School of Chemistry - Theses
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ItemUltrasonic synthesis of advanced photocatalytic materials for use in continuous flow-through reactors( 2017)Water purification is a problem around the world. Every year, authorities introduce new standards on the amount of pollutants that could be released into the environment, and new compounds are continuously being added to the list of toxic waste. Some of these substances can be defined as persistent organic pollutants (POPs), and their degradation is generally difficult to achieve. In these cases, advanced oxidation processes are required for the decomposition of these hazardous molecules. In this thesis, the advanced oxidation process used is photocatalysis, which can be used to decompose almost any kind of organic pollutant. To enhance the photocatalytic activity of photocatalysts, researchers have focused their attention on reducing the size of the particles, reaching just a few nanometers. Unfortunately, no sufficient efforts were made in continuous flow studies, which are necessary if future industrial implementations are desired. In these systems, the photocatalyst can be used in two ways: dispersed or coated onto a surface. The former offers high activity, but its recovery at the end of the reaction can be difficult. Instead, the latter permits almost no effort in the catalyst retrieval, but its efficiency is significantly low. For these reasons, this thesis aims to investigate the conversion of nanosized catalysts into micron sized powders without a loss in activity. In this manner, the catalyst could be used as a dispersion, enhancing the degradation and reducing the costs involved in the filtration procedures. In order to accomplish this aim, microspheres were used as a template material. The studies on continuous flow systems and their comparison to batch systems, carried out in this thesis, could be useful for future industrial implementations. For the generation of microspheres, ultrasonic emulsification technique was utilized, and the fundamental principles of ultrasound, along with those of microspheres, photocatalysis, and continuous flow reactors are discussed in Chapter 1. In Chapter 2, a structured literature review examining ultrasonic emulsification, microencapsulation, photocatalysts, and studies on continuous flow reactors, is discussed. In this thesis, chitosan (a natural amino-polysaccharide used in a wide range of applications) was chosen as a shell material for the generation of the microspheres, while nano sized TiO2 and ZnO were used as model nanosized photocatalysts. In Chapter 3, materials, analytical methods, and e experimental details used in this thesis are discussed. Three continuous flow reactors are presented, along with a new type of ultra-bright LEDs used as a light source for the photocatalytic degradation of rhodamine B, metanil yellow and methylene blue. Chapter 4 is the first chapter of result and discussion section. The role of counter ions on controlling the properties of ultrasonically generated chitosan microspheres, produced via oilin- water emulsion technique, was investigated. Various acids were used to dissolve chitosan, and it was found that the conjugate bases of the acid used (which acted as counter ions to neutralize the positive charges of ammonium ions present in the chitosan backbone) played a significant role in controlling the size, size distribution, and stability of the chitosan microspheres. In Chapter 5, the development of micron sized photocatalysts was studied. Chitosan microspheres were used for the conversion of nano sized TiO2 and ZnO (25-50 nm) into micron sized particles, possessing a size of about 10 μm. The micron sized photocatalysts possessed a photocatalytic efficiency similar to that of the nano sized powders, which was investigated in both aqueous and gas phases. In addition, the mechanism on the formation of the micron sized structures was proposed. In Chapter 6, the comparison of the photocatalytic activity of batch and continuous flow systems was investigated, using the micron sized catalyst (TiO2) previously studied. It was found that the continuous flow system is able to increase the amount of decomposable dye of up to 110% compared to that reached by the batch system. In addition, the catalyst used was found to be suitable for such continuous flow studies, with no loss in activity over a period of 42 hours. In Chapter 7, the use of ultra-bright LEDs on continuous flow systems, and the ability to apply the theory of such systems on photocatalytic reactions, were studied. It was found that the consideration of the kinetics of the photocatalytic reaction being pseudo-first order is not entirely correct, and that the new type of light source is suitable for photocatalytic degradations. In Chapter 8, some concluding remarks have been provided.
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ItemUltrasonic modification of micelle structures( 2015)The tremendous attention given to micelle systems is due to its potential uses in many scientific, biomedical and industrial applications. Micellar aggregations possess unique ability to exhibit different physicochemical properties owing to their dynamic and reversible structural transformation. This flexibility is controllable by different stimuli. In this study, the possibility of designing a variety of micelle nanostructures using ultrasound is investigated. Using ultrasound as a stimulus is an advantage as it eliminates the needs of adding external chemicals to the micellar system, and experimental parameters could be easily controlled. The fundamental properties of micelles and various forces generated by ultrasound in liquids are discussed in Chapter 1. The second chapter on Literature Review is structured in line with Results and Discussion chapters. In the first section, the use of a fluorescence based technique for the determination of critical micelle concentration as well as reported attempts in monitoring micelle structural changes are outlined. In the second section, literature dealing with structural changes in micelle systems by different stimuli is reviewed. The Reptation Reaction Model is discussed in detail in the following section. This section provides theoretical arguments on the reptation process of micelle, as well as the different reaction (recombination) routes that result in the formation of different structures of micelle. In the last section, sonochemical synthesis of gold nanoparticles is reviewed. In Chapter 3, materials used, solution preparation methods, experimental approaches and analytical methods used are described. The micelle used in this study is cetyltrimethylammonium salicylate (CTASal) prepared by an ion exchange process between cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). The ultrasonic settings chosen include different sonication reactors, frequency and applied power. In Chapter 4, a fluorescence based technique using fluorescein isothiocyanate (FITC) as a probe to monitor structural changes in micelle was successfully implemented. The method was found to be successful in detecting the critical micelle concentration (cmc) as well as for monitoring the concentration dependent structural growth of CTAB micelle system. It was then tested to the sonicated CTASal micelle system. The limitation of this technique is also discussed in this chapter. In Chapter 5, the first ultrasound-driven transformation of CTASal micelle structures is reported. The wormlike micelle formed from CTAB and NaSal was chosen due to the increasing interest of such viscoelastic micelles in recent technological applications. The sonication was carried out with a plate-type transducer at 211 kHz frequency. The wormlike micelle was found to transform to long threadlike micelle and vesicles/tubular micelle, simultaneously. These were confirmed by the cryo-TEM and rheological measurements. A mechanism for ultrasound induced micelle structural changes is proposed based on the Reptation Reaction Model. This study was also aimed to understand, and hence efficiently control CTASal micelle structural changes. This is accomplished by comparing the effect of sonication using different ultrasonic reactors as well as applying different frequencies and sonication power. This work is discussed in Chapter 6. Three types of ultrasonic transducers were used: (i) horn-type, (ii) plate-type, and (iii) high intensity focused ultrasound (HIFU). In Chapter 7, the sonochemical synthesis of gold nanoparticles using HIFU at 463 kHz is reported. This work was carried out to characterize the HIFU unit. Characterization of HIFU using snochemiluminescence imaging was also carried out. This system was then used to study the effect of HIFU on CTASal micelle structures. In Chapter 8, some concluding remarks have been provided.
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ItemEnergy efficiency and advantages of ultrasonic synthesis of nanomaterials( 2015)The physico-chemical effects of ultrasound (US) have been used widely for synthesising various materials. The focus of this project is to evaluate the energy efficiency and advantages of ultrasonic synthetic process. Poly(methyl methacrylate) and poly(methyl methacrylate)-CaCO3 nanocomposites were synthesised by conventional and US-assisted (USK) emulsion polymerization. Although the conversions obtained were similar for both processes, nanocomposites produced by USK were smaller with a narrower particle size distribution. In another study, the photocatalytic activity of CdS nanoparticles synthesized using US were compared with those synthesized using mechanical agitation on the basis of energy input. Samples synthesized using a US horn (USH) and a high shear homogeniser (HSH) showed a lower photocatalytic activity compared to those synthesized in an US bath (USB) and using mechanical stirring (NUS). However, when the power input per unit volume (W/L) is considered, the order of effectiveness of the catalysts is USB>NUS>HSH>USH, suggesting that the mild cavitation conditions generated in the USB process are sufficient to produce an efficient photocatalyst. Overall, US assistance provides improvement in conversions/yields and the dispersive effects help obtain smaller particle sizes and narrower size distributions. However, when the increased energy requirements are taken into account it is obvious that when combining US with conventional material synthesis techniques, it is imperative to choose not only the right amount of energy input but also, the right mode of US input in order to synthesize the most efficacious nanomaterials.
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ItemUltrasonic formation of stable food emulsions for the delivery of nutrients( 2014)Growing market trend for the consumption of healthy food has led to production of foods rich in bioactives. This thesis reports on the outcomes of an investigation into the formation of stable emulsions of a bioactive, flaxseed oil, in two complex liquid food matrices, namely, skim milk and carrot juice by employing low frequency high intensity ultrasound, particularly in the absence of external food additives, pre-emulsification procedures and external heat and/or pressure. Initially, the study focussed at analysing the process-induced changes to skim milk at sono-emulsification experimental conditions. Sonication of skim milk for up to 8 min (maximum emulsification time) caused minor modifications to about less than 20% of total whey proteins. The modifications to whey proteins is also an advantage since they can act as emulsifiers to stabilise ultrasonically generated flaxseed oil emulsion droplets. The minimal change noticed at short sonication time indicates the safety of the technique. The changes to whey proteins resulted in a decrease in the turbidity of milk but did not affect the viscosity of milk. The casein micelles showed stability against disruption even until 60 min of sonication, except for the very minor changes in their sizes. This work is published in Innovative Food Science and Emerging Technologies and presented in Chapter 3. The feasibility of making finer and stable 7% flaxseed oil-skim milk emulsions was explored at different power levels ranging from 88 to 176 W and processing time from 1 to 8 min. Highly stable emulsions were obtained at 3 min & 176 W and 5 min & 132 W in the absence of external food additives. The emulsion stability test reported a shelf life of 9 days at 4±2ºC against phase separation of emulsions. The efficiency of sono-emulsification was checked up to oil loading of 21%. The factors responsible for emulsion stability were identified to be partially denatured whey proteins and the optimum power-process time combinations employed in the process. The study also drew comparisons between conventional and sono-emulsification processes. High shear mixing and pressure homogenization processes were compared against ultrasound at equivalent energy densities. On comparison, it was observed that at similar energy densities, high shear process of 12.5 min at 70 W & 17500 rpm and pressure homogenization at 200 bar (5 pass) & 655 W did not produce finer and stable emulsions as observed with 5 min of ultrasound process at 176 W. This work is published in Food Hydrocolloids and presented in Chapter 4. After characterization of emulsions, the functionality studies of 7% flaxseed oil-skim milk emulsions were conducted primarily to understand the nature/characteristics of downstreamed products, viz., beverage emulsions (ready-to-drink) and yoghurts (lactic acid gel). Results showed that sonication process did not induce oxidation in emulsions and did not change their viscosity until 9 days of storage at 4±2ºC. Also, the acid gels obtained from milk emulsions showed improved functionalities such as higher gel strength, reduced syneresis, higher storage modulus and reduced gelation time against controls. The evidences obtained suggested that the newly formed fat globular surface (by sonication) stabilized by denatured whey proteins played a major role in functionality, mainly by interacting with other whey proteins and caseins during gelation and formation of network. All the observations helped at arriving at a complete mechanism of gelation of those sono-emulsions. This work is published in Ultrasonics Sonochemistry and presented in Chapter 5. Finally, the feasibility of sonication to disperse flaxseed oil in carrot juice was explored. Stable emulsions of 1% bioactives were obtained at 4 min & 176 W and the size of emulsions were 0.6 μm until 8 days of storage at 4±2ºC. The pectins and/or proteins of carrot juice were responsible for emulsion stability, either by electrostatic repulsion or steric stabilization. The emulsions showed high negative zetapotential values of above -30 mV until 9 days of storage at 4±2ºC. The results of oxidative stability of emulsions study suggested the need for inert atmospheric conditions while processing. It showed that carotenoids exhibited pro-oxidant behaviour mainly because of its concentration, presence of oxygen rich atmosphere and formation of few process-induced free radicals during sonication. The study also looked at delivering flaxseed oil in an aqueous mixture of milk and carrot juice system. The sonication of carrot juice under proposed emulsification conditions also led to many advantages in the juices, such as improvement in cloud stability, increased carotenoid content, reduction in pectin methyl esterase activity, absence of non-enzymatic browning, etc. This work is submitted in Food and Bioprocess Technology and presented in Chapter 6.
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ItemUltrasonic synthesis and characterization of multifunctional nano/microcapsules( 2013)Liquid and air-filled nano/microcapsules have potential applications in diagnostic and therapeutic medicine. Air and liquid cores are generally coated by a protein, lipid or polymer shell. Air-filled nano- and microcapsules, commonly referred to as nano- and microbubbles (NBs and MBs), are ultrasound responsive colloidal particles with a strong potential to become theranostic agents, combining the contrast and therapeutic functionalities. In the last decades, they have played a significant role as ultrasound contrast agents in diagnostic imaging. Similarly, liquid and solid encapsulated nano/ microcapsules have potential applications in targeted drug delivery in medicine. This thesis investigates the use of high energy ultrasound processing (batch and flow-through) to synthesise nano- and microcapsules using proteins and biocompatible polymers (e.g., polymethacrylic acid) in aqueous solutions. The ultrasound-assisted self-assembly and cross-linking of lysozyme at water–air and water–perfluorohexane interfaces are shown to produce lysozyme-shelled hollow microbubbles (LSMBs) and microcapsules (LSMCs), respectively. The arrangement of lysozyme at the air–liquid or oil–liquid interfaces is accompanied by changes in the bioactivity and conformational state of the protein. A comprehensive study of the microspheres (LSMBs and LSMCs) size distribution, degradability, mechanical and surface properties, acoustic response and biofunctional properties has been performed. A novel flow-through sonication technique for synthesizing stable and monodispersed nano- and micrometer-sized bubbles has been developed. It has been demonstrated that the size and size distribution of the bubbles are controlled by the active cavitation zone generated by ultrasound. The cytoxicity of LSMBs and LSMCs on adherent cell line derived from a human breast adenocarcinoma cells (SKBR3) has been evaluated. SKBR3 shows capability to phagocyte LSMBs and LSMCs in vitro. In addition, a one-pot ultrasonic procedure has been developed as a versatile route for synthesizing polymerv coated microspheres, PMASH MB, that have potential application as drug delivery vehicles. The use of biocompatible thiolated poly(methacrylic acid) as the shell material offers control over size, morphology and functionality of the microspheres. LSMBs and PMASH MBs are shown to possess echogenic properties and can be used for loading oligonucleotides. Finally, LSMBs were engineered as a support for the immobilization of gold nanoparticles and an enzyme, alkaline phosphatase, in order to develop micro-antimicrobial and biosensing devices.
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ItemUltrasonics as a new platform technology in dairy processing( 2013)It has been shown recently that the use of ultrasound (US) on dairy whey systems can lead to a reduction in the size of whey protein aggregates and a consequent reduction in viscosity. Further, application of ultrasound after heat treatment can lead to a heat stable dairy product. However, a number of unanswered questions remain regarding this approach and this thesis attempts to answer some of these questions. Prior to commercialisation, a comparison with other physical shear and high pressure processes is required to determine the viability of US in the dairy industry. The thesis shows that at identical energy density of 153 J/ml, both sonication at 20 kHz and homogenisation at 80 bar of a 5 wt% whey protein concentrate (WPC80) solution provided similar reductions in whey protein aggregate size and viscosity. Smaller reductions were observed in samples subjected to high shear mixing at the same energy density, which may be the result of excessive foaming. The work shows that free radicals are absent in both the high shear mixing and homogenisation processes, implying that high shear forces is responsible for the observed changes, rather than acoustic collapse events. In addition, heat stability was achieved in all systems, with the best results again obtained for both homogenisation and sonication. Hence, the combination of a heat treatment followed by any suitable high shear process is capable of producing a low viscosity, heat stable product. A further concern was whether ultrasound impacted only the aggregate size or whether there were more subtle changes to the secondary structure of the protein. In this thesis, the changes in aggregation and secondary structure of the individual whey proteins and their mixtures upon sonication and the combination of heat and sonication were studied. No structural changes were observed in any native protein solution upon sonication at 31 J/mL using a 20 kHz sonicator. Prolonged sonication led to minor structural changes in pure β-lactoglobulin (β-LG) and α-lactalbumin (α-LA) solutions, as shown using fourier transform infrared spectroscopy (FTIR) analysis, but these proteins remained predominantly in their native β-sheet and α-helical structure respectively. None of the heated and sonicated protein samples showed any large increases in β-sheet content. Hence, with the conditions performed for heat and US treatments in this thesis, no drastic alteration in the secondary structure of the protein were observed in US of dairy whey solutions. In the dairy industry, membrane ultrafiltration (UF) is used for the concentration of whey to produce whey protein concentrate. A further aim of the thesis was to determine the impact of ultrasound if applied upstream of the UF unit to reduce membrane fouling and increase productivity. In addition, the combination of heat and US pre treatment is investigated as it is a promising approach to produce heat stable powders while alleviating membrane fouling. The use of sonication on 5 to 10 wt% WPC80 solutions had a small but significant effect on membrane fouling – for the 10 wt% solution, the cake growth factor fell from 0.66 to 0.44 × 10 11 m/kg and the steady state flux increased from 16.8 to 17.7 L/m2.h. Similar subtle effects were observed with fresh whey, with the cake growth factor falling from 3.7 to 3.0 × 10 11 m/kg. This may reflect the low solids concentration used in these experiments and the use of more concentrated protein solutions might lead to more positive results. Conversely, a pronounced effect was observed in the heat-treated feeds: with increasing solids concentration, both pore blockage and cake growth grew for all heat-treated feeds but these two parameters remained low for the pre-heated and sonicated feed. Sonication was also found to delay the ‘gelling’ of proteins as indicated by the higher gel concentration obtained in the pre heated and sonicated feed (27 wt%) relative to a solution exposed only to heat (21 wt%). However, 100% flux recoveries upon cleaning were not achievable in heat treated feeds and surface charge measurements indicated that protein deposit remained attached to the membrane surface. This may be due to the inability of the chemical cleaning agents to break down large, denatured protein aggregates formed during heating. microfiltration (MF) of skim milk on its selectivity and productivity was also investigated. In skim milk MF, the best selectivity was obtained at the lowest TMP of 55 kPa. A pre heated and sonicated feed provided the lowest whey protein and highest casein rejections, with values of 85.8 and 97.9 % respectively, and the greatest absolute flux of approximately 10 L/m2.h. However, the selectivity and flux obtained were considerably lower than that generally observed with the use of ceramic membranes.
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ItemEffect of sonication treatment on the physico-chemical properties of starch-based suspensions( 2011)Starch, one of the most abundant polysaccharides in nature, is used in a wide range of applications, particularly in Food Industry. Ultrasound treatment has been identified as a potential Food Processing Technology. The aim of my thesis is to investigate how the application of ultrasound affects the physico-chemical properties of starch-based suspensions. Acoustic cavitation-induced structural modifications were achieved by varying ultrasonication parameters (temperature, time, sonication power and frequency) and the characteristics of the starch dispersion (botanical origins, amylose/amylopectin ratio, granular or macromolecular states and granule surface properties).