Non-covalent and Ion-specific Interactions in Charged Macromolecules

Sadman, Kazi

doi:https://doi.org/10.21985/n2-dwnj-qs69

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Non-covalent and Ion-specific Interactions in Charged Macromolecules

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Non-covalent and ion-specific interactions in the context of charged polymers are ubiquitous in nature and in synthetic applications. For example, mussels utilize metal-coordinate bonds to form tough underwater adhesion to a wide range of substrates. The sandcastle worm uses coacervation of oppositely charged polymers to build robust structures for self-defense. In new antimicrobial macromolecule design, the balance of cationic charge and hydrophobicity can allow broad spectrum specificity to bacteria while preserving non-toxicity to mammalian cells. These biological inspirations motivate a deeper understanding of non-covalent interactions between charged polymers, or polyelectrolytes, such that these interactions may be carefully engineered for synthetic applications such as surfactants, electro-deposited coatings, and fouling-resistant surfaces. This thesis will explore a number of non-covalent interactions in model systems of polyelectrolyte complexes (PECs), which are materials composed of oppositely charged polyelectrolytes. PECs were deemed as intractable materials from a processing standpoint since they cannot be melt processed like traditional thermoplastics, and are not soluble in any organic solvents. To overcome these significant limitations, the layer-by-layer assembly technique was developed where alternating monolayers of polycations and polyanions were deposited sequentially on a substrate. A further advancement came with the discovery that PECs can be dissolved into associating polymer solutions using concentrated salt solutions, which permits traditional polymer casting techniques to be used. The associative behavior of PECs are affected by hydrophobicity, ion-pairing affinity, molecular weight, topology, and hydrogen bonding interactions among other factors, making these materials an excellent test case to assess the role of individual non-covalent interactions. Furthermore, since PECs respond to the solution ionic strength, they are also a great platform to test the interaction of different salts with charged macromolecules. This thesis will focus on strategies to control the deposition of PECs into thin-films, quantify- ing their mechanical behavior, and elucidating the role of salt identity in modulating their response. Chapter 2 will introduce a one-step electrochemical approach for depositing PEC films which overcomes the low-throughput nature of the commonly used layer-by-layer technique. Chapter 3 will focus on the role of hydrophobic effects in dictating the mechanical behavior of PECs, and Chapter 4 will focus on the response of very strongly associating complexes that are nearly insoluble in any context. Together, Chapter 3 and 4 will build a comprehensive picture of hydrophobic and ion-specific interactions in PEC materials and will emphasize the importance of the complex’s water fraction for governing the rheological properties. To achieve these goals, the quartz crystal microbalance (QCM) will be developed and utilized as an advanced rheometer to simultaneously measure the areal mass and viscoelastic properties of PECs. This approach will provide deep physical insights into the behavior of these materials because the QCM is ideally suited to quantifying the response of materials where a change in mass is associated with a change in the mechanical properties. In the case of PECs, rheological properties are primarily governed by the water content of the materials, which can be measured accurately on the QCM in response to added salt which swells the complex. Since this technique operates at a single high frequency, it also allows exquisite resolution of time-dependent changes in properties. However, using the QCM is often a challenge for casual users since it requires an understanding of thickness limits for sample preparation, and requires one to use the correct viscoelastic analysis. This work will provide the reader with a guideline for best practices to use the QCM as a rheometer, outline the thickness limitations for accurate viscoelastic analysis, and provide a MATLAB data analysis GUI (found here) that can directly analyze data from the popularly used QCM-D instrument. Therefore, another aim of this work is to provide any users of the QCM with intuition and framework behind how to design, perform, execute experiments, and finally analyze the obtained results. Lastly, this work will investigate the adsorption of highly charged polyelectrolytes to oil/water interfaces. The behavior of charged molecules at interfaces is of significant technological importance due to its relevance in surfactant science. While the behavior of amphiphiles at interfaces has been well-studied, that of highly charged macromolecules has not. Yet quantifying the behavior of highly charged molecules near hydrophobic interfaces is of paramount importance for many chemical and biological contexts, for example, to inform the design of new antibiotics for drug-resistant bacteria. The significant role of ion solvation in mediating the adsorption of charged macromolecules will be quantified and discussed.

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