Structure and Frictional Properties of Self-Assembled Surfactant

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Langmuir 1996, 12, 1235-1244

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Structure and Frictional Properties of Self-Assembled Surfactant Monolayers Yihan Liu and D. Fennell Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455

Qun Song and David W. Grainger Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received June 23, 1995. In Final Form: October 26, 1995X Frictional properties of monolayers formed from 11 double-chain quaternary ammonium surfactants self-assembled onto mica were measured using lateral force microscopy. Frictional forces differ by orders of magnitude and can increase or decrease with increasing humidity, and frictional force vs velocity curves in some cases display maximum. These differences correlate at a molecular level with variations in the surfactant’s chemical composition, degree of unsaturation, chain length, and ω functional group. These differences can also be directly related to the monolayer’s structure, phase transition temperature, compressibility and surface hydrophobicity as determined by X-ray diffraction, scanning calorimetry, contact angle, and simulation data. Comparison between lateral force measurements using a standard AFM tip and measurements modified by replacing the tip with a sphere provide a relationship between nanoscale and microscale frictional properties. These observations establish a relation between frictional properties and molecular properties of thin films which are important in many applications including lubrication and tribology.

Introduction Ultrathin molecular films show considerable technological importance in areas involving electronic and optical devices, sensors and transducers, protective and lubricate layers, and patternable materials, etc.1,2 In recent years, considerable effort has been directed toward establishing relationships between structure, forces, and electrical and mechanical properties of organic thin films on surfaces. Self-assembled monolayers of surfactant molecules constitute model systems which permit incorporation of diverse chemical and physical properties and ease of preparation. Insights gained from these model studies can be codified and applied to more complex technological and biological systems. Our present understanding of friction and boundary lubrication is mainly based on empirical correlation. However, the development of modern instrumentation, particularly the surface force apparatus3-7 and the scanning force microscopy,8-12 provides a way to characterize frictional properties of systems at a molecular level. In X Abstract published in Advance ACS Abstracts, February 1, 1996.

(1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (2) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, J. K.; Yu, H. Langmuir 1987, 3, 932. (3) Bailey, A. I.; Courtney-Pratt, J. S. Proc. R. Soc. London 1955, A227, 500. (4) Israelachvili, J. N.; Tabor, D. Nature 1973, 241, 148. (5) Briscoe, B. J.; Evans, D. C. B. Proc. R. Soc. London 1982, A380, 389. (6) Yoshizawa, H.; Chen, Y.-L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128. (7) Hirz, S. J.; Homola, A. M.; Hadziioannou, G.; Frank, C. W. Langmuir 1992, 8, 328. (8) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Luthi, R.; Howald, L.; Guntherodt, M. J.; Fujihira, M.; Takano, M.; Gotoh, Y. Nature 1992, 359, 133. (9) Meyer, E.; Overney, R.; Brodbeck, D.; Howald, L.; Luthi, R.; Frommer, J.; Guntherodt, H.-J. Phys. Rev. Lett 1992, 69 (12), 1777. (10) Meyer, E.; Overney, R.; Luthi, R.; Brodbeck, D.; Howald, L.; Frommer, J.; Guntherodt, H.-J.; Wolter, O.; Fujihira, M.; Takano, H.; Gotoh, Y. Thin Solid Films 1992, 220, 132. (11) Liu, Y.; Wu, T.; Evans, D. F. Langmuir 1994, 10, 2241.

this paper, we present a systematic study of frictional properties of self-assembled surfactant monolayers using lateral force microscopy.13,14 The surfactants are doublechain quaternary ammonium salts, differing in composition, chain length, degree of unsaturation, and functional groups. Our goal is to develop a structure-property relationship. We establish that frictional force can be related to the structure of constituent surfactant molecules in the monolayer film, which in turn provides unique insights into the physics and chemistry of the surfactants as well as into the thermodynamics of two-dimensional systems. Experimental Details Surfactants. Ditetradecyldimethylammonium acetate (2C14N2C1OAc), dihexadecyldimethylammonium acetate (2C16N2C1OAc), and dioctadecyldimethylammonium acetate (2C18N2C1OAc) were the same compounds as those used in ref 15 (2C14) and ref 16 (2C16 and 2C18). The surfactants were prepared by ion exchanging from the corresponding bromide salts (Sogo Pharmaceuticals, Japan) and recrystallized. Dieicosyldimethylammonium bromide (2C20N2C1Br) and didocosyldimethylammonium bromide (2C22N2C1Br) were a gift from Professor Robert Moss and were used as received. Dodecyloctadecyldimethylammonium bromide (C12C18N2C1Br) was synthesized by Dr. J. E. Brady, using the method described in ref 17. Two ω-hydroxysubstituted surfactants, (16,16′-dihydroxydihexadecyl)dimethylammonium bromide (2(HOC16)N2C1Br) and (16-hydroxydihexadecyl)dimethylammonium bromide (HOC16C16N2C1Br), were a gift from Dr. John Trend and were used as received. A fluorinated (12) Haugstad, G.; Gladfelter, W. L.; Weberg, E. B.; Weberg, R. T.; Jones, R. R. Langmuir, in press. (13) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. (14) Marti, O.; Colchero, J.; Mlynek, J. Nanotechnology 1990, 1, 141. Also, Marti, O.; Colchero, J.; Mlynek, J. In Nanosources and Manipulation of Atoms Under High Fields and Temperatures: Applications; Binh, V. T., et al., Eds., Kluwer Academic Publishers: Kluwer, 1993; p 253. (15) Miller, D. D.; Bellare, J. R.; Kaneko, T.; Evans, D. F. Langmuir 1988, 4, 1363. (16) Tsao, Y.-H.; Yang, S. X., Evans, D. F.; Wennerstro¨m, H. Langmuir 1991, 7, 3154. (17) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. 1988, 92, 774.

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Table 1. Deposition Conditions for the Self-Assembled Monolayers surfactant

solution

incubation time

rinse solution

2C14 2C16 2C18 2C20 2C22 C12C18 2(C9dC9) C9dC9C18 2(HOC16) HOC16C16 FC

cyclohexane, ∼10-4 M cyclohexane, ∼10-4 M cyclohexane, ∼10-4 M, heated cyclohexane, ∼10-4 M, heated cyclohexane, ∼10-4 M, heated cyclohexane, ∼10-4 M cyclohexane, ∼10-4 M cyclohexane, ∼10-4 M aqueous, ∼10-5 M, heated aqueous, ∼10-5 M, heated chloroform, ∼10-3 M

5 min 5 min 5 min 5 min 5 min 5 min 5 min 5 min 20 min 20 min 70 h

cyclohexane cyclohexane warm water warm water warm water cyclohexane cyclohexane cyclohexane warm water warm water chloroform

compound (FC), N-(R-(trimethylammonio)acetyl)-O,O′-bis(1H,1H,2H,2H-perfluorodecyl)-L-glutamate chloride (2C8FC2-L-GluC1N3C1Cl), was purchased from Sogo Pharmaceuticals, Japan, and was used as received. Two unsaturated compounds, dimethyldioleylammonium iodide (2(C9dC9)N2C1I) and dimethyloctadecyloleylammonium iodide (C9dC9C18N2C1I), were synthesized in the present work as described below. Synthesis of Dimethyldioleylammonium Iodide and Dimethyloctadecyloleylammonium Iodide. Oleoyl chloride (1) was prepared by stirring 35 mmol of oleic acid (Aldrich, 99%) and excess thionyl chloride (ca. 30 mL, Mallinckrodt) for 3 days at room temperature. Excess thionyl chloride was then removed under reduced pressure to yield oleoyl acid chloride (1). Oleyl oleoyl amide (2) was prepared as follows. Oleoyl chloride (1.6 g, 20 mmol) was dissolved in 200 mL of dichloromethane and cooled to 0 °C. After purging with nitrogen for 1 h, a solution containing 20 mmol of oleylamine (Janssen Chemica, 97%), 3.3 mL of triethylamine (Aldrich, 99%), and 100 mL of dichloromethane was slowly added using a dropping funnel under nitrogen purge. The mixture was stirred for 3 days at room temperature. The color of the mixture changed from yellow to brown. Vacuum evaporation of the solution provided a yellow solid which was recrystallized from ethanol three times. Yield: 2.9 g (27%). Dioleylamine (3) was prepared as follows. Amide 2 (5.4 mmol) was dissolved in 80 mL of THF (Aldrich, 99%) and slowly added to a suspension of LiAlH4 (36.7 mmol) in THF (80 mL), and the mixture was stirred at 50 °C for 5 days. After the solution was cooled to 0 °C, excess LiAlH4 was destroyed by addition of water. THF was removed through vacuum evaporation, and the residue was extracted with diethyl ether. The organic phase was separated and dried over MgSO4. After diethyl ether was removed, a yellowish solid was obtained. Yield: 1.16 g (70%). Dimethyldioleylammonium iodide (4) was prepared as follows. Dioleyl amine (3) (0.94 g, 1.8 mmol) was dissolved in 50 mL of acetone. Di-tert-butyl-p-cresol (di-tert-butyl-4-methylphenol, Aldrich, 99%) was added in trace quantities as a polymerization inhibitor. This solution was cooled to -5 °C, and 0.86 g (6.1 mmol) of CH3I (Aldrich, 99%) was added. This mixture was stirred first at -5 °C for several hours and then at room temperature for 2 days. Vacuum evaporation of the acetone provided a yellow solid crude product which was recrystallized from acetone several times to yield the pure yellow solid. Yield: 0.23 g (18%). NMR (CDCl3): ppm 0.85 (CH3(CH2)7), 1.3 ((CH2)7), 2.0 (CH2-CdC), 3.0 (+N-CH3), 4.4 (CdC-H). Dimethyloctadecyloleylammonium iodide (5) was synthesized using a procedure identical to that described for compound 4 with the exception that octadecylamine (Aldrich) was substituted in place of oleylamine in synthesizing the precursor compound (2). Yield: 0.24 g (19%). NMR (CDCl3): ppm 0.85 CH3(CH2)7), 1.3 ((CH2)7), 2.0 (CH2-CdC), 3.0 +N-CH3), 4.4 (CdC-H). Sample Preparation. Monolayer samples were prepared by dipping a piece of freshly cleaved muscovite mica (Union Mica Corp.) in a solution containing the surfactant and then rinsing in pure solvent to remove excess surfactant.16 Table 1 lists the conditions for deposition. Cyclohexane and chloroform (Fisher Scientific, ACS grade) were used without further purification. Water was millipore filtered and further processed with a Water Prodigy (from Labconco), reaching a resistivity of better than 18

MΩ cm. The samples were kept in a desiccator before the experiment and used within a week. Lateral Force Microscopy. A lateral force microscope (LFM, Nanoscope III, Digital Instruments) was used in this study. Quantitative data for the normal and the lateral forces was obtained by a calibration procedure described in the Appendix. Two cantilevers/tips (Digital Instruments) designated as V1 and V2 were used in collecting most of the data presented in this paper except for those shown in Figures 1, 6, 7, and 10.11 The two cantilevers were V-shape silicon nitride beams microfabricated with integrated pyramidal tips which have a radius of curvature in the range of 20-40 nm, as determined by a scanning electron microscope (SEM). Prior to an experiment, the tip was washed in nitric acid (30%) for 2 min, rinsed in water followed by rinsing in ethanol, and then dried in a laminar flow hood. After the experiment, the tip was saved and used again in the next experiment. When a series of experiments were completed, the cantilever/tip was then imaged using a SEM to determine the dimension of the cantilever and the tip, and also to see if there was damage to the tip. The measured dimension parameters were then used to calculate the force constants (bending and twisting) of the cantilever spring. The calculation was done by finite element analysis. In a separate set of experiments, we modified the scanning probe by attaching a silica glass sphere (Polysciences, Inc.), 15 µm in diameter, to the end of a cantilever, a method first introduced by Ducker and co-workers.18 We used a different procedure to attach the probe, as described in the following. An AFM with a silicon single-crystal beam cantilever (Digital Instruments) was placed under a top-view optical microscope, and the hand screws and the back motor on the AFM were used to control the tip’s position. The original tip was moved over to a “pond” of wet epoxy resin (M-Bond 610 from Measurements Group, Inc.) spread over a corner of a clean silicon wafer and dipped in the glue only briefly and gently in order to prevent uptake of too much glue. Next, the tip was raised and moved to another region of the silicon surface (without glue) where glass beads had been previously spread out. The tip was then precisely positioned above a chosen bead, and the latter was attached by bringing the tip into contact with the bead. The steps must be completed within 5 min before the epoxy hardens. The advantage of this procedure is that the routine can be very efficient. The epoxy was cured by baking the cantilever/probe in a vacuum oven at 175 °C for 1.5 h. Some cantilevers have a reflective metal coating on the back which, depending on the material and thickness, may have just enough mismatch in thermal expansion with the beam material that an appreciable amount of bending in the cantilever may occur, in which case this procedure would not be useful. After curing, the probe was cleaned by water plasma etching (13.5 MHz, 300 mTorr water vapor, 5 W for 5 min). This was mainly to remove any excess glue that possibly had migrated to the bottom of the ball where contact with the sample surface would be made in force measurement. The spherical probe was inspected by SEM after force measurements to ascertain that there was no excess glue observable, though we cannot exclude the possibility of a thin layer of glue still remaining on the surface undetected by SEM; see later discussion. Using a spherical ball in force measurements increases the contact area between the probe and the sample surface and enables us to characterize frictional behavior at a level bridging the macroscopic and the nanoscopic. Force Measurements. Frictional force was measured by sliding the probe repeatedly across the sample surface in the x direction with the y scan disabled. The length of sliding ranged from 300 nm to 1 µm, and our result was independent of the length of sliding within this range. As the tip slides, frictional force values at differnt locations along the track were recorded over one scan cycle with load and sliding velocity fixed, and an average frictional force was found. The scatter around the average force value in a scan cycle was generally less than 10% for a given monolayer system. All measurements were made in air at room temperature. In order to control the humidity, the measurements were carried (18) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239.

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Properties of Self-Assembled Monolayers out in a closed chamber equipped with an inlet for nitrogen. Before the measurements, the chamber was purged with nitrogen, and during force measurements, the humidity in the chamber was monitored and maintained at a constant level between 20 and 30%, unless stated otherwise. Before and after each force measurement, AFM images of the sample surface were taken to ascertain the absence of spurious contamination or damage resulting from the force measurement. We found that force measurements with load up to 40 nN by the regular AFM tip on a V-shape cantilever left no detectable damage to the monolayers. All data presented in this paper, except those in Figure 10, were collected from completely intact surfaces. Differential Scanning Calorimetry (DSC), X-ray Diffraction, and Light Microscopy. DSC, X-ray diffraction, and light microscopy were employed to determine the surfactant bulk structure and phase transition. DSC (Perkin-Elmer, DSC 7) measurements were made with heating and cooling rates of 2 °C/min. X-ray diffraction data were collected with a Rigaku D-Max B diffractometer for wide-angle measurements, and for small-angle measurements, with a Rigaku 12 kW rotating anode generator and a modified Dratky camera (details can be found in ref 19); both machines were equipped with a sample stage temperature control. A light microscope with cross polarizers was also used to examine surfactant bulk texture in aqueous solution. The microscope was also equipped with a temperature variation stage.22

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Figure 1. Frictional force vs sliding velocity for bare mica, under two different loads. Reprinted with permission from ref 11.

Results Film Structure and Stability. AFM imaging of all 11 of the self-assembled monolayers established that they are close packed and molecularly smooth over microns or tens of microns, in agreement with previous observations.11,16 Analysis of the AFM images yields an area of 50 Å2 per surfactant molecule consistent with electrostatic bonding to the negative charge sites on the mica surface (ca. 48 Å2/charge). The monolayers were stable in air for a certain period of time and were robust against scanning. The lifetime of the monolayers in ambient air ranged from weeks to several months, after which deterioration occurred. Monolayers from long chain saturated surfactants exhibited longer lifetime than those from unsaturated ones or ones with shorter chains. For example, 2C14, 2(C9dC9), and (C9dC9C18) monolayers deteriorated after about 3 weeks in ambient air while 2C20, 2C22, the ω-hydroxysubstituted 2(HOC16), and HOC16C16, as well as FC, were intact after 4 months. The other surfactant monolayers had lifetimes somewhere in between. (For the ditetradecyldimethylammonium (2C14) monolayer, occasionally, defects like holes of nanometer size were found in some samples. The 2C14 monolayer was unstable in water which was manifested by desorption while being scanned in situ by the AFM tip in a fluid cell. This same phenomenon of instability in water was also found with the unsaturated compounds, dimethyldioleylammonium (2(C9dC9)) and dimethyloctadecyloleylammonium (C9dC9C18) monolayers. The rest of the surfactant monolayers were all stable against scanning both in water and in air.) We return to this later in the discussion. Dependence of Frictional Force on Sliding Velocity. Results of frictional forces as a function of sliding velocity measured with regular AFM tips are shown in Figures 1-9. (Figures 1, 6, and 7 are taken from a previous publication11 for ease of comparison.) Our measurements were reproducible, and each graph represents at least two sets of independent experiments. Measurements using the spherical glass probe 15 µm in diameter on dihexadecyldimethylammonium (2C16) monolayer are shown in Figure 10. Before the force (19) Kaler, E. W. Ph.D. Thesis, University of Minnesota, 1982. (20) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: New York, 1994.

Figure 2. Frictional force vs sliding velocity for the ditetradecyldimethylammonium (2C14) monolayer. Measurement made with tip V1. The curved lines are meant to guide the eyes.

measurements, we imaged the sample surface using a regular sharp AFM tip to establish that the surface was clean and defect free. During the measurement with glass probe, we found that the data was noisier than those collected with a regular AFM tip. Especially at large loads and moderate to high velocities, frictional forces recorded along a scan line sometimes exhibited spikes, a phenomenon that is often associated with the probe damaging the surface. After the force measurements, the surface was imaged with the spherical probe in the AFM height mode and showed a barely discernible scratch trace produced by the measurement at the lower load and a distinctive scratch trace with excess material accumulated in the vicinity produced from the measurement at the larger load (Figure 10). This extra material could be the surfactant pulled out of the monolayer by the probe and redeposited onto the surface. There seemed to be a threshold in velocity (or frictional force) beyond which damage occurs. We could not pinpoint exactly at what velocity severe damage started except that the noise level in the frictional force during data recording severed at velocities near where the arrow points in Figure 10, indicating discontinuity on the monolayer surface.

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Figure 3. Frictional force vs sliding velocity for dieicosyldimethylammonium (2C20) and didocosyldimethylammonium (2C22) monolayers. Measurement made with tip V1. The curve lines are meant to guide the eyes.

Figure 4. Frictional force vs sliding velocity for dodecyloctadecyldimethylammonium (C12C18) monolayer at two different loads. Measurement made with tip V1. The curved lines are meant to guide the eyes.

Figure 5. Frictional force vs sliding velocity for the unsaturated dialkyl monolayers, dimethyldioleylammonium (2(C9dC9)) and dimethyloctadecyloleylammonium (C9dC9C18). Measurement made with tip V2. The curved lines are meant to guide the eyes.

After AFM measurements, the glass sphere was imaged by a SEM. At low magnification (×2K) the glass surface appeared smooth, while at higher magnification (×20-

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Figure 6. Frictional force vs sliding velocity for dihexadecyldimethylammonium (2C16) monolayer under different loads. The curved lines are meant to guide the eyes. Reprinted with permission from ref 11.

Figure 7. Frictional force vs sliding velocity for dioctadecyldimethylammonium (2C18) monolayer under different loads. The curved lines are meant to guide the eyes. Reprinted with permission from ref 11.

Figure 8. Frictional force vs sliding velocity for the ω-hydroxysubstituted dihexadecyldimethylammonium (2(HOC16) and HOC16C16) monolayers. Measurement made with tip V2. The curved lines are meant to guide the eyes.

50K) the surface looked porous. We believe that the porous surface is responsible for the damage which resulted from abrasive scrapping of the surfactant monolayer off of mica. Repeated experiments with another spherical glass bead produced similar results.

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Figure 9. Frictional force vs sliding velocity for the FC monolayer at two different loads. Measurement made with tip V1.

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Figure 11. Frictional force vs applied load for the dihexadecyldimethylammonium (2C16) monolayer measured at two humidity levels. Sliding velocity was kept fixed at 6.12 µm/s. Measurement made with tip V2. The fittings are as follows: solid line at RH ) 10%, Fric ) 0.038(Nload + 8.2); dashed line at RH ) 30%, Fric ) 0.046(Nload + 13). Note that the frictional force magnitude is significantly less here than the corresponding data shown in Figure 6. This is because the data in Figure 6 was taken with a different type of cantilever/tip11 which had a much larger tip, resulting in a much larger contact area, and was also operated at higher applied loads.

Figure 10. Frictional force vs sliding velocity for the dihexadecyldimethylammonium (2C16) monolayer measured with a glass sphere. At the smaller load, the monolayer survived the measurement with a minor scratch, while at the larger load, major damage to the monolayer was discerned from the AFM image after the force measurement, with damage starting at moderate to high velocity.

Effect of Humidity on the Magnitude of Frictional Force for Hydrophilic and Hydrophobic Surfaces. We also measured the frictional force at different levels of humidity on hydrophilic and hydrophobic surfaces. As illustrated in Figures 11 and 12, frictional forces between these two types of surfaces differ in magnitude and also show an opposite dependence on humidity. For the hydrophilic surfaces prepared from the ω-hydroxysubstituted HOC16C16 and 2(HOC16) (contact angle ) 20° and 7 ( 1°, respectively), frictional forces decrease with increase in humidity; for the hydrophobic surfaces prepared from the rest of the surfactants, such as 2C16 (contact angle ) 62°), frictional forces increase with increase in humidity. Measurements of Surfactant Bulk Structure and Chain Melting Temperature. The velocity dependence measurements show a wide spectrum of response. There is a strong correlation between the frictional force curves, the surfactant chain compressibility, and chain melting temperature. Since it is difficult to measure phase transition directly on a monolayer of ca. 20 Å thick on a mica surface, we instead measured the surfactants’ bulk phase chain melting temperature in water, using an optical microscope, X-ray diffraction, and DSC.

Figure 12. Frictional force vs applied load for the (16,16′dihydroxydihexadecyl)dimethylammonium (2(HOC16)) monolayer measured at two humidity levels. Sliding velocity was kept fixed at 6.12 µm/s. Measurement made with tip V2. The fittings are as follows: solid line at RH ) 10%, Fric ) 0.96(Nload + 89); dashed line at RH ) 47%, Fric ) 0.24(Nload + 2.2 × 102).

The double-chained surfactants form a lamellar bilayer structure20 in water for a wide range of concentrations. The lamellar structure was established by examination in the optical microscope with crossed polarizers and also by measurement with X-ray diffraction. The transition temperature was found by DSC and confirmed by a change in the X-ray diffraction pattern. In the small angle X-ray measurement, chain melting was indicated by the weakening and disappearance of the second-order peak associated with lamellar planes and by a stepwise increase in the interlamellar repeat distance. In the wide angle X-ray measurement, the diffraction peak at ca. 4.2-4.4 Å corresponding to the nearest neighbor distance between hydrocarbon chains21 (or 4.9-5.2 Å for the fluorocarbon chains) in a crystalline (frozen) state disappears upon heating past the chain melting temperature. Our values (21) Small, D. M. The Physical Chemistry of Lipids; Plenum Press: New York, 1986; Chapter 3.

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Table 2. Correlation between Structure, Chain Melting Temperature, and Frictional Force Maximum for the Surfactant Monolayers m

s

Tm

p

along with those from the literature16,22-25 are summarized in Table 2. Note that although the surfactant chain melting temperature in aqueous solution depends on the type of counterions, our main concern is to compare relative values among the different surfactants in order to establish a correlation with the frictional measurement on the monolayer, where in the latter, the counterions are absent as we have verified by X-ray photoelectron spectroscopy. Discussion Most studies of frictional properties of solid surfaces and thin films have focused on measurement of frictional coefficients, comparison of relative frictional forces on different materials, or wear patterns. Several recent studies related frictional properties with specific physicochemical and mechanical properties of the molecular species on surfaces.5,6,12 While dependence of frictional force upon applied load on thin organic films5-7,11 generally follows Amonton’s law with an added term accounting for adhesion,26 dependence of frictional force on velocity can be very different for different molecules even in monolayer films, as evidenced from our results. Figures 2 and 3 show that for monolayers of 2C14, 2C20, and 2C22 frictional force initially increases with velocity and then reaches a plateau, a phenomenon resembling shear thinning in a bulk system. This differs from that measured on the bare substrate mica (Figure 1) and indicates that the adsorbed film has a viscoelastic nature. A very different behavior was observed for monolayers of (22) Tsao, Y.-H.; Evans, D. F.; Rand, R. P.; Parsegian, V. A. Langmuir 1993, 9, 233. Also, Tsao, Y.-H. Ph.D. Thesis, University of Minnesota, 1991. (23) Laughlin, R. G.; Munyon, R. L.; Fu, Y.-C.; Fehl, A. J. J. Phys. Chem. 1990, 94, 2546. (24) Dubois, M.; Zemb, Th. Langmuir 1991, 7, 1352. (25) Okahata, Y.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 789. (26) Derjaguin, B. V. Wear 1988, 128, 19.

C12C18 (Figure 4), 2(C9dC9) and C9dC9C18 (Figure 5), 2C16 (Figure 6), and 2C18 (Figure 7) where the frictional force displays a peak at some critical velocity. The two ω-hydroxy-substituted molecules 2(HOC16) and HOC16C16 are identical to 2C16 in chain length and basic structure except for the terminal groups, yet contrary to 2C16, their monolayers exhibit a monotonic increase of frictional force with velocity. Finally, the fluorinated compound which has a somewhat different chain structure from the rest of the surfactants shows that its frictional force varies in the same manner as that of bare mica. In order to understand these various forms of velocity dependence, we need to compare the surfactants in terms of their molecular properties. In the next two sections, we discuss two basic types of frictional behavior separately, one corresponding to no peak in the frictional force vs velocity curve and the other corresponding to the existence of a peak. Effect of Chain Length. Comparison between 2C14 (Figure 2) and 2C20 and 2C22 (Figure 3) illustrates the effect of different chain lengths. Monolayers from 2C20 and 2C22 have frictional forces 1 order of magnitude larger than that of 2C14. (As indicated in the figure caption, these data were collected with the same tip. Comparison of absolute frictional force should not and will not be made between samples measured by different tips, as tip geometry may vary.) The difference correlates with their chain melting temperatures (Table 2). At room temperature, where our measurements were made, 2C14 chains are expected to be in a melted state whereas 2C20 and 2C22 are in a frozen state. The melted chains are more compliant and therefore yield a smaller resistance. As an aside, we mention that molecular image of the 2C14 monolayer was difficult to obtain and the images were not as resolved as those from frozen chains, probably due to melted chains being more mobile. As a general rule, frictional force increases with increasing chain length which can be associated with rigidity of the monolayer. (We point out here a common misconception that a supposedly sharp ARM tip rides on a single molecule and that it can penetrate between surfactant chains. On the contrary, a tip of 20 nm radius of curvature, corresponding to the sharpest tips used in most AFM experiments, still lands on at least several molecules, provided that no asperity exists. Molecular dynamics27 and Monte Carlo28 simulations indicate that under small to moderate load a monolayer would be elastically compressed by an indenting tip without destruction. Our observation and force measurement data support this picture. By shearing between the tip and the monolayer, we measure the collective mechanical properties of the surfactant chains.) Rigidity of the chains also lessens shear thinning. While the frictional force on the 2C14 monolayer reaches a plateau at relatively low velocities, the curves for 2C20 and 2C22 level out at moderate velocity. For 2(HOC16) and HOC16C16 (Figure 8), slopes in the frictional forces vs velocity curves drop only at very high velocity. For FC which has the highest melting temperature, the frictional force increases linearly with the logarithm of velocity, just like that on the crystalline solid mica (Figures 1 and 9). These trends are in complete accordance with the progressive increase in chain melting temperature. Surfactant chain length also affects the stability of the monolayer. Strong adsorption of oriented arrays depends on two criteria: substrate-head group bonding and lateral dispersion force between neighboring chains. Since all of (27) Tupper, K. J.; Colton, R. J.; Brenner, D. W. Langmuir 1994, 10, 2041. (28) Siepmann, J. I.; McDonald, I. R. Phys. Rev. Lett. 1993, 70 (4), 453.

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our monolayers form via ionic bonding to the substrate, the difference in the monolayer stability observed reflects a difference in the lateral cohesion. Longer chains comprising more methylene units strengthen total lateral cohesion and thereby provide monolayer crystallinity and stability. This is in accordance with our observation that monolayers from longer chain surfactants were more stable than the ones from shorter chain surfactants. Phase Transition in the Monolayer. In 1993, Yoshizawa and co-workers reported a measurement using a surface force apparatus (SFA) of frictional force between two dihexadecyldimethylammonium (2C16) monolayers as a function of temperature, where a maximum friction was observed near room temperature.6 The fixed sliding velocity with which their measurement was made was 0.175 µm/s,29 which is within the range where we observed the frictional peaks (Figure 6). In a previous study,11 we found a peak in the friction vs velocity measurement for 2C16 and 2C18 monolayers and also showed that hexane vapor adsorption in the monolayer of 2C16 shifted the peak position. We concluded that the phenomenon must be related to the physical state of the chains in the monolayer. In the present work, we observed that three other surfactant monolayers also exhibit such a maximum in the friction vs velocity curves. Inspection of Table 2 shows that these five surfactants all have chain melting temperatures that are 10-30 °C above room temperature, whereas the rest of the surfactants have a Tm which is either below or more than 40 °C above room temperature. So a peak in the friction-velocity spectrum was observed for those surfactants with chain melting temperatures which are higher than, but within a small range from, the temperature at which the measurements were made. This suggests a correlation between existence of the peak and chain melting. Complete disordering of the monolayer can be excluded, however, since the surfactant head groups are anchored to the substrate lattice sites, and no destruction such as a void was ever found as a result of friction measurement at the peak velocity. Repeated measurements along the same track gave the same result, indicating that the monolayer must have returned to its initial state after going through the peak. We also verified that this phenomenon is independent of the scanning probe. As Figure 10 shows, the spherical glass ball produced the same result at a small load as the regular AFM tip on 2C16. Note also from the figure that at the larger load the frictional force curve reached a local maximum at the critical velocity before major damage occurred which was caused by the porous glass surface. The monotonic increase at the higher velocity portion of the curve is expected to be associated with friction on exposed mica surface covered by a mash of loose surfactant molecules from the broken monolayer. At the molecular scale, what happens to the surfactant chains as the tip scans across the monolayer surface is not known from our experiments. However, we can gain some insight from simulation studies. Tupper27 and Siepmann28 showed that as an opposing flat surface or a tip approaches and compresses on a surfactant monolayer, the mean tilt angle of the alkyl chains increases, and as the tip withdraws from the surface, the adhesion between the tip and the monolayer results in a “straightening up” of the chains from their normal equilibrium position.28 We can therefore argue that as the tip moves across the sample surface, it elastically compresses the chains in the new position ahead while pulling the chains in the (29) Jacob Israelachvili, private communication.

Langmuir, Vol. 12, No. 5, 1996 1241

previous position behind due to adhesion. Thus for microscopically smooth and well-defined long chain monolayer surfaces, at least part of the work done by frictional force, or strictly speaking, the force in the lateral direction, is consumed by compressing and stretching the alkyl chains. A peak in the frictional force implies a maximum amount of work resulting in maximum energy transferred to the monolayer assembly. We hypothesize that this unusual amount of energy reflects a local phase transition in the monolayer. Several studies on phase transformations in ultrathin organic films have been reported. In 1985, Naselli and co-workers described the first detailed study on the orderdisorder transition in a Langmuir-Blodgett film of cadmium arachidate using infrared and Raman spectroscopies.30,31 They observed a two-step melting process in the LB film; in the first step, a reversible pretransitional disordering of the hydrocarbon tails occurred progressively over a range of temperatures starting substantially below the bulk melting point, and this was followed by an irreversible breakup of the head group lattice above the melting point. Their result was confirmed by Kobayashi and co-workers.32 Electron diffraction33 and X-ray diffraction34 studies also showed that both mono-33 and multilayers33,34 of cadmium arachidate underwent reversible disordering below the bulk melting temperature. Sasanuma et al.34 further divided the disordering process below the bulk melting point into two steps; at 30-50 °C below Tm, onset of the pretransitional disordering occurred, and at 30 °C below Tm, conformational changes from trans to gauche state in the hydrocarbon chain began. FTIR measurement of a progressive disordering in the chains of a self-assembled dioctadecyldimethylammonium (2C18) monolayer at elevated temperatures was reported by Rabinovich et al.35 In a different study using a quartz crystal microbalance (QCM), Garrell and co-workers36 measured the change in the viscoelastic response of a selfassembled octadecanethiol monolayers. They found that the monolayer underwent a phase transition at 18 °C but showed no melting while a thicker layer exhibited both a phase transition at 18 °C and melting at the bulk Tm of 29 °C. Different mechanisms have been suggested for the disordering and phase transitions observed above, including conformational change from trans to gauche in the alkyl chains31,34 and rotator phase transition,33 although a good model does not yet exist. The condition under which Yoshizawa and co-workers found a maximum in frictional force between two monolayers of 2C16 using the SFA coincides with ours; however, the mechanism of chain interdigitation and disentanglement which they proposed6 cannot explain our data since our measurement was made between a monolayer and a bare solid surface. Whether the phase transitions seen in the cadmium arachidate LB films, in the thiol monolayer, and in the self-assembled dialkyl monolayers are of the same nature and whether they indicate a phenomenon more general are unknown. It is clear that more experimental and theoretial work in (30) Naselli, C.; Rabe, J. P.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, (2), 2136. (31) Naselli, C.; Rabe, J. P.; Rabolt, J. F.; Swalen, J. D. Thin Solid Films 1985, 134, 173. (32) Kobayashi, K.; Takaoka, K.; Ochiai, S. Thin Solid Films 1989, 178, 453. (33) Bohm, C.; Steitz, R.; Riegler, H. Thin Solid Films 1989, 178, 511. (34) Sasanuma, Y.; Kitano, Y.; Ishitani, A.; Nakahara, H.; Fukuda, K. Thin Solid Films 1991, 199, 359. (35) Rabinovich, Ya. I.; Guzonas, D. A.; Yoon, R.-H. Langmuir 1993, 9, 1168. (36) Garrell, R. L.; Chadwick, J. E. Colloids Surf., A 1994, 93, 59.

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this area is needed in order to understand phase transitions on surfaces. Effect of Humidity: Influence of Surface Functional Groups upon Frictional Force. We now consider the influence of humidity upon frictional force on hydrophilic and hydrophobic monolayer surfaces. Dihexadecyldimethylammonium (2C16) and ω-hydroxysubstituted (16,16′-dihydroxydihexadecyl)dimethylammonium (2(HOC16)) are identical in chain length and basic structure except for the terminal groups. Two major differences were found between these two types of surface. First is the difference in the magnitudes of the frictional force. For the 2(HOC16) monolayer, frictional force is about 2 orders of magnitude larger than that for 2C16. The two sets of data shown in Figures 11 and 12 are also collected with the same tip, and thus comparison is legitimate. Also note that these measurements were carried out with a fixed sliding velocity of 6.12 µm/s, which is outside the peak region in the frictional force-velocity spectrum for 2C16, so one needs not worry about the probeinduced phase transition discussed above. We can fit the data according to26

F ) µ(N + N0) where F is frictional force, N is load, N0 is adhesion force, and µ is the frictional coefficient. We see (figure captions) that both the frictional coefficient and the adhesion term are an order of magnitude larger for 2(HOC16) than for 2C16. This large difference in frictional force arises from two contributions. The hydroxy groups introduce a large degree of rigidity by hydrogen bonding and thus make the monolayer less compliant. The other contribution comes from the adhesion term. The hydroxy groups present on the surface of the 2(HOC16) monolayer result in a higher surface energy as compared to the methyl-terminated monolayer from 2C16. Therefore, the adhesion is larger between the 2(HOC16) surface and the tip than between 2C16 and the tip. The above equation indicates that at a small load, as in our case, adhesion contributes significantly to the frictional force. The second major difference between 2C16 and 2(HOC16) surfaces is the opposite directions in which frictional force changes with change in humidity. For 2C16, the frictional coefficient increases slightly from 0.038 to 0.046 when the humidity increases from 10% to 30%; whereas for 2(HOC16), it decreases from 0.96 to 0.24, a factor of 4, when humidity increases from 10% to 47%. These opposite behaviors can be understood in terms of the effect of adsorption and capillary condensation of water molecules on the monolayer surface. For 2C16, water does not adsorb on the monolayer surface, and the capillary condensation in the contact zone37 between the monolayer and the scanning probe is expected to be small. For 2(HOC16) on the other hand, water molecules can be strongly bound to the hydroxyl groups on surface by hydrogen bonding,38 and the capillary condensation in the contact zone can be significant. Simulation data39 indicates that a small difference in the surface composition can result in marked difference in surface wettability. For a weakly wetting surface, the hydration layer remains diffuse and is less than a single layer even at high humidity approaching saturation; however, for a strongly wetting surface, the hydration layer is dense and can exceed a single layer at humidity (37) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991; pp 222-223. (38) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982; p 35. (39) Delville, A. J. Phys. Chem. 1995, 99, 2033.

Liu et al.

above 30%.39 Thus we suspect that there is a uniform layer of water molecules bound to the surface of 2(HOC16) and that interaction between the monolayer and the tip is mediated by the water film. In this case, the water film can act as a lubricant. The higher the humidity, the thicker and, hence, the more lubricating the water layer becomes. This accounts for the reduction of the frictional coefficient with an increase in humidity for the 2(HOC16) monolayer. Our data is also consistent with results from Binggeli and Mate40 who studied the effect of water condensation on silicon oxide surface, though in the latter the authors did not observe significant influence of adsorbed water until at high humidities (>75%). The difference in the onset of the water lubrication effect between their system and ours can be attributed to the 2(HOC16) surface being more wettable than silicon oxide or to the difference in cantilever sensitivity, or both. The water molecules between the tip and the hydrophobic surface of the 2C16 monolayer, by comparison, are significantly less, and they form a discontinuous layer. This not only suppresses water’s lubricating effect but increases the water meniscus due to capillary condensation in the contact zone as a result of the increase in humidity resulting in an increase in the adhesive contact area and hence an increase in the frictional force. Indeed, we see that both types of monolayer demonstrate an increase in the adhesive force with increase in humidity (Figures 11 and 12, second term in the linear fitting). The difference between 2(HOC16) and 2C16 surfaces is that, for the former, the reduction in the frictional coefficient due to an increase of water lubrication at higher humidity offsets by a significant margin the increase in friction due to an increase in adhesive force, whereas the hydrophobic surface of 2C16 appears to obliterate the lubricating effect of an aliquot of water between the sliding surfaces and frictional force increases with an increase in capillary condensation at higher humidity. Comparison to Macroscopic Measurements. We have shown that frictional measurement at a nanoscopic level using AFM/LFM not only yields frictional information but also allows detailed chemistry and physics of surface species to be characterized. The sensitivity of frictional behavior to specific structure of adsorbed long chain molecules is well-demonstrated here. Would we gain the same structural information if measurements were conducted between macroscopic bodies, a situation which is of more practical convenience in many cases? To answer this question, we repeated the measurements with the spherical ball at a micrometer scale, as compared to the nanometer scale of the regular AFM tip, and showed that basically we detected the same behavior except when damage occurred due to the abrasiveness of the glass surface. As mentioned in the experimental section, we cannot exclude a possibility that a thin layer of glue still remained on the glass surface which was not observed by SEM, and which would likely affect the adhesion and magnitude of the frictional force. Nevertheless, from our data, it is clear that the major effect on the shape of the force curves came from the roughness of the probe; the detailed chemistry of the sphere surface seems to have played a less dominant role in this case. We had also carried out an experiment (result not shown here) using a sliding microindentor where friction between a diamond tip with a radius of curvature of 50 µm and a 2C16 monolayer on mica was measured. We applied the load in the range of several mN, as compared to several nN in AFM. We found that severe damage to the (40) Binggeli, M.; Mate, C. M. Appl. Phys. Lett. 1994, 65 (4), 415.

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monolayer and even to the mica underneath was produced. Instead of a peak, a general increase of frictional force with increasing velocity was observed, but the data showed significant scatter. We conclude therefore that physicochemical properties of molecular species in adsorbed thin films can influence and be characterized by fictional measurements so long as the measurements preserve the integrity of the film. Sliding between rough surfaces, or under a large load, runs the risk of damaging the surface. Once damage occurs, structural information can be lost and the frictional force obtained becomes some averaged value not necessarily characteristic of the original film. That is probably why we have not seen much detailed structural information like those obtained in our measurements being extracted from macroscopic frictional measurements done conventionally. Conclusion Our main results can be summarized as follows. (1) Frictional force depends on the surfactant chain length. Longer chain molecules can provide a crystalline structure and enhance lateral cohesion; thus they are better candidates for stable and robust coatings than the shorter chain molecules. However, the longer chain surfactants are less compliant toward shearing and yield higher frictional force as compared with shorter chain molecules in a melted state. (2) Frictional force also depends on functional groups at the solid/air interface. Terminal functional groups can affect the wettability of the surface. Our experiments showed that magnitude of frictional force changes with a change in humidity in opposite ways for hydrophilic and hydrophobic surfaces. In higher humidity, water capillary condensation can either increase friction through increased adhesion in the contact zone or reduce friction through enhancing water’s lubricating effect. (3) Shearing on the surface of monolayer from certain types of surfactant may induce local phase transition in the film. The phenomenon is related to the surfactants’ structure and bulk chain melting, but the nature of the phase transition is unclear. (4) While sensitive microscopic measurements with smooth surfaces can give us important structural information, macroscopic measurements with rough and abrasive surfaces often result in surface damage and may smear out the individual characteristics of the species on surface. These results have important implications for designing lubrication systems. In the classical theory of boundary lubrication, reduction in friction between metal surfaces arises from incorporating an adsorbed thin film. Care must be taken in maneuvering the properties of the film in order to achieve maximum lubrication. Different factors that influence the film’s stability and low friction may have to be compromised; induced phase transitions in the film that may cause lubrication failure should be avoided. Another important point we have learned in this study is that, from a more fundamental point of view, measurements of frictional properties of thin films can provide unique insight into the physics and chemistry of the constituent molecules and the thermodynamics of twodimensional systems. Acknowledgment. We like to thank Dr. Yi-Hua Tsao for helpful discussions, Dr. Herbie He Huang for calculating the force constants of the cantilevers, Professor John Evans for assistance in plasma etching of the modified scanning probes, and Dr. John Nelson for carrying out the microscratch test with a nanoindentor. The use of a

Langmuir, Vol. 12, No. 5, 1996 1243

Figure 13. Schematic set up of AFM/LFM cantilever and optical system illustrating force calibration.

Hitachi S-900 field emission scanning electron microscope in the Department of Cell Biology & Neuroanatomy at the University of Minnesota is gratefully acknowledged. This work is financially supported by the Center for Interfacial Engineering, a National Science Foundation engineering research center, a grant from 3M (Y.L. and D.F.E.), the NSF-EPRI joint research program (NSF Grant MSS-9212496, EPRI contract RP 8019-07), and a NSF Grant DMR-9357439 (Q.S. and D.W.G.). Appendix A Practical Procedure for Calibration and Data Conversion in Numerical Frictional Force Measurement with the AFM/LFM. The outputs from the photodiodes (Figure 13) in AFM/LFM are voltage signals. Our goal is to calibrate and convert them to force values. In the normal direction,

F⊥ ) β⊥(V⊥ - V⊥0)

(1)

and in the lateral direction,

F| ) β|(V| - V|0)

(2)

where V⊥ and V| are voltages corresponding to the deflected cantilever in the normal and the lateral directions, respectively, and V⊥,|0 are voltages corresponding to the undeflected cantilever with no force exerted. β⊥,| are conversion factors, which depend on the laser beam power, the detector amplification, the cantilever, and the relative positions of the cantilever and the photodiode detector. So they need to be determined for each experiment. We can single out the dependence of β on the cantilever force constants by noting

β⊥ ) k⊥S⊥

(3)

β| ) k|S|

(4)

where k⊥,| are cantilever force constants in N/m for bending and twisting, respectively, and S⊥,| are sensitivity parameters relating the voltage signal to the distance the tip moves (m/V). S⊥ can be readily obtained from the force calibration curve in AFM. As illustrated in Figure 14a, in the linear compliance region where the tip and the piezo are in contact and move together, the slope is just 1/S⊥. There are two methods to find S|. The first method employs the same idea as in finding S⊥. One can use static friction to obtain a LFM scope curve which contains a region where the tip sticks to the sample surface (Figure

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Instruments, the quadrant photodiode detector has the diamond shape with windows divided as shown in Figure 13, and the differential signals are normalized by the sum signals, i.e.,

V⊥ ∝

A-B A+B

(10)

V| ∝

C-D C+D

(11)

where A-D denote areas projected by the laser beam onto the corresponding photodiode windows. Simple algebra shows

2bδ dV⊥ ∝ 1 2 /2b dV| ∝ Figure 14. Schematic diagrams showing a (a) force calibration curve in AFM and a (b) scope curve in LFM. Inverse slopes give sensitivity parameters.

14b). This method is simple but can be inaccurate if the sample surface slips relative to the tip. Another method is to relate S| to S⊥. We note that suppose the cantilever bends and twists by an infinitesimal angle, dθ⊥ and dθ|, respectively, so that the reflected laser spot on the photodiodes moves a distance δ in both the vertical and horizontal directions (Figure 13), we have

δ ) 2ldθ⊥ ) 2ldθ|

dθ⊥ )

3dz 2L

(6)

where dz is the distance the end of the cantilever bent, and

dθ| )

dx t

(7)

where we assume that the base of the tip acts as a rotation pivot, and t is the tip height, and dx is the distance the tip moves in the lateral direction. From (5) we then get

3dz dx ) 2L t

(8)

Since dz ) dV⊥S⊥ and dx ) dV|S|,

S| 3t dV⊥ ) S⊥ 2L dV|

(9)

The voltage generated in each photodiode window is directly proportional to the area of laser spot projected in that window. For the LFM, Nanoscope III from Digital

2bδ (ba - 1/2b2)

(13)

Thus

dV⊥ (ba - 1/2b2) C + D ) ) 1 dV| A+B / b2

(14)

2

The ratio of the sums A + B to C + D are readily obtainable from the microscope’s total voltage displays. Substituting (14) into (9) then gives

S| )

(5)

where l is the distance between the cantilever and the photodiode. For a beam cantilever of length L and tip height t,

(12)

3t C + D S 2L A + B ⊥

(

)

(15)

Finally once the cantilever’s force constants k⊥ and k| are known, absolute force values can be obtained from eqs 1-4. For a rectangular beam cantilever, k⊥ and k| can be easily calculated from the cantilever and tip dimensions (see for instance refs 11 and 14). For a V-shape cantilever, lateral twisting is more complicated. The silicon nitride V-shape cantilevers used in our experiments have the following dimensions: length L ) 110 µm, width ) 20 µm, V opening angle ) 51°, and tip height t ) 3.57 µm. We calculated the bending and twisting force constants by the finite element method to be k⊥ ) 0.136 N/m (contrary to the manufacture’s claim of 0.58 N/m) and k| ) 54.3 N/m. In summary, to obtain absolute normal and frictional force values, one needs to determine the dimensions of the cantilever and tip used and to record the voltage display A + B and C + D in each experiment. The formulas above are derived as applied to the LFM from Digital Instruments; one can easily make changes to adjust to different types of microscopes. Registry Numbers supplied by author: ditetradecyldimethylammonium acetate, 115652-56-7; dihexadecyldimethylammonium acetate, 71326-37-9; dioctadecyldimethylammonium bromide, 3700-67-2; didocosyldimethylammonium bromide, 4283118-5; dodecyloctadecyldimethylammonium bromide, 35674-625; N-(R-(trimethylammonio)acetyl)-O,O′-bis(1H,1H,2H,2H-perfluorodecyl)-L-glutamate chloride, 88185-38-0.

LA950504O