Investigation Of Molecular Orientation And Structure At Air/Aqueous Solution Interfaces Of Electrolytes And Dye Molecules Using Sum Frequency Generation Spectroscopy

Vapor/liquid interface extensively exists in the nature. Many important reactions or processes in environment, catalysis, biology, geology take place at vapor/liquid interfaces. The study of the structures and dynamics at vapor/liquid interface at molecular level is crucial for understanding these reactions. Recently, the surface-specific second order nonlinear optical spectroscopy - sum frequency generation vibrational spectroscopy has been widely used to study the vapor/liquid interfaces. In this dissertation, polarization and configuration dependent sum frequency generation vibrational spectroscopy (SFG-VS) was used to study the molecular structure, orientation, and adsorption at various vapor/liquid interfaces. The information we obtained is important for understanding the electronic excited state, orientation, and dynamics of interfacial molecules. The dissertation contains eight chapters.Chapter 1, briefly reviews SFG studies at air/liquid interfacesChapter 2 presents the detailed description of basic theory of single and double resonant sum frequency generation spectroscopy and data analysis.Chapter 3 studies the orientaional structure of vapor/ionic liquid interface with the SFG polarization and experimental configuration analysis. Previous studies orientational structure of cation at ionic liquid surface lacked of consistencies between different experimental techniques. In this chapter, the systematic polarization and configuration SFG study on vapor/[bmim][PF6] interface revealed the coexistence of two distinct orientation angles for the methyl group of the butyl chain of the interfacial [bmim] cations. These spectral observations partially agree with the previous molecular dynamic studieds and provide the direct evidences of revealing the complexity and inhomogeneity of the ionic liquid surfaces.Chapter 4 discusses the reconstruction and reorientation of cations adsorbed at air/[bmim][BF4] aqueous solution of low concentrations. At very low concentrations, the butyl chain exhibited a significant gauche defect, indicating a disordered conformation; and the cation ring oriented at the surface with a fairly small tilt angle.When the concentration increased, the cation ring tended to lie flat at the surface; and the gauche defects of the butyl chain decreased due to a chain-chain intermolecular interaction and the consequent more ordered interfacial molecular arrangement. Additionally, the anti-symmetric stretching mode in the PPP and SPS spectra exhibited a peak shift, indicating that there exists more than one kind of orientation or chemical environment for the butyl CH3 group.Chapter 5 studies the adsorption of cation and anion of ionic liquid [bmim][MS] at air/water interface. At low concentrations, the cation that adsorbed to the surface increased dramatically as the concentration increased, in the meanwhile the surface tension of the solution decreased significantly. When the surface density of the cation approached its maximum, the decrease of the surface tension became less obvious. When the concentration of the solution approached 0.1 mole fraction, the anion began to adsorbe to the surface and the orientational angle of the cation changed from 20-30°to 40-50°. The data indicating that at the concentration of 0.1 mole fraction, the cation and the anion began to form micelle-like aggregates. These findings provided important information for understanding the surface behavior of water-miscible ionic liquid and surface tension of ionic liquid aqueous solutionsChapter 6 reports the specific ion effect of alkaline earth chloride at air/water interface. The sum frequency vibrational spectra indicate that the influence of the alkaline cations on interfacial hydrogen-bond follow the decreasing ranking order:Ca2+, Sr2+, Ba2+, exibiting specific cation effect. We used the polarization null angle method to measure the orientation angle of free O-H at air/alkaline earth chloride aqueous solution interface. The results showed that as the concentration increased the orientation angle of free O-H became more tilted. The surface tension also varied in the same way indicating that the surface tension of electrolyte aqueous solution is somehow related to the orientation of the free O-H.Chapter 7 applies double resonance sum frequency generation spectroscopy (DR-SFGS) to study the coupling between electronic excited state and the high vibrational states of the ground state. The knowledge about electronic excited state and its’coupling between vibrational state is still limited. In this chapter we observed strong visible-IR SFG process in DR-SFGVS of Coumarin 314 adsorbed at air/water interface. The observed visible-IR process was much stronger than the IR-visible process. The electronic SFG spectra of Coumarin 314 adsorbed at air/water interface also indicate the existence of strong coupling between electronic excited state and the high vibrational states of the ground state. These results provide important information for understanding the electronic and vibrational structures of interfacial molecules.Chapter 8 is the overall review of this dissertation and the perspective of vapor/liquid studies.