Characterization of lubricant liquid confined in nano space

(A)　J. Watanabe, M. Mizukami, K. Kurihara, “Resonance Shear Measurement of Confined Alkylphenyl Ether Lubricants”, Tribol. Lett. 56, 501-508 (2014).

The tribological properties of four phenyl ether lubricants, monoalkydiphenyl ether (MADE), dialkyldiphenyl ether (DADE), m-phenoxyphenoxy m-biphenyl (m-4P2E), and m-bis(m-phenoxyphenoxy) benzene (m-5P4E), confined between mica surfaces have been investigated using the resonance shear measurement (RSM). The RSM study revealed that the viscous parameter of these lubricants increased when they were confined in a nano-space. The viscous parameter b2 values of these lubricants confined in a gap smaller than ca. 2-3 nm were in the order of MADE [DADE [m-4P2E & m-5P4E, while their bulk viscosities were in the order of m-5P4E [m-4P2E [ DADE[MADE. Further, the MADE, which has better hydrodynamic lubrication property because of the lowest bulk viscosity, was relatively easily squeezed out from the gap compared to others. These results have demonstrated that phenyl ether lubricants confined in a nano-space behave quite differently from those of the bulk phase, and the direct characterization of confined lubricants is imperative for designing efficient lubricants, especially for boundary lubrication.

Electrochemical surface forces apparatus and characterization of electrode interfaces

(A)　Kamijo, T.; Kasuya, M.; Mizukami, M.; Kurihara, K. “Direct Observation of Double Layer Interactions between the Potential-controlled Gold Electrode Surfaces Using the Electrochemical Surface Forces Apparatus” Chem. Lett. 40, 674-675 (2011).

We have designed a new apparatus, an electrochemical SFA, for measuring the forces between symmetric gold electrode surfaces under electrochemical potential control. The surface separation was determined by two-beam (twin-path) interferometry. The potential was applied to the gold surfaces (working electrode) in 1 mM aqueous KClO4 using Ag/AgCl as the reference and Pt as the counter electrode. We observed the van der Waals attraction and the double layer repulsion which decreased with the increasing potential from -0.1 to 0.2 V (vs. Ag/AgCl).

(B)　Kasuya, M.; Kurihara, K. “Characterization of Ferrocene-Modified Electrode Using Electrochemical Surface Forces Apparatus” Langmuir, 30, 7093-7097 (2014).

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 evaluated from the double-layer repulsions between the Fc+-SAM electrodes in 1 mM aqueous KClO4 was 0.0027 C/m2, which was 2.5 times greater than that of the Fc-SAM, at 0.0011 C/m2. The charge density values of the Fc+-SAM were evaluated for various counteranions using the same method, which were 0.0048, 0.0040, and 0.0104 C/m2 for NO3-, SO42-, and CF3SO3-, respectively. The degrees of dissociation between the ferrocenium cation and these counteranions were obtained from charge density and the density of the ferrocenium on the electrode. The degrees of dissociation value of CF3SO3-, 4.1%, was the highest, followed in the order, SO42- > NO3- > ClO4-, indicating that most of the positive charges of the ferrocenium cation were compensated by formation of an ion pair with counteranions.

Effect of confinement on electric field induced orientation of a nematic liquid crystal

(A)　S. Nakano, M. Mizukami, K. Kurihara, “Effect of Confinement on Electric Field Induced Orientation of a Nematic Liquid Crystal”, Soft Matter 10, 2110-2115 (2014).

We report the effect of confinement on the electric field induced orientation of a nematic liquid crystal, 4-cyano-4′-hexylbiphenyl (6CB), between mica surfaces. The resonance shear measurement was employed for monitoring changes in the viscosity of 6CB at various surface separation distances (D) with and without an applied electric field. The viscosity depended on the surface separations, and the behaviour for D < ca. 20 nm was quite different from that at D >ca. 20 nm. For D > ca. 20 nm, the viscosity values obtained in the presence of the electric field (ac 1 kHz, 1.87 kV mm-1, homeotropic orientation) were ca. 2 times higher than the values obtained without the electric field (0 kV mm-1, planar orientation) due to the difference in the molecular orientation, and were nearly constant. At D < ca. 20 nm, the viscosity of 6CB both with and without the electric field sharply increased and they merged into an identical value at D = 12.5 ± 1.3 nm (Dc), then exhibited a plateau up to D = 6 nm. With the decreasing distance below 6 nm, the viscosity of confined 6CB both with and without the electric field further increased up to more than 100 N s m-1 at the hard wall thickness of D = ca. 4.0 nm. These results indicated that 6CB molecules both with and without the electric field had the same orientation at D < Dc. The most likely orientation of 6CB was parallel to the surfaces because 6CB was originally in a planar orientation on the mica surface. These results demonstrated for the first time that the effect of confinement exceeded the electric field, thus 6CB molecules could not change their orientation under the electric field at the surface separation below Dc.

Characterization of nano-confined ionic liquids

(A)　K. Ueno, M. Kasuya, M. Watanabe, M. Mizukami, K. Kurihara, “Resonance Shear Measurement of Nanoconfined Ionic Liquids”, Phys. Chem. Chem. Phys. 12, 4066-4071 (2010)

Two types of imidazolium-based ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim][NTf2]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), 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 [C4mim][NTf2] and 6.9 nm for [C4mim][BF4]. The more pronounced solvation force found in [C4mim][NTf2] 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.

(B)　F. F. Canova, H. Matsubara, M. Mizukami, K. Kurihara, A. L. Shluger, “Shear Dynamics of Nanoconfined Ionic Liquids”, Phys. Chem. Chem. Phys. 16, 8247-8256 (2014)

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 [C4mim], 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 [C4mim][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 [C4mim][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.

X-ray diffraction study on liquid nanofilm

(A)　S. Nakano, M. Mizukami, N. Ohta, N. Yagi, I. Hatta, K. Kurihara, “Structural Change in Smectic Liquid Crystal Nanofilm under Molecular-Scale Confinement Measured by Synchrotron X-ray Diffraction”, Jpn. J. Appl. Phys. 52, 035002-1-4 (2013).

The determination of liquid structures under nanometer-scale confinement is important in advanced sciences and technologies. Synchrotron X-ray diffraction measurement was performed to investigate the structure of a smectic liquid crystal, 4-cyano-4'-octylbiphenyl (8CB), nanofilm (thickness: 1.7 ±0.5 nm) confined between mica surfaces. A diffraction peak at q = 1.99 nm-1, corresponding to the lamellar spacing of 8CB, appeared immediately after 8CB was injected between the surfaces. This diffraction peak gradually decreased with time, indicating the structural change in 8CB from the ordered to the disordered state. The relaxation time was ca. 60 min and the diffraction peak almost disappeared at 100 min after 8CB injection. We could directly monitor the time course of the structural change in the smectic liquid crystal under molecular-scale confinement.

Interfacioal water

(i) Viscosity and lubricity of nano-confined water between silica

(A) Motohiro Kasuya, Masaya Hino, Hisho Yamada, Masashi Mizukami, Hiroyuki Mori, Seiji Kajita, Toshihide Ohmori, Atsushi Suzuki, Kazue Kurihara, “Characterization of Water Confined between Silica Surfaces Using the Resonance Shear Measurement", J. Phys. Chem. C, 117, 20738-20744 (2013).

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.

(ii) Viscosity and lubricity of nano-confined water between mica

(A) H. Sakuma，K. Otsuki, K. Kurihara, "Viscosity and lubricity of aqueous NaCl solution confined between mica surfaces studied by shear resonance measurement", Phys. Rev. Lett., 96, 046104 (2006).

This paper reported the viscosity charge of the thin film of aqueous NaCl solution confined between mica surfaces measured by shear resonance measurement. The viscosity started to increase at separations below 2 nm, then sharply increased, 2-4 orders of magnitude larger than the bulk value below 1 nm while maintaining high lubricity. Our study employing a novel shear measurement provided a comprehensive picture for the dynamics of confined water thinner than a few nanometer.

(iii) Interfacial macrocluster formation of water

(A)　M. Mizukami, Kobayashi, and K. Kurihara, “Structuring of Interfacial Water on Silica Surface in Cyclohexane Studied by Surface Forces Measurement and Sum Frequency Generation Vibrational Spectroscopy”, Langmuir 28, 14284-14290 (2012).

We investigated interfacial water, formed by adsorption or phase separation (prewetting transition), on a silica surface in water–cyclohexane binary liquids using a combination of colloidal probe atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy. At 33 ± 9 ppm water, the long-range attraction extending to 19.4 ± 2.9 nm appeared, which was caused by the contact of water layers formed on silica surfaces. The attraction range increased with increasing water concentration and reached 97 ± 17 nm at the saturation concentration of water in cyclohexane (C*), indicating that the thickness of the water layer formed on silica was ca. 50 nm. The interfacial energy between the water adsorption layer and bulk solution (γ = 79.3 ± 2.0 mN/m) was estimated from the pull-off force, and was significantly larger than the value for the bulk water/cyclohexane interface (γ = 50.1 mN/m). SFG spectroscopy demonstrated that the interfacial water formed an icelike structure at C*. These results indicated that the interfacial water molecules formed an icelike ordered structure induced by the hydrogen bonding with surface silanol groups, resulting in the free OH groups being more exposed to the bulk solution. On the other hand, the water adsorption layer induced by phase separation at water concentrations above C* was found to be less ordered and its structure at the adsorption layer/bulk interface was almost the same as that of bulk water, although its thickness was almost the same as that formed at C*. To our knowledge, this is the first report of the observation of liquid adsorption layers formed by chemical interaction up to saturation and by the wetting transition above saturation, and their differences in the structure and properties at the molecular level.

The mechanism of high viscosity of precipitate calcium carbonate nano particle dispersion

(A)　Yoshisada Kayano, Hiroshi Sakuma, and Kazue Kurihara "Nanorheology of Dioctyl Phthalate Confined between Surfaces Coated with Long Alkyl Chains ", Langmuir, 23, 8365 - 8370, (2007)

(B)　Yoshisada Kayano, Hiroshi Sakuma, and Kazue Kurihara "Effect of Water on Nano-rheology of Dioctylphthalate Confined between Surfaces Coated with Long Alkyl Chains ", Trans. MRS-J., 32[2], 367 - 370, (2007)

To shed light on the mechanism related to the high viscosity of a precipitated calcium carbonate (PCC) dispersion in dioctyl phthalate (DOP), the viscosity of DOP in a nanometer space was investigated using the shear resonance measurement. The high viscosity of DOP observed by the shear reasonance measurement was in good agreement with bulk viscosity in a confinement space, indicating that increase in the solvent viscosity indeed contributed to the high viscosity of DOP dispersion of PCC. This is the first study of connecting the macroscopic and nanoscopic rheological data. This study shows that the nanoscopic measurement is a powerful tool for understanding macroscopic behavior, which has a significant bearing on both basic science and engineering.

Direct measurement of forces between the enzyme and substrate

(A) T. Suzuki，Y.-W. Zhang, T. Koyama, D. Y. Sasaki, K. Kurihara, "Direct observation of substrate-enzyme complexation by surface forces measurement ", J. Am. Chem. Soc., 128, 15209-15214 (2006).

(B) T. Suzuki，Y.-W. Zhang, T. Koyama, D. Y. Sasaki, K. Kurihara " Direct observation of specific interaction between enzyme-substrate complexes using colloidal probe atomic force microscopy ", Chem. Lett., 33, 536-537 (2004).

We have been developing the direct approach for studying the interaction between biological molecules using colloidal probe AFM for force measurement and Langmuir-Blodgett (LB) method for surface modification. In this paper, we studied the enzyme reaction of HepPP synthase catalyzing the carbon chain elongation reaction. This enzyme is composed of two dissociable subunits (I and II) exhibit a catalytic activity only when they are associable together in the presence of Mg²⁺

and Farnesyl diphosphate (FPP, the substrate of HepPP synthase). The glass surface was modified with oriented subunit I and subunit II of the enzyme respectively, then the interactions between these subunits and substrate were directly measured using colloidal probe AFM. This is the first direct demonstration of the specific interaction involved in the enzyme reaction. Elementary processes of the enzyme reaction were studied by quartz crystal microbalance in addition to the force measurement.

Sensitive molecular recognition by using a novel conductive 2D template film

(A) T. Miyahara，K. Kurihara, " Electroconductive Langmuir-Blodgett films containing a carotenoid amphiphile for sugar recognition ", J. Am. Chem. Soc., 126, 5684-5685 (2004).

(B) T. Miyahara，K. Kurihara " Two-dimensional molecular imprinting: Binding of sugars to boronic acid functionalized, polymerized Langmuir-Blodgett films ", Chem. Lett., 1356 (2000).

The fabrication of various molecular devices has attracted much interest in nanotechnology. Especially, the conducting molecule incorporated in a two-dimensionally organized matrix could function as an electric wire in nanoscale electric circuits. If molecular recognition sites could be attached to the conducting molecules, the molecules can then be connected to each other or wired to the desired joints via the guest molecules, and further be applicable to the chemical sensors or the nanoelectronic devices.

In the present study, we synthesized a novel electroconductive amphiphile functionalized with a boronic acid. The electroconductive amphiphile was incorporated into the LB film on an electrode. Using this electrode, the redox current of the redox-active sugar derivative was selectively observed, indicating that the electroconductive amphiphile functioned as a molecular wire. This provides a key step for designing molecular electronic devices based on molecular assembling.

Interfacial molecular macrocluster formation

(A) M. Mizukami, M. Moteki, K. Kurihara, "Hydrogen-Bonded Macrocluster Formation of Ethanol on Silica Surfaces in Cyclohexane", J. Am. Chem. Soc., 124, 12889-12897 (2002).

(B) M. Mizukami, K. Kurihara, "EthanoI Cluster Formation on Silicon Oxide Surface in Cyclohexane-EthanoI Binary Liquids", Chem. Lett., 248 (2000);

(C) M. Mizukami, K. Kurihara "Long Range Attraction between Glass Surfaces in Cyclohexane-Ethano I Binary Liquids", Chem. Lett., 1005 (1999);

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 longrange 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 °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). Same phenomenon was observed also for methanol, 1-propanol, 2-propanol, and carboxylic acid-cyclohexane binary liquids.

Density Dependent Jump in Compressibility of Polyelectrolyte Brush Layers

(A) T. Abe, N. Higashi, M. Niwa, K. Kurihara, "Density Dependent Jump in Compressibility of Polyelectrolyte Brush Layers as Revealed by Surface Forces Measurements", Langmuir, 15, 7725 (1999).

Interactions between apposed brush layers of polyelectrolytes ionized poly(L-glutamic acid) (polymerizationdegree n)21, 44, 48) were investigated in water at pH 10 by the direct surface forces measurement. Brush layers were prepared by the Langmuir-Blodgett (LB) deposition of amphiphiles bearing a poly(L-glutamicacid) chain and two octadecyl groups. The obtained surface force and stress profiles consisted of a longrange electrostatic repulsion and a short-range steric repulsion. The distance where the steric repulsion appeared was in good agreement with twice the length of the polyelectrolyte chains in the extended form. The stress profiles of the polyelectrolyte brushes at n ) 21, 44, and 48 produced the identical curve when the distance was scaled by the length corresponding to twice the thickness of an undeformed polyelectrolyte layer. Interactions between the layers of a poly(L-glutamic acid) (n ) 48) were also studied as a function of the polyelectrolyte chain density in the brush layers. The density was varied by mixing the poly(Lglutamic acid) amphiphile with dioctadecylphosphoric acid. The sudden increase in the short-range repulsion attributed to the steric component was found at the critical density of 0.20(0.07 chain/nm2 with decreasing chain density in the brush layers, indicating the existence of the transition in the interaction mode of polyelectrolytes. The steric repulsion was quantitatively analyzed to provide the elastic compressibility modulus of the polyelectrolyte brushes. The obtained modulus of 0.6(0.1 pN/chain was in the high-density region, and 4.4 ( 0.7 pN/chain was in the low-density region. The transition in the counterion binding to polyelectrolytes may account for the density-dependent change in the interactions of the polyelectrolyte chains.

A novel method for the study for the structuring of confined liquids

(A) C. D. Dushkin, K. Kurihara, "Resonance Shear Force Rheometer Modeled as Simple Oscillating Circuit", Rev. Sci. Instrum., 69, 2095 (1998).

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 filmfromthe background oscillation of the mechanical parts. The method is applied here to investigate the elastic contact between two solid surfaces.