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Regular Stripe Patterns in Skeletonized Langmuir-Blodgett Films of Arachidic Acid J. Mahnke,† D. Vollhardt,*,‡ K. W. Sto¨ckelhuber,† K. Meine,‡ and H. J. Schulze† Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, D-14424 Potsdam/Golm, Germany, and Research-Group for Colloids and Interfaces Supported by the Max-Planck-Society at the Institute for Ceramic Engineering, Freiberg University of Mining and Technology, Chemnitzer Strasse 40, D-09599 Freiberg, Germany Received February 26, 1999. In Final Form: June 28, 1999 Regular stripe patterns are formed in skeletonized LB films of two arachidic acid monolayers deposited on Si wafers from Cd2+-containing aqueous subphase after monolayer transfer at pH 5.7. These stripe patterns are very different from those less regular patterns observed at lower or higher pH values of the aqueous subphase. Atomic force microscopy (AFM) and phase shift interference microscopy (PSIM) studies have shown that these stripes are grooves aligned in definite distances along the meniscus and perpendicular to the dipping direction of the monolayer transfer. There exist deep straight grooves of about 6 nm depth in a regular distance and between them less marked stripelike defect lines. These regular stripe patterns occur only in the skeletonized LB films, but they are already preformed during the monolayer transfer onto the solid substrate. The formation of the regular stripe patterns is decisively affected by the arachidic acid/cadmium arachidate ratio but also by the dipping rate. The direct correlation to the autooscillations of the meniscus suggests an electrohydrodynamic instability mechanism as cause for the nonuniform ordering and composition of the LB film.
Introduction First basic knowledge about handling monolayers at the air/water-interface and building up multilayer systems on solid substrates by a periodic dipping process was published in two famous papers by Blodgett and Langmuir.1,2 They also showed that the density and the refractive index of Langmuir-Blodgett (LB) films of fatty acids can be changed in a defined way by dipping them into benzene. Due to the fact that the solubility of protonized acid and fatty acid metal salts in organic solvents is strongly different, the protonized molecules can be removed from the film by this method. The molecular ratio of both forms can be adjusted dependent on the pH of the subphase and the kinds of metal cations.3,4 Therefore several attempts have been made to use this method for the preparation of optical layers with a defined refractive index (e.g. as antireflex layer in optics).5,6 All these attempts failed, and the scientific interest in skeletonized LB films faded for over a decade. The development of highly sensitive techniques, such as the AFM, made it possible to look inside the processes during skeletonization on a microscopic and molecular level. Recently Kurnaz et al.7 reported on a microphase separation of acid and metal salt on a 100 nm length scale. Evenson et al.8 also studied the structure of skeletonized LB films using an AFM. They found terraces of bilayer
steps and islands of cadmium arachidate in a 11-layer mixed film that was washed in ethanol. In the following study, we present studies of skeletonized LB films on a higher length scale in the range up to 100 µm. The results show that regular stripe patterns are evolved in skeletonized LB films of arachidic acid deposited on Si wafers from the Cd2+-containing aqueous subphase only after the monolayer transfer at very definite pH values. Already during the monolayer transfer process, the ratio of the protonized fatty acid and the fatty acid metal salt affects decisively the pattern formation after skeletonization. Stripe patterns of a different type were found in nonskeletonized monolayers of the phospholipid DPPC (L-R-dipalmitoylphosphatidylcholine) deposited on a solid substrate.9,10 The authors explained their results by a substrate-mediated condensation. Our experimental results show, however, that a substrate-mediated condensation cannot be the reason for the pattern formation in the skeletonized fatty acid salt systems. It can be experimentally shown that the formation of the remarkable defect lines in the skeletonized LB films obviously correlate with autooscillations of the meniscus during the monolayer transfer process. Our studies on skeletonized LB films were originally motivated by investigations on their wetting behavior and the stability of thin aqueous wetting films formed on these substrates.11,12 Experimental Section
* Corresponding author. † Freiberg University of Mining and Technology. ‡ Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. (1) Blodgett, K. B. J. Am. Chem. Soc. 1935, 57, 1007. (2) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937, 51, 964. (3) Spink, J. A. J. Colloid Interface Sci. 1967, 23, 9. (4) Petrov, J. G.; Kuleff, I.; Platikanov, D. J. Colloid Interface. Sci. 1982, 88, 29. (5) Tomar, M. S. J. Phys. Chem. 1974, 78, 947. (6) Hasmonay, H.; Dupeyrat, M.; Dupeyrat, R. Opt. Acta 1976, 23, 665. (7) Kurnaz, M. L.; Schwartz, D. K. J. Phys. Chem. 1996, 100, 11113. (8) Evensson, S. A.; Badyal, J. P. S.; Pearson, C.; Petty, M. C. Adv. Mater. 1997, 9, 58.
Preparation of LB Layers. The LB films were prepared on a KSV5000 trough (KSV Instruments) placed under a laminar flow box (UniEquip, 99%, Merck, Darmstadt, Germany) and chloroform (p.a., Merck) were used without further purification. All experiments were performed at room temperature. The spreading solution (1 mg/mL arachidic acid dissolved in chloroform) was spread on the trough surface. After the evaporation of the solvent, the monolayer was then brought to the selected surface pressure (π ) 30 mN/m) with a constant compression rate of 0.06 nm2 molecule-1 min-1. The used glass substrates (microscopic glass slides, 20 × 20 × 0.1 mm) were carefully cleaned in a 7:3 v/v mixture of 30% H2O2 and 95% H2SO4 at 110 °C for about 30 min. Afterward the substrates were rinsed with MilliQ+ water and stored under water. For methylation the glass slides were dried and put in pure hexamethyldisilazane (HMDS, Merck) for several minutes and rinsed with cyclohexane (Merck). The substrates were methylated to achieve a greater stability of the LB layers for the study of the stability of wetting films.12 Skeletonization. The skeletonization of the LB layers was performed in cyclohexane (p.a., Merck) by dipping the substrate into a vessel with the solvent for 20 s. Although the process is supposed to continue for hours, our ellipsometric investigations have shown that the main part of the soluble film material is removed within the first few seconds and the stability of the films strongly decreases afterward.13 We have chosen a quite short time for the skeletonization to keep the films as stable as possible for the following wetting studies on these systems. However, the films might not be fully skeletonized in accordance with the molecular content of either component. Phase Shift Interference Microscopy (PSIM). A phase shift interference microscope (Micromap 512, delivered by Atos, Pfungstadt, Germany) was used to visualize the structure of the skeletonized LB layers on a 100-µm scale. This system consists of an interference microscope (Nikon Optiphot) equipped with Mireau interference objectives (5×, 10×, 20×, and 40×) mounted on a piezoelectric translation turret (pzt). A CCD camera (640 × 480 pixels) on the microscope transfers the interference optical image to a personal computer. For illumination, monochromatic light with a wavelength of 632.6 nm was used. During the measurement the pzt shifts the objective a distance corresponding to the wavelength of the used light toward the sample, controlled (13) Mahnke, J. Diploma Thesis, University of Regensburg, 1995.
by special software. Out of the intensity change during this process the computer software calculates height information with very great accuracy (subnanometer) for every pixel. So this method allows a height resolution of 0.1 nm at the normal lateral resolution of light microscopy of about 0.5 µm. Atomic Force Microscopy (AFM). AFM measurements were performed with a NanoScope III (Digital Instruments, Inc., DI) in air at room temperature, using the scanners 10 µm × 10 µm and 125 µm × 125 µm. For the AFM studies in contact mode, cantilevers with silicon nitride tips and spring constants of 0.06 N/m were used. The height calibration was performed with the 180 nm standard of DI. Investigation of Wetting Film Stability. The stability of aqueous wetting films on the skeletonized LB layers was investigated using a Derjaguin-Scheludko film balance, which is described in detail elsewhere.12 The wetting film is formed by pressing an air bubble under a aqueous KCl solution (10-3 M) onto a solid surface covered with the skeletonized LB film. The intervening liquid film between the bubble and the solid substrate is then observed from beyond using a Nikon inverse light microscope. The thickness of this thin water film (in the range of 50 nm to some hundreds nanometers) can be measured using a interferometric method. The fluid film is thinning, and when the thickness reaches a critical value hcrit, rupture takes place. The rupture process starts with formation of a small hole showing a three-phase contact line (TPC) between the solid (LB-film), fluid (water film), and gaseous phases. The shape of the expanding TPC during dewetting of the substrate was followed using a highspeed video system (Speedcam, Weinberger AG, Dietikon, Switzerland). The possible resolutions with this system were 1024 frames/s (128 × 128 pixel), 2048 frames/s (128 × 64 pixel), and 4096 frames/s (128 × 32 pixel). Usually the first combination was used.
Results The preparation of skeletonized LB films was motivated by studies on the wetting film rupture for which were needed hydrophobic surfaces with a controlled degree of heterogeneity as substrates. The idea was to relate the number of defects in the surface after skeletonization of the transferred layer with the proportion of protonated fatty acid to the fatty acid salt in the floating monolayer. The lateral distribution of the defects was routinely checked by using the phase-shift-interference microscope (PSIM). It is interesting to note that only in a narrow pH region at about pH 5.7 striking stripelike defect patterns are
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Figure 3. (a) AFM image of a skeletonized LB film (2 monolayers of arachidic acid on hydrophobized glass; subphase, pH 5.7, 2.5 × 10-4 M Cd2+; vertical dipping at 5 mm/min). Besides the deep groove, numerous additional small defects can be seen. (b) Cross-sectional profile along the line of (a). The depth of the groove is about 6 nm.
formed. These defect patterns are very different from those observed at lower or higher pH values of the aqueous subphase. Figure 1 shows two typical PSIM images of such stripewise structures in a 2 monolayer LB film of arachidic acid transferred from a pH 5.7 subphase and skeletonized in cyclohexane for 20 s. The stripelike defect lines correspond to grooves which run always perpendicular to the dipping direction during the transfer process, i.e., parallel to the meniscus line. The distance between the main defect lines observable in the PSIM images is about 150 µm and depends mainly on the dipping rate. The values given in Table 1 are averages of 10 PSIM images, as shown in Figure 1. The distance decreases, when the dipping rate is reduced from 5 to 3 mm/min, but at high dipping rates of >8 mm/min the distance between the main defect lines is so large that it cannot be observed. The grooves are not yet formed after the monolayer transfer, but rather they are developed during the skeletonization process similar to a photographic development process (Figure 2). The stripe patterns are obviously preformed or initiated during the transfer process since the grooves always end at the horizontal plane of the cyclohexane. In addition, it can seen that the immersed part of the substrate appears lower than the adequate original layer, which clearly indicates the removal of film material due to the skeletonization. Figure 2 also shows that the orientation of the stripes is not affected by the direction in which the substrate was dipped into the cyclohexane. This is corroborated by different experiments which show that the orientation of the resulting stripes depends only on the monolayer transfer direction but not on the dipping direction during the skeletonization. The corresponding AFM images show more clearly that between the deep grooves observable by the interference microscope additional grooves exist aligned with the same direction; see for example Figure 1b. These additional defect lines are less marked and, therefore, hardly visible in the resolution of the interference microscope (Figure 1a). A representative AFM image and the corresponding cross section profile are presented in Figure 3. It can be seen in the cross section profile that the depth of the stripes is about 6 nm which is in the range of the double layer thickness of cadmium arachidate.14 The depth measured with the PSIM amounts to 2-4 nm concluded from the gray value of the stripes in Figure 1a. The discrepancy in the defect depth might be caused by the fact that the sensitivity of the interference microscopy is affected by changes in the optical properties due to the density change (14) Petty, M. C. Langmuir Blodgett Films: An Introduction; Cambridge University Press: Cambridge, U.K., 1996.
Figure 4. Damages in a skeletonized LB film (2 monolayers of arachidic acid on hydrophobized glass; subphase, pH 5.7, 2.5 × 10-4 M Cd2+; vertical dipping at 5 mm/min) caused by an AFM tip.
of the layer. As consequence, the thickness measured by PSIM is apparently reduced. The resolution of the PSIM technique is too low for microstructures below 0.5 µm. The skeletonization reduces obviously the stability of the LB layer system so that damages by the AFM tip can be easier caused than in usual LB films. Figure 4 shows an example for such damaging. The stripe patterns run parallel to the meniscus during the LB transfer over distances of at least several hundreds of micrometers. Its occurrence is clearly correlated with a typical “slip-stick” behavior of the meniscus observable on a video frame sequence. In the case of a continuous LB transfer, the stripes were not formed during the skeletonization. Similar to many other dynamic processes, it is quite difficult to depict this behavior in single images. Therefore Figure 5 shows a cartoon, because single video frames picked out of the whole film would be less informative. In this perspective, the meniscus should ideally be a straight line, when receding with a constant speed from the substrate surface and forming a homogeneous LBlayer. Therefore the observation of the meniscus is an often used criterion to characterize the quality of the transfer. The situation changes in those cases when stripes were observed in the skeletonized layers. The three-phase contact line is withdrawn for a while by the substrate, sticking to it on one line (P1 in Figure 5). Periodically it breaks at one point resulting in an fast movement of the meniscus to the left and right side of the substrate. The distance between two sticking points (P1 and P2 in Figure 5) of the meniscus as estimated from a video film is roughly equal to the average distance between the main stripes visible in the microscope leading to the schematic molecular arrangement of salt and free acid shown in Figure 5c.
Langmuir-Blodgett Films of Arachidic Acid
Figure 5. Schematic representation of the meniscus behavior during the LB transfer: (1a,b) front view of the breaking meniscus (slip-stick mechanism between points P1 and P2); (2a-c) side view of the meniscus slipping down from point P1 to point P2; (3) transfer behavior (shown in 1 and 2) leads to a stripelike molecular distribution of the free fatty acid.
The skeletonization of LB films was done in connection with some other works to study the wetting behavior of heterogeneous or microstructured surfaces.13,19,20. The defect lines of skeletonized LB films change dramatically the rupture mechanism of an aqueous thin liquid wetting film. These studies were performed with the DerjaguinScheludko film balance equipped with a high-speed video system. An expanding hole in a wetting film on the hydrophobic surface of the LB film forms a three-phase contact line. Figure 6 shows a sequence of images during the rupture process on a skeletonized LB system. It can be clearly seen that the rupture is strongly accelerated at once when the TPC reaches a defect stripe (usually by a factor of 10-15). This is corroborated by some vTPC data for different substrate surfaces presented in Table 2. On the other hand, it should be mentioned that, unlike in other cases of heterogeneous LB-layers,12 the critical thickness of the wetting films hcrit at which rupture commences is not affected by the skeletonization. So far, however, it is not quite clear whether the presence of the stripes initiates the formation of a first hole in the film. (15) Kistler, S. F., Schweizer, P. M., Eds. Thin Liquid Film CoatingScientific principles and their technological implications; Chapman & Hall: London, 1997. (16) Fruhner, H.; Kra¨gel, J.; Kretzschmar, G. J. Inf. Rec. Mater. 1989, 17, 273. (17) Zholkovskij, E. K.; Vorotyntsev, M. A.; Staude, E. J. Colloid Interface Sci. 1996, 181, 28. (18) Kovalchuk, V. I.; Kamusewitz, H.; Kovalchuk, N. M.; Vollhardt, D. Phys. Rev. E 1999, 60, 2029. (19) Mahnke, J.; Sto¨ckelhuber, K. W.; Schulze, H. J.; Radoev, B. Colloids Surf. A, in press. (20) Mahnke, J.; Mu¨ller, P.; Schulze, H. J.; Sto¨ckelhuber, K. W.; Weber, E. Colloids Surf. A 1998, 142, 275.
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Discussion Morphology and local ordering of ultrathin organic films deposited on solid surfaces depend not only on the system (substances, substrate surface preparation) but also strongly on the conditions of their formation (surface pressure, transfer rate).10 During the dynamic wetting of solid substrates, the processes in the vicinity of the threephase contact line play an important role.15,16 Despite the extreme importance for technological processes, dynamic wetting processes in the vicinity of the three-phase contact line have not yet been sufficiently understood. It is generally accepted that dynamic Langmuir wetting complies with the basic condition for unstable behavior. Although regular striations running normal to the tipping direction were recently found for nontreated LB films of phospholipids, their formation is coupled with the substrate-mediated condensation of the monolayer material which occurs only within a certain pressure range. The causes for the pattern formation in LB films of DPPC are obviously very different from those observed in skeletonized arachidic acid/cadmium arachidate LB films. It is well-known that the fraction of the cadmium salt formed in long chain fatty acid (arachidic acid) LB films deposited from a dilute CdCl2 subphase increases from 0 to 1 within the pH range of about 4.8-6.2. Over the whole pH range, more or less defined defects can be observed in the skeletonized LB films, see for example ref 7, but only at the definite pH 5.7, the well-developed defect stripes are formed. The experiments have shown that the stripe formation is obviously correlated with the autooscillations of the meniscus. Consequently the stripe patterns are preformed or initiated by complicated processes in the meniscus region due to the transfer dynamics. Two questions remain to be answered: (i) Which mechanism gives rise to the autooscillation of the meniscus? (ii) In which way does the oscillation result in the formation of the stripes? A theoretical consideration of the problem does not yet exist so that the origin of the instability can only be discussed. It can be expected that the local monolayer state (structure, composition, interfacial energies) in the region of the three-phase contact line differs from that at the plane surface.10 At the transfer of the second monolayer, complicated interactions exist in the contact region between the receding substrate surface and the partially ionized monolayer at the air/water interface. The substrate surface is already covered with the first monolayer partially ionized. Under these dynamic conditions, electrohydrodynamic instability should occur during the Langmuir wetting. The electrical double layers of both monolayers interfere with each other in the meniscus region so that a nonuniform surface potential and a nonuniform surface charge can be induced. The electrical interaction affects the strong interfacial energies, the local ordering, and the composition of LB films. In any case, the electrical interactions in the dynamics of the threephase contact line should be a significant reason for the instability mechanisms during the Langmuir wetting process. Nonuniform distribution of the composition of the LB film is indicated by the occurrence of stripe patterns perpendicularly aligned to the dipping direction after the transfer of arachidic acid/cadmium arachidate monolayer at a definite pH value (Figures 1-3). According to the appearance of the stripelike grooves after the skeletonization, arachidic acid and cadmium arachidate are segregated due to the electrohydrodynamic instability during the monolayer transfer. At the skeletonization, the stripes are formed by dissolution of the enriched arachidic acid from the LB film. The fact that well-
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Figure 6. Image series of a rupturing wetting film on a skeletonized LB system (2 monolayers of arachidic acid on hydrophobized glass; subphase, pH 5.7, 2.5 × 10-4 M Cd2+; vertical dipping at 5 mm/min). Table 2. Average TPC Velocities on Different Hydrophobic Surfacesa system 2 ML Cd-AA by methylation 2 ML skeletonized 2 ML Cd-AA hydrophibized Cd-AA between the skeletonized glass plate pH 6.8 grooves on the groove average velocity (µm/ms)
65
49
10
111
a AA: arachidic acid. ML: monolayer. Typical standard deviation about 50%.
developed stripe patterns are only formed in a narrow range of about pH 5.7 indicates the significance of the arachidic acid/cadmium arachidate ratio. The intrinsic pKa of the arachidic acid monolayer has been estimated at about 5.4, but it seems to be specifically affected by individual cations.7 Concerning the reasons for the instability mechanism, the correlation between the appearance of the stripe patterns and the autooscillations of the meniscus are of special interest. Under certain conditions, hydrodynamic instability can be caused by electrical interactions in surface regions.17 The analysis of the hydrodynamic instability can provide an important contribution for the understanding of autooscillations, as recently found for autooscillations of the surface tension at the air/water interface.18 Conclusions Regular stripe patterns are evolved in skeletonized LB films of arachidic acid deposited on Si wafers from the Cd2+-containing aqueous subphase only after the monolayer transfer at a definite range of about pH 5.7. As
evidenced by AFM and PSIM studies, these stripes are deep grooves aligned in more or less definite distances along the meniscus and perpendicular to the dipping direction of the monolayer transfer. Although these regular stripe patterns occur only in the skeletonized LB films, they are preformed already during the monolayer transfer onto the solid substrate. The formation of the regular stripe patterns in the skeletonized LB films investigated is correlated to the autooscillations of the meniscus during the transfer of the arachidic acid/cadmium arachidate monolayer at pH 5.7. Under the dynamic transfer conditions of the second monolayer, the interference of the electrical double layers in the meniscus region can induce a nonuniform surface charge. Due to the electrohydrodynamic instability during the monolayer transfer, arachidic acid and arachidate are segregated during the transfer process. This is indicated by the appearance of the stripelike grooves after skeletonization. The arachidic acid/cadmium arachidate ratio affects decisively the instability mechanism. Theoretical work is in progress to analyze the nonlinear regime. For the consideration of the hydrodynamic meniscus instability, it is necessary to develop a theory of an electric double layer near the three-phase contact line. Acknowledgment. We thank Dr. V. Kovalchuk from the Institute of Biocolloid Chemistry, Kiev, Ukraine, for valuable discussions and Helmut Partzscht, Freiberg, Germany, for the high-speed video measurements. Financial support by the German Research Community (SFB 285 “Particle Interactions” at the Technical University Freiberg; SFB 312 “Vectorial Membrane Processes”, Berlin) is gratefully acknowleged. LA990230V
