Integration or segregation: how do molecules behave at oil/water interfaces?

It has been over 250 years since Benjamin Franklin, fascinated with the wave-stilling effect of oil on water, performed his famous oil-drop experiments; nevertheless, the behavior of water molecules adjacent to hydrophobic surfaces continues to fascinate today. In the 18th century, the calming of the seas seemed the most pertinent application of such knowledge; today, we understand that oil-on-water phenomena underlie a range of important chemical, physical, and biological processes, including micelle and membrane formation, protein folding, chemical separation, oil extraction, nanoparticle formation, and interfacial polymerization. Beyond classical experiments of the oil-water interface, recent interest has focused on deriving a molecular-level picture of this interface or, more generally, of water molecules positioned next to any hydrophobic surface. This Account summarizes more than a decade's work from our laboratories aimed at understanding the nature of the hydrogen bonding occurring between water and a series of organic liquids in contact. Although the common perception is that water molecules and oil molecules positioned at the interface between the immiscible liquids want nothing to do with one another, we have found that weak interactions between these hydrophilic and hydrophobic molecules lead to interesting interfacial behavior, including highly oriented water molecules and layering of the organic medium that extends several molecular layers deep into the bulk organic liquid. For some organic liquids, penetration of oriented water into the organic layer is also apparent, facilitated by molecular interactions established at the molecularly thin region of first contact between the two liquids. The studies involve a combined experimental and computational approach. The primary experimental tool that we have used is vibrational sum frequency spectroscopy (VSFS), a powerful surface-specific vibrational spectroscopic method for measuring the molecular structures of aqueous surfaces. We have compared the results of these spectroscopic studies with our calculated VSF spectra derived from population densities and orientational distributions determined through molecular dynamics (MD) simulations. This combination of experiment and theory provides a powerful opportunity to advance our understanding of molecular processes at aqueous interfaces while also allowing us to test the validity of various molecular models commonly used to describe molecular structure and interactions at such interfaces.

Sum frequency generation surface spectra of ice, water, and acid solution investigated by an exciton model.

A new computational scheme is presented for calculation of sum frequency generation (SFG) spectra, based on the exciton model for OH bonds. The scheme is applied to unified analysis of the SFG spectra in the OH-stretch region of the surfaces of ice, liquid water, and acid solution. A significant role of intermolecularly coupled collective modes is pointed out. SFG intensity amplification observed for acid solutions in the H-bonded OH-stretch region is reproduced qualitatively and accounted for by enhanced orientational preference "into the surface" of the H(2)O bisectors within the hydronium solvation shell.

Water at hydrophobic surfaces: weak hydrogen bonding and strong orientation effects.

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.

Assembly of long chain phosphatidylcholines at a liquid-liquid interface.

The molecular-level organization of mixed and pure saturated symmetric chain 1,2-diacyl-sn-glycero-3-phosphocholines (PCs) adsorbed at a carbon tetrachloride-aqueous interface is explored by probing the hydrocarbon chain conformation within the adsorbed layer. PCs of the chain lengths found most frequently in biological systems, which in pure form are seen to form either very well-ordered or disordered layers, are observed in these studies to assemble into interfacial layers ranging from disordered to ordered states when mixed in various proportions. Independently, while C(16) and shorter chain PCs tend to form disordered layers, a strong increase in ordering is observed for C(18) and longer chain PCs in which the hydrocarbon chains are found to be primarily in an all trans conformation. Pure C(17)-PCs adsorbed at the interface produce layers with an intermediate degree of chain ordering. The ability to tune interfacial layer properties in mixed systems as a function of molecular composition, including PC chain length as demonstrated here, is an important mechanism by which surface characteristics of oil-water emulsion systems can be controlled both in vivo and in numerous commercial applications.

Tunable picosecond infrared laser system based on parametric amplification in KTP with a Ti:sapphire amplifier.

A picosecond laser system that will generate high-power tunable IR pulses with bandwidths suitable for spectroscopic applications is discussed. The system is based on white-light continuum generation in ethylene glycol and optical parametric amplification in potassium titanyl phosphate. The nonlinear-optical processes are driven by a regeneratively amplified Ti:sapphire laser that produces 1.7-ps pulses at a repetition rate of 1 kHz. Energies as high as 40 and 12 microJ have been achieved over the signal (1.02-1.16-microm) and idler (2.6-3.7-microm) tuning ranges, respectively. The IR beam temporal and spatial characteristics are also presented.

Second harmonic generation for in situ analysis of electrode surface structure.

The nonlinear optical technique of second harmonic generation has been used as an in situ probe of surface structure and symmetry at the electrode-electrolyte interface. Rotational anisotropy in the second harmonic (SH) signalprovides valuable information on the degree of structural order at the metal surface. For Ag(111), the observed patterns in the p-and s-polarized SH output agree with current theories and demonstrate that an ordered surface is present. These patterns change as the electrode is electrochemically roughened.