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Stability of the Parallel Layer during Alkane Spreading and the Domain Structures of the Standing-Up Layer Lingbo Lu, Kari J. Zander, and Yuguang Cai* Department of Chemistry, University of Kentucky, 505 Rose Street, Lexington, Kentucky 40506 Received November 19, 2009. Revised Manuscript Received March 8, 2010 The spreading of liquid alkanes over surfaces plays an important role in applications such as lubrication, painting, and printing. To make significant advances in these fields, it is essential to increase our understanding of the interactions between alkanes and surfaces. Long-chain alkanes form two typical adsorption structures on a surface;the parallel phase and the standing-up phase. The most thermodynamically stable structure is the parallel phase, in which the alkane molecules lie flat on the surface. If the temperature is slightly below the bulk melting point, then alkanes form a thermodynamically stable standing-up phase on top of an existing parallel layer. At lower temperatures, the standing-up phase becomes metastable. Using atomic force microscopy, we have found that the standing-up alkane layer consists of multiple domains, indicating that the standing-up layer forms through a multinucleation process during the liquid-solid transition on the surface. If, however, the temperature is above the melting point, then we have found that the standingup layer shrinks to a droplet and leaves a residue on its original position. During the spreading of an alkane droplet, the parallel layer forms on the substrate surface surrounding the droplet by adsorption from the vapor, which precedes the arrival of the liquid. There has been uncertainty, however, as to whether the parallel layer moves with the liquid alkane or remains stationary during spreading. In this study, we used the residue left on the parallel layer as a landmark to monitor the movement of the parallel layer during the spreading of an alkane droplet. Using this landmark, we found that the parallel layer remained stationary on the substrate, indicating that the liquid alkane spreads on a stationary parallel layer surface. Therefore, this study reveals that the surface properties of the parallel layer;not the surface properties of the substrate;control the spreading and wetting of a liquid alkane.
I. Introduction Alkanes are the major component of lubricants. The structures, spreading, and wetting of alkanes on solid interfaces are fundamental issues in tribology.1-4 The surface structures of long-chain n-alkanes have been extensively studied by X-ray diffraction,5-7 neutron diffraction,8-10 ellipsometry,11 optical microscopy,12,13 and atomic force microscopy (AFM).14-16 In a narrow temperature range near the bulk alkane’s melting point, long-chain alkane molecules adsorb on the hydrophilic surfaces according to the bilayer model, which is illustrated in Figure 1.7,11,17,18 This model *Corresponding author. E-mail:
[email protected].
(1) Gellman, A. J. Tribology Lett. 2004, 17, 455. (2) Wasan, D. T.; Nikolov, A. D. Nature 2003, 423, 156. (3) Dedkov, G. V. Phys. Status Solidi A 2000, 179, 3. (4) Bhushan, B. Proc. Inst. Mech. Eng., Part J 2001, 215, 1. (5) Wu, Z.; Ehrlich, S. N.; Matthies, B.; Herwig, K. W.; Dai, P.; Volkmann, U. G.; Hansen, F. Y.; Taub, H. Chem. Phys. Lett. 2001, 348, 168. (6) Schollmeyer, H.; Struth, B.; Riegler, H. Langmuir 2003, 19, 5042. (7) Basu, S.; Satija, S. K. Langmuir 2007, 23, 8331. (8) Taub, H.; Herwig, K. W.; Matthies, B.; Hansen, F. Y. Inorg. Mater. 1999, 35, 847. (9) Herwig, K. W.; Matthies, B.; Taub, H. Phys. Rev. Lett. 1995, 75, 3154. (10) Diama, A.; Matthies, B.; Herwig, K. W.; Hansen, F. Y.; Criswell, L.; Mo, H.; Bai, M.; Taub, H. J. Chem. Phys. 2009, 131, 10. (11) Volkmann, U. G.; Pino, M.; Altamirano, L. A.; Taub, H.; Hansen, F. Y. J. Chem. Phys. 2002, 116, 2107. (12) Lazar, P.; Schollmeyer, H.; Riegler, H. Phys. Rev. Lett. 2005, 94, 116101. (13) Kohler, R.; Lazar, P.; Riegler, H. Appl. Phys. Lett. 2006, 89, 241906. (14) Bai, M.; Knorr, K.; Simpson, M. J.; Trogisch, S.; Taub, H.; Ehrlich, S. N.; Mo, H.; Volkmann, U. G.; Hansen, F. Y. Europhys. Lett. 2007, 79, 26003. (15) Trogisch, S.; Simpson, M. J.; Taub, H.; Volkmann, U. G.; Pino, M.; Hansen, F. Y. J. Chem. Phys. 2005, 123, 154703. (16) Van, L. P.; Kyrylyuk, V.; Polesel-Maris, J.; Thoyer, F.; Lubin, C.; Cousty, J. Langmuir 2009, 25, 639. (17) Mo, H.; Taub, H.; Volkmann, U. G.; Pino, M.; Ehrlich, S. N.; Hansen, F. Y.; Lu, E.; Miceli, P. Chem. Phys. Lett. 2003, 377, 99. (18) Nozaki, K.; Saihara, R.; Ishikawa, K.; Yamamoto, T. Jpn. J. Appl. Phys. 2007, 46, 761.
5624 DOI: 10.1021/la904387d
was presented by Volkmann et al. on the basis of their ellipsometry measurement of the solution-deposited dotriacontane (C32) film on a SiO2 surface.11 The bilayer model was confirmed by a synchrotron X-ray reflectivity study17 and a contact-mode AFM study.15 In addition, Basu and Satija found that the longchain alkane films on SiO2 formed from vapor deposition also adopted the bilayer structure.7 Recently, our group used ac-mode AFM to reveal that 1-hexatriacontane (C36) on hydrophilic surfaces also had the same bilayer structure.19 The bilayer adsorption structure has been firmly established by these experimental results from different groups. According to the bilayer model, alkane molecules adsorbed on the hydrophilic surface have two basic structures;the parallel layer and the standing-up layer. Alkane molecules lie flat on the surface in the first few layers, which are called the parallel layers and are thermodynamically stable. Additional alkane molecules form the thermodynamically metastable standing-up layer on top of the parallel layer. The alkane molecules in the standing-up layer expose the CH3 group whereas the alkane molecules in the parallel layer expose the CH2 group. As a result, under AFM characterization, the parallel layer and the standing-up layer show different friction and phase contrast. A submonolayer of the standing-up alkane usually takes the shape of the seaweed. Such seaweed shape originates from the diffusion-limited aggregation (DLA) growth mechanism.20,21 Reference 14 presents a phase diagram for a film of intermediate-length alkanes adsorbed on the SiO2 surface. The phase diagram revealed that the standing-up alkane layer (monolayer (19) Cai, Y. G. Langmuir 2009, 25, 5594. (20) Holzwarth, A.; Leporatti, S.; Riegler, H. Europhys. Lett. 2000, 52, 653. (21) Kn€ufing, L.; Schollmeyer, H.; Riegler, H.; Mecke, K. Langmuir 2005, 21, 992.
Published on Web 03/18/2010
Langmuir 2010, 26(8), 5624–5631
Lu et al.
Article
II. Material and Methods
Figure 1. Bilayer model depicting the adsorption structure of long-chain n-alkanes on a hydrophilic surface.
phase) is metastable except in a narrow temperature range near the bulk melting point. When the liquid alkane is undercooled (the chemical potential of the standing-up layer is lower than that of the liquid alkane), alkane molecules in the liquid phase turn into ordered standing-up layers. In contrast, when the alkane is heated to above its melting point (the chemical potential of the standing-up layer is higher than the liquid alkane), the alkane molecules in the standing-up layer turn back into the liquid phase, which is called delayering.14 The standing-up alkane structure is thermodynamically stable only in a narrow temperature range around the alkane melting point. If the temperature is higher than this narrow range, the standing-up layer will delayer to liquid droplets (L phase) whereas if the temperature is lower than this range the standing-up layer will eventually change into the surface rotator phase (R0 ). Such phase changes occur reversibly. Recently, we discovered that the parallel layer on the hydrophilic surface could be formed from the vapor adsorption before the liquid alkane spread, which also brings up a new question about the stability of the parallel layer during the spreading: does the parallel layer move outward along with the liquid during spreading?22 If the parallel layer indeed moves along with the liquid or it is pushed outward by the liquid, then the liquid should spread directly onto the solid substrate. If the parallel layer is stationary, then the liquid droplet would spread on top of the parallel layer. Under this circumstance, the interactions that control the spreading, wetting and capillary properties of the liquid would be solely between the liquid and the parallel layer. The surface properties of the substrate would have very little role in controlling the liquid’s behavior. Therefore, the stability of the parallel layer during spreading determines what controls the liquid’s behavior, which is a significant issue in studying the liquid spreading. However, it is difficult to use AFM to characterize the motion of the featureless parallel alkane layer because no reference point can be found for tracking the motion and for comparison. The motion of the parallel layer has not previously been investigated. Using the prefabricated surface pattern to study the spreading and alkane structures offers an alternative approach to tackling this technical difficulty. The prefabricated pattern has known geometric dimensions and chemical properties. The shape and chemistry of the pattern can serve as references for tracking the movement and comparing the change in surface properties. Through comparison over the same region before and after the spreading, we can measure the change in the featureless films. In this article, we report our study of the stability of the parallel layer and the fine structure of the seaweed-shaped standing-up layer using the surface-patterning approach. We found that the parallel layer did not move during the spreading. In addition, we found that the seaweed-shaped standing-up alkane layer was composed of many domains with different heights, suggesting they are not formed by the DLA mechanism alone. (22) Lu, L. B.; Cai, Y. G. Langmuir 2009, 25, 13914.
Langmuir 2010, 26(8), 5624–5631
Materials. Silicon (100) wafers (KC electronics) were polished to the ultraflat level with an rms roughness of
