Unusual Structure and Dynamics at Silica/Methanol and Silica/Ethanol Interfaces--A Molecular Dynamics and Nonlinear Optical Study.

Vibrational sum frequency (VSF) spectroscopy and molecular dynamics simulations are used to investigate ethanol-silica and methanol-silica interfaces. We describe the subtle differences in molecular organization that result in the observed differences in the VSF spectra for methanol and ethanol at the alcohol-silica interface. Alcohol molecules hydrogen-bonded to the silica surface induce orientational opposition in an adjacent low-population region, which implies VSF signal reduction. This low population region is essentially of zero density in the ethanol system, implying less signal cancelation. Simulated silica defect sites increase the population of this region in both systems. Interestingly, the induced orientation in this region influences subsequent molecular orientation only in the ethanol-silica system, preserving the interfacial anisotropy. These effects suggest a stronger VSF response from the ethanol-silica system versus the methanol-silica system, where more methanol molecules reside in the low-population region, and this region does not induce order in subsequent solvent layers.

Cyclodextrin-functionalized chromatographic materials tailored for reversible adsorption.

Novel dendronized silica substrates were synthesized. First- and second- generation polyaryl ether dendrons were appended to silica surfaces. Using Cu(I) mediated cycloaddition "click" chemistry, ß-cyclodextrin was tethered to the dendronized surfaces and to a nondendronized surface for comparison purposes. This synthesis strategy affords a modular, versatile method for surface functionalization in which the density of functional groups can be readily varied by changing the generation of dendron used. The surfaces, which are capable of adsorbing target analytes, have been characterized and studied using X-ray photoelectron spectroscopy (XPS) and vibrational sum frequency spectroscopy (VSFS). Fluorescence spectroscopy was used to study the surfaces' ability to retain coumarin 152 (C152). These studies indicated that the ß-cyclodextrin functionalized surfaces not only adsorbed C152 but also retained it through multiple aqueous washes. Furthermore, these observations were quantified and show that substrates functionalized with first-generation dendrons have a more than 6 times greater capacity to adsorb C152 than slides functionalized with monomeric ß-cyclodextrin. The first-generation dendrons also have 2 times greater the capacity than the larger generation dendrons. This result is explained by describing a dendron that has an increased number of ß-cyclodextrin monomers but, when covalently bound to silica, has a footprint too large to optimize the number of accessible monomers. Overall, both dendronized surfaces demonstrated an increased capacity to adsorb targeted analytes over the slides functionalized with monomeric ß-cyclodextrin. The studies reported provide a methodology for characterizing and evaluating the properties of novel, highly functional surfaces.

Binary Solvent Organization at Silica/Liquid Interfaces: Preferential Ordering in Acetonitrile-Methanol Mixtures.

Nonlinear vibrational spectroscopy experiments examined solvent organization at the silica/binary solvent interface where the binary solvent consisted of methanol and acetonitrile in varying mole fractions. Data were compared with surface vibrational spectra acquired from silica surfaces exposed to a vapor phase saturated with the same binary solvent mixtures. Changes in vibrational band intensities suggest that methanol ideally adsorbs to the silica/vapor interface but acetonitrile accumulates in excess relative to vapor-phase composition. At the silica/liquid interface, acetonitrile's signal increases until a solution phase mole fraction of ∼0.85. At higher acetonitrile concentrations, acetonitrile's signal decreases dramatically until only a weak signature persists with the neat solvent. This behavior is ascribed to dipole-paired acetonitrile forming a bilayer with the first sublayer associating with surface silanol groups and a second sublayer consisting of weakly associating, antiparallel partners. On the basis of recent simulations, we propose that the second sublayer accumulates in excess.

Liquid organization and solvation properties at polar solid/liquid interfaces.

Second order nonlinear optical spectroscopy has been employed to examine the organization of four different liquids at the hydrophilic silica/liquid interface. The liquids - cyclohexane, methylcyclohexane, 1-propanol, and 2-propanol - were chosen to isolate how intermolecular forces between the liquid and the substrate competed with steric effects to control liquid structure and solvating properties across the interfacial region. Vibrational sum frequency generation (VSFG) data showed that cyclohexane structure at the silica/liquid cyclohexane interface closely resembled the structure of a cyclohexane monolayer adsorbed to the silica/vapor interface. Methylcyclohexane, however, showed evidence of large structural reorganization between the silica/liquid and silica/monolayer/vapor interfaces. 1-Propanol at a silica/vapor interface formed a well-ordered, Langmuir-like monolayer due to strong hydrogen bonding with the surface silanols and cohesive van der Waals interactions between carbon chains. 1-Propanol at the silica/liquid interface retained the same ordered structure. In contrast, 2-propanol adopted different structures adsorbed to the solid/vapor and at the solid/ liquid interfaces. Specifically, the plane defined by 2-propanol's three carbon atoms changed orientation from being perpendicular to the surface (silica/vapor) to parallel to the surface (silica/liquid). Surface mediated liquid structure affected the solvation of adsorbed solutes. Resonance enhanced second harmonic generation (SHG) data showed that silica/alkane interfaces were significantly more polar than would be expected based on a solute's bulk solution solvatochromic behavior. Both silica/alcohol interfaces exhibited alkane-like polarity, a result that was interpreted in terms of a reduction in hydrogen bonding opportunities for adsorbed solutes.