Two types of imidazolium-based ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(4)mim][NTF(2)]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF(4)]), confined between silica surfaces were investigated by surface force apparatus (SFA)-based resonance shear measurements together with surface force measurements. The surface force profiles in the ILs showed oscillatory solvation forces below the characteristic surface separations: 10.0 nm for [C(4)mim][NTf(2)] and 6.9 nm for [C(4)mim][BF(4)]. The more pronounced solvation force found in [C(4)mim][NTf(2)] suggests that the crystal-forming ability of the IL contributes to the stronger layering of the ILs adjacent to the surface. The resonance shear measurement and the physical model analysis revealed that the viscosities of the confined ILs were 1-3 orders of magnitude higher than that of the bulk IL. This paper also focused on the correlation between the resonance shear behaviour and the lubrication property of the ILs, and the suspension rheology in the ILs. An understanding of the solid-IL interface and of ILs confined in nanospace will facilitate the further development of novel applications employing ILs.

Adsorption of ethanol onto silica surfaces from ethanol-cyclohexane binary liquids was investigated by a combination of colloidal probe atomic force microscopy, adsorption excess isotherm measurement, and FTIR spectroscopy using the attenuated total reflection (ATR) mode. An unusually long-range attraction was found between the silica (glass) surfaces in the presence of ethanol in the concentration range of 0.1-1.4 mol % at room temperature. At 0.1 mol % ethanol, the attraction appeared at a distance of 35 +/- 3 nm and turned into a repulsion below 3.5 +/- 1.5 nm upon compression. Half of the attraction range agreed with the adsorption layer thickness estimated from the adsorption excess amount by assuming that the adsorption layer was composed only of ethanol. This indicated that the observed long-range attraction was caused by the contact of opposed adsorption layers of ethanol on the silica surfaces and that the sharp increase of repulsion at shorter distance was caused by the overlap of structured ethanol clusters adjacent to the surface. ATR-FTIR spectra demonstrated that ethanol adsorbed on the silica (silicon oxide) surfaces formed hydrogen-bonded clusters (polymers). Practically no ethanol clusters were formed on the hydrogen-terminated silicon surface. These results indicated that the cluster formation involved hydrogen-bonding interactions between surface silanol groups and ethanol hydroxyl groups in addition to those between ethanol hydroxyl groups. At higher temperatures (30-50 degrees C), the range and the strength of attraction decreased owing to the decrease in the hydrogen-bonded clusters monitored by FTIR spectroscopy, reflecting the nature of hydrogen bonding. The range and the strength of the attraction also changed when the ethanol concentration increased: The long-range attraction started to decrease at 0.6 mol % ethanol at room temperature and disappeared at 1.4 mol % while the adsorption excess amount remained almost constant as did the FTIR peak intensity of the hydrogen-bonded OH group of adsorbed ethanol. In the bulk solution, ethanol clusters appeared at 0.5 mol % ethanol; thus, this change in the attraction could be accounted for in terms of the exchange of ethanol molecules between the surface clusters and bulk clusters. The novel self-assembled structure of alcohol on the surface, found in this study may be called a "surface molecular macrocluster" because the hydrogen-bonded clusters extend to distances of ca. 20 nm longer than the typical sizes of common clusters, 2-4 nm, of alcohol (e.g., ethanol).

We performed the resonance shear measurement (RSM) for evaluating the properties of water confined between silica surfaces with and without water vapor plasma treatment, which was used to increase the density of the silanol groups on the surfaces. We compared the properties of the confined water, such as viscosity and lubricity, by controlling the surface separation at a 0.1 nm resolution. The observed resonance curves for water between the plasma-treated and untreated silica surfaces showed the following results: (1) The viscosity of the water confined between the plasma-treated silica surfaces increased due to water structuring at separations less than 3 nm, while the value for the water between the untreated silica surfaces was 8 nm. (2) The water confined between the plasma-treated surfaces could maintain lubricity under the normal pressure of more than 1.7 MPa; however, the water confined between the untreated surfaces lost lubricity under the normal pressure of more than 0.4 MPa. To discuss these properties in terms of water structures on the silica surfaces, we performed sum frequency generation (SFG) vibrational spectroscopy for water on the plasma-treated and untreated silica surfaces. The main peak of SFG spectra for the water on the plasma-treated silica was around 3200 cm −1 , and that for water on the untreated silica was around 3400 cm −1 , indicating that the hydrogen bonding network of the water on the plasma-treated silica surface was stronger than that on the untreated one due to the higher silanol density. The strongly networked water could exhibit higher lubricity with the increased silanol density.

This paper describes a new physical model for resonance shear measurement. The resonance shear method developed by us provides a tool for investigating the rheological and tribological properties of liquids confined between two surfaces as a function of the surface distance from micrometer to zero (contact) with nanometer level resolution with high sensitivity and stability. The properties of the confined liquid can be quantitatively studied by analyzing the resonance curve using a physical model. However, the quantitative analysis using the previously developed model was applicable only for the condition of the relatively low liquid viscosity (below approximately 100 Pa s). A new physical model described in this paper enabled us to continuously analyze the properties of confined liquids at all distances, which was not possible by the previous model. It became possible to calculate the movement of a lower surface and the shear rate applied on the confined sample using the parameters obtained from the resonance curves.

We designed a new surface forces apparatus for measuring the interactions between two nontransparent substrates and/or in nontransparent liquids. The small displacement of a surface, the bottom one in this study, was measured by the two-beam (twin path) interferometry technique using the phase difference between the laser light reflected by the fixed mirror and that by the mirror on the back of the bottom surface unit. It is possible to determine the distance with a resolution of 1 nm in the working range of 5 microm. This apparatus was successfully applied to measure the forces between mica surfaces in pure water and aqueous KBr solutions.

We used molecular dynamics simulations to study the structure and shear dynamics of two ionic liquids (ILs) featuring the same cation 1-butyl-3-methyl-imidazolium or [BMIM], paired with bis(trifluoromethanesulphonyl)amide [NTF2] and tetrafluoroborate [BF4] anions, confined between two hydroxylated silica surfaces. The results demonstrate how the shape of IL molecules affects their layering structure at hydroxylated silica surfaces and how the layered structure of nanoconfined liquids determines their dynamical properties at the molecular level. When [BMIM][NTF2] is sheared, larger molecular fluctuations in the inner layers are required to stabilise the system, and the resulting dynamics is irregular. The alternating charged layers in [BMIM][BF4] allow the system to stabilise through smaller oscillations, and the layers appear to shear on top of each other in a laminar fashion. The simulated dynamics explains qualitatively the relative change in viscosity that the two ILs exhibit when confined, as has been observed in previous experiments.

Recently, we have succeeded in identifying the structure of the adsorption layer of ethanol on a silica surface in cyclohexane to be a hydrogen-bonded linear aggregate (polymer), which we call a surface molecular macrocluster. In this work, we studied the effect of the miscibility of liquids on the formation of the surface molecular macroclusters for confirming that this is a surface induced phenomenon. We investigated the interaction and the structure of methanol adsorbed on a silica surface in methanol-cyclohexane binary liquids by a combination of colloidal probe atomic force microscopy, adsorption excess isotherm measurement, and FTIR spectroscopy using the attenuated total reflection (ATR) mode, and compared the results with those of the ethanol-cyclohexane and 1-propanol-cyclohexane binary liquids. The former system is immiscible at methanol concentrations of ca. 8-90 mol %, and the latter two are miscible at any composition. At 0.03 mol % methanol, which is far from the critical concentration for the phase separation, the contact of the methanol macrocluster layers formed on the silica surface brought about the attraction from a distance of 42 +/- 5 nm which was similar to that observed in ethanol-cyclohexane. At a methanol concentration of 9.0 mol %, above bulk phase separation, completely different force profiles were observed. These results demonstrated that the molecular macrocluster formation was different from the wetting induced by the bulk.

X-ray diffraction measurement at the SPring-8 synchrotron was employed to investigate the structures of two types of imidazolium-based ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][NTF2]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), confined between silica surfaces by varying the surface separation distances of ca. 500 nm (bulk liquid), ca. 10 nm, and ca. 2 nm (hard wall thickness). The obtained diffraction profiles and intensities were discussed by considering the structures and properties of the nano-confined ILs between the silica surfaces investigated by resonance shear measurement (RSM) and molecular dynamics simulation (MD) in our previous reports. [C4mim][NTf2] showed two diffraction peaks at q = 8.8 nm-1 (spacing d = 0.71 nm) and at q = 14.0 nm-1 (spacing d = 0.45 nm) at the greatest distance (D = ca. 500 nm), which were assigned to the interval between the same ions (anion-anion or cation-cation) within the polar network of [C4mim][NTf2] and the interval between the neighboring anion-cation, respectively. The positions of these two peaks remained the same at D = ca. 10 nm and at the hard wall (D = ca. 2 nm) and their intensity factor increased, indicating that both the cation and anion existed in the same layer. This result was consistent with the checkerboard structure of [C4mim][NTf2] on the silica surface computer simulated in our previous studies. On the other hand, [C4mim][BF4] showed a peak at q = 15.4 nm-1 (spacing d = 0.41 nm) corresponding to the anion-cation interval at the greatest distance (D = ca. 500 nm). This peak became broader and weaker at D = ca. 12 nm and at D = ca. 2 nm.