Complementary experimental and theoretical studies presented in this work examine the structure, organization, and solvating properties of methanol at a silica/methanol, solid/liquid interface. Findings from these experiments illustrate how strong association between a silica substrate and methanol solvent creates a distinctly nonpolar solvation environment for adsorbed solutes. Resonance-enhanced second-harmonic spectra and time-resolved fluorescence emission in a total internal reflection geometry both show that adsorbed solutes sample an interfacial environment having properties resembling those of a nonpolar solvent. Molecular dynamics simulations identify the origin of this effect. Strong hydrogen bonding between the first layer of methanol and silica’s silanol groups creates what is effectively a methyl-terminated surface that leads to a second layer having significantly reduced density and hydrogen bonding compared to bulk solution. The calculated solvent reorientation times in these first two layers is significantly slower than in bulk, implying slow dielectric relaxation and supporting both second-harmonic and time-resolved fluorescence results. Collectively, these studies illustrate how surface-induced changes in solvent structure change the chemistry at strongly associating solid/liquid interfaces as compared to bulk solution limits.

b S Supporting Information ' INTRODUCTIONInterfacial solvation will depend sensitively on a subtle balance of soluteÀsubstrate, soluteÀsolvent, and substrateÀsolvent interactions. Here, the term solvation is used to describe the local environment experienced by a solute. Interfacial solvation has often been described in terms of averaged contributions from the two adjacent phases. 1À6 Such models have proven successful at describing solvation across weakly associating interfaces characterized by large excess free energies. When experimental findings differ from predictions of this "averaged" model of interfacial solvation, results can usually be rationalized in terms of complex solvent structure that reorganizes under the constraint of reduced mobility and strong, directional forces imposed by the surface. 1,5,7À13 These interphase substrateÀsolvent interactions form local domains having properties that do not arise in bulk solution. Only recently have researchers begun to report how more subtle effects such as solute orientation and solute solubility in bulk solution can influence interfacial solvation. 9,14À19 Strongly associating solid/liquid interfaces are characterized by low interfacial energies and strong, directional interactions between the substrate and the solvent. These forces impose longrange structure on the adjacent solvent that can extend several solvent diameters into bulk solution. 17,20À24 The effects of solid surfaces on solvent structure remain an area of active investigation. Fundamental studies attempt to isolate and quantify asymmetric, intermolecular interactions, while empirical investigations parametrize interphase forces between solid substrates and different solvent mixtures to improve chromatographic separations and to tune surface reactivity. 25À33 The affinitiy of a solute for a surface strongly influences adsorption energetics and the resulting environment that a solute samples. In the case of strong soluteÀsubstrate associations, solvent identity may not play a significant role in interfacial solvation. However, if solventÀsubstrate interactions are significantly stronger than those between the solute and substrate, then solute molecules might not accumulate at an interface at all. An example of how these competing energetics affect interfacial phenomena comes from detailed studies of adsorption and retention in chromatographic systems. Wirth and co-workers reported tailing and line broadening in model liquid chromatography measurements and assigned this effect to strong associations between solutes and the silica substrates. 29 They noted that this behavior occurs at both low and neutral pH values with neutral pH values leading to longer retention times and more broadening. Rendering the mobile phase acidic reduces retention time but still results in eluent tailing due to strong adsorption sites on the silica composed of acidic ABSTRACT: Resonance-enhanced second harmonic generation (SHG) spectroscopy was used to probe the electronic structure of p-nitroanisole (pNAs) adsorbed...

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.