Dissertation/Thesis Abstract

Vibrational Sum Frequency and Raman Spectroscopic Studies of Air-Aqueous Interfaces and Solutions of Nitrate Salts, and Air-Silica Surface Adsorption Studies
by Xu, Man, Ph.D., The Ohio State University, 2008, 150; 10631392
Abstract (Summary)

Nitrate ions are ubiquitous in aqueous-phase atmospheric aerosols as well as surface and ground waters, and are involved in a variety of reactions in the atmosphere, hydrosphere, and biosphere. A fundamental understanding of nitrate ions at air-aqueous interfaces is of prime importance with respect to understanding heterogeneous atmospheric chemistry and geochemistry. In the studies presented in this dissertation, the nature of nitrate ions and metal-nitrate ion pairing at air-aqueous interfaces is investigated by an interface specific technique, vibrational sum frequency generation (VSFG) spectroscopy.In order to understand ion pairing in the interfacial region, the first approach is to investigate ion pairing in aqueous solutions. Raman and infrared spectroscopies were employed for the bulk study. Results show that Pb2+ has a strong tendency to form ion pairs with nitrate, and the formation of ion pairs increases with increasing cation size: Mg 2+ < Ca2+ < Sr2+</sub> < Pb2+ .

At the air-aqueous interface, the vibrational symmetric stretch mode of nitrate was directly probed by VSFG. Analysis of the VSFG spectra reveals the perturbation from cation-anion interactions on interfacial nitrate ions. Mg(NO3)2 is particularly interesting at air-aqueous interfaces. At low concentration of Mg(NO3)2, interfacial nitrate anions are relatively free from Coulombic effects of Mg2+ cations, which indicates that the number of water molecules available for solvation significantly affects the extent of interfacial ion pairing.

The addition of nitrate salts also has considerable effects on the interfacial water structure, as observed in our VSFG study of the OH stretching modes. Results reveal that the presence of ions and the ion pairs perturbs the interfacial water hydrogen-bonding network, and the perturbation varies with cation. We propose an increasing interfacial depth as the cation size increases: Mg 2+ < Ca2+ <; Sr2+.

In additional studies, distinct from the ion-pairing work, the surface acidity of silica was determined using a surface acidity probe molecule, ethylenediamine. The VSFG study of ethylenediamine adsorption onto silica surfaces indicates that the adsorption occurs via protonation of an amine group by a silica surface silanol site. The surface acidity of silica is therefore between two pK a values of ethylenediamine: 7.56 to 10.71.

Indexing (document details)
Advisor: Allen, Heather C.
Commitee: Coe, James, Lenhart, John
School: The Ohio State University
Department: Environmental Science
School Location: United States -- Ohio
Source: DAI-B 78/11(E), Dissertation Abstracts International
Subjects: Chemistry, Geochemistry
Keywords: Air-aqueous interfaces, Air-silica surface adsorption studies, Nitrate salts, Raman spectroscopic studies, Solutions, Vibrational sum frequency
Publication Number: 10631392
ISBN: 978-0-355-01639-0
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