Kazue Kurihara

Nanotribology of thin liquid-crystal films studied by the shear force resonance method

Abstract We present the first resonance study on the tribology of molecularly thin liquid films. For this purpose we utilize a shear force apparatus, constructed as an attachment to the surface force apparatus, and measure the response of a film pressed between two solid surfaces and subjected to lateral oscillations with differing frequencies. There is a dramatic change in the amplitude and the phase of the output signal in the vicinity of the resonance frequency of the mechanical system. The resonance curves obtained at various normal loads are highly sensitive to the dissipation processes in the sample. The tribological parameters are extracted from the experimental data using an original theoretical model. In this way one can measure not only the friction coefficient, but also the damping coefficient, which provides information on the energy loss in the film. Attributing this loss to the work done against friction, one can calculate the energy of binding of a liquid molecule to the substrate. The samples are thin liquid crystals, 4-cyano-4- n -alkylbiphenyls ( n = 5, 6, 8) in various states, which can also serve as model lubricants. It is found that the films of these compounds exposed to friction are of different mechanics and energetics depending on their structure and on the intermolecular interactions.

Hydrophobicity, Specific Ion Adsorption and Reactivity

Long-range hydrophobic interactions and ion co-ion and counterion adsorption specificity in interactions and in self-assembly have posed a real puzzle for theories of colloid science. Also, a role for dissolved gas has hardly been considered. A variety of observations apparently in conflict with established theories are discussed and analysed. These point to defects in the present theoretical framework and to ways in which a reconciliation can be achieved.

A resonance shear force rheometer modeled as simple oscillating circuit

A novel resonance method for studying the viscoelasticity of very thin liquid films and elastic materials is developed using a shear force apparatus. The shear stress created by an oscillating piezo unit attached to leaf springs is recorded as the lateral displacement by capacitance probe. The oscillation frequency is varied around the resonance frequency of the mechanical system in order to trace the amplitude and the phase of the resonance peak. Two reference states are obtained: the resonance of free oscillations in air and one under constrain introduced by the cantilever spring in contact with the shear mechanical unit. The presence of a liquid film changes these resonance states depending on the film thickness and the cantilever load. A simple mechanical model is proposed entrapping the contribution of different parts in effective spring, mass, and damping constants. The model separates the effect of the liquid film from the background oscillation of the mechanical parts. The method is applied here t...

New surface forces apparatus using two-beam interferometry

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.

A new physical model for resonance shear measurement of confined liquids between solid surfaces

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.

Two types of two-component gels formed from pseudoenantiomeric ethynylhelicene oligomers.

Two-component gels formed from pseudoenantiomeric ethynylhelicene oligomers in toluene exhibited two different properties depending on difference in numbers of helicenes in the two components. The combinations (M)-5/(P)-4, (M)-6/(P)-4, and (M)-7/(P)-4, which contained oligomers with comparable numbers of helicenes, formed transparent gels (Type I gels). The combinations (M)-6/(P)-3, (M)-7/(P)-3, and (M)-8/(P)-3, which contained oligomers with considerably different numbers of helicenes, formed turbid gels (Type II gels). Negative Cotton effects were observed for the Type I gels in the region between 350 and 450 nm, and were positive for the Type II gels, despite the use of (M)-oligomers for the longer components. UV/vis exhibited absorption maxima at 350 nm for the Type I gels and at 338 nm for the Type II gels. Different behaviors in gel formation processes were observed by fluorescence studies. Atomic force microscopy analysis showed fiber structures of 25-50 nm diameter for Type I gels and bundles of 100-150 nm diameter for Type II gels. The stoichiometry in gel formation also differed: The Type I gels showed 1:1 stoichiometry of the two components; the Type II gels showed no 1:1 stoichiometry, likely 1:2 stoichiometry. Using the Type I and II gels, two-layer gel systems were constructed.

Photosensitized charge separation and hydrogen production in reversed micelle entrapped platinized colloidal cadmium sulphide

Irradiation of platinized colloidal cadmium sulphide, generated in situ in Aerosol-OT reversed micelle entrapped water pools in iso-octane sensitized water photoreduction by thiophenol, dissolved in the organic phase.

Binding of sugars and water-soluble polymers to a monolayer of cyclic resorcinol tetramer at the air-water interface

Abstract Resorcinol-dodecanal cyclotetramer forms stable monolayers at the air-water interface. Molecular interaction between this monolayer and substrates bearing hydrogen-bonding groups is studied. Sugars and water-soluble polymers bind to the monolayer selectively, resulting in its expansion. Selectivity of the sugar binding is different from that of extraction into CCl4: ribose gives significant effects in both systems, while fucose, which is readily extracted in CCl4, is less effective in the monolayer system. Xylose and galactose, which are complexed weakly in CCl4, provide strong effects on the πA isotherms. Binding of riboflavin is also examined by monitoring the absorption of the deposited monolayer and by deposition on the piezocrystal and frequency measurement. Complexation of the monolayer is made efficient owing to the dense packing of the hosts.

Hydrogen-Bonded Macrocluster Formation of 1-Propanol and 2-Propanol on Silica Surfaces

We have investigated the adsorption of 1- and 2-propanol on silica surfaces from their mixtures with cyclohexane using a combination of colloidal probe atomic force microscopy, adsorption excess isotherms, and FTIR spectroscopy in the ATR mode. The adsorption isotherm indicated that a similar amount of each alcohol was adsorbed on the silica surfaces. FTIR spectra revealed that 1-propanol adsorbed on the surface employing hydrogen-bonding between the surface silanol groups and the hydroxyl groups of 1-propanol as well as between the hydroxyl groups of 1-propanol in the form of a linear zig-zag structure. This structure is similar to the linear hydrogen-bonded structure of ethanol, which we have found on silica and called a ‘surface molecular macrocluster’ (M. Mizukami, M. Moteki, K. Kurihara, J. Am. Chem. Soc. 2002, 124, 12 889). The contact of adsorbed layers of 1-propanol on the opposed silica surfaces brought about the long-range attraction extending to 69 ± 9 nm at 0.1 mol-% 1-propanol. 2-Propanol was also adsorbed on the surface by the hydrogen-bonding, however, in the form of a cyclic structure. No long-range attraction was observed in the 2-propanol/cyclohexane binary liquids at 0.1–6.0 mol-%. The absence of a long-range attraction can be explained by the cyclic aggregation structure of 2-propanol on the surface.

Characterization of Ferrocene-Modified Electrode Using Electrochemical Surface Forces Apparatus

A electrochemical surface forces apparatus (EC-SFA) was employed to measure the interactions between gold electrodes modified with self-assembled monolayers of ferrocene alkyl thiol (Fc-SAM) and oxidized ferrocene (ferrocenium cation, Fc(+)-SAM) in a 1 mM aqueous electrolyte. The double-layer repulsion in both cases of the Fc-SAM and Fc(+)-SAM electrodes was observed. The surface charge density (σ) evaluated from the double-layer repulsions between the Fc(+)-SAM electrodes in 1 mM aqueous KClO4 was 0.0027 C/m(2), which was 2.5 times greater than that of the Fc-SAM, at 0.0011 C/m(2). The σ values of the Fc(+)-SAM were evaluated for various counteranions using the same method, which were 0.0048, 0.0040, and 0.0104 C/m(2) for NO3(-), SO4(2-), and CF3SO3(-), respectively. The degrees of dissociation (αd) between the ferrocenium cation and these counteranions were obtained from σ and the density of the ferrocenium on the electrode. The αd value of CF3SO3(-), 4.1%, was the highest, followed in the order, SO4(2-) > NO3(-) > ClO4(-), indicating that most of the positive charges of the ferrocenium cation were compensated by formation of an ion pair with counteranions.