9390
Langmuir 2000, 16, 9390-9395
Friction Anisotropy and Asymmetry Related to the Molecular Tilt Azimuth in a Monolayer of 1-Monopalmytoyl-rac-glycerol Kenji Hisada‡ and Charles M. Knobler* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569 Received May 30, 2000. In Final Form: August 18, 2000 The molecular tilt azimuth in domains of a condensed phase in monolayers of 1-monopalmytoyl-racglycerol at the air/water interface exhibits a regular star texture. Lateral-force microscopy has been used to investigate the friction behavior of a monolayer that has been transferred onto a silicon substrate. The condensed domains revealed strong friction anisotropies and asymmetries. The angular dependence of these properties correlates well with the tilt azimuth of the alkyl tails in the monolayer.
Introduction As a result of the research on ultrathin films of organic molecules over the past decade, it is now possible to prepare monomolecular layers with known microscopic and macroscopic organization on solid supports.1 Such monolayer films can be first produced at the air/water interface and then transferred to a solid surface by the LangmuirBlodgett (LB) technique or they can be prepared from solutions by self-assembly. The relatively easy availability of such ordered two-dimensional systems has led to their use in investigations of a number of physical phenomena, among them tribology. Although the phenomenological study of friction at the macroscopic scale has a long history,2-4 frictional processes at the atomic scale have been very difficult to study. With the use of wellcharacterized monolayer films, it has become possible to correlate mechanical properties (adhesion, friction and wear) with the molecular structure of the sliding interfaces. A key element in the tribological studies of monolayers is the use of scanning force microscopy (SFM), which provides the ability to visualize electrically insulating films in situ with high lateral resolution.5-10 SFM measurements reveal the topology of a surface and, when operated in the lateral force mode, they can provide information * Corresponding author. E-mail:
[email protected]. † Current address: Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Fukui University, Fukui 910-8507, Japan. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (2) Bowden, F. P.; Tabor, D. Friction and Lubrication of Solids; Oxford University Press: Oxford, 1986. (3) Handbook of Micro/Nanotribology; Bhushan, B. Ed.; CRC Press: Boca Raton, 1995. (4) Persson, B. N. J. Sliding Friction-Physical Principles and Applications; Springer: New York, 1998. (5) Dai, Q.; Vurens, G.; Luna, M.; Salmeron, M. Langmuir 1997, 13, 4401. (6) Srinivasan, U.; Houston, M. R.; Howe, R. T.; Maboudian, R. J. Microelectromech. Syst. 1998, 7, 252. (7) Tripp, C. P.; Veregin, R. P. N.; McDougall, M. N. V.; Osmond, D. Langmuir 1995, 11, 1858. (8) Radmacher, M.; Tillmann, R. W.; Fritz, M.; Gaub, H. E. Science 1992, 257, 1900. (9) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J.; Fujihara, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (10) Meyer, E.; Overney, R.; Brodbeck, D.; Howald, L.; Lu¨thi, R.; Frommer, J.; Gu¨ntherodt, H.-J. Phys. Rev. Lett. 1992, 69, 1777.
about the friction as well. Such studies have examined the effects of chain length11-13 and terminal group14 on the friction and they also have revealed that the friction can sometimes depend on the scan direction. This frictional anisotropy has been observed, for example, in LB films of a polydiacetylene15 and of an organic bilayer16 and has been associated with different molecular orientations with respect to the scan direction in ordered films. In films in which there is a uniform tilt of the molecular backbone with respect to the surface normal, there may be a spontaneous large-scale organization of the molecular tilt azimuth into regular patterns.17 Such tilt patterns provide a unique opportunity for studies of the relation between tilt direction and sliding friction. Duschl and coworkers18-20 found such tilt order in monolayers of a thiolipid transferred to mica by the LB technique. Lateralforce microscopy (LFM) images revealed regular 6-fold star patterns that could be associated with different molecular tilt directions. They also observed an asymmetry in the frictional force: the friction observed in a scan of the AFM tip from left to right differed from that in a scan from right to left. Condensed-phase domains containing seven-arm star patterns have been observed in Langmuir monolayers of glycerol esters,21 Figure 1. Optical studies demonstrate that the stars survive LB transfer to solid supports, and, in a previous brief communication, we described qualitative measurements of the relation between friction and (11) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (12) McDermott, M. T.; Green, J.-B.; Porter, M. D. Langmuir 1997, 13, 2504. (13) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800. (14) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (15) Carpick, R. W.; Sasaki, D. Y.; Buirns, A. R. Tribol. Lett. 1999, 7, 79. (16) Overney, R. M.; Leta, D. P. Tribol. Lett. 1995, 1, 247. (17) Qiu, X.; Ruiz-Garcia, J.; Stine, K. J.; Knobler, C. M.; Selinger, J. V. Phys. Rev. Lett. 1991, 67, 703. (18) Santesson, L.; Wong, T. M. H.; Taborelli, M.; Descouts, P.; Liley, M.; Duschl, C.; Vogel, H. J. Phys. Chem. 1995, 99, 1038. (19) Gourdon, D.; Burnham, N. A.; Kulik, A.; Dupan, E.; Oulevey, F.; Gremaud, G.; Stamou, D.; Liley, M.; Dienes, Z.; Vogel, H.; Duschl, C. Tribol. Lett. 1997, 3, 317. (20) Liley, M.; Gourdon, D.; Stamou, D.; Meseth, U.; Fischer, T. M.; Lautz, C.; Stahlberg, H.; Vogel, H.; Burnham, N. A.; Duschl, C. Science 1998, 280, 273. (21) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloid Surf., A 1993, 76, 187.
10.1021/la000743y CCC: $19.00 © 2000 American Chemical Society Published on Web 10/12/2000
Molecular Tilt Azimuth in Monopalmytoyl Monolayer
Langmuir, Vol. 16, No. 24, 2000 9391 carried out at constant surface pressure by vertical dipping at a rate of 5 mm min-1. On this hydrophilic substrate, the MPG molecules are oriented with the polar headgroup toward the surface and the alkyl chains exposed to the air. Scanning Force Microscopy. Contact-mode measurements were performed with a scanning force microscope (Park Scientific Instruments, Autoprobe CP) on the transferred monolayers. Triangular cantilevers (Park Scientific Instruments Microlevers) with a normal spring constant of 0.05 N m-1 and a pyramidal Si3N4 tip were used in this study. All measurements were performed with a 100-µm scanner at loading forces between 0.5 and 6 nN.
Results
Figure 1. Brewster-angle microscope image of condensedphase domains in a monolayer of 1-monopalmytoyl-rac-glycerol (MPG) at the air/water interface. The domains are surrounded by an isotropic (liquid-expanded) phase. There are seven wedgeshaped segments within each domain, which are visible in the domain at the upper right. Only six segments are visible in the domain at the lower left because, with this setting of the microscope analyzer, two segments have the same contrast. The segments correspond to different directions of the molecular tilt azimuth.
tilt direction in such domains that had been transferred to silicon wafers and mica;22 Meine et al. carried out similar experiments.23 The monoglycerides are well-suited for such experiments, because the organization of the tilt azimuth in the stars has been established by Brewster-angle microscopy21 and the underlying microscopic structure has been determined by diffraction at grazing incidence.24,25 We report here a detailed quantitative scanning force microscopy study of the frictional force in such films. Experimental Section Langmuir monolayers of 1-monopalmytoyl-rac-glycerol (MPG) were prepared in a NIMA-type 611 trough. Solutions of the amphiphile (Sigma, purity >99 mol %) were spread from n-heptane/ethanol (Fischer, spectranalyzed) solutions (9:1 by volume) onto water purified with a Millipore Milli-Q system (F > 18 MΩ). The surface pressure-area isotherms were identical to those previously observed.22,23 The condensed-phase domains on the water surface are typically 300-500 µm in diameter. Because the maximum scan range of our scanning force microscope is 100 µm, we added a small amount (
