Molecular dynamics simulations and vibrational sum frequency generation (VSFG) experiments in the methyl-stretching spectral region have been used to study acetonitrile at the silica/liquid, silica/vapor, and liquid/vapor interfaces. Our simulations show that, at the silica/liquid interface, acetonitrile takes on a considerably different structure than in the bulk liquid. The interfacial structure is reminiscent of a lipid bilayer, and this type of ordering persists for tens of Ångstroms into the bulk liquid. This result has important implications for processes involving solid/acetonitrile interfaces, such as heterogeneous catalysis and chromatographic separations. Fitting of VSFG data that have an extremely low nonresonant background contribution provides strong evidence for interfacial populations pointing in opposite directions at these interfaces, in agreement with our simulations. The picture developed from our simulations and experiments reconciles conflicting interpretations of data from previous experimental studies of interfacial acetonitrile.

Using a transmission-spectrum-based method, the refractive index of a 50 μm thick sample of poly(methyl methacrylate) (PMMA) was measured as a function of wavelength. To mitigate the effects of nonplane-parallel surfaces, the sample was measured at 16 different locations. The technique resulted in the measurement of index at several thousand independent wavelengths from 0.42 to 1.62 μm, with a relative RMS accuracy <0.5×10(-4) and absolute accuracy <2×10(-4).

Resonance enhanced second harmonic generation (SHG) has been used to identify solvation mechanisms at different solid/liquid interfaces. Solvation interactions are characterized as being either nonspecific and averaged over the entire solute cavity or specific, referring to localized, directional interactions between a solute and its surroundings. SHG spectra report the electronic structure of solutes adsorbed to silica/organic solvent interfaces, and different solutes are chosen to probe either interfacial polarity or interfacial hydrogen bond donating/accepting opportunities. SHG results show that interfacial polarity probed by p-nitroanisole depends sensitively on solvent structure, whereas hydrogen bonding interactions probed by indoline are insensitive to solvent identity and instead are dominated by the hydrogen bond donating properties of the polar silica substrate. The bulk solvation interactions were modeled with a series of ab initio calculations that characterized solute electronic structure within a dielectric continuum and in the presence of explicit, individual solvent molecules. Collectively, these measurements and calculations create a comprehensive picture of how solvation mechanisms vary at different polar, solid surfaces.

Propionitrile has been studied at the liquid/ vapor, silica/vapor, and silica/liquid interfaces using vibrational sum-frequency generation (VSFG) spectroscopy and optical Kerr effect (OKE) spectroscopy. VSFG studies show that the alkyl tail of propionitrile tends to point into the vapor phase at the liquid/vapor and silica/vapor interfaces. At the silica/liquid interface, all vibrational resonances except for the methylene symmetric stretch exhibit strong cancellation in the VSFG signal. This result supports the existence of a lipid-bilayer-like organization at the silica/liquid interface, in agreement with simulation. OKE data for propionitrile confined in porous sol−gel glasses indicate that there is a surface layer, with a thickness of roughly 4 Å, that experiences inhibited orientational dynamics. The OKE data thus corroborate the picture of interfacial organization suggested by the VSFG results.

We report the development of a counter-propagating, broadband vibrational sum-frequency generation spectrometer based on a Ti:sapphire regenerative amplifier. We present simple procedures for aligning the spectrometer and for setting the timing of the IR and visible pulses. We demonstrate that the use of this geometry offers a number of important advantages over a co-propagating geometry, including a high dynamic range, reduced nonresonant background signal at buried interfaces, and minimal beam deviation upon changing samples.

Static contact-angle measurements have been used to study water and five organic liquids (n-decane, benzene, acetonitrile, octyl cyanide, and sebaconitrile) on bare silica and on a range of silanized silica surfaces. We have used these measurements to determine the polar and dispersive components of the surface energies of these liquids and of the modified silica surfaces. These data offer clues into the microscopic structuring of the liquids and how this structuring is influenced by solid interfaces. We have also observed a strong relationship between the polar and dispersive components of the surface energies of the modified silica surfaces.

The structures of medium-length n-alkane species (C(8)-C(11)) adsorbed to a hydrophilic silica/vapor interface were examined using vibrational sum frequency spectroscopy. Experiments sampling out-of-plane orientation show a clear pattern in vibrational band intensities that implies chains having primarily all-trans conformations lying flat along the interface. Further analysis shows that the methylene groups of the alkane chains have their local symmetry axes directed into and away from the surface. Spectra acquired under different polarization conditions interlock to reinforce this picture of interfacial structure and organization. Variation in signal intensities with chain length suggests that correlation between adsorbed monomers weakens with increasing chain length. This result stands in contrast with alkane behavior at neat liquid/vapor interfaces where longer length alkanes show considerably more surface induced ordering than short chain alkanes.

Resonance-enhanced, second harmonic generation (SHG) is used to measure the electronic structure of solutes sensitive to specific solvation adsorbed to liquid/liquid and liquid/solid interfaces. Here, specific solvation refers to solvent-solute interactions that are directional and localized. N-methyl-p-methoxyaniline (NMMA) is a solute whose first allowed electronic transition wavelength remains almost constant (approximately 315 nm) in non-hydrogen-bonding solvents regardless of solvent polarity. However, in hydrogen-bond-accepting solvents such as dimethylsulfoxide, NMMA's absorbance shifts to longer wavelengths (320 nm), whereas in hydrogen-bond-donating solvents (e.g., water), the absorbance shifts to shorter wavelengths (approximately 300 nm). SHG experiments show that at alkane/silica interfaces, surface silanol groups serve as moderately strong hydrogen-bond donors as evidenced by NMMA's absorbance of 307 nm. At the carbon tetrachloride/water interface, NMMA absorbance also shifts to slightly shorter wavelengths (298 nm) implying that water molecules at this liquid/liquid interface are donating strong hydrogen bonds to the adsorbed NMMA solutes. In contrast, experiments using newly developed molecular ruler surfactants with NMMA as a model hydrophobic solute and a hydrophilic, cationic headgroup imply that, as NMMA migrates across an aqueous/alkane interface, it carries with it water that functions as a hydrogen-bond-accepting partner.