Doctor of Philosophy (PhD), Ohio University, 2020, Chemistry and Biochemistry (Arts and Sciences)

Investigating the specific local environment and molecular interactions at liquid surfaces is crucial in comprehending physical, chemical, and biological processes. Probing the interfacial molecular conformations will provide an insight into the relationship between surface structure and the governing interactions at the surface. Herein, this dissertation used the approach of the effect of substituents on the interfacial conformation of a methacrylate backbone, to observe such surface structure interaction relationships. The ethyl end of the methacrylate monomer was substituted with bulky groups and electron-withdrawing groups. These substituted monomers were synthesized via nucleophilic addition elimination reaction and characterized using sum frequency generation (SFG) spectroscopy at the air-liquid interface. The spectroscopic results were correlated with surface tension measurements and the overall dipole moment of the molecules. The presence of bulky substituents affected the orientation distribution of the interfacial molecules. On the other hand, in the presence of electron-withdrawing groups, the intensity of vibrational modes was affected, suggesting the changes in interfacial molecular conformations and existing intermolecular interactions. In another project, quaternary ammonium surfactants were utilized to assess their conformation and orientation at the air-water interface using SFG spectroscopy. Herein, the approach of solvent isotopic substitution was used to investigate the surfactant water interactions. The results showed the addition of deuterated water rearranges its head group parallel to the interface and straightens its chain with reduced gauche defects. The change in the conformation of the surfactant molecules at the air-liquid interface showcased the difference in intermolecular interactions for water and deuterated water. In summary, these studies revealed the importance of SFG spectroscopy as a tool to probe surface structures in gauging m (open full item for complete abstract)

Committee: Katherine Cimatu (Advisor) Subjects: Chemistry; Molecules; Optics; Physical Chemistry; Polymers

Doctor of Philosophy, University of Akron, 2018, Polymer Science

Interfaces play a crucial role in phenomena such as wetting, adsorption, adhesion, friction, heterogenous ice-nucleation, and biocompatibility. The interfacial molecules exhibit unique behavior due to missing interactions at the surface (or differing interactions across an interface). Designing molecules for targeted applications demands a thorough understanding of the connection between molecular-level interfacial interactions and macroscopic observables, which is currently limited due to the difficulties in accessing the buried solid/liquid and solid/solid interfaces in situ. In this dissertation, we employ interface-sensitive infrared-visible sum frequency generation (SFG) spectroscopy to probe the interfacial structure of simple liquids (or liquid mixtures) as well as complex proteins next to a sapphire substrate and discuss the ramifications for macroscopic phenomena such as adsorption, solidification, and adhesion. SFG, being a second order non-linear optical technique, provides insights into the interfacial structure, orientation, and concentration of molecules. First, the competitive adsorption to sapphire from three binary liquid mixtures, acetone-chloroform, tetrahydrofuran (THF)-benzene, and N,N-dimethylformamide (DMF)-benzene, has been investigated using SFG. The preferential adsorption of one component over another forms the basis for a variety of applications such as separation or purification using membranes or column chromatography, as well as biological implant acceptance or rejection. The relative interfacial concentrations of the two components from binary mixtures are determined by analyzing the shape of the sapphire hydroxyl peak. By fitting the adsorption isotherm with the thermodynamic Everett model, the differences in interfacial energies (Δγ) of the two components with the sapphire substrate are determined. These are then compared with the Δγ values calculated using the Dupre-Fowkes approach. The calculated Δγ values are consistent with the (open full item for complete abstract)

Committee: Ali Dhinojwala (Advisor); Mesfin Tsige (Committee Chair); Mark Foster (Committee Member); Toshikazu Miyoshi (Committee Member); Jutta Luettmer-Strathmann (Committee Member) Subjects: Materials Science; Polymers