© Copyright 1999 American Chemical Society
APRIL 27, 1999 VOLUME 15, NUMBER 9
Letters Characterization of Phase Separation in Mixed Surfactant Films by Liquid Tapping Mode Atomic Force Microscopy Yonghui Yuan and Abraham M. Lenhoff* Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received July 14, 1998. In Final Form: December 11, 1998 Two-dimensional phase separation in Langmuir films comprising mixtures of charged and neutral surfactants transferred onto graphite was observed by liquid tapping mode atomic force microscopy (LTMAFM). Resolution of the different phases was based on the difference in surface charge density, which resulted in a difference in tip-sample electrostatic interaction and a consequent apparent difference in height despite the identical tail lengths of the two surfactants. Exploration of the effects of temperature and subphase ionic strength showed that phase separation tended to occur at low temperature and high ionic strength, while at high temperature and low ionic strength the monolayer was homogeneous. The method offers a sensitive new approach to identifying distinct phases in Langmuir films.
Introduction Two-dimensional phase separation in mixtures of surfactants spread into Langmuir films can be observed directly using fluorescence microscopy and Brewster angle microscopy.1,2 Limitations of these methods are that only phase features larger than about 1 µm are visible and that a dye probe that is preferentially soluble in one of the phases must be added for fluorescence microscopy. Recently, atomic force microscopy (AFM) and its derivative friction force microscopy (FFM) have been used to study phase separation of Langmuir-Blodgett (LB) films by imaging them after transfer to solid substrates. Due to the high lateral resolution of the methods and the simplicity of sample preparation, as well as the practical applications of LB films transferred onto solid substrates, this new class of methods has found increasing use. AFM has been used to study phase separation of mixed phos* To whom correspondence should be addressed. Phone: (302) 831-8989. Fax: (302) 831-4466. E-mail:
[email protected]. (1) Mo¨hwald, H. Annu. Rev. Phys. Chem. 1990, 41, 441. (2) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.
pholipids,3 stratum corneum lipids,4 and a mixture of arachidic acid and 5,10,15-triphenyl-20-(4-DL-R-phenylalanylamino)phenylporphyrin (TPPP),5 on the basis of the difference in height of the two phases, which resulted from the different tail lengths of the surfactants used. Condensed and expanded liquid phases of a single surfactant have also been observed by this method.6 FFM is more useful than AFM in such applications because of its ability to reflect surface friction properties, allowing it to provide clearer contrast even in systems in which the substrate is rough, which decreases the resolution of the height images. In studies of two-dimensional phase separation and morphology of a variety of LB films (3) Yang, X. M.; Xiao, S. J.; Lu, Z. H.; Wei, Y. Surf. Sci. 1994, 316, L1110. (4) Grotenhuis, E. T.; Demel, R. A.; Ponec, M.; Boer, D. R.; van Miltenbur, J. C.; Bouwstra, J. A. Biophys. J. 1996, 71, 1389. (5) Wu, H. M.; Xiao, S. J.; Tai, Z. H.; Wei, Y. Phys. Lett. A 1995, 199, 119. (6) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213.
10.1021/la980879k CCC: $18.00 © 1999 American Chemical Society Published on Web 03/11/1999
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Figure 1. LTM AFM height images of a transferred SAME and STAB mixture on graphite from deionized water at (a) 20, (b) 10, and (c) 6.5 °C. Surface pressure, 30 mN/m; scan size, 10 µm; z range, 15 nm. chloroform to a total concentration of 1.92 mM in the molar ratio using FFM,7-11 the friction between the AFM tip and the 3:1. The subphase was either distilled, deionized (DI) water from sample surface was found to differ for different phases, a Millipore Milli-Q system, or an NaCl solution. A computerallowing the two phases to be distinguished. However, a controlled, user-programmable minitrough (Model 100DE, KSV disadvantage of all the above AFM and FFM imaging is Instruments, Trumbull, CT) was used for holding the subphase that it was conducted in air, so the samples had to be and spreading the monolayer. The trough, of dimensions 450 dried before imaging. In the drying process, capillary forces mm × 150 mm, was jacketed, allowing the temperature of the could have affected the structure of the transferred LB subphase to be controlled by circulating water from a water bath; films. the actual temperature of the subphase was measured imWe have studied the phase separation of two-dimenmediately after transfer of the monolayer. Dust was removed from the subphase by sucking with a clean pipet, and the platinum sional surfactant mixtures of nonionic and charged Wilhelmy plate was flame-burned before each experiment. The surfactants by liquid tapping mode (LTM) AFM imaging.12 surfactant-chloroform solution (50 µL) was slowly added onto In this method, the transferred surfactant monolayer was the subphase surface using a microsyringe. After the chloroform imaged in a liquid environment, thus eliminating the effect had evaporated and the surfactant mixture had spread on the of the drying process. In tapping mode AFM, the AFM tip subphase, the monolayer was compressed by moving the two oscillates as it scans across the sample surface, with the barriers to a surface pressure that was kept constant at 30 mN/ amplitude and phase angle of the oscillation changing as m. The monolayer was transferred to a freshly peeled graphite the tip scans different areas of the sample surface due to substrate (diameter 1.5 cm) by the “touching” method: the differences in the tip-sample interaction. The height and substrate was lowered in a horizontal position until it touched the monolayer surface, and then lifted away quickly. The graphite phase images can be obtained simultaneously. The substrate was hydrophobic, so the monolayer would stick to the amplitude is generally affected mainly by the steric graphite with the surfactant headgroups exposed and a thin water repulsion between the tip and the surface and so provides film coating it. The sample was placed in the AFM liquid cell and the height data of the AFM image. However, long-range imaged in a liquid environment immediately, before it had had interaction may also play a role, and in this work we found time to dry. electrostatic interaction to be significant. The two 2-D The sample was imaged using a Nanoscope III scanning probe phases were distinguishable in two different scanning microscope (SPM) (Digital Instruments, Santa Barbara, CA). 13 as a modes, namely height mode and phase imaging, The imaging was performed under liquid conditions by tapping result of differences in the surface charge density of the mode AFM with the liquid in the AFM liquid cell being the same two phases. The electrostatic origin of the height difference aqueous solution as the subphase. Triangular, thin silicon nitride in liquid tapping mode was verified by AFM force curve AFM tips (150 µm) (Digital Instruments) were used in the LTM imaging. Typical experimental parameters in the LTM AFM measurement and by imaging the same sample by other imaging were: drive amplitude, 150 mV; set point, 0.7 V; scan AFM techniques, including tapping mode in air, contact rate, 1.3 Hz. mode in liquid, and lift mode in liquid. The method enables
us to study phase separation of surfactants even with the same tail length, as well as some systems in which the drying process would cause surface aggregation. Materials and Methods Stearic acid methyl ester (SAME) (Sigma Chemical Company, St. Louis, MO) and stearyltrimethylammonium bromide (STAB) (TCI America, Portland, OR) were dissolved in analytical grade (7) Yuba, T.; Yokoyama, S.; Kakimoto, M.; Imai, Y. Adv. Mater. 1994, 6, 888. (8) Meyer, E.; Overney, R.; Lu¨thi, R.; Brodbeck, D.; Howald, L.; Frommer, J.; Gu¨ntherodt, H. J.; Wolter, O.; Fujihira, M.; Takano, H.; Gotoh, Y. Thin Solid Films 1992, 220, 132. (9) Fujihira, M.; Takano, H. Thin Solid Films 1994, 243, 446. (10) Xiao, S.; Wu, H.; Yang, X.; Li, N.; Wei, Y.; Sun, X.; Tai, Z. Thin Solid Films 1995, 256, 210. (11) Yokoyama, S.; Kakimoto, M.; Imai, Y. Synth. Met. 1996, 81, 265. (12) Hansma, P. K.; Cleveland, J. P.; Radmacher, M.; Walters, D. A.; Hillner, P. E.; Bezanilla, M.; Fritz, M.; Vie, D.; Hansma, H. G.; Prater, C. B.; Massie, J.; Fukunaga, L.; Gurley, J.; Elings, V. Appl. Phys. Lett. 1994, 64, 1738. (13) Brandsch, R.; Bar, G.; Whangbo, M.-H. Langmuir 1997, 13, 6349.
Results and Discussion The characteristics of the films were studied as a function of both temperature and subphase ionic strength. Figure 1 shows typical LTM AFM height images of the surfactant monolayer transferred onto newly peeled graphite substrates from deionized water at 20, 10, and 6.5 °C. Because of imperfect peeling, some graphite steps of height 1-2 nm can be seen. At 20 °C, these steps are the only features, suggesting that the surfactant monolayer was homogeneous, with only a single phase present. At 10 °C, several small, well-defined bright areas appear, corresponding to heights about 2.5 nm above the background. At 6.5 °C, more and larger bright areas appear, about 2.1 nm higher than the dark areas. In Figure 2 the phase images, obtained simultaneously, are shown for the areas in Figure 1a and c. The first image is that in the absence of phase separation (20 °C), and again only graphite steps are seen, with no second phase
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Figure 2. LTM AFM phase images corresponding to Figure 1a, homogeneous single phase, and c, two distinct phases, showing inversion of the bright and dark areas. Z range: 15°.
present. The image at 6.5 °C confirms the phase separation visible in Figure 1c. The bright and dark domains are inverted compared to the height data, but the contrast is sharper, and for this reason subsequent images are shown in the form of phase data. The appearance of different domains is also dependent on the subphase ionic strength. The film was seen in Figure 1a to be homogeneous at 20 °C when the subphase was deionized water. Figure 3 shows phase images of the surfactant monolayer transferred from subphases of different ionic strengths with the subphase temperature fixed at 20 °C. For a subphase ionic strength of 1 mM (Figure 3a), the result is similar to that in Figure 1b, with only small patches of the second phase appearing. Because of the contrast inversion, the minor phase here appears dark. At 10 and 100 mM (parts b and c of Figure 3, respectively), the domains of the second phase are larger, indicating that phase separation is promoted by a higher subphase ionic strength. The heights of the second phase in the corresponding height images (not shown) are 1.3, 3.0, and 1.3 nm greater than the background, respectively. The alkyl chains of the two surfactants are equally long, and because their headgroups are comparatively small, the molecular lengths are similar, about 2.4 nm. Our films were transferred at 30 mN/m, at which surface pressure the surfactant molecules were densely packed, so it is unlikely that the observed behavior is due to aggregation on the substrate, leaving areas of bare graphite exposed. On the other hand, the pressure was too low for the behavior to be explained as formation of multiple layers. Instead the features seen can be explained by monolayer phase separation. It should be extremely difficult to distinguish two surfactants of equal length in height mode in air, a
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situation made more difficult for two phases that may be composed of both surfactants. The explanation lies in the nature of the tip-sample interaction upon which the height data are based. The height image in tapping mode AFM reflects the reduction in oscillation amplitude of the cantilever. When the imaging is performed in aqueous solution at low ionic strengths, the long-range electrostatic interaction between the tip and the sample surface is especially significant. A surface of higher charge density reduces the cantilever oscillation amplitude to a greater extent, thus appearing as the region that appears higher in the height data (brighter with the gray scale convention used here). The AFM tip (pK 7-8) should be positively charged under the conditions of our experiments (pH around 6),14 as are the STAB headgroups, so electrostatic interactions between the tip and the surface should be repulsive. Therefore we assign the bright areas in Figures 1b and c to be a phase of higher surface charge density (rich in STAB) and the dark areas to be a phase of lower surface charge density. To verify the electrostatic origins of the apparent height difference, we used AFM to obtain the force curves for the interaction between the AFM tip and the surfactant monolayer transferred from a subphase of DI water at 8 °C (under which conditions phase separation was seen by LTM-AFM). The experiment was performed in DI water (pH 5.8), so the AFM tip was positively charged. The curves shown in Figure 4 map the force measured as the tip approaches the respective surfaces, for the bright and the dark areas seen in the height images, respectively. In these force curves, the long-range electrostatic and shortrange van der Waals interactions between the tip and the surface are clearly seen. The repulsive electrostatic force is dominant at long range for the interaction between the tip and the bright area but absent for that between the tip and the dark area, indicating that the surface charge density for the bright area is higher. This confirms our assignment of the two phases and the role of electrostatic interaction in distinguishing the two phases. The explanation of the phase images is more complicated. Recent studies have shown that contrast in phase imaging is related not only to surface properties, but also to the experimental parameters, such as driving amplitude and set point.15-17 Because the imaging parameters were varied little in our experiments, we can disregard their effects on the images. The phase images can then be explained by recognizing that the phase shift increases with the stiffness of the surface, leading to a brighter area within the gray scale convention used.17 For the STABrich phase, the long-range electrostatic repulsion between the tip and the surface acts as a cushion and decreases the effective surface stiffness, which reduces the phase shift (darker region). For the STAB-lean phase, the repulsion is predominantly steric and thus more abrupt, leading to a bright region. Thus in the phase images, in contrast to the height images, the darker areas correspond to the phase of higher surface charge density. To show the essential role of tapping mode, we also performed AFM imaging in other modes for comparison. Figure 5 shows liquid-phase contact mode height and friction images for a sample prepared at 10 °C, under which conditions LTM AFM images showed phase sepa(14) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446. (15) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H.-J.; Whangbo, M.-H. Langmuir 1997, 14, 3807. (16) Cleveland, J. P.; Ancykowski, B.; Schmid, A. E.; Elings, V. B. Appl. Phys. Lett. 1998, 72, 1. (17) Magonov, S. N.; Elings, V.; Whangbo, M. H. Surf. Sci. 1997, 375, L385.
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Figure 3. LTM AFM phase images of a transferred surfactant film on graphite from NaCl solution of concentration (a) 1 mM, (b) 10 mM, and (c) 100 mM at 20 °C. Surface pressure, 30 mN/m; scan size, 10 µm; z range, 20°.
ration. In the contact mode image, the only surface features visible are the graphite steps, whereas the friction image shows the presence of a second phase. We also found that the different phase domains could be seen in lift mode images when the interleave scan was enabled; that is, the scan was performed with the tip several nanometers above the sample surface. Although the image quality was not as good as that for the tapping mode images, the lift mode images (not shown) confirmed the existence of distinct regions, which could not be seen when the tip was scanned in true contact mode (Figure 5a). We believe that this experiment, as well as the fact that the difference between the two phases was seen only for LTM AFM imaging and not for tapping mode AFM imaging of the same samples in air, further confirms the electrostatic origins of the apparent difference in height. The basis for the distinction can also be seen in the force curves (Figure 4), where the two phases display similar interaction forces at close separation but markedly different ones beyond a few nanometers. The structure and shape of the domains of the two phases can be clearly seen in the images. The domain shape and structure are thought to be determined by the balance between the line tension and long-range dipole (electrostatic) interactions.18-20 If the line tension domi-
nates, the domains tend to be circular, but if the dipole force becomes important, a variety of domain shapes may appear. In our experiments, at 30 mN/m surface pressure, both phases were in the liquid-condensed or the solid phase, especially at low temperature. Because of the identical tail lengths of the two surfactants, the line tension was low, and the domains did not always adopt a circular shape. In addition, the domains may not have been in an equilibrium state because the time between the spreading and transfer was very short (about 5 min). There were, however, systematic trends in the characteristics seen; for example, Figure 1 shows that the size of the STABrich phase domains increased as the subphase temperature was lowered. Although a phase diagram for this system is not available in the literature, our results are consistent with an upper critical temperature, which is to be expected. The phase separation seen at higher ionic strengths (Figure 3) presumably results from double-layer screening of the electrostatic repulsion among the ionized STAB molecules in the monolayer. The increased screening as the ionic strength in the subphase increases allows denser packing of charged surfactant molecules at constant pressure, so they may produce larger areas of high surface charge density. The issue of whether phase separation occurs before or after film transfer remains unresolved, as the deposition process has been found to lead to phase separation for certain monolayers and substrates.21,22 When simple fatty acids are transferred to a mica substrate at a very low surface pressure (liquid-expanded phase), the transferred surfactants have been found to condense into close-packed islands, leaving the substrate exposed. Such condensation was seen on mica and on silica substrates at high pH but not on silica at low pH, so the surface charge density has been argued to be an important factor in the condensation. Also important is the water layer between the film and the substrate, which can facilitate movement of the surfactant molecules after transfer. Many factors affect the structure and morphology of the film on the substrate, for example, properties of the surfactant(s), substrate, and ions in the subphase. In our experimental system, it is unlikely for several reasons that phase separation occurred after the transfer. First, the monolayer was transferred to a hydrophobic substrate with the surfactant tails in contact with the substrate, so there was no water
(18) de Koker, R.; McConnell, H. M. J. Phys. Chem. 1993, 97, 13419. (19) de Koker, R.; McConnell, H. M. J. Phys. Chem. 1994, 98, 5389. (20) Mayer, M. A.; Vanderlick, T. K. J. Chem. Phys. 1994, 100, 8399.
(21) Sikes, H. D.; Woodward, J. T.; Schwartz, D. K. J. Phys. Chem. 1996, 100, 9093. (22) Sikes, H. D.; Schwartz, D. K. Langmuir 1997, 13, 4704.
Figure 4. AFM extending force curves (average of 7) for the interaction between the tip and the different surface domains. The spring constant of the AFM cantilever used was 0.32 N/m.
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Figure 5. Contact mode AFM (a) height and (b) friction images of a transferred surfactant film on graphite from DI water at 20 °C, 30 nN/m. Scan size: 10 µm.
film present between the tails and the substrate. Second, the experiments were conducted at a surface pressure of 30 mN/m, so the surfactant molecules were densely packed before the transfer, with little free area for condensation. Third, imaging was performed in the same aqueous solution as the subphase, so there was no driving force such as capillarity to drive aggregation. Although the surfactants on the substrate may have had some residual mobility, it would not be significant enough to cause phase separation during our 2-3 h of imaging. We imaged the same area on a sample for 2 h and observed no significant change of the phase domains. For these reasons, we believe that our AFM images reflect the actual structure of the surfactant films. In summary, we have introduced a new method of characterizing phase separation of mixed surfactant films,
liquid tapping mode AFM imaging, based on the difference in surface charge density of the two phases. This method is suitable for mixtures of charged and neutral surfactants, even when these surfactants have equal tail lengths. The method does not require addition of a label, and it is capable of the very high resolution typical of scanning probe methods. For the particular surfactant system that we have examined, the technique readily allowed characterization of the effects of subphase temperature and ionic strength on the formation of the second phase. Acknowledgment. This work was supported by NASA (Grant Number NAG8-1242). We thank Orlin D. Velev for useful discussions. LA980879K
