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Probing surface properties at the nanoscale: effects of physicochemical heterogeneity on membrane wettability
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- Author / Creator
- ISMAIL, MD FARHAD
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Water purification by polymeric membranes has been abundantly applied to address global challenges of water scarcity and pollution of aquatic environments. Polymeric membranes are excellent candidates for low cost and energy-efficient high-quality water purification process. However, progress in polymeric membranes has been constrained due to their fouling propensity during the filtration process. It is anticipated that next-generation membranes will be highly selective and fouling-resistant. These two parameters can be controlled by three surface parameters, i.e., wettability, roughness, and surface charge due to surface functional groups. Given that, the development of novel materials and proper surface modification approaches are imperative to substantially advance the water purification technology. Thus, proper characterization and tuning of the surface properties are paramount to develop novel material for next-generation membrane.
This work aims at developing novel methodologies to study the membrane surface physicochemical properties through wettability analysis at room and elevated temperature. It is worth noting that techniques to measure the physicochemical properties at the interfacial region with high sensitivity are significantly limited, considering the number of new materials being developed. Such measurement and characterization techniques include contact angle (θ), X-ray photoelectron spectroscopy (XPS), and Atomic force microscopy (AFM). Each of these available techniques provides information about the constitution of a layer of different thicknesses at the polymer surface, which is known as interphase. For example, “θ- interphase” may refer to the outer ~1 nm of a solid surface that contributed to the surface wettability. Likewise, the "XPS interphase" may refer to the outer ~10 nm of a solid surface, which is accessible to the XPS measurement technique. In some cases, surface functional group does not influence wetting as it remains outside of the θ-interphase and thus nevertheless accessible to reagents in solution. The portion of the solid where these types of interfacial interactions are possible can be referred to as the "sub- θ interphase". This interphase is less well-defined than those associated with the available characterization techniques. A detailed understanding of the solid-liquid interfacial interactions at the "sub- θ interphase" can be useful in evaluating the permeation properties of membranes such as selectivity and rate of fouling.
The initial phase of this thesis will study how the wettability analysis determines the physical and chemical heterogeneity. Theoretical frameworks have been developed to predict the physical and chemical heterogeneity based on the quantification of interfacial energy. This further provides useful insights on the membrane permeation properties, e.g., perm-selectivity, and rate of fouling. AFM and streaming potential methods were also employed to compare the obtained results from the wettability analysis. Our experimental results reveal that the wettability method can provide fast and appropriate understandings on the effects of physical and chemical heterogeneity over membrane permeation properties. The second phase of the study is focused on the surface adaptation and responsive behavior due to the change of the surrounding medium. Many polymeric surfaces reversibly change their properties due to the contact and molecular interaction with another liquid. The developed theoretical framework can predict the experimental observation of solid-liquid-liquid wettability within ±~15% deviation. The experimental data reveal that solid surface tension cannot be an intrinsic property which may alter due to the change of the surrounding liquid. Finally, the role of the surrounding medium temperature over the solid surface tension has been studied systematically. The outcome of this study reveals that surface tension of the polymeric surfaces alters due to the increase of surrounding medium temperature. This alteration becomes significant if all the three phases (solid-liquid-liquid) exhibit some polarity.
Overall, the specific goal of this research is to provide a detailed insight regarding the effect of membrane surface heterogeneity in air and aqueous media on membrane wettability, and its subsequent impacts on membrane performance. This work further provides a novel methodology for fast and accurate characterization of polymeric membranes by wettability analysis. The outcome of the present research can also be readily extended to many other related engineering processes. With the relationship between interfacial energy and permeation properties, this work paves the way for designing novel materials with desired transport properties for the efficient separation process. -
- Graduation date
- Fall 2020
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.