No abstract

Vibrational studies that selectively probe molecular structure at CCl4/H2O and hydrocarbon/H2O interfaces show that the hydrogen bonding between adjacent water molecules at these interfaces is weak, in contrast to generally accepted models of water next to fluid hydrophobic surfaces that suggest strong hydrogen bonding. However, interactions between these water molecules and the organic phase result in substantial orientation of these weakly hydrogen-bonded water molecules in the interfacial region. The results have important implications for understanding water adjacent to hydrophobic surfaces and the penetration of water into hydrophobic phases.

The surfaces of aqueous solutions of NaF, NaCl, NaBr, and NaI have been examined using vibrational sum-frequency spectroscopy. Spectra of these salts in mixtures of HOD/H2O/D2O have been used to provide insight into how simple salts alter the hydrogen bonding structure of water in the surface region. As the anion is changed, the observed interfacial hydrogen bonding also changes, indicating the presence of anions in the interfacial region. The isotopic dilution experiments performed on each solution enable separation of the contributions arising from interfacial water with differing degrees of hydrogen bonding. Frequency shifts in the peaks attributed to tetrahedrally coordinated water molecules within the interfacial region display the structure-making characteristics of F- and the structure-breaking characteristics of Cl-, Br-, and I-. However, water molecules residing in the topmost surface layer show minimal perturbation by the presence of these anions as probed by the characteristics of the donor OH mode of water molecules that straddle the air/water interface. These results indicate a significantly diminished population of the anions at the uppermost layer of the surface region.

The molecular structure and orientation of interfacial water molecules at the air/water interface in the presence of soluble cationic and anionic surfactants has been characterized. We have employed vibrational sum frequency generation (VSFG) to probe the orientation of interfacial water molecules as a function of both bulk surfactant concentration and solution ion strength. The observed ordering of interfacial water molecules is manifested by an enhancement of the OH stretching modes in the VSFG spectra. We attribute the observed enhancement to an alignment of the interfacial water molecules induced by the large electrostatic field present at these charged interfaces. The nature of this enhancement is further explored by studying mixed (cationic and anionic) surfactant systems as well as surfactant systems at different ionic strengths. From these studies we find that the interfacial water molecules attain their highest degree of alignment at surface surfactant concentrations well below maximum surface coverage.

Vibrational sum-frequency spectroscopy (VSFS) studies of a series of HOD/H 2 O/D 2 O mixtures ranging from pure D 2 O to pure H 2 O have been performed at the vapor/water interface. The various concentrations allow an iterative fitting procedure to be applied, resulting in a set of resonant peaks which consistently describe the vibrational modes of water molecules present in the interfacial region. The resonant sum-frequency response from the contributing vibrational modes allows more definitive characterization than in previous studies of the bonding interactions between surface water molecules. Comparison of the resonant spectrum of the vapor/ H 2 O interface with the sum-frequency spectrum obtained at the CCl 4 /H 2 O interface reveals more similarity between the interfacial hydrogen-bonding environments than previously determined. Recent molecular dynamics simulations of VSF spectra of the vapor/H 2 O interface are in good agreement with the experimentally obtained spectra, and give insight into the molecular interactions in the interfacial region, as well as an estimate of the interfacial depth probed.

Measuring the molecular properties of the surface of acidic and basic aqueous solutions is essential to understanding a wide range of important biological, chemical, and environmental processes on our planet. In the present studies, vibrational sum-frequency spectroscopy (VSFS) is employed in combination with isotopic dilution experiments at the vapor/water interface to elucidate the interfacial water structure as the pH is varied with HCl and NaOH. In acidic solutions, solvated proton species are seen throughout the interfacial region, and they alter the hydrogen bonding between water molecules in ways that reflect their depth in the interfacial region. At the higher frequencies of the OH stretch region, there is spectral evidence for solvated proton species residing in the topmost layers of the interfacial region. As reported in previous VSF studies, more strongly bound solvated proton species are observed at lower OH stretching frequencies. The solvated proton species that have stronger hydrogen bonding are similar in structure to those found in bulk acid solutions and likely reside somewhat deeper in the interfacial region. There is also evidence of OH stretching from solvated protons and relatively strong hydrogen bonding in the solvation sphere that is similar to other solvated ions. In contrast, water molecules solvating OH(-) ions show relatively weak hydrogen bonding and significantly less interfacial order. VSF spectra are acquired under multiple polarizations to provide crucial information for the interpretation of the spectra and for the determination of interfacial structure.

The structure and hydrogen bonding of water molecules provides this unique solvent with properties essential to many physical, chemical, and biological processes. The intermolecular hydrogen bonding between water molecules in the bulk medium is disrupted at the surface, imparting the surface with unique structural and thermodynamic properties. We provide an overview of a range of experimental studies from this laboratory in which the structure, orientation, and hydrogen bonding of interfacial water molecules at liquid interfaces are directly probed by resonant vibrational sum frequency spectroscopy. The studies provide insight into the difference in water structure and hydrogen bonding at an air/water interface relative to the interface between two bulk immiscible liquids, namely the CCl 4 /H 2 O interface. Also described are studies aimed at understanding how the presence of a charged alkyl surfactant alters the structure of water at these two interfaces. In both cases field-induced alignment of water molecules in the double layer region is prevalent. This induced alignment has been examined under a variety of experimental conditions. A series of isotopic dilution studies conducted for the first time at liquid surfaces are also described. In these studies the intermolecular and intramolecular coupling of vibrational modes in the water molecules are diminished. The results of these and above-mentioned studies provide valuable information for those interested in developing theoretical descriptions of water at surfaces and interfaces.

The conformational order of sodium dodecyl sulfate (SDS) adsorbed at the D2O-CCl4 interface has been examined by total internal reflection sum-frequency vibrational spectroscopy. A change in conformation of the alkyl chain with increased surface coverage at the liquid−liquid interface is observed. A series of aqueous surfactant concentrations have been examined in order to determine the effect of surface coverage on the conformation of the alkyl chains at the interface. Polarization studies indicate that, for the concentration range examined, the symmetry axis of the terminal methyl group on the alkyl chain is oriented primarily along the surface normal. Identification of spectral features in the C−H region of the infrared region is facilitated by examination of the sum-frequency spectrum from an analogous deuterated compound.