Vibrational sum-frequency spectroscopy (SFS) and total internal reflection Raman scattering (TIR Raman) have been used to study the adsorption of hexadecyltrimethylammonium bromide (CTAB) to hydrophilic silica. These two complementary techniques permit the determination of the adsorbed amount with a sensitivity of approximately 1% of the maximum surface coverage, changes in the average tilt of the adsorbed molecules, the presence of asymmetric aggregates in the adsorbed film, and the structure and orientation of the water molecules in the interfacial region. The TIR Raman spectra show a monotonic increase with CTAB concentration with no measurable changes in the relative intensities of the different polarization combinations probed, implying that no significant changes occur in the conformational order of the hydrocarbon chain. In the sum-frequency (SF) spectra, no detectable peaks from the surfactant headgroup and hydrophobic chain were observed at any surface coverage. Major changes are observed in the water bands of the SF spectra, as the originally negatively charged silica surface becomes positively charged with an increase in the adsorbed amount, inducing a change in the polar orientation of the water molecules near the surface. The detection limits for hydrocarbons chains in the SF spectra were estimated by comparison with the SF spectrum of a disordered octadecyltrichlorosilane monolayer. The simulations demonstrate that the asymmetry in the adsorbed CTAB layer at any concentration is less than 5% of a monolayer. The results obtained pose severe constraints on the possible structural models, in particular at concentrations below the critical micellar concentration where information is scarce. The formation of hemimicelles, monolayers and other asymmetric aggregates is ruled out, with centrosymmetric aggregates forming from early on in the adsorption process.

The lubricating properties of an ionic liquid on gold surfaces can be controlled through application of an electric potential to the sliding contact. A nanotribology approach has been used to study the frictional behavior of 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py(1,4)]FAP) confined between silica colloid probes or sharp silica tips and a Au(111) substrate using atomic force microscopy. Friction forces vary with potential because the composition of a confined ion layer between the two surfaces changes from cation-enriched (at negative potentials) to anion-enriched (at positive potentials). This offers a new approach to tuning frictional forces reversibly at the molecular level without changing the substrates, employing a self-replenishing boundary lubricant of low vapor pressure.

Atomic force microscopy on hydrophobic microspheres in water reveals a strong attraction with a range of 20 -200 nm, following an initial steep repulsion at long range. The data are consistent with a single submicroscopic bubble between the surfaces, with the attraction due to its attachment and lateral spread, and the repulsion dependent on film drainage and the electric double layer. The results provide direct experimental evidence of the existence of long-lived submicron bubbles, and of their bridging as the cause of the measured long-range attractions between macroscopic hydrophobic surfaces.[S0031-9007(98)06357-1] PACS numbers: 61.16.Ch, 68.10.Cr, 68.15. + e, 82.65.Dp In the early 1970s Blake and Kitchener [1] measured the rupture of the water film between a hydrophobic surface and an approaching bubble, and concluded that a long-ranged attraction existed. The force between two macroscopic hydrophobic surfaces has since been directly measured, and, although the quantitative details vary, the measurements confirm a strong attraction that is much larger than the van der Waals force (see Ref.[2]). The extreme range of the force (measurable at 300 nm [3]) challenges conventional theories of surfaces forces and the liquid state. Comparisons with polywater are not entirely uncalled for, following the early suggestion [4] that the force was due to extended, surface-induced, water structure.Most consensus for the underlying physical mechanism has focused on long-range electrostatic forces, following the proposal by Attard [5] that the two surfaces coupled via correlated fluctuations. This idea and its various modifications [6-9] all predict a strong dependence on the electrolyte concentration, which experiments variously confirm [10 -12] and refute [3,[13][14][15].Alternatively, it has been suggested [3,16] that the force is due to the presence of submicroscopic bubbles adhering to the surfaces (Harvey nuclei), with the attraction due to the attachment to the other surface and subsequent lateral spreading. The proposal was based on the observation of steps or discontinuities in the force data at large separations [3], which were taken to be due to the bridging of multiple bubbles. The idea is supported by the fact that the force tends to be more short ranged when measured in de-aerated water [15,17], and when measured between surfaces that had never been exposed to the atmosphere [17], presumably due to the attachment of bubbles to defects in the surfaces when they were taken through the air-water interface.What is attractive about bridging bubbles as a mechanism for these long-ranged forces is that the range of the force is set by the physical size of the bubble, and one avoids a putative surface-induced structure in the liquid that extends over thousands of molecular diameters. The main difficulty with the proposal is that, according to macroscopic thermodynamics, bubbles are metastable [16]; the Laplace equation predicts a high internal gas pressure for submicroscopic bubbles that should make them dissolve [18...

Atomic force microscopy measurements reveal that superlubricity can be "switched" on and off in situ when an ionic liquid is used to lubricate the silica-graphite interface. Applying a potential to the graphite surface changes the ion composition of the boundary layer and thus the lubricity. At positive potentials, when the interfacial ion layer is anion rich, friction falls to ultra-low levels.

The OH stretching region of water molecules in the vicinity of nonionic surfactant monolayers has been investigated using vibrational sum frequency spectroscopy (VSFS) under the polarization combinations ssp, ppp, and sps. The surface sensitivity of the VSFS technique has allowed targeting the few water molecules present at the surface with a net orientation and, in particular, the hydration shell around alcohol, sugar, and poly(ethylene oxide) headgroups. Dramatic differences in the hydration shell of the uncharged headgroups were observed, both in comparison to each another and in comparison to the pure water surface. The water molecules around the rigid glucoside and maltoside sugar rings were found to form strong hydrogen bonds, similar to those observed in tetrahedrally coordinated water in ice. In the case of the poly(ethylene oxide) surfactant monolayer a significant ordering of both strongly and weakly hydrogen bonded water was observed. Moreover, a band common to all the surfactants studied, clearly detected at relatively high frequencies in the polarization combinations ppp and sps, was assigned to water species located in proximity to the surfactant hydrocarbon tail phase, with both hydrogen atoms free from hydrogen bonds. An orientational analysis provided additional information on the water species responsible for this band.