- 100 views
- 145 downloads
Studying the Molecular Structure of Aqueous/Mineral Oxide Interfaces by Vibrational Sum Frequency Generation Spectroscopy and Zeta Potential Measurements
-
- Author / Creator
- Rashwan, Mokhtar
-
Water/mineral oxide interfaces are ubiquitous in various atmospheric, geological, biological, and industrial systems. Hence, studying such abundant interfaces is essential for understanding the macroscopic interfacial reactions, such as adsorption/desorption, dissolution, flocculation, coagulation, and other chemical reactions. However, given that solid/liquid interfaces are buried, they are not easily accessible by conventional characterization techniques. Therefore, employing a surface/interface-sensitive characterization technique that can provide a molecular picture of the chemistry at mineral oxide/aqueous interfaces is necessary for gaining insight into the electrical double layer structure, as well as ion-mineral and mineral-mineral interactions under different bulk solution conditions. Such a molecular understanding is essential for improving the efficiency of industrial and geochemical processes involving aqueous mineral oxides, such as mineral dissolution and aggregation, mineral beneficiation and agglomeration, and oil sand tailing treatment. Vibrational sum frequency generation spectroscopy (VSFG), an interface-sensitive spectroscopic technique, has been widely used for studying aqueous/mineral oxide interfaces. VSFG gives interfacial molecular information through monitoring changes in vibrational features of SFG-active vibrational modes of water molecules and mineral (hydr)oxides hydroxyl species at the studied interface. Hence, probing the water and other molecular species with mineral oxides can shed light on the structure of the electric double layer. Furthermore, combining VSFG with -potential measurements, such as streaming potential and electrophoretic measurements, can provide a comprehensive molecular picture of the structure of the electrical double layer, mineral-mineral interactions, and surface reactions under different bulk solution conditions at mineral oxide/aqueous interfaces. Using VSFG together with -potential measurements, we investigated the electrical double layer structure at the silica/aqueous interface under different pH conditions in the presence of divalent calcium ions. We observed charge neutralization of the silica surface upon increasing the pH from 6 to 10.5, corresponding to a minimum in the interfacial water SFG signal intensity and an isoelectric point at pH 10.5 from streaming current measurements, followed by charge inversion at higher pH. By correlating the -potential measurements with the presence of a peak attributed to CaOH+ we were able to shed light on the mechanism of overcharging for this system. We also investigated the effect of bulk solution pH and the nature of the alkali medium on silica-kaolinite interactions as they are the most significant constituent of oil sands tailings. For the first time SFG provided a spectral signature of the kaolinite mineral. Furthermore, SFG measurements of pH-variation experiments on the silica/kaolinite interface showed the sensitivity of the silica/kaolinite interface to the nature of the alkaline medium. Our results suggest that lime promotes disordering of water at the silica/kaolinite particle interface at pH 12 and above. With NaOH addition, however, the interfacial water SF intensity is still significant even under highly alkaline conditions. Furthermore, the nature of silica-kaolinite binding was highly attractive at very high pH with the addition of both lime and NaOH solutions. Providing molecular information about clay adsorption and water structure should improve the efficiency of mineral processing systems involving silica and clay minerals, such as dewatering of oil sand tailings.
The effect of bulk solution pH and surface morphology on the aqueous/titania interface was also investigated using sum frequency generation spectroscopy. The surface structure of the titania/aqueous interface was found dependent on morphology, as shown in the noticeable differences in the vibrational features of the two surfaces for planar and nanoporous titania. The SFG spectral intensity in the water stretching region of the nanoporous surface was higher than the planar substrate at all pHs suggesting a greater amount of adsorbed water molecules at the nanoporous surface, attributed to the different surface charge densities and surface areas, in addition to the different methods of preparation of the titania surfaces. Moreover, the SF spectral intensity was modulated by pH, where a minimum was observed at pH 4 on both surfaces, corresponding to the isoelectric point of the studied titania surfaces. The pH where the maximum water intensity was observed differed for the two surfaces highlighting the sensitivity of the titania/aqueous interface to the morphology of the prepared surface under different pH conditions. -
- Subjects / Keywords
-
- Graduation date
- Fall 2021
-
- Type of Item
- Thesis
-
- Degree
- Doctor of Philosophy
-
- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.