THE LIQUID ACETONITRILE/SILICA INTERFACE: A MODEL SYSTEM FOR REINTERPRETING THE VIBRATIONAL SUM-FREQUENCY GENERATION (VSFG) SPECTRA OF LIQUID/SOLID INTERFACES
Souna, Amanda J
Fourkas, John T.
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The interfacial properties of liquid acetonitrile at solid surfaces play an essential role in processes that occur in applications such as heterogeneous catalysis, chromatography, and batteries. In this thesis, the acetonitrile liquid/silica (LS) interface is used as a model system to develop a more complete approach to the interpretation of vibrational sum-frequency generation (VSFG) spectroscopy, a nonlinear optical technique that is indispensable for exploring interfacial organization and dynamics. VSFG experiments, in combination with molecular dynamics (MD) simulations, indicate that the acetonitrile structure at this interface has properties similar to a supported lipid bilayer. In the first part of this thesis, I use this system to explore the influence of dynamics and intermolecular vibrational coupling on VSFG spectra of the methyl stretch. Time-resolved experiments examine the spectral contribution of the more dynamic second sublayer upon introducing an IR-probe delay. The resulting spectra could not be fit with a static line shape, indicating that there is a time-dependent vibrational frequency that results from reorientation-induced spectral diffusion (RISD), a phenomenon in which molecules experience different dielectric environments as they reorient. The temperature dependence of VSFG spectra was consistent with the influence of more rapid reorientation and an earlier onset of RISD. Close proximity and specific intermolecular configurations can allow transition dipoles to couple, allowing vibrational energy to be shared collectively between nearest neighbors via an excitonic mechanism. The influence of this coupling on VSFG spectra was quantified by performing an isotopic dilution study on the acetonitrile LS interface. In the last part of this thesis, VSFG spectral trends are shown to predict the onset and degree of ion current rectification in electrolyte solutions in acetonitrile through glass nanopores. VSFG studies on LiClO4, LiBF4, NaClO4, and LiPF6 solutions in acetonitrile at an LS interface indicate how ions are incorporated into and modify the bilayer structure. The conclusion that charge distributions at the structured interfaces of nonaqueous electrolyte solutions cannot be interpreted using the standard models for charged interfaces will have wide-ranging applications and guide us in the interpretation of third-order contributions to VSFG spectra.