Measurement method of physical properties of adsorbed layers

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Measurement method of physical properties of adsorbed layers Kentaro Tanakaa and Katsumi Iwamoto a ' Department of Marine Electronics and Mechanical Engineering, Tokyo University of Marine Science and Technology 2-1-6, Etchujima, Koto-ku, Tokyo 135-8533, Japan Lubrication by a molecularly thin film has become very important in micro machines, magnetic recording disks and so on. Molecularly thin film is considered a good lubricant for these micro devices. When, the thickness of the lubricant film is thinned to several nanometers, the tribological properties of the film become different from that of the bulk. Therefore it is important to know these properties. Quartz crystal microbalance (QCM) is common in thin film growth, where it is used for film thickness measurements. QCM allows one to evaluate the physical properties of viscous liquid layer on the quartz crystal surface. In this study, we investigated the effect of additives on the viscosity. When the additives formed the adsorbed layer on the quartz crystal, the decreases in the resonant frequency of the quartz crystal were observed. It is found that QCM can detect the properties of adsorbed layers.

1. INTRODUCTION The lubrication by thin film has become very important in micro machines, magnetic recording disks and so on. Molecularly thin films are considered a good lubricant for it. However, when the thickness of the lubricant film is thinned to several nanometers, the tribological properties of the film become different from that of the bulk. Recently, many experimental approaches have been carried out to elucidating how molecularly thin film lubricates. Atomic force microscopy (AFM) can measure the factional properties of polymer thin film [1-3]. Properties of shearing thin liquid film between two solid surfaces have been investigated by using a surface force apparatus (SFA) [4,5]. Although these approaches elucidated that the properties of molecularly thin liquid film confined to solid surfaces are dramatically different from those of the corresponding bulk liquid, the nature of the thinning liquid film on the solid surface is not understood. In order to understand it, it is important to know how 'thinning' the film thickness on the surface effects on physical properties of film itself. Quartz crystal microbalance (QCM) has been known as a sensitive mass-measuring device [6]. And, QCM has been used extensively to monitor the physical properties of thin liquid film on its surface. The resonant frequency of the quartz crystal is dependent on the viscosity and density of the liquid film [7-9]. In this study, physical properties of liquid films were investigated by QCM.

2. EXPERIMENTS 2.1. Oscillation frequency of QCM in contact with a liquid Quartz crystal microbalance (QCM) is known to provide very sensitive mass measuring device. Deposition of a given mass on the QCM electrode produces shifts in the resonance frequency of the quartz crystal. The relationship between the changes in mass and the frequency are given by the Sauerbrey equation [6]. The resonant frequency also can be affected by the viscosity and density of the liquid. In this case, a simple relationship has been derived which expresses the change in oscillation frequency (AF) in contact with a viscous liquid [8]. (1) where F« is the resonant frequency, pq and fxq are the density and the shear modulus of the quartz crystal, Pi and tji are the density and the viscosity of liquid. There is a decrease in frequency that is dependent upon the viscosity and density of the liquid film on the quartz crystal. 2.2. Apparatus The experimental apparatus comprised at 6-MHz, AT-cut quartz crystal, which oscillates in transverse shear motion, and an oscillating circuit designed to

I I 1 I Liquid ( M h i l l Wm

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1i111

Figure 1. Parameters in the Equation 1

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Cell

2.4. Procedure

Liquid AT-cut quartz crystal

The quartz crystal with Au electrodes was mounted in a Daifron cell, as represented in Figure 2. After the frequency stabilized, the base oil was added to the cell. The volume of the base oil in the cell was 10ml, and the frequency monitored. Outputs from the oscillating circuit were feed to the frequency counter interfaced to a personal computer providing data collection in real time. All data were acquired at room temperature (~26C°). 3. RESULTS AND DISCUSSION

Frequency counter

Computer

Figure 2. Scimatic of the quartz crystal microbalance (QCM) apparatus

Au electrode Quartz crystal

3.1. Frequency change with hexadecane Figure 4(a) shows a typical frequency decrease with passage of time (black line), when the QCM cell was filled with a 10ml hexadecane solution. First of all, as a hexadecane solution was injected, the frequency abruptly decrease by ~2500Hz and then decreased slowly up to ~2800Hz for 300 minutes. In this case, the adsorbed layer is hardly formed, because the interaction between the hexadecane molecule and the surface of quartz crystal is weak. It seems that the frequency decrease about 300Hz corresponds the formation of physically piled up layers of hexadecane. 3.2. Effects of adsorbed layers

Figure 3. Au electrode were deposited on the quartz crystal surface drive the crystal at its resonant frequency. Figure 2 gives a schematic representation of the experimental apparatus. 2.3. Materials A 6MHz AT-cut quartz crystal with Au electrodes was used. Au electrodes (13mm in diameter) were deposited on both sides of quartz crystal (Figure 3). Hexadecane was prepared as typical base oil. And we used hexadecanoic acid and hexadecanetiol as additives. Hexadecanetiol was obtained from Wako Pure Chemicals, and other chemicals were obtained from Kanto Kagaku. Table 1. Chemical formula Hexadecane CH3(CH2)14CH3 Hexadecanoic acid CH3(CH2)14COOH Hexadecanetiol CH3(CH2)i4CH2SH

To investigate the effects of adsorbed layers, we monitored the frequency change of quartz crystal after injecting lOmM hexadecanoic acid and lOmM hexadecanetiol into a hexadecane-filled QCM cell. Hexadecanoic acid has methyl-terminated groups (CH3) and acid terminated groups (COOH). COOH groups can form physically adsorbed layer to the surface. Hexadecanetiol has methyl-terminated groups and mercapto-terminated groups (SH). SH groups can form chemically adsorbed layer to the surface. It is well known that highly ordered molecular monolayer of alkylthiols formed on to a gold surface by chemisorption of SH groups and aggregation of the alkyl chains. This is a kind of self-assembled monolayers. Figure 4(b) shows a QCM frequency change with passage of time after injecting lOmM hexadecanoic acid into a 10ml hexadecane solution. The solution was injected, the frequency abruptly decrease by ~2500Hz as seen in Figure 4(a), and then decreased slowly up to ~3200Hz for 400 minutes. This suggests that the adsorbed layers are formed on to the surface. Figure 4(c) shows the frequency change with passage of time after injecting lOmM hexadecanetiol into a 10ml hexadecane solution. The solution was injected, the frequency abruptly decrease by ~2500Hz as seen in other cases, and then decreased

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(a) Hexadecane - ^

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-1000J- (b) Hexadecane + Hexadecanoic acid (10mM) — -2000 k ^ Second

him -3000

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-3000 -4000 200

400 Time [min]

+10mM Hexadecanetiol

Figure 5. Change in frequency averaged between 700-750min. Error bar indicates standard deviation. mentioned before, hexadecanetiol formed selfassembled monolayer, which is highly ordered, this results mean that (p-rjf12 of hexadecanetiol layer is smaller than that of hexadecanoic acid layer. When the quartz crystal is in contact with viscoelastic film, AF is not linear with (p-Tj)ll2[\0]. In order to measure the properties of adsorbed layers for tribological issues, it is necessary to add some modifications to the equation 1. 3.3. Effects of rinsing with ethanol

(c) Hexadecane + Hexadecanetiol (10mM)

0

7 Hexadecane

600

Figure 4. Time dependencies of frequency decrease with (a) hexadecane, (b) +10mM hexadecanoic acid, (c) +10mM hexadecanetiol slowly up to ~3000Hz for 400 minutes. In this case, it is considered that the adsorbed layer was formed on to the surface. But the frequency decrease was rather rapid compared with case of hexadecanoic acid. Most of the frequency decreasing occurred in first 100 minutes. It means that the adsorbed layer is rapidly formed. Figure 5 also shows change in frequency in the steady state (700-750min.) with solutions. By injecting the additives, the frequency changed. The frequency change is transformed into (p-rf)xa increase by referring to equation 1. Although, as we

All the experiments were repeated two times. New quartz crystals were used at first time. Then, some kinds of molecular layers were formed on to the quartz surface. So, we tried to rinse off these layers from the surface with ethanol solutions for following experiments. There are the black line and the gray line respectively in Fig. 4 (a)-(c). The black lines show the results of first experiments as mentioned in sec. 3.1 and 3.2. The gray lines show the results of second experiments. In the cases of hexadecane (Fig. 4(a)) and hexadecane with hexadecanoic acid (Fig. 4(b)), the second experiments shows the almost same result with first experiments. However, in the case of hexadecanetiol (Fig. 4(c)), the frequency does not show the tendency of slow decreasing. In other words, the period of adsorbed layer formation does not observed. These results indicate that the chemically adsorbed layers, which had been formed by first experiment, could not be removed by rinsing with ethanol. 4. CONCLUSIONS In this work, the quartz crystal microbalance was applied to study the properties of the adsorbed layers on the quartz crystal surface. The relation between the resonant frequency of the quartz crystal and the

134 134 viscosity and density of adsorbed layer were used. When the adsorbed layer was formed on the quartz crystal, the decreases in the resonant frequency of the quartz crystal were observed. It means that (p-tj)m of the adsorbed layer was larger than the base oil. It was found that QCM could detect the properties of adsorbed layers. This work showed QCM technique will provide a measurement method of tribological properties of thin film. ACKNOWLEDGEMENTS This work is supported by the grant of TTRF (Taiho Kogyo Tribology Research Foundation). REFERENCES [1] C. M. Mate, "Atomic force microscope study of polymer lubricants on silicon surfaces," Phys. Rev. left., 68 (1992) 3323-3326. [2] X. Xiao, J. Hu, D. H. Charych and M. Salmeron, "Chain length dependence of the frictional properties of alkylsilane molecules self-assembled on mica studied by atomic force microscopy," Langmuir, 12 (1996) 235-237. [3] H. Liu, B. Bhushan, "Nanotribological characterization of molecularly thick lubricant films for applications to MEMS/NEMS by AFM," Ultramicroscopy, 97 (2003) 321-340. [4] S. Granick, "Motions and relaxations of confined liquids," Science, 253 (1991) 1374-1379. [5] M. L. Gee, P. M. McGuiggan, J. N. Israelachivili and A. M. Homola, "Liquid to solidlike transitions of molecularly thin films under shear," J. Chem. Phys., 93 (1990) 1895-1906. [6] G. Z. Sauerbrey, "The use of quartz oscillators for weighing thin layers and for microweighing," Physik, 155 (1959) 206-222. [7] T. Nomura and M. Okuhara, "Frequency shifts of piezoelectric quartz crystals immersed in organic liquids," Anal. Chim. Ada. ,142 (1982) 281-284. [8] K. K. Kanazawa and J. G. Gordon I I , "The osillation frequency of a quartz resonator in contact with a liquid," Anal. Chim. Ada., 175 (1985) 99-105. [9] H. Muramatsu and K. Kimura, "Quartz crystal detector for microrheological study and its application to phase transition phenomena of Langmuir-Blodgett films," Anal. Chem., 64 (1992) 2502-2507. [10] H. Muramatsu, E. Tamiya and I. Karube, "Computation of equivalent circuit parameters of quartz crystal in contact with liquids and study of liquid properties," Anal. Chem., 60 (1988) 21422146