Adsorption studies using optical reflectometry
Date
2014
Authors
Howard, Shaun Christopher
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The modification of a material's surface properties is critical to many modern industries. The chemical modification of surfaces is usually achieved through the adsorption of surfactant or polymer molecules onto a surface for the purpose of altering rheology, molecular interactions, optical properties, etc. An understanding of the mechanism underpinning the adsorption process has the potential to optimise properties with minimal waste. The work presented in this thesis seeks to uncover additional information about the mechanism of adsorption using a model surfactant cetyl trimethylammonium bromide (CTAB) and a variety of electrolytes to explore some of the nuances involved in the physical-chemical processes. To accomplish this, an Optical Reflecometer (OR) was designed and constructed to improve on resolution of prior instruments, and was used in conjunction with complementary measurement techniques (Quartz Crystal Microbalance, Atomic Force Microscopy) to obtain new insight into the mechanism of adsorption. Isotherms for the adsorption of CTAB onto silica are presented for a family of electrolytes (NaBr, KBr and CsBr) at varying concentrations, allowing for the determination of the common intersection point (CIP) for each system. The CIP represents the point on the isotherm where the dominating driving force in adsorption changes from electrostatic to hydrophobic interactions. In the absence of specific-ion affinities between the solution and the surface, the CIP should be independent of the background electrolyte used. The results of this work indicated that the Cs+ ion shows a preferential affinity for the surface. Time scales of adsorption are explored in great detail for the CTAB + NaBr and CTAB + NaSal systems through the construction of isotherms derived from both OR and QCM studies. These isotherms, along with a series of concentration cycling (ziggurat) experiments elucidate more information regarding the true nature of equilibrium surface excess and identify a range of concentrations over which adsorption is found to occur much slower than previously thought. A mechanism is proposed by which surface bound aggregates grow through monomer addition, not by bulk aggregate collision with the surface. OR is shown to have uses above and beyond simple adsorption, as demonstrated by the successful observation of the swelling-collapse behaviour of a stimulus responsive microgel film. By combining OR data with analogous results obtained from QCM, we are able to infer information regarding the viscoelastic properties of the microgel at various points in its conformational lifecycle. The conclusion is that swelling occurs much more rapidly than collapse, which we attribute to the formation of a dense skin layer on the surface which hinders diffusion of water out of the film.
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