School of Chemistry - Theses
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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.