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Studies of charged molecules at the air/water interface by sum frequency generation vibrational spectroscopy. Hu, Dan


This dissertation studies the surface chemistry of water and charged molecules using phase-sensitive sum frequency generation (SFG) vibrational spectroscopy. The studied molecules include surfactants, polyelectrolytes, bitumen, and ionic liquid, which are related to technological processes, such as surface modification, catalysis, and bitumen production. Interactions of the polyelectrolyte partially hydrolyzed polyacrylamide with water and cations at air/liquid interfaces were studied. The polyelectrolyte caused water molecules to re-orient with the hydrogen pointing toward the air. The addition of Na⁺ counteracted the negative charges of the polyelectrolyte. Divalent cation Ca²⁺ formed a polymer-ion complex with the polymer and completely destroyed the ordered water structure. The addition of polyelectrolytes to a surfactant solution caused a complex behavior of the surface tension. SFG studies showed that the complex surface tension behavior was the result of a surface charge reversal. A better ordered interfacial molecules produced low surface entropy, which counteracted the surface enthalpy decrease and kept the surface tension nearly unchanged at a low surfactant concentration. The ordering of water was found to play a role in surface tension. Four types of surfactants were studied: nonionic, zwitterionic, anionic, and cationic surfactants. Particularly, ionic surfactants decreased the surface entropy to near zero or even negative, which was associated with a surfactant-induced ordering of surface water molecules and an increase in hydrogen bond formation. Both effects lead to the reduction of water’s surface entropy. Studies of bitumen/water interfaces using phase-sensitive SFG showed that the bitumen surface carried negative charges, which induced a well-ordered water structure at the bitumen/water interface. The presence of salt neutralized the surface charge and nearly destroyed the ordered water structure. Both anionic and cationic surfactants interacted with the bitumen surface. Finally, the water structure at the air/1-butyl-3-methylimidazolium tetrafluoroborate aqueous solution interface was studied. The orientation of water molecules indicated that a charge reversal occurred at the interface when the concentration of the ionic liquid (IL) changed. The imidazolium cations resided at the water surface at a low IL mole fraction. However, with an increased IL mole fraction, the surface number of anions increased making the surface negatively charged.

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