Langmuir 1994,10, 1251-1256
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Formation of Bilayer Disks and Two-Dimensional Foams on a Collapsing/Expanding Liquid-Crystal Monolayer Matthew C. Friedenberg, Gerald G. Fuller,’ Curtis W. Frank, and Channing R. Robertson Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received July 26,1993. I n Final Form: November 1 7 , 1 9 9 P We have visualized the collapse of a monolayer film of the thermotropic liquid-crystal 4’-n-octyl-4cyanobiphenyl(8CB)at the air-water interface using Brewster angle microscopy. First, the film collapses into small circular domains that represent fluid multilayer structures under the influence of a line tension. These multilayer disks grow in both area and number with further compression. Coalescence of domains occursat high area fraction and is coupledwith flow of the underlyingmonolayer. At still higher compression, additional coalescenceleads to a homogeneousmultilayer film. When the multilayer is allowed to expand, circular holes form. As these holes grow on further expansion,a two-dimensional foam is formed, stabilized by thin lamellae of the collapsed 8CB phase. The foam eventually breaks apart as the f i i expands back into the monolayer regime. Our results confirm the proposal by Xue et al. [Phys. Rev. Lett. 1992,69,4741 that this collapse is a transition between the monolayer and a uniform trilayer composedof an interdigitated bilayer on top of a monolayer.
Introduction The study of insoluble monolayers at the air-water interface has received considerable attention in recent years.lI2 These molecular assemblies have been proposed for a wide range of applications, including nonlinear optics, information storage, display technologies, and biological sensorse3 The development of highly sensitive in situ experimental techniques has played a key role in uncovering the structure and organization of these systems. To design new materials with appropriate properties for the proposed applications of these films, a fundamental understanding is required of the cohesive and adhesive interactions that define their behavior at the air-water interface. Therefore, one area of interest is the collapse process, by which material is forced out of the monolayer at high degrees of compression to form multilayer or bulk structures. A number of experimental approaches have been utilized to study the mechanism of monolayer collapse, including analysis of pressure-area isotherms,4PS modeling of the kinetics of film collapse a t constant pressure or area,8J and visualization of collapsed film structures by electron micro~copy.~J’ The picture that emerges from these and other monolayer collapse experiments is often contradictory. Generally, the collapse process is irreversible and shows considerable hysteresis -on cycles of compression and
* To whom all correspondence should be addressed.
e Abstract published in Advance ACS Abstracts, February 15, 1994. (1) Ulman, A. An Introduction to Ultrathin Organic F i l m ; Academic Press: New York, 1990. (2) Roberta, G. Langmuir-Blodgett F i l m ; Plenum Press: New York, 1990. (3) Swalen, J.D.;Allara,D.L.;Andrade,J. D.; Chandross,E.A.;Garoff, S.; Israelachvilli, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3, 932. (4) Diep-Quang, H.; Ueberreiter, K. Colloid Polym. Sei. 1980, 258, 1056. McFate, C.;Ward, D.; O h t a a d , J., I11 Langmuir 1993, 9, 1036. ( 5 ) Rapp, B.;Gruler, H. Phys. Reu. A 1990,42, 2215. (6) Cary, A.; Rideal, E. K. h o c . R. SOC.London, A 1925, A109,318. Smith,R. D.;Berg, J. C. J. Colloid Znterfoce Sei. 1980,74,273. De Keyser, P.; Jooe, P. J. Phys. Chem. 1984,88, 274. (7) Baglioni, P.; Gabrielli, G.; Guarini, G. G. T. J. Colloid Interface
Swift,
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expansion. The details of collapse often depend strongly on experimental conditions such as compression rate, temperature, and P H . ~In some cases, monolayer collapse can be effectively modeled as the nucleation and growth of a bulk phaseas In others, collapse is viewed as the folding up and sliding of molecular layers on top of one another.* The collapse of monolayers of liquid-crystalline molecules is of interest because of the intrinsic orientational order that these systems possess. In the case of smectic liquid-crystals, the presence of both orientational and positional order leads to a layered structure in the bulk, and it is of interest to understand to what extent this structure is preserved as the film thickness approaches molecular dimensions.10 Pressurearea isotherms of the collapse of the ferroelectric liquid-crystal p-hexyloxybenzylidene-p’-amino-2-chloro-c~-propylcinnamate (HOBACPC)S exhibit multiple peaks, and the collapse areas suggest transitions between structures that differ in thickness by exactly one molecular layer. In addition, numerous studies of the collapse of the a-helical polyglutamates strongly suggest a transition from monolayer to bi1ayer.l’ A similar transition to multilayer structures is proposed in a surface balance investigation of the collapse of a pyramidic liquid-crystal monolayer.12 Xue et al.13 utilized surface balance measurements, ellipsometry, and optical second-harmonic-generation to investigate the collapse of a monolayer of the thermotropic liquid-crystal 4’-n-octyl-4-cyanobiphenyl(8CB), which occurs as a smectic-A at room temperature in the bulk. The results from these complementary techniques were consistent with a model in which the 8CB monolayer collapses through a transition to a homogeneous threelayer film composed of a monolayer plus an interdigitated bilayer. A similar model of organized collapse was (9) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gae Interfaces; John Wiley & Sons, Inc.: New York, 1966. (10) Daillant, J.; Zalczer, G.; Benattar, J. J. Phys. Reo. A 1992, 46, Rfil - --- -LR. -. (11) Malcolm, B. R. Polymer 1966, 7,595. Takeda, F.; Mataumob, M.; Takenaka, T.; Fujiyoshi, Y. J. Colloid Interface Sci. 1981,84, 220. Tanizaki,T.; Hara, K.; Takahara,A.; Kajiyama, T. Polym. Bull. 1993,30, 119. (12) ElAbed,A.;Hachapfel, A.;Hasmonay,H.;Billard,J.;Zimmerman, H.; Luz, 2.;Peretti, P. Thin Solid F i l m 1992,210/211,93. (13) Xue, J.; Jung, C. 5.;Kim, M. W. Phys. Rev. Lett. 1992,69,474.
0743-7463/94/2410-1251$04.50/0 0 1994 American Chemical Society
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1252 Langmuir, Vol. 10, No. 4,1994 proposed by Dent et al.14 in studies of a deuterated derivative of a related molecule [4'-n-pentyl-4-cyanoterphenyl (T15)I by in situ neutronreflectivity. The neutron reflectivity data suggested a transition from a monolayer of T15 to a monolayer of interdigitated T15 dimers, followed by a further transition to a bilayer of dimers. Direct visualization of monolayer collapse is a powerful means of gaining insight into the physics of this complex process. However, due to the scarcity of suitable experimental techniques, such studies are rare. Mann et al.15 have recently demonstrated the utility of Brewster angle microscopy (BAM)16J7for visualizing monolayer collapse. The principle of BAM is as follows: The presence of a monolayer at the air-water interface modifies the local reflectivity of the interface. By imaging a laser beam reflected off the surface at the Brewster angle, where contrast is highest, one can observe structures in the monolayer due to their differential reflectivities. Unlike epifluorescence microscopy,ls this technique does not rely on the partitioning of impurity molecules for contrast; therefore, it can be used to visualize transitions between condensed phases such as those that exist during collapse. Recent investigations using BAM include phase transitions in fatty acid and alcohol m ~ n o l a y e r s , l ~the J ~ growth ~l of solid domains in monolayers of the phospholipid dimyristoylphosphatidylethanolamine (DMPE),16and deformation of polymer domains of poly(dimethylsiloxane).'5 Here, we apply this technique to the collapse of a monolayer of the thermotropic liquid-crystal 8CB into a homogeneous trilayer.
Experimental Section Collapse experiments were performed on a 11 cm X 70 cm symmetric-compression KSV-5000 Langmuir-Blodgett trough (KSV, Helsinki). The subphase was deionized water, purified by the Milli-Q system (Millipore Corp.). The subphase temperature was 23.0 f 0.2 "C,and the pH was 5.5 f 0.1. A spreading solution of 8CB (BDH Ltd.) in hexane (Burdick and Jackson UV grade) was prepared at a concentration of 0.75 f 0.03 mg/mL. Approximately 90 pL was spread at the airwater interface. After allowing 15 min for the residual solvent to evaporate, the barriers were compressed at a rate of 10.0 mm/ min (1.60~2/(molecule min)). The surfacepressure was recorded throughout the compression using the Wilhelmy plate method. After compressionthrough the collapseplateau, the barriers were expanded at the same rate to study the transition back to a monolayer. A Brewster angle microscope was constructed to image structures in the collapsedmonolayers. Our design follows that of HBnig, Overbeck,and MBbius.10 Briefly,p-polarized light from a 632.8-nm, 8-mW HeNe laser (MellesGriot) is reflected off the airwater interface at the Brewster angle (53.1O). The reflected beam passes through a 50 mm focal length lens (Newport), into an analyzer at known angle to the incident polarization, and finallyto aCCTV camera (SanyoVDC3825,0.05 lux sensitivity). Rotation of the analyzerallowsthe image contrast to be adjusted by varying the reflected polarization that is passed to the camera. The Brewster angle microscope was positioned at the center of the trough. BAM images were recorded on videotape with a VCR (MagnavoxVHS)and subsequently digitizedwitha DT2862 frame grabber (Data Translation). Digitized images were enhanced in contrast and corrected for geometric distortion on a (14) Dent,N.;Grundy,M. J.;Richdson,R.M.;Roser,S.J.;McKeown, N. B.; Cook, M. J. J. Chim. Phys. 1988,85, 1003. (15) Mann, E. K.; HBnon, 5.;Langevin, D.; Meunier, J. J. Phys. IZ 1992, 2, 1683. (16) Hbnig, D.; Mbbius, D. J. Phys. Chem. 1991,95,4590. (17) HBnon, 5.; Meunier, J. Reo. Sci. Instrum. 1991, 62, 936. (18) von Tscharner, V.; McConnelI, H. M. Biophys. J. 1981,36,409. (19) Hbnig, D.; Overbeck, G. A.; MBbius, D. Adu. Mater. 1992,4,419. (20)Hoeoi, K.; Ishikawa, T.; Tomiolta, A.; Miyano, K. Jpn. J . Appl. Phys. 1993,32, L135. (21) Overbeck, G. A.; Hbnig, D.; MBbius, D. Langmuir 1993, 9,555.
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Figure 1. Pressure-area isotherms obtained during (a) compression and (b) reexpansion of a monolayer of the liquid-crystal 8CB through the collapse region. Roman numerals correspond to subdivisionsof the isotherm, as described in the text. Letters point to the pressure-area state of the monolayer for Brewster angle micrographs in Figures 2 and 3.
SPARCstation 2 using SunVISION image processing toole (Sun Microsystems).
Results and Discussion The pressure-area isotherms obtained during compression and expansion of the 8CB film are shown in Figure 1. We follow the notation of Xue et al.'3 in dividing the isotherm into five regions, as noted on these figures. Region I represents submonolayer coverage at near-zero surface pressure. Region I1 corresponds to the rise in pressure as the monolayer becomes complete. Region I11 is the plateau region encountered after compressing the monolayer beyond the collapse point. Region IV represents the upturn in surface pressure after compression through the plateau. Region V corresponds to the second plateau after further compression. The significance of the lettered arrows in these figures is explained below. The compression isotherm (Figure la) is in excellent agreement with that obtained by Xue et al. The 8CB monolayer is seen to collapse a t a relatively low surface pressure (4.5 mN/m), which suggests that the surface activity of this material arises from a balance of molecular interactions that is readily perturbed. This is confirmed by recent experiments by Xue and co-workers,n who found that stable monolayers of cyanobiphenyls are only formed from derivatives with alkyl chains between 5 and 10 carbons in length. We find that under our experimental conditions, the expansion isotherm (Figure lb) shows a slight hysteresis with respect to the compression isotherm: the projected area of the monolayer, obtained by extrapolating the pressure-area curve in region I1 to zero surface pressure, is about 5 % smaller. The collapse is therefore irreversible over the time scaleof these experiments. We have observed hysteresis in the 8CB collapse isotherm with compression rates as low as 2.5 mm/min (0.4 A2/(molecule min)). Compression. The Brewster angle micrographs obtained during the collapse of an 8CB monolayer under conditions of continuous compression are reproduced in (22) Xue, J.; Jung, C. S.;Kim, M. W. Presented at the Langmuir 6 Conference.
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Collapse of a Monolayer Film
(g)
(h)
Figure 2. Brewster angle micrographs of multilayer disk formation and coalescence during the continuous compression of 8CB through the collapse plateau. The scale of the images is 2.1 mm X 1.0 mm. For (a-g), the analyzer angle is 50'; for (h), the analyzer angle is 8 0 ' . Dark areas represent the monolayer phase, and light areas correspond to the collapsed phase.
Figure 2. The scale of each image is approximately 2.1 mm by 1.0 mm. Each corresponds to a position on the compression isotherm as indicated by an arrow in Figure la. The analyzer position is 50" for Figure 2a-g and 80" for Figure 2h, the latter for enhancedcontrast in this image. We note certain features of these images that will assist
in their interpretation: First, the illumination of the images is nonuniform due to the Gaussian intensity profile of the laser. Second, the images show a radially-banded texture due to the quality of the laser beam and slight imperfections in our optics. Third, the images are only partly infocus because of the viewing angle of the microscope.
1254 Langmuir, Vol. 10, No. 4, 1994 Convectiondue to slight air currents and thermalgradients prevented us from following an individual domain throughout the entire collapse process. Therefore, we do not discuss the kinetics of the collapse process in this paper, although such work is in progress. Despite these deficiencies, the domain structures are clearly visible. In region I, where the coverage is submonolayer, we observe a wide range of morphologies,including condensed monolayer islands and foamlike structures (not shown). As these pack together into a complete monolayer, the surface pressure begins to rise (region 11) and the layer takes on a homogeneous appearance (Figure 2a). Xue et al.13have postulated a vertical orientation for the monolayer phase based upon the projected molecular area of the monolayer and the known smectic-A ordering of the bulk material a t room temperature. We find that the reflected intensity of the layer can be uniformly extinguished by rotating the analyzer to 90°, signifying that the reflected beam is predominantly p-polarized. Therefore, we believe that the 8CB monolayer behaves as a uniaxial anisotropic film with the optical axis approximately perpendicular to the interface, consistent with the near-vertical8CB orientation proposed by Xue at al. Soon after the film is compressed past the collapse point and into the plateau region (region 111),we observe the appearance of small, bright circular domains (Figure 2b). These domains are not immediately visible at the collapse point, as they must first grow larger than the lateral resolution of our microscope (approximately 10 pm). In addition, since only a small fraction of the total film area is sampled with the microscope, we cannot comment on the spatial homogeneity of the domain formation. The collapsed domains appear to be of uniform reflectivity, so we consider them to be multilayer structures of uniform thickness. The circular form of these domains reveals that they are fluid, their shape determined by line tension acting a t the domain perimeter. We have verified the presence of this line tension by followingthe elongation and relaxation of the domain shapes after applying a deformation witha jet of air (results not shown). Although the appearance of the domains is similar to that observed in the liquid-expandedlliquid-condensedphase transition of pentadecanoic acid monolayers,lg there are two important differences that arise from the perpendicular versus tilted orientation of the molecules in these two systems. First, domains of pentadecanoic acid show a contrast inversion when the analyzer is rotated, due to the presence of a tilt-anisotropy, while collapsed 8CB domains are uniformly extinguished. Second, domains of pentadecanoic acid do not coalesce because of the existence of disclinations between individual domains. We observe coalescence in the 8CB multilayer domains, as described below. As the compression continues, there is an increase in both the number and size of multilayer domains (Figure 2c). In many images, the collapsed domains appear to be organized. For example, the domains are observed to fall on a line in some cases, while in others they are seen to surround patches of monolayer that are free of collapsed structures. We speculate that collapse may be occurring at preferred sites in the monolayer, such as a t domain boundaries, and that the domain structure of the underlying monolayer acts as a template for the growth of collapsed domains. On further compression, the multilayer domains pack into close proximity without coalescing (Figure 2d). The lack of observed coalescence could arise from mechanisms which prevent close contact of domains, such as the
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presence of repulsions between domains or low domain diffusivities. Alternatively, coalescence may be inhibited by mechanisms that stabilize individual domains at contact, such as the presence of line-active impurities at the domain perimeter, or the existence of a thin interfacial zone in which the 8CB orientation deviates from that found in the domain interior. We intend to investigate these possibilities in greater detail. As the individual multilayer disks grow into contact, they eventually begin to fuse (Figure 2e). We observe that this coalescence is accompanied by convection of nearby domains toward the coalescing structures, though this is not apparent from the static images, and suggest that this is due to a strong coupling between the collapsed domains and the underlying monolayer. Domain coalescence has also been described by Mann et al.16in a recent BAM study of the collapse of poly(dimethy1siloxane) monolayers. The authors observed the coexistance of circular polymer domains with several discrete thicknesses in the same collapsed layer. In contrast, collapsed domains of 8CB in region I11 appear to be of homogeneous reflectivity and, therefore, of equal thickness. Thus, we are observing the transition from a uniform monolayer to a uniform multilayer. As the surface pressure begins to increase (regionIV), coalescenceis nearly complete (Figure 20. In this figure, the dark linear feature on the left of the image arises from a coalescence in progress between two large domains. On further compression, the collapsed layer becomes uniform in appearance (Figure 2g) and remains so as the pressure increases. This suggests that the film in region IV is a homogeneousmultilayer structure. As was the case in region 11, the reflected beam can be uniformly extinguished by rotating the analyzer to 90°, suggesting that the multilayer has no significant in-plane anisotropy. The appearance of the film is similar to that in region I1 (Figure 2a), except the intensity is greater due to the increase in thickness. Information about the number of layers in this collapsed film can be inferred from the pressurearea isotherm, Figure la. The mean molecular area of a theoretical uncompressed close-packed monolayer of 8CB can be obtained by extrapolatingthe initial rise in surface pressure (region 11)to zero surface pressure. This yields a value of 48.8 A2. Applying the same analysis to region IV gives a value of 14.7 A2 for the molecular area of the multilayer film. We assume that this multilayer is composed of an 8CB monolayer, identical to that in region 11, covered by a second layer. From the projected areas, the fraction of 8CB molecules remaining in the monolayer is readily seen to be 0.301. The molecules in the second layer must therefore have a mean molecular area of 21.0 A2,too small to be a second monolayer. From considerations of the molecular packing in bulk phases, Leadbetter et aL23 have estimated that a densely-packed interdigitated bilayer of 8CB would take up an area of 20-22 A2/molecule in the plane normal to the long axis. Therefore, our datasupport a three-layer film: monolayer plus interdigitated bilayer. Xue et al.,13 performing a similar analysis, estimated a mean molecular area of 22.6 A2 and first proposed an interdigitated bilayer for the structure of this collapsed layer. Further compressionof the f i i leads to a second plateau in the isotherm (regionV) and the formation of new circular domains (Figure 2h). In order to enhance the contrast between the newly collapsed domains and the underlying multilayer, the analyzer has been rotated to 80° in this (23)Leadbetter, A. J.; Durrant, J. L.A.; Rugman, M.Mol. Cryst. Liq. Cryst. 1977, 34,231.
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Figure 3. Bremter angle micrographs of the formationof foam structures during the reexpansionof a collapsed 8CB multilayerback to a monolayer. The scale of the images is 2.1 mm X 1.0 mm. The analyzer angle is 50’. Dark areas represent the monolayer phase, and light areas correspond to the collapsed phase.
image. The domains appear to assume a greater distribution of intensities than seen in the first collapse, suggesting heterogeneity in thicknesses. Expansion. Structures obtained during continuous expansion of the collapsed 8CB film are reproduced in Figure 3. The approximate pressure-area state for each
image is indicated by an arrow in the expansion isotherm (Figure Ib). The analyzer position is 50’ for all images. The layer is uniform in appearance in region IV of the isotherm (Figure 3a), similar to Figure 2g of the compression. As the film expands back into the plateau (region 111),large circular holes form in the multilayer (Figure 3b).
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These polydisperse holes are typically observed in clusters, though isolated holes are also seen. On further expansion of the multilayer, the holes increase in size (Figure 3c). We find that the number density of holes is much smaller than that of the collapsed domains seen in compression, and their average size is larger. As the collapsed film continues to return to the monolayer, neighboring holes come into contact and occasionally deform (Figure 3d). A t areas near 32 A2/ molecule (Figure 361, we observe that the holes in the multilayer are fairly close-packed, and the layer takes on the appearance of a two-dimensional foam structure. Similar foams have previously been observed in the gasliquid coexistence region of stearic acid monolayer^?^ the submonolayer regime of poly(dimethyl~iloxane),~~ and other systems, but we believe that this is the first report of foam formation on a collapsed monolayer. The foam cells are seen to deform considerably as the intervening multilayer continues to be depleted (Figure 30. The multilayer fluid accumulates in the interstices between cells, and the cells are stabilized from rupture over the time period of the expansion by thin septa of collapsed 8CB, thinner than the resolution of our microscope. When the fluid walls between the holes become sufficiently thin, rupture occurs. A rupture in progress is captured in Figure 3g. The fluid septum, now broken, is driven toward the central pool of fluid (Plateau border) (24) Moore, B.; Knobler, C. M.; Broeeta, D.; Rondelez, F. J. Chem. SOC.,Faraday Trans. 2 1986,82, 1753.
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by capillary flow. As the system expands back into region 11, the images are predominantly that of the uniform monolayer (Figure 3h). A small number of multilayer droplets are observed, however, even as the surface pressure decreases to zero. This residual multilayer content explains, in part, the hysteresis that we observe on the compressionlexpansioncycle.
Conclusions We have visualized the collapse of the liquid-crystalline material 8CB from a monolayer to a homogeneous multilayer and its subsequent reexpansion to a monolayer. The images obtained with Brewster angle microscopy reveal the formation and coalescence of circular bilayer disks above the monolayer as the mechanism of this collapse. Expansion of the multilayer results in the creation of a two-dimensional foam, in which foam cells are separated by thin lamellae of collapsed 8CB. Brewster angle microscopy is shown to be a powerful tool for elucidating the mechanism of monolayer collapse, providing high contrast between monolayer and collapsed structures, as well as information about molecular orientation in these structures. In addition, this technique has the potential to be applied to kinetic studies of collapse, including structure formation, evolution, and stability. Acknowledgment. M.C.F. acknowledges the support of the Fannie and John Hertz Foundation. Additional partial support was provided by the NSF-DMR MRL Program administered through the Center for Materials Research at Stanford.
