Solvation, Structure and Organization at Liquid Surfaces

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This dissertation presents the results of nonlinear spectroscopic studies whose goal is to understand how the asymmetric nature of interfaces and intermolecular interactions give rise to interfacial solvation properties and solvent structure. The first part of this thesis uses resonance enhanced second harmonic generation to examine the polarity and hydrogen bonding opportunities at interfaces formed between hydrophilic silica and both weakly and strongly associating organic liquids. Measuring interfacial electronic spectra of probe molecules that exhibit solvatochromic sensitivity to polarity and hydrogen bonding, we saw that small changes in solvent structure affect interfacial polarity, and strongly associating alcohols solvents create a region of heterogeneous polarity at the interface. Silica appears to donate hydrogen bonds to adsorbates no matter what solvent (protic or aprotic) was chosen.

The second part of this dissertation uses another nonlinear spectroscopic technique, vibrational sum frequency generation, to determine the structure and orientation of solvent molecules adsorbed to silica/vapor, silica/liquid, and neat liquid/vapor interfaces.  By comparing spectral features appearing under different experimental polarization conditions, we have determined average solvent orientations and degree of organization.  Our initial studies of alkanes adsorbed to the silica/vapor interface show that despite strong substrate-adsorbate interactions, molecules at the interface show some degree of long range order and organization.

In order to examine how the strength of intermolecular forces between adsorbates and either the substrate or neighboring molecules affect interfacial organization, we measured vibrational spectra of octanol isomers as well as different functional group containing n-alkyl molecules at silica/vapor and silica/liquid interfaces.  The octanol studies show that strongly associating molecules form ordered monolayers at the silica/vapor interface, but that strength of lateral interactions is important for preserving that order when the liquid is brought into contact.  Branched isomers appeared very disordered at solid/liquid interfaces.

Further examining this change in order between solvents at silica/vapor and silica/liquid interfaces using equal length but different functional group containing solvents, we see that the energetics of adsorption and solvation are likely to be responsible for the degree of order both at the solid/vapor surface (adsorption) and solid/liquid interface (both adsorption and solvation).