration protocols.<br></div>

M. Tech. (Department of Chemistry, Faculty of Applied and Computer Sciences), Vaal University of Technology. / The ligand 2, 2-dithiobisbenzothiazole and it metal complexes have been a subject of interest in various fields but they have found to exhibit remarkable and prevalent biological and pharmacological activities. The ligand tends to coordinate to complexes through the sulfur atom and hence the metal-sulphide bond are good precursor to generate metal sulfide nanoparticles using single-source precursor route. The complexes are generally prepared by reflux for 1 to 2 hours depending on the solvent used to produce very stable solid products and some form in crystalline form. All the prepared nickel and cadmium complexes were characterized using techniques such as elemental analyzer, IR, 13C NMR spectroscopy and thermogravimatric analysis. The data obtained from the spectroscopic analysis was consistent of the coordination of the ligand with the metal ions through the sulphur atoms of the 2,2-dithiobisbenzothiazole moiety. The thermal analysis of the prepared complexes gave a final residue of metal sulphide for both metal complexes. Characterization techniques showed the formation of bidentate complexes for both nickel complex and cadmium complex. The prepared complexes were then used to synthesize metal sulphide nanoparticles .The nanoparticles were prepared by thermal decomposition method of the single source precursor in a solution of oleylamine (OLA). Two different parameters were investigated temperature and time to study their effect on the size and shape of the nanoparticles. The synthesized nanoparticles were characterized using techniques such as UV-Vis spectroscopy, photoluminescence spectroscopy, and X-ray diffraction analysis and transmission electron microscopy. The temperatures of the reaction have a significant effect on the rate of the reaction that will affect the size and shape of the nanoparticles. This effect was confirmed by the optical properties of the synthesized nanoparticles prepared at different reaction temperatures. The spectra shows that absorption maximum and band edge shift to lower wavelength as the temperature of reaction was progressively increased. This trend is associated to the decrease in particles size of the prepared nanoparticles. TEM images further confirmed that the particles size of the prepared nanoparticles was progressively decreased as the temperature was increased. The time of the reaction is one of the most significant factors in the synthesis of the nanoparticles. The investigation of the time of the reaction yield results that depicted that with increase in time of the reaction, the band edge increases, but relatively at short wavelength to the bulk. Hence, the band edges of the nanoparticles were blue shifted significantly to the bulk. The results show that with an increase in the time of the reaction, the nanoparticles increases in their size due to Ostwald ripening. The optimum complexes and optimum nanoparticles were used to further study their electrochemical properties using cyclic voltammetry and electrochemical impedance spectroscopy (EIS) graphs were fitted using the randles circuit and they confirm that the NiS nanoparticles GCE greatly increase the electron transfer rate, probably due to the nanostructured surface property of the NiS nanoparticles. Differential pulse voltammetry (DPV) was used to study the electrochemical behavior and the DPV showed that the current response of Rhb was higher for the optimum temperature NiS nanoparticles compared to all the materials used. There was an increase in the Rhb current response with an increase in pH and pH 7 was used as the optimum pH when Ni- complex was used as a modifier and pH 8 was used as optimum when NiS nanoparticles were used as a modifier. Effect of concentration showed that the NiS nanoparticles for the optimum temperature had a wide linear range and a low detection limit. The method has good accuracy, acceptable precision, and reproducibility. This method provides a novel electrochemical method for determination of RhB.