pubs.acs.org/Langmuir © 2009 American Chemical Society
Imaging of Collapsed Fatty Acid Films at Air-Water Interfaces Sangjun Seok,† Tae Jung Kim,‡ Soon Yong Hwang,‡ Young Dong Kim,*,‡ David Vaknin,§ and Doseok Kim*,† †
Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul, Korea, ‡Nano-Optical Property Laboratory and Department of Physics, Kyung Hee University, Seoul, Korea, and §Ames National Laboratory and Department of Physics, Iowa State University, Ames, Iowa 50011 Received January 9, 2009. Revised Manuscript Received July 8, 2009 In situ imaging ellipsometry is employed to monitor the morphology of collapsed films of fatty acid Langmuir monolayers on pure water and on CaCl2 solution. The ellipsometry images reveal the existence of multilayer domains in the collapsed region, and analysis of the images yields the thicknesses of these domains. The multilayer films formed on water are mainly trilayers, while those on CaCl2 solution are mainly bilayers. The structure of the collapsed films also changes sensitively depending on the history of compression of the molecular layer.
Introduction Langmuir monolayer and the multilayer Langmuir-Blodgett films of surfactant molecules have been extensively studied by various experimental techniques including surface pressure/area measurement (to get π-A isotherm),1 electron microscopy,2 ellipsometry,3-6 X-ray reflectivity and diffraction,7-15 Brewster angle microscopy (BAM),14-16,18-20 atomic force microscopy (AFM),16-18,20 and nonlinear optical spectroscopy.21-24 These studies allowed in-depth understanding of the molecular conformation in the layer, phase transitions, and the structure of the multilayer. On the other hand, collapsed multilayer films made by compression of the monolayer beyond the in-plane closely packed *Corresponding khu.ac.kr.
authors.
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(1) See, for example: Roberts, G. Langmuir-Blodgett Films; Plenum: New York, 1990. (2) Neuman, R. D. J. Colloid Interface Sci. 1976, 56, 505. (3) Rasing, T.; Hsiung, H.; Shen, Y. R.; Kim, M. W. Phys. Rev. A 1988, 37, 2732. (4) Xue, J.; Jung, C. S.; Kim, M. W. Phys. Rev. Lett. 1992, 69, 474. (5) Knobloch, H.; Pe~nacorada, F.; Brehmer, L. Thin Solid Films 1997, 295, 210. (6) Ducharme, D.; Tessier, A.; Russev, S. C. Langmuir 2001, 17, 7529. (7) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippman-Krayer, P.; M€ohwald, H. J. Phys. Chem. 1989, 93, 3200. (8) Malik, A.; Durbin, M. K.; Richter, A. G.; Huang, K. G.; Dutta, P. Phys. Rev. B 1995, 52, 16. (9) St€ommer, R.; Englisch, U.; Pietsch, U.; Holy, V. Physica B 1996, 221, 284. (10) Kaganer, V. M.; M€ohwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779. (11) Lee, K. Y. C.; Gopal, A.; Nahmen, A. V.; Zasadzinski, J. A.; Majewski, J.; Smith, G. S.; Howes, P. B.; Kjaer, K. J. Chem. Phys. 2002, 116, 774. (12) Vaknin, D.; Bu, W.; Satija, S. K.; Travesset, A. Langmuir 2007, 23, 1888. (13) Bu, W.; Vaknin, D. Langmuir 2008, 24, 441. (14) H€onig, D.; M€obius, D. J. Phys. Chem. 1991, 95, 4590. (15) Kurnaz, M. L.; Schwartz, D. K. Phys. Rev. E 1997, 56, 3378. (16) Schief, W. R.; Touryan, L.; Hall, S. B.; Vogel, V. J. Phys. Chem. B 2000, 104, 7388. (17) Gourier, C.; Knobler, C. M.; Daillant, J.; Chatenay, D. Langmuir 2002, 18, 9434. (18) Yun, Y.; Ahn, K.; Kim, M. W. Europhys. Lett. 2005, 70, 555. (19) Zou, L.; Wang, J.; Basnet, P.; Mann, E. K. Phys. Rev. E 2007, 76, 031602. (20) Teixeira, A. C. T.; Brogueira, P.; Fernandes, A. C.; Silva, A. Chem. Phys. Lipids 2008, 153, 98. (21) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. Rev. B 1999, 59, 12632. (22) Watry, M. R.; Tarbuck, T. L.; Richmond, G. L. J. Phys. Chem. B 2003, 107, 512. (23) Roke, S.; Schins, J.; M€uller, M.; Bonn, M. Phys. Rev. Lett. 2003, 90, 128101. (24) Nishida, T.; Johnson, C. M.; Holman, J.; Osawa, M.; Davies, P. B.; Ye, S. Phys. Rev. Lett. 2006, 96, 077402. (25) Alonso, C.; Alig, T.; Yoon, J.; Bringezu, F.; Warriner, H.; Zasadzinski, J. A. Biophys. J. 2004, 87, 4188.
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monolayer are relatively less investigated as yet,17,25-30 in spite of their relevance in various biological processes. Research activity in this area has been most intense on lung surfactant films.25-27 As reversible collapse and revival of the surfactant film in the alveoli is essential for pulmonary action, Langmuir monolayers whose composition mimics that of the lung surfactant have been studied in depth to understand the underlying mechanism for reversible collapse. These studies found out that different lipid molecules and the proteins constituting the lung surfactant system (and their relative composition) are crucial for reversible collapse behavior. The more common system of fatty acid Langmuir monolayer, on the other hand, forms very rigid films at high pressures, and one can imagine its collapse behavior would be different from that of lung surfactant monolayers.27 The microscopy and rheology study of the collapse phenomenon in a single-component system of fatty acid allowed modeling of the folding and collapse phenomenon.28,29 Change in the structure of the collapsed film of fatty acid molecules having different chain lengths under different compression rates, temperatures, and subphase conditions (pH and salt species) have also been investigated.30 The ionic species in the subphase seem to play a key role in the collapse process, as even a minute concentration of divalent cation (Ca2þ) in subphase water was found to promote the folding and membrane fusion processes.26,31,32 The techniques used as yet although useful do not provide a complete picture of the collapsed film and the collapsing mechanism. Electron microscopy and atomic force microscopy techniques of the system require the transfer of the collapsed film to solid support,2,16,17,20,33,34 a process that may alter the structure of the films. Fluorescence microscopy, light scattering microscopy, and Brewster angle microscopy techniques have been used successfully for in situ imaging of the films on water subphase undergoing collapse.16,27,30 However, these techniques are not (26) Lipp, M. M.; Lee, K. Y. C.; Takamoto, D. Y.; Zasadzinski, J. A.; Waring, A. J. Phys. Rev. Lett. 1998, 81, 1650. (27) Lee, K. Y. C. Annu. Rev. Phys. Chem. 2008, 59, 771. (28) Ybert, C.; Lu, W.; Mller, G.; Knober, C. M. J. Phys. Chem. B 2002, 106, 2004. (29) Lu, W.; Knobler, C. M.; Bruinsma, R. F. Phys. Rev. Lett. 2002, 89, 146107. (30) Angelova, A.; Vollhardt, D.; Ionov, R. J. Phys. Chem. 1996, 100, 10710. (31) Vogel, S. S.; Zimmerberg, J. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 4749. (32) Summers, S. A.; Guebert, B. A.; Shanahan, M. F. Biophys. J. 1996, 71, 3199. (33) Vollhardt, D.; Kato, T.; Kawano, M. J. Phys. Chem. 1996, 100, 4141. (34) Vollhardt, D. Adv. Colloid Interface Sci. 2006, 123, 173.
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suitable for quantitative analysis essential for determination of the detailed structure of the collapsed films. Recently, molecular dynamics simulations have been applied for studying monolayer collapsing process and vesicle conformation.35,36 Grazing-angle X-ray diffraction (GIXD) found out the detailed structure of the lung surfactant film undergoing collapse.25 The collapsed films of fatty acid were investigated recently by using X-ray and neutron reflectivity and GIXD,12,13 where the authors reported that the structure of the collapsed film depends sensitively on the presence of salt in the subphase water. For monolayers on pure water the collapsed domains were predominantly trilayers, while the bilayer domains were dominant on CaCl2 solutions. In those studies, X-ray and neutron beams used to measure the reflectivity necessarily average over a large footprint (∼1 cm2) of the surface. Since the collapsed monolayer is inherently inhomogeneous,4 a microscopic probe that can map out the local domains on a finer length scale as compared to X-ray or neutron scattering techniques would be complementary to study this system. Herein, we report on imaging ellipsometry (IE) to probe films of fatty acid molecules on water and on CaCl2 solutions in situ before and after the collapse of the monolayer.37,38 In contrast to conventional ellipsometry that measures average property of the surface over typically millimeter size of the probe beam, imaging ellipsometry can get local ellipsometric angle values of the image with a spatial resolution of a few micrometers determined by the optics used. It is noninvasive, in situ technique, unlike electron microscopy or AFM. Whereas fluorescence microscopy or BAM can provide two-dimensional images of the collapsed films, the ellipsometric method we utilize in this study yields information on the third dimension of the individual domains, namely their thicknesses. A Langmuir trough was put in the sample position of the imaging ellipsometer, and the surface was imaged in situ in the process of fatty acid film changing from monolayer into collapsed multilayer. As expected, the surface after collapse was found to be very inhomogeneous with coexisting regions of bare water surface, monolayer, and multilayer films. The thickness of the main portion of the collapsed domains corresponded to trilayer and bilayer when they were prepared on water and on CaCl2 solution, respectively, supporting the earlier proposition that the thickness of the collapsed region depends sensitively on the existence of salt in the water subphase. Changing the protocol of the compression altered the resulting structure of the collapsed films significantly, demonstrating that the compression and collapse of the monolayer proceeds through metastable states away from equilibrium.30,33,34,39
Experimental Section Arachidic acid (CH3(CH2)18COOH, Tokyo Chemical Industry, AA for short) was spread from a 3:1 mixture of chloroformmethanol (HLPC grade, Aldrich) solution onto ultrapure water (18.2 MΩ 3 cm, pH 5.7) or onto 1 mM CaCl2 (99.9þ%, Aldrich) solution in a home-built Langmuir trough. The barrier of the Langmuir trough was pushed at a rate of 0.58 A˚2/min per molecule, similar to the previous experiments.6,7,12 The barrier was controlled to maintain a constant surface pressure during the IE measurement. All the IE experiments were carried out at room temperature (21 °C). (35) Lorenz, C. D.; Travesset, A. Langmuir 2006, 22, 10016. (36) Baoukina, S.; Monticelli, L.; Risselada, H. J.; Marrink, S. J.; Tieleman, D. P. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 10803. (37) Bae, Y. M.; Oh, B. K.; Lee, W.; Lee, W. H.; Choi, J. W. Anal. Chem. 2004, 76, 1799. (38) Howland, M. C.; Szmodis, A. W.; Sanii, B.; Parikh, A. N. Biophys. J. 2007, 92, 1306. (39) Kato, T.; Hirobe, Y.; Kato, M. Langmuir 1991, 7, 2208.
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Figure 1. Schematic diagram of the imaging ellipsometry setup. The imaging ellipsometer (Beaglehole Instruments, New Zealand) used in this work is composed of a quartz-halogen source, monochromator, polarizer, retarder, zoom lens, analyzer, and charge-coupled device (CCD) detector.40 The schematic is shown in Figure 1. The illumination assembly is composed of a lamp housing and a condenser. The aspheric lens in front of the lamp makes the illumination beam parallel throughout the polarizer and the retarder positioned afterward in the same arm. A heat filter that cuts out the infrared radiation from the light bulb is placed before the polarizer to protect it from overheating damage. The light from the lamp is polarized at 45° angle so that it consists of equal in-phase amplitudes of s- and p-waves. The light then passes through computer-controlled rotating retarder, which adds a phase shift between the s- and p-waves. Following the retarder, a condensing lens converges the beam to provide suitable illumination of the surface. Light reflected from the sample makes an image through a zoom microscope objective onto the CCD. An analyzer is placed between the lens and the detector to convert the phase shift in the reflected light to intensity information. The CCD detector collects the images of the ellipsometric signal over an extended area (∼0.05 cm2) of the sample in real time. Usually the analyzer angle is chosen to be either þ45° or -45°. The analyzer is also computer-controlled during the measurement. The spectral range can be varied from 400 to 800 nm. The incidence angle and the wavelength are set to 55° and 633 nm, respectively, and the measured image covers a sample area of 1.8 mm 2.7 mm. To determine the ellipsometric angles in the image with least experimental error, 11 CCD images are taken at two different analyzer angles of þ45° and -45° to obtain the average value, with typical exposure time of 100 ms for each CCD image. Unlike usual microscopy, the numerical aperture (NA) and the lateral resolution of the imaging ellipsometer cannot be increased up to the diffraction limit. Large-numerical aperture objective lens would collect the light beams from the sample over wider angular range. However, it increases the uncertainty of the incidence and reflection angles thus decreases the resolution in vertical direction. To compromise between lateral and vertical resolutions, the numerical aperture was kept at ∼0.18, from which the lateral resolution was estimated to be ∼4.3 μm with 633 nm wavelength. With the above optics used and the typical precision of the ellipsometric angle values, the thickness accuracy of ∼0.5 nm and the refractive index accuracy of ∼0.01 (for SiO2/Si sample measured with 633 nm light) could be achieved. To actually confirm the resolutions of the instrument, we prepared a test pattern as shown in Figure 2. On a p-type silicon wafer with 300 nm of SiO2 overlayer, 60 nm deep straight channels of varying widths (2, 3, 4, 6, 8, and 10 μm) were made. The refractive indices of the substrate and the overlayer were n = 3.86 (κ = 0.015) and n = 1.46, respectively. Figure 2 shows the (40) Seok, S.; Kim, D.; Kim, T. J.; Kim, Y. D.; Vaknin, D. J. Korean Phys. Soc. 2008, 53, 1488.
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Figure 2. (top) Ellipsometric image of the test sample. (bottom) Height change (taken along the line in the above image) as measured by IE, shown together with the actual sample topography measured by AFM.
Figure 3. π-A isotherms of the arachidic acid on pure water and 10-3 M CaCl2 solution. ellipsometry images from this test pattern. As the refractive indices are all known, the raw data (ellipsometric angle values) were converted to channel height for direct comparison. The trace in the graph below (blue line) is the topography of the sample following the line in the image showing the clear existence of all the channels, and the extracted depth value from the IE image follows the contour of wider channels reasonably well, reaching the actual channel depth of the 60 nm at the center. However, for the channels narrower than 4 μm, the extracted depth value did not reach full 60 nm. Thus, the resolution of the IE setup is considered to be between 4 and 6 μm, in fair agreement with the optical resolution of 4.3 μm estimated from the numerical aperture of the objective used.
Results and Discussion A. Langmuir Isotherms. Figure 3 shows π-A isotherms of the arachidic acid Langmuir monolayer on pure water (black line) and 1 mM CaCl2 solution (red line) measured by the Wilhelmy plate method at constant compression rate of 0.58 A˚2/min per molecule. As the isotherm and the film structure are known to change with the monolayer compression history (e.g., different compression speed),30,34,39 we chose to investigate the structure of the film compressed very differently as a comparison to the compression with a constant rate. The blue isotherm curve was taken for the AA/CaCl2 solution with the same compression rate of 0.58 A˚2/min per molecule until the close-packed monolayer (at ∼19.5 A˚2) was formed and then allowed to relax for 1 h while keeping the pressure constant at 31.6 ( 0.5 mN/m using the computer-controlled feedback.12 As the barrier was pushed again, 9264 DOI: 10.1021/la900096a
Figure 4. Layer thickness versus Δ from calculation and the Δ value mapping of the images: blue, green, red, and gray regions correspond to monolayer, bilayer, trilayer, and multilayer, respectively.
the pressure quickly increased to the previous value of 53 mN/m, and the film area was shown to be reduced by 8.8%. The structure of the film compressed in the latter way will be shown later. Overall, the isotherms are in good agreement with the ones in previous reports.6,7 The existence on Ca2þ in the subphase affects the isotherm both before and after the collapse,30,41,42 and the slight differences between the isotherms on pure water and on CaCl2 solutions observed in Figure 3 are reproducible.12 It seems that the Ca2þ ion adsorption on the headgroup had an influence on the isotherm. The previous report that investigated the isotherm and film structure of DMPA lipid at different concentrations of Ca2þ in the subphase also found out the onset of the pressure buildup changes with Ca2þ concentration and proposed that the reduction of electrostatic repulsion by ion binding is responsible for the change.41 Imaging ellipsometry measurements were performed in situ from the onset of monolayer formation to the film collapse to yield images of the molecular thin film undergoing monolayer collapse. B. Single-Layer Model for Analysis of the Ellipsometry Data. In conventional ellipsometry, two ellipsometric angles (Ψ and Δ) are determined from the ratio of complex reflectivities of sand p-polarized light as rp/rs = tan ΨeiΔ, from which the refractive index and the thickness of the film can be deduced. In imaging ellipsometry, 2-dimensional array of the pairs of numbers (Ψ, Δ) over the sample surface are determined to represent the local property of the sample, making this technique suitable for probing the third dimension of the inhomogeneous surface.38,43 It has been known that for layers appreciably thinner than the wavelength of the probing light the Δ value is sensitive to the change in the layer thickness, while the Ψ value is hardly affected.6,38 Since the films in the current study are much thinner than the wavelength, all the IE images in the present study are Δ value images of arachidic acid/water under different conditions. With reported values of refractive indices n1 = 1.44 (arachidic acid)6 and n2 = 1.33 (water and 1 mM salt solution, refractive index value hardly changes with concentration in this range),44 the measured Δ value can be converted into film thickness, as shown in Figure 4. For better visualization of the thickness of the collapsed regions, mono-, bi-, and trilayer regions are color-coded differently (Figure 4) with the boundaries set at the midpoint values between the thickness values of monolayer (28 A˚, Δ = 0.087), bilayer (50 A˚, Δ = 0.16), and trilayer (75 A˚, Δ = 0.23). These thickness values were taken (41) L€osche, M.; M€ohwald, H. J. Colloid Interface Sci. 1989, 131, 56. (42) Kundu, S.; Langevin, D. Colloids Surf., A 2008, 325, 81. (43) Reiter, R.; Motschmann, H.; Orendi, H.; Nemetz, A.; Knoll, W. Langmuir 1992, 8, 1784. (44) See: David, R. L. Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2000.
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Figure 5. Δ mapping images before collapse: (a-c) monolayer on pure water with corresponding molecular areas at 24, 20.5, and 19.5 A˚2; (d-f ) monolayer on 1 mM CaCl2 solution, with corresponding molecular areas at 22.5, 20.5, and 19.5 A˚2. The insets in (e) and (f ) show the small dotted regions with the Δ value larger than those of the monolayer indicating the spontaneous collapse of the monolayer on the Ca2þ solution.
Figure 6. Histograms of the Δ values corresponding to each Δ map in Figure 4. Numbers at the top axis of each graph are the film thicknesses in A˚. The histograms (a) and (b) can be fitted by two Gaussian functions, and all the other histograms could be fitted satisfactorily by single Gaussian function. The histograms in (g) show the regions of higher Δ values in (c) (red bars) and (f ) (open rectangles).
from the X-ray and neutron reflectivity12,40 and corresponded to the length of the all-trans AA molecule for the monolayer and slightly smaller than twice (three times) the AA length for bilayer (trilayer). C. Monolayer before Collapse. Figure 5 shows Δ images of the AA Langmuir monolayers at several different values of molecular areas before collapse. Parts a-c are for monolayers on pure water, while parts d-f are for monolayers on 1 mM CaCl2 solution. The molecular areas (as indicated in the figure caption) in each case correspond to (a, d) onset of the tilted condensed phase, (b, e) beginning of the untilted phase, and (c, f ) just before collapse. In Figure 5a,d the overall blue color in both images indicates that the surface is covered by a monolayer. Upon close examination, change in brightness across the image is seen, indicating that Langmuir 2009, 25(16), 9262–9269
the layer thickness is not uniform over our detection area of 1.8 mm 2.7 mm. This can be understood to be from the coexistence of the gaseous phase (light-blue region where the arachidic acid molecules are less populated thus more tilted) and the condensed phase (deep-blue region). The above assignment becomes more plausible with the statistical treatment of the distribution of the Δ values in the corresponding areas. Figure 6 shows the pixel histograms of the corresponding images in Figure 5 consisting of 0.1 million pixels (of the CCD). The number assigned for each pixel is the unique Δ value for that position, and the histogram from this set of Δ values indicates the relative abundance of the specific Δ value or, equivalently, the distribution of film thickness within the probed area. As shown in Figure 6a,d, the histogram in both cases DOI: 10.1021/la900096a
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Figure 7. Δ mapping images after collapse: (a-c) collapsed monolayer on pure water; (d-f ) collapsed monolayer on 1 mM CaCl2 solution, with molecular areas of (a) 18.5, (b) 17.1, (c) 14.5, (d) 18.5, (e) 17.5, and (f ) 13.5 A˚2.
(on pure water and on CaCl2 solution) could be fitted by two Gaussian functions, where the center values correspond to the average Δ values (denoted by Δ0 hereafter) of the two domains, and the peak widths (∼0.5 and ∼ 0.8 nm) correspond to the thickness variation over the beam area. The Δ0 values of 0.050 and 0.062 in Figure 6a correspond to the films thicknesses of 16.1 and 20.0 A˚ in Figure 4 for AA/water. The Δ0 values from Figure 6d correspond to 14.1 and 21.5 A˚ for AA/CaCl2 solution. These results agree well with the expected thickness values for the gaseous phase and the condensed phase of the arachidic acid monolayer.6 Figure 5b,e was taken at an area of 20.5 A˚2/molecule, the onset of the untilted phase. The images became much more homogeneous as compared to the images in Figure 5a,d, and the corresponding histograms in Figure 6b,e could be fit adequately by a single Gaussian with Δ0 = 0.080, corresponding to a film thickness of 25.8 A˚ (Δ0 = 0.079 and 25.4 A˚ for AA/CaCl2). The result matches well with the full extension of the arachidic acid molecule as well as with the thickness reported from X-ray and neutron reflectivity.12 Careful examination of Figure 5e,f (shown as an inset) shows that small dots with the Δ value appreciably higher than that of monolayer begin to appear. As such a large Δ value (∼0.134 and the thickness of 43.2 A˚) in Figure 6e can only be associated with that of the multilayer, these dots (∼10 μm in diameter) were assigned to spontaneous nucleation to the multilayer.33,34 To see whether this spontaneous nucleation is influenced by the existence of Ca2þ in the subphase, the domains of which the Δ value is larger than 0.12 over the size of ∼5 μm or lager are counted in the images: they were less than 10 in Figure 5b,c while in Figure 5e,f the numbers were in the range of 35-40. To investigate the effect of Ca2þ more quantitatively, the histograms in the Δ value range corresponding to multilayers were drawn in Figure 6g for images in Figure 5c,f. There are significant number of pixels having large Δ values up to 0.16 for AA/CaCl2 solution (open rectangles), while those on water (red bars) are only a few. This clearly suggests that the spontaneous formation of the multilayer is facilitated by the existence of Ca2þ in the subphase. It might be related to the observation in a DPPG monolayer in which folding transition was induced by Ca2þ cation in the subphase.26 Recent study using phase-contrast microscopy (PCM) also found out (45) Hatta, E. Langmuir 2004, 20, 4059.
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that the present of Cd2þ in the water promotes spontaneous collapse of the monolayer.45 The ellipsometry images taken just before collapse of the monolayer (Figure 5c,f, both at 19.5 A˚2/molecule) are almost the same as Figure 5b,c. The histograms in Figure 6 corresponding to these images also show the same behavior, yielding distributions with center values at 25.4 and 25.8 A˚. D. Collapse of the Monolayer. Figure 7 is a collection of Δ images in the collapsed phase; (a-c) for AA on pure water and (d-f ) for AA on 1 mM CaCl2 solution. The molecular areas correspond to (a) 18.5, (b) 17.1, (c) 14.5, (d) 18.5, (e) 17.5, and (f ) 13.5 A˚2. Starting from the beginning of the induced collapse, the image in Figures 7a (AA/water) shows only a few, large collapsed domains, while there are many small collapsed domains scattered across the whole area in Figure 7d (AA/CaCl2 solution). As the nucleation in monophasic film is expected to occur at the boundary of 2-D crystalline domains,29 this difference suggests that the size of the domain would be larger in case of AA/water as compared to that in AA/CaCl2 solution. Or one can consider the spontaneous multilayer domains in Figure 5 as nucleation centers for further collapse process; as there exist more spontaneous nucleation domains in AA/CaCl2 solution (see Figure 5e,f ), it would be natural that there are more collapsed domains scattered across the surface for AA in CaCl2 solution. It is interesting to note that the collapsed regions in Figure 7a,b are accompanied by light-blue regions (assigned to bare water surface) of similar shape, indicating the molecules in nearby monolayer domains are used as reservoir to form multilayer domains. The inset of Figure 5f shows that even in case of spontaneous nucleation, the small region of bare water is visible near nucleation region. Bilayer (green) and trilayer (red) regions increase as the film is further compressed for both AA/water (Figure 7a-c) and for AA/CaCl2 solution (Figure 7d-f ). Figure 8 shows the corresponding histograms for the images in Figure 7. The histograms exhibit increase in width and shift to the larger Δ value with the decrease in the surface area. Figure 9 shows the fully collapsed morphology of the films with surface area reduced to 9.5 A˚2/molecule in Figure 9b,d. This molecular area is approximately half of the area of the intact monolayer (the area could not be reduced further due to the limitation in the through and the beam geometry in the imaging ellipsometry setup). The image is indeed very inhomogeneous and is covered with multilayer domains. We find that the average Langmuir 2009, 25(16), 9262–9269
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Figure 8. Histograms of the Δ values corresponding to each Δ map in Figure 6.
Figure 9. Δ mapping images after collapse: (a, b) collapsed monolayer on pure water; (c, d) collapsed monolayer on 1 mM CaCl2 solution at molecular areas of (a) 12.5, (b) 9.5, (c) 12.3, and (d) 9.5 A˚2.
Figure 10. Histograms of the Δ values corresponding to each Δ map in Figure 8. Numbers at the top axis of graphs (b) and (d) are the film thicknesses in A˚. The Gaussian functions used to fit the histrograms (fitting parameters listed in Table 1) are shown in (b) and (d). The graph (e) shows the fitted results from (b) (in solids lines) and (d) (in dotted lines).
domain size is slightly smaller on AA/CaCl2 solution than that on pure water. Presumably, the different underlying domain strucLangmuir 2009, 25(16), 9262–9269
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molecule (Figure 10b,d) are fitted with multipeak Gaussian functions. Initially, we tried to use only three peaks with peak positions (Δ0 values) fixed at the reported values of bilayer and trilayer. It became clear after the first attempt that another peak at Δ0 = 0 is needed, reasonably so as the multilayer formation necessarily exposes bare water surface. The histogram in Figure 10b,d needed another peak at higher Δ value, corresponding to the layers that are even thicker than the trilayer. We also allowed the change of the peak positions (Δ0 values) a bit around the published values to get the final fitting shown in the solid lines in Figure 10b,d. The widths of the Gaussian peaks used were around 1.3-2.3 nm (listed in Table 1). These widths are quite large, as the Δ value at each pixel value is necessarily an average over the beam area (limited by the spatial resolution of the setup), and the collapsed sample is expected to be very inhomogeneous. The fitting parameters are listed in Table 1. Notable in Table 1 is the relative area for each case. For AA/water the area corresponding to trilayer
Figure 11. π-A isotherm of the arachidic acid on 10-3 M CaCl2 solution obtained by keeping the pressure constant (at 31.6 mN/m) after the film was compressed to a full monolayer. The arrows in the graph show the points where the ellipsometry images in Figure 12 were taken.
is the largest, while for AA/CaCl2 solution the bilayer region occupied the largest area. The fitting results from Figure 10b,d compared in Figure 10e show clear difference due to Ca2þ concentration in the subphase. This supports the recent proposition that the collapsed regions depends sensitively on the existence of divalent salt in the water subphase.12,13,35 In the model suggested, the formation of calcium diarachidate in the bilayer domain in case of AA/CaCl2 solution lowers the energy as compared to more common trilayer. Thickness of multilayers from the fitting is smaller than the integer multiples of monolayer, suggesting the molecules could be tilted from the surface normal (more than the molecules in a single layer) as they form multilayers.6,7,12,13,35 E. Effect of Relaxation (Aging) during Film Compression. As the compression process of the Langmuir monolayer is not an equilibrium process,30,33,34,39 π-A isotherm and the structure of the film are also expected to change depending on the compression history. To study all the possible ways of compressing the film is obviously impossible, thus another way very different from the previous one of compression at constant rate was chosen as a sample to demonstrate the history-dependent structural change. As described before, the film on CaCl2 solution was compressed with the speed of 0.58 A˚2/min per molecule until the close-packed monolayer (at ∼19.5 A˚2) was formed and then allowed to relax for 1 h while keeping the pressure constant at 31.6 ( 0.5 mN/m using the computer-controlled feedback.12 Afterward, it was pushed with the same compression rate as before. The ellipsometry images were taken at (a) 18.5, (b) 17.4, (c) 14.5, (d) 13.1, (e) 13.3, and (f ) 11.3 A˚2 (positions shown as arrows in Figure 11). The ellipsometric images taken before it reached full monolayer (not shown) were very similar to Figure 5d-f. The structural change caused by the relaxation process is already obvious in Figure 12a, the image taken while the pressure is kept around 31.6 mN/m. The average Δ0 value of this image is 0.057 (18.3 A˚), appreciably smaller than that in the condensed
Table 1. Parameters Used To Fit the Distributions Shown in Figure 9b,da water subphase
area (%)
Δ0
width
ds (A˚)
salt subphase
water 6.7 0.06 water monolayer 28.0 0.090 0.06 28.8 monolayer bilayer 22.1 0.147 0.04 47.1 bilayer trilayer 39.4 0.211 0.07 67.1 trilayer multilayer 3.8 0.3 0.05 95.3 multilayer a The area of each Gaussian indicates the relative coverage of a certain domain of the area.
area (%) 7.8 16.6 52.2 13.2 10.2
Δ0 0.090 0.142 0.221 0.3
width
ds (A˚)
0.06 0.03 0.06 0.05 0.07
28.8 45.4 70.0 95.3
Figure 12. Δ mapping images of the aged film on 1 mM CaCl2 solution, with molecular areas of (a) 18.5, (b) 17.4, (c) 14.5, (d) 13.1, (e) 13.3, and (f ) 11.3 A˚2 (marked in Figure 11). 9268 DOI: 10.1021/la900096a
Langmuir 2009, 25(16), 9262–9269
Seok et al.
phase at the same pressure (condition similar to Figure 5e). As we resume compressing the film, spontaneous collapses previously observed in Figure 5e,f became a lot more prevalent (shown in Figure 12b). As the surface is occupied more by the bilayer in Figure 12c and the pressure decreased at this point, these points are considered to “induce” the collapse upon compression. Further compression is shown to increase the multilayer domains in the image, but the detailed shape is very different. On the “aged” film, the multilayer domains are much finer in size, and their Δ values indicate they are mostly bilayers. The fully collapsed film shown in Figure 12f is also very different from its counterpart Figure 9d;the film on CaCl2 solution collapsed by continuously compression. It clearly indicates the collapse is a kinetic process, and the compression history of the film changes the structure substantially.
Conclusions Using imaging ellipsometry, we obtained detailed pictures of arachidic acid molecular film before and after collapse on pure water and on 1 mM CaCl2 solution. The monolayers before collapse were found to be homogeneous with average film thickness corresponding to the size of the molecule. The images show that small regions in the monolayer undergo spontaneous collapse more readily when spread on CaCl2 solutions. As the film is further compressed, multilayer domains are formed and grow with the decrease in the surface area available for the film. The
Langmuir 2009, 25(16), 9262–9269
Article
analysis of the image at the smallest nominal molecular area (9.5 A˚2/molecule) indicates the collapsed film is rich in trilayer domains for arachidic acid molecules on water, while the bilayer structure is preferred for arachidic acid on CaCl2 solution. The structure of the collapsed films varies significantly with the history of the compressed monolayer, demonstrating the compression of the monolayer proceeds through metastable states away from equilibrium. Finally, it is hoped that we demonstrate successfully that imaging ellipsometry is an able technique to study Langmuir monolayers and related systems. It is an in situ technique to monitor the change of the system in real time. As an ellipsometric technique, the depth information it yields is much more accurate without resorting to calibration processes as in Brewster angle microscopy.19 Its imaging capability is most suitable to study inhomogeneities in the system, and it can find many future applications to study wide range of subjects such as nucleation, domain structure, and film collapse. Acknowledgment. This work is supported by the grant from KOSEF No. R01-2008-20884-0 and the World-Class University program. The work at KHU was supported by KOSEF through Nano-Optical Property Laboratory. Ames Laboratory is supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, through Ames Laboratory under contract under Contract No. DE-AC02-07CH11358.
DOI: 10.1021/la900096a
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