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Title: The supramolecular structure of surfactant formulations
Author: Hussain, Mariam
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2020
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Powdered detergent products are formulated with various components, such as aqueous polymers and surfactant solutions, to which powdered components such as salts and zeolites are added and these are mixed at a high shear rate for a few hours. The interactions between the surfactant and polymer directly influences the supramolecular structure (particularly the phase and colloidal structure) of the formulation. This in turn influences the final detergent powder structure, properties and performance. Therefore, a thorough understanding of the structural changes resulting from the addition of polymer to aqueous NaLAS dispersions, as well as the structural changes arising from shearing the formula is vital when considering cost effective formulation decisions, but the effect of both of these factors are poorly explored. This PhD is designed to understand the influence of the addition of polycarboxylate polymer on the phase and colloidal structure of linear akylbenzene sulphonate (NaLAS) surfactant dispersions in aqueous conditions and saturated sodium sulphate solution conditions. For the aqueous system, a phase diagram was constructed using centrifugation and optical polarised microscopy. Following this, a more in-depth understanding of the phase behaviour was realised using Small Angle X-Ray scattering (SAXS), as well as the influence of polymer on the colloidal structures using 2H nuclear magnetic resonance NMR. The combination of these studies led to the conclusion that the polymer behaves as a flocculant in the system, causing phase separation in the micellar regime. Additionally, a decrease in the bilayer spacing in the lamellar phase was observed due to the addition of the polymer compressing the bilayers. Also, the the osmotic effects resulting from the polymer addition caused multilamellar vesicle fusion, which then sub-sequently increased the average multilamellar vesicle size. This change in vesicle size was measured using 2H (NMR). Following this, the influence of the addition of the polymer on microstructure of NaLAS in saturated sodium sulphate solution were explored. Firstly, by constructing a phase diagram and then followed by pulsed-gradient simulated train echo (PGSTE) diffusion experiments to characterise the microgeometry of the system. How this microstructure changed with polymer concentration was also explored. A phase diagram of the system which was constructed using centrifugation, showed that the addition of the polymer increased the ratio of the lamellar phase to the isotropic phase until there was just a single lamellar phase remaining. Surface area to volume ratio, pore size and tortuosity measurements using NMR,as well as SAXS experiments helped show that this change in ratio was not due to microgeometry changes. Hence, it was likely that the ratio change observed was a result of the polymer sterically stabilising the vesicles. Finally, the structural changes over time resulting from the application of a high constant shear rate on a model formulation system were explored. Upon the application of high constant shear rates (above 1800 s−1), an irreversible increase in the viscosity was observed. In-situ cryo-SEM experiments were carried out to conclude that in this system prior to the high shear mixing, kinetically stable vesicles were present due to the low bubble rise velocity resulting from the high viscosity of the system without the bubbles. The application of high shear rates for extended periods of time increased the Reynolds number of the system, which promoted bubble coalescence, and this consequently significantly increased the bubble rise velocity. This bubble rise velocity increase in time reduced the bubble fraction in the system, and hence the measured viscosity. This is because the high shear rates significantly increases the Reynolds number within the system to a turbulent regime, promoting the creaming of the bubbles to the surface and subsequently bubble breakdown. The addition of the polycarboxylate polymer prevented all the time dependent viscosity changes as inhibited the formation of kinetically stable bubbles in the system.
Supervisor: Bayly, Andrew ; Cayre, Olivier ; Nagaraj, Mamatha Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available