Langmuir 1994,10, 2311-2316
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Multilayer Formation in Thin Films of Thermotropic Liquid Crystals at the Air-Water Interface Marc N. G. de Mul* and J. Adin Mann, Jr. Chemical Engineering Department, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106 Received October 21, 1993. In Final Form: May 10, 1994@ Molecules that form thermotropic liquid crystals can form well-ordered thin films when spread at the air-water interface. This ordering is due to their high dipole moment and rodlike structure. We report a high degree of order compared to other monolayer systems in thin films of 4'-octyl[l,l'-biphenyl]-4carbonitrile (8CB)spread at the air-water interface by correlating isotherm measurements and observations by Brewster angle microscopy. A first-order phase transition from a monolayer to a trilayer is shown to occur, during which circular domains are formed consisting of an interdigitated bilayer on top of the monolayer. The trilayer collapses to dense homogeneous domains, which are interpreted as stacked interdigitated bilayers with a structure similar to that of the smectic layers formed in bulk nematic 8CB near the surface. Dynamical phenomena during coalescenceof domains with a different number of layers are described which may have analogs in other multilayered systems.
Introduction The molecular order at the surface of a thermotropic liquid crystalline film is often markedly different from that in the bulk of the film. However, in the absence of external fields the average molecular orientation in the bulk liquid crystalline phase depends on the nature of the surface or the interface ofthe film.1 On solid surfaces the bulk orientation of the film can be induced by the surface through the anchoring of the liquid crystalline molecules, a phenomenon which is of major importance for the performance of liquid crystal display devices. A large research effort has been directed to the design of aligning surfaces that are able to control the molecular orientation in a reproducible way, while optimizing the display operation characteristics. The molecular mechanism of bulk phase anchoring to a surface, however, is still not well understood. Both the mesostructure, Le., the lateral structure on a characteristic length scale of 1pm, and the molecular structure of the surface are important. In nematic phases the substrate is found to induce layer formation at the surface, with a structure similar to that of the layers in smectic phases, while further away from the surface the smectic layering gives way to nematic order, characterized by a nonzero mean molecular orientation, the anchoring direction. Layer formation also occurs a t free surfaces or interfaces with a liquid. The surface layering at free surfaces has been studied by X-ray reflection and surface light scattering experiment^.^-^ A different research path is to measure the intermolecular interactions in the first few monolayers at a surface, which cause the surface-induced layering. The waterair interface is an ideal model surface for this purpose because it is easy to prepare in a pure state and because the surface coverage can be smoothly adjusted using the Langmuir trough techniqueOsSince many compoundsthat form thermotropic liquid crystals are amphiphilic and insoluble in water, monolayers can be formed readily by
* Abstract published in Advance ACS Abstracts, June 15,1994.
(1)JBrbme, B. Rep. Prog. Phys. 1991,54,391. (2)Pershan, P.S. J. Phys. Colloq. 1989,50,C7-1. (3) Bbttger, A.; Joosten, J. G. H. Europhys. Lett. 1987,4,1297. (4) Gierlotka, S.; Lambooy, P.; de Jeu, W. H. Europhys. Lett. 1990, 12,341. (5) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to SelfAssembly, 1st ed.; Academic Press, Inc.: Boston, 1991.
spreading on the water s ~ r f a c e . Much ~ , ~ work has been reported on monolayers of lyotropic liquid crystal-forming compounds, whereas little is known about spread films of materials that form thermotropic liquid crystals. Fundamental information on the thermodynamics of monolayers at the air-water interface can be obtained from the surface phase diagram, while a number of recently developed optical techniques, such as optical second harmonic generation8 and synchrotron X-ray diffraction, may be used to gain insight into the molecular packing in the various surface phases. Due to its similarity to the layered smectic phase, a monolayer or multilayer at the air-water interface can be used as a well-defined model to study interactions within the layers. Particularly the effect of temperature changes and mesomorphic phase transitions on molecular packing may be determined from the two-dimensional phase diagram. The mesostructure of a monolayer or multilayer at the air-water interface can be monitored by a Brewster angle microscope (BAM).9J0 The operating principle of this instrument is the almost zero reflectivity of the water surface for light polarized in the plane of incidence and incident a t the Brewster angle. If a polarized laser beam is directed to a monolayer-covered surface at the Brewster angle, a weak beam is reflected by only the monolayer. The reflected beam can be imaged on a CCD camera with contrast provided by local differences in the thickness and optical dispersion properties of the monolayer. Therefore, the state of a liquid crystalline monolayer or multilayer can be visualized a t any point on the phase diagram. In this paper we report measurements of the surface pressure vs area isotherm and observations by Brewster angle microscopy of thin films of 4'-octyl[ 1,l'-biphenyl]4-carbonitrile (8CB) at the air-water interface. (Note: It has recently come to our attention that BAM studies of bilayer formation on 8CB monolayers have been carried out simultaneously by a group a t Stanford Uni~ersity.'~) This material is the most widely used and studied thermotropic liquid crystal. Surface isotherm measure(6) Daniel, M.F.;Lettington, 0. C.; Small, S. M. Thin Solid Films
1RF1.9 ---, -9s - , 81 - -.
(7) Albrecht, 0.;Cumming, W.; Kreuler, W.; Laschewsky, A.; Ringsdorf, H. Colloid Polym. Sci. 1986,264,659. (8)Rasing, Th.; Berkovic, G.; Shen, Y. R.; Grubb, S. G.; Kim, M. W. Chem. PhysrLett. 1986,130, 1. (9) HBnon, S.;Meunier, J. Rev. Sci. Znstrum. 1991,62,936. (10)Honig, D.; Mobius, D. J.Phys. Chem. 1991,95,4590.
0743-7463/94/2410-2311$04.50/00 1994 American Chemical Society
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ments6reveal afirst-order phase transition at a relatively low surface pressure, which has been ascribed to smooth collapse of the monolayer t o form multilayers. This interpretation was supported by evidence from ellipsometric a n d optical second-harmonic generation measurements.11J2 Multilayer formation at the air-water interface has similarities with that of the smectic layers that exist at the surface of a free-standing nematic 8CB film.3,4 Drainage of the free films is observed to decrease their thickness in steps of a size that is close to the thickness of the interdigitated bilayer observed in the bulk smectic A phase13 of 8CB. Apart from its relevance to liquid crystal ordering, the collapse of 8CBfrom monolayers to multilayers is of direct significance to other systems where bilayers and multilayers occur. We will present direct observations of monolayer collapse to form multilayers. The classical view of multilayer formation at the air-water interface is the surface pressure-induced folding model followed by sliding of the layers on top of each other.14 While w e find evidence to support this model, a much richer set of dynamical phenomena occurs at high surface pressures which has not been reported before.
Experiments In this work we employed a Brewster angle microscope built in our laboratory from inexpensive standard components. Although it has the advantage that the field kept in focus is substantially larger, we chose not to use the elaborate focusing method designed by Meunier and co-w~rkers.~ Instead we used an arrangement with one or two lenses, similar to the Mtibius setup,1° which allows image acquisition in real time. The light source was a 10-mW HeNe laser (Melles-Griot) which emits a polarized beam with a wavelength of 632.8 nm. The image was registered by a Pulnix TM-7CN CCD camera and captured by a Perceptics Pixelbuffer frame grabbing card, interfaced to an Apple Macintosh computer. Captured images were processed with NIH Image and HIPG DIP Station image processing software. The image processingprocedure included an expansion of the image to correct for the incident angle and a filtering operation to reduce interference fringes and noise. Furthermore, the brightness of each image was scaled t o improve contrast. Contrast in the images is provided by gradients of the thickness, density, and optical dispersion properties of the film. Moreover, the polarization state of the reflected light, and therefore the optical anisotropy of the film, may be determined by adding an analyzer. All images reported here were recorded without using an analyzer in the path of the reflected beam. However, separate experiments were done to analyze the optical anisotropy in the films and hence the molecular tilt variations, in which case an analyzer was used. The microscope was mounted over a commercial film balance (Lauda) equipped with a floating barrier for surface pressure measurement with a precision of f O . l mN/m. A second moving barrier was operated by an IBM PC computer, so that isotherms could be recorded automatically. The temperature ofthe trough could be controlled with a precision of f0.5 K by circulating water from a water bath. However, the experiments reported here were recorded at room temperature to avoid thermal circulation in the subphase which disrupts the monolayer domains, an effect that occurs even at trough temperatures a few degrees higher or lower than ambient. Note also that keeping a specific section of the film under the laser footprint to observe its evolution in time was not possible in our setup, since the domains drift with a velocity of up to 1d s , partially depending on the speed ofthe movingbarrier. The microscope,film balance, (11)Xue, J.; Jung, C. S.;Kim, M. W. Phys. Rev.Lett. 1092,69,474. (12)Guyot-Sionnest,P.;Hsiung, H.;Shen, Y. R.Phys.Rev.Lett.1988, 57. 2963. (13)De Gennes, P.G.; Prost, J. The Physics ofLiquid Crystals, 2nd ed.; Clarendon Press: Oxford, 1993. (14)Ries, H. E.,Jr. Nature 1979,281, 287. (15)Friedenberg, M.C.;Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994,10,1251.
7
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Figure 1. Surface pressure vs area isotherm of an 8CB monolayer at the air-water interface at 22 f 0.5 "C. The chemical structure and transition temperatures of 8CB are shown. Letters a-h indicate the conditions at which the corresponding images in Figure 2 were recorded. and computers were located in the Polymer Microdevice Laboratory, a large class 100 clean room at Case Western Reserve University. The liquid crystal 8CB was obtained from BDH Chemicals with a purity of better than 99%. The spreading solvent was HPLC grade chloroform, which was used as received. After deposition of a large amount of this solvent on the surface and subsequent compression, the surface pressure was not significantly affected. Very pure water was prepared by a Millipore Milli-QPlusultrapure water system. This water was equilibrated with air and therefore with carbon dioxide. The trough and the glassware were cleaned by treatment with a strongbase, followed by dilute nitric acid, and with methanol (HPLC grade, used as received) and chloroform, after which they were extensively flushed with pure water. Before each experiment the trough was cleaned with methanol and chloroform to remove residual organic components and dust, after which it was flushed three or more times with water. The floating barrier was cleaned by ultrasonic treatment in a chloroform solution. After this procedure blank isotherm measurements showed no increase in surface pressure down to a very small surface area. Monolayers and multilayers of 8CB were spread by slowly depositing a small amount, typically 100pL, of a dilute solution on the water surface using a Hamilton syringe. After evaporation of the chloroform solvent a homogeneous film was obtained. The 8CB films were comparatively stable: a contraction in molecular area of less than 10% was observed when the film was kept at a constant surface pressure of 2 mN/m for 2 h.
Results Despite their short alkyl chain lengths compared to longchain fatty acids, which are known to form stable monolayers, cyanobiphenyls are easily spread on the water surface. The cyanobiphenyl compounds have a dipole moment16 of about 6 D a n d are therefore highly polar. Their liquid crystallinity arises from the biphenyl rings, which behave as a rigid rod a n d induce layering in the bulk fluid phase at suitable conditions. The transition temperatures a n d chemical structure of 8CB are shown in Figure 1. Mesomorphic phase transitions in 8CB occur at or around temperature, although the effect of bulk transitions on the structure at a surface, if any, is not known. As we will show, the rodlike nature of the molecules causes formation of multilayers at the airwater interface that are highly ordered compared to collapse structures observed in thin films of conventional monolayer forming compound^.'^ (16)Kawski, A.; Kukielski, J. Mol. Cryst. Liq. Cryst. 1990,182B, 209. (17)Siegel, S.;Honig, D.; Vollhardt, D.; Mobius,D. J.Phys. Chem. 1092,96,8157.
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We carefully measured the surface pressure while compressing and expanding the 8CB film on the water surface. A typical isotherm is shown in Figure 1. The isotherm is characterized by a steep rise in surface pressure up to an area of 0.40f 0.02 nm2/moleculeand a surface pressure of 4.8f0.1mN/m, followed by a sharp kink and a plateau region, characterizing a first-order phase transition. The pressure then increases again to 6.2 f 0.1mN/m, and a second kink is observed at 0.09 f 0.02 nm2/molecule,after which a second plateau region occurs. The isotherm is relatively independent of the temperature, and alkylcyanobiphenyls with a shorter chain length such as 4’-hexyl[l,l’-biphenyll-4-carbonitrile (6CB) and 4’-heptyl[l,l’-biphenyll-4-carbonitrile(7CB) have similar or virtually identical isotherms. Direct observation of the monolayer during isotherm data collectiongreatly clarifies understanding of the phase behavior. The physical significanceofthe various features of the isotherm can be unambiguously determined using the Brewster angle microscope. For the 8CB monolayer, at high areas per molecule, larger than 0.50 nm2, dense liquid-phase islands exist in equilibrium with a gas phase (Figure 2a). These liquid domains appear with homogeneous intensity and thus do not show variations in molecular tilt direction, within domains and between domains, on a large enough length scale to be observed by rotating the analyzer. On compressionto smaller areas per molecule the domains coalesce and form a homogeneous monolayer. The surface pressure then increases steeply up to the first plateau as the monolayer is further compressed. Immediately at the kink, a first-order phase transition is initiated and dense circular domains appear with a homogeneous thickness (Figure 2b). We often observed the domains to be arranged in strings, suggesting that the flocculation kinetics favors linear aggregates. However, a rigorous statistical analysis of the images is required to determine the details of the aggregate structure. Similar to the liquid domains at lower pressures, the dense domains do not show variations in molecular tilt such as observed in, e.g., fatty acid monolayers.18 Domain growth is observed when the area of the film is further reduced (Figure 2c). When the domains are sufficiently large, they deform and coalesce as the stress of the boundary line between domains reaches a certain threshold value (Figure 2d). Coalescence proceeds on a time scale of seconds, demonstrating a large line tension and a high degree of fluidity of the domains. At the second rise in surface pressure the film is essentially homogeneous but with occasional defects in the form of small holes. The second phase transition at molecular areas smaller than 0.09 nm2occurs in a muchless ordered way. Initially very dense round islands are nucleated (Figure 2e),which are later joined by other domains with varying brightness and therefore a variety of thicknesses (Figure 20. Again, these domains are homogeneous and do not show molecular tilt variations. Interestingly, the coexisting phase for the domains is the condensed film formed during the first-order phase transition between 0.40 and 0.09 nm2. This film therefore supports domains with different thicknesses that do not coalesce or form on top of each other. On further reduction of the area per molecule, the thickness of the domains does not change and the domains are pressed together, although they do not deform significantly. We observed a number of dynamical phenomena equalizing the domain thickness such as the spreading of a layer over a less thick domain (Figure 2g,
upper left corner) and the formation of contact points between three regions with different thicknesses (Figure 2h). While nucleation of the domains with the highest thickness observed continues during compression, coalescence progresses by these mechanisms and tends to make the domain thickness uniform. In this region ofthe isotherm dense monolayer domains can be seen visually on the water surface.
(18)Ruiz-Garcia, J.;Qiu, X.; Tsao, M.-W.; Marshall, G.; Ihobler, C. M.; Overbeck, G. A.; Mobius, D. J . Phys. Chem. 1993,97,6955.
(19) Leadbetter, A. J.; Richardson, R. M.; Colling, C. N. J. Phys. Colloq. 1976, 36,Cl-37.
Discussion Isotherms. A model for the structure of the interfacial film during the first phase transition has been proposed in the literature which we believe to be correct.’l According to the model, which was based on the isotherm and on ellipsometric data, the first-order phase transition starting at 0.40nm2 occurs by formation of a bilayer of 8CB molecules on top of a monolayer. This argument is supported by the bounding molecular areas of the transition, i.e., the areas per molecule at which the phase transitions are initiated. It has been shown12 that molecules in the first monolayer of 8CB on the water surface possess an average molecular tilt angle from the surface normal of 71 f 2”. Assuming that the area per molecule in the monolayer is dominated by the cyanobiphenyl head group, which has a length of 1.1 nm and a width of 0.43nm, and taking the tilt angle into account, we calculate a molecular area of 0.41nm2, which agrees well with the measured value. A probable structure of the first monolayer on the water surface is shown in Figure 3a. A similar calculation can be done to find the mean area per molecule of the trilayer. For this purpose we use literature data on the organization of 8CB bilayers. The molecular structure in the bulk smectic A phase of 8CB is known to consist of interdigitated bilayers with an antiparallel arrangement of the molecules. Indeed, X-ray diffraction measurements on the bulk nematic phase of 7CB have that the molecular packing on a microscopic scale, i.e., on a characteristic length scale of a few nanometers, can also be described as smectic-like, interdigitated bilayers, with an average molecular area of 0.25 nm2. Likewise, a smectic-like, layered structure exists near the surface of a free-standing nematic 8CB film in contact with air.3 The stability of the smectic bilayers arises from the strong dipole-dipole attraction in the antiparallel orientation. Since the 8CB bilayer on top of the first monolayer on the water surface can be regarded as a free 8CB surface, its structure is almost certainly an interdigitated bilayer as well. Its area per molecule can be reliably approximated by 0.25 nm2, the molecular area of the 7CB bilayer. The total area per molecule ofthe trilayer is then calculated to be 0.096 nm2, which again is comparable to the measured value of 0.09 nm2. Note that this area is not the absolute area per molecule, but rather the available area on the water surface divided over all the molecules present. On the basis of these arguments, a tentative trilayer structure can be constructed (Figure 3b). Brewster Angle Microscope Images. The trilayer structure can be used to explain qualitatively some features of the BAM images. During the first-order phase transition the reflected intensity is equal for each domain, and the domains are homogeneous and do not show molecular tilt variations on a mesoscopic length scale, which would be observed as intensity gradients over each domain as in fatty acid monolayers.ls These observations show that the domains have a constant thickness, while
2314 Langmuir, Vol. 10, No. 7, 1994
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1
b
C
C
f
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d
g
h
Figure 2. Images of the structure of 8CB monolayers and multilayers. The conditions under which each image was taken are shown in Figure 1. A description of the images is given in the text. Notes: Brighter areas are more dense. The brightness of each image was separately scaled; i.e., domains with the same density may appear with a different brightness in separate images. Diagonal stripes are diffraction fringes. The length of the bars is 100 pm.
the magnitude of the reflected intensity indicates that they consist of multiple layers of 8CB molecules. A n
explanation for the absence of an average molecular tilt direction is suggested by the observation that 8CB bilayers
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the surface normal. Observations by X-ray reflectivity on free-standing films of 8CB support the existence of a tilted monolayer as the top layer.4 However, the evidence is inconclusive, and it appears that both odd and even numbers of layers may occur. Following this reasoning, the multilayer domains are proposed to consist of a number of stacked interdigitated bilayers, which may or may not support an additional tilted monolayer. The stacked interdigitated bilayer model is shown in Figure 3c. This model poses several interesting questions. First, why do the stacked bilayers not spread out to form a macroscopically uniform multilayer? Spreading is energetically more favorable since the bilayers are quite fluid as discussed below and do not have a rigid semicrystalline structure. One possible explanation is that the stacked layers are strongly coherent and resist deformation, thus remaining in a metastable position. Second, why do no partially covered bilayers exist? From simple energetic arguments there is no explanation for the observation that the stacked interdigitated bilayers fully cover the ones below. As before, this may be a kinetic phenomenon linked to the formation mechanism of the stacked bilayers. Several other factors may affect the bilayer structure, such as the water content of the bilayers. To summarize, the stacked interdigitated bilayer model accounts well for the observed structures. However, it does not provide an explanation as to why these structures form. An answer to this problem may be found by appropriate molecular dynamics computations. The model can also be tested by synchrotron X-ray diffraction and ellipsometry measurements on the spread films at the air-water interface. A combination of these techniques and atomic force microscopy on films deposited on Figure3. Schematicrepresentation of the molecular structure solid substrates by Langmuir-Blodgett techniques should of a monolayer of 8CB at the air-water interface (a), a trilayer provide additional information on the structure of this (b),and a multilayer with stacked interdigitated bilayer domains class of multilayers. (c). Note that the multilayer may have an even or odd number of monolayers. The lateral dimensions of the stacked bilayer Monolayer and Multilayer Collapse Dynamics. domains are not drawn to scale. The collapse of the 8CB monolayer to a trilayer is remarkable since it proceeds in a very ordered way as a in free-standing films are oriented normal to the ~ u r f a c e . ~ first-order phase transition. Very bright three-dimenHowever, it is also possible that nonzero average tilt sional nuclei do not occur, in contrast to the collapse regions exist with a size smaller than the microscope observed in monolayers of stearic acid1' and poly(diresolution limit, for example, forming a mosaic structure. methylsiloxane).20 Rather, the monolayer buckles This structure is likely to exist in the monolayer phase smoothly according to the classical foldingmodel,14which where mesoscopic molecular orientation gradients are is represented in Figure 4a, with simultaneous interlockknown to exist but were not detected with the BAM. A ing of the head groups to form the interdigitated bilayer. similar molecular arrangement has been found in the bulk Although the initial nucleation of monolayer collapse nematic phase of 7CB, where the orientation order extends proceeds on a length scale smaller than the resolution of over a characteristic length scale of 100 molecule^.^^ our BAM, the bilayer domains on top of the first monolayer Bilayer domains were also observed to be very fluid. at the water surface are seen to assume quickly a circular Coalescence and subsequent relaxation of fused domains shape and to grow uniformly in diameter until they are to a circular shape occur rapidly, probably due to large closely packed. At any time during compression of the van der Waals forces and the attractive dipole-dipole 8CB film the domains have a relatively narrow size interactions in the interdigitated bilayer. These forces distribution, although the average diameter varies slightly account for a large line tension that initially resists domain in different regions of the surface film. This may show deformation before coalescence. that, after initiation of the collapse at the first kink in the isotherm, the nucleation rate is lowered considerably due After the second phase transition of the film at 0.09 to continuing folding in each bilayer domain. nm2, circular domains are observed that are as homogeneous as the trilayer domains but show a wide distribution At the end of the first-order phase transition, when all of reflected intensities. Again, the domains do not show domains have coalesced into a uniform trilayer film, mesoscopic molecular tilt variations, so that the different collapse to multilayers takes place more chaotically, brightness must be due to domains with a different although similarin some respects to the first-order collapse thickness. The homogeneity of the reflected intensity for process. The bright multilayer domains that are nucleated each domain shows that the domains are not threehave a variety of thicknesses, so that their formation can dimensional collapsed regions, but multilayers with a wellno longer be explained by the classical folding model. Due defined line tension tending to make the domains circular. to the interdipole association, the bilayers may remain An even number of layers is energetically unfavorable coherent longer than the single monolayer previously, compared to an odd number as the top layer would be a monolayer, which is subject to large repulsive dipole(20) Mann, E. IC;H h o n , S.; Langevin, D.; Meunier, J. J. Phys. ZZ 1992,2,1683. dipole forces,unless the dipoles are sufficiently tilted fiom
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Figure 4. Schematic overview of the collapse mechanism of an 8CB monolayer to a trilayer (a) and of a trilayer to a ninelayered stacked interdigitated bilayer structure (b) according to the folding model. Note that following the mechanism in (b) each interdigitated bilayer completely covers the one below.
leading to the formation of extra folds as shown in Figure 4b. This model would account for the observation that no partially covered stacked interdigitated bilayers exist in the films studied. Alternatively, the collapsemay destroy temporarily the local structure of the monolayer, after which the fragments self-assembleinto different numbers of stacked bilayers. As before nucleation of the collapse takes place on a length scale smaller than 1pm. While the observed collapse phenomena have direct relevance to other systems that contain monolayers or multilayers, such as micelles and biological membranes, more significantare the dynamics of the coexistingstacked bilayer state. Initially, the multilayer domains are separated and maintain a circular shape dictated by the line tension. Contrary to the trilayer domains, the circular shape is preserved when the domains are compressed to a close-packedstate. Coalescencedoes not take place until the domains are pressed together hrther. At this point a two-stage mechanism for coalescence is observed. First, if the coalescing domains have different thicknesses, a layer from the thicker domain will spread rapidly over the less thick domain. The spreading proceeds radially until the edges of the domain are reached. Noncircular domain shapes are observed at a later stage, which give
de Mu1 and Mann way to two-dimensional foamlike structures, indicating that coalescence only occurs when domains have equal thicknesses. However, the domain shape and size distributions are not uniform over the entire surface area, and different regions of the film show different domain structures. Therefore, the structures seen at this stage are likely not equilibrium structures. Similar dynamical phenomena are expected to occur in other systems that consist of ordered multilayers. For instance, in self-assembled structures such as phospholipid bilayers and lamellar micelles, the multilayer collapse processes will occur when the structures are subjected to stress. Stacked bilayers may be found in biological cell structures, which may give further informationabout their stability. The uniquely well-ordered structure of thin films of the thermotropic liquid crystals makes them suitable model systems for the study of the intermolecular interactions, thermodynamics, and formation dynamics of monolayers and multilayers.
Conclusions We demonstrated that 8CB forms stable monolayers and multilayers a t the air-water interface with a high degree of order compared to other monolayer systems. Two phase transitions occur along the pressure-area isotherm during which circular domains coexist with a continuous phase. These domains were interpreted as multilayered structures akin to surface-induced smectic layers in bulk liquid crystals and free-standing liquid crystalline films. The trilayer model for the first-order phase transition has been shown to be consistent with observations of the fluidity and homogeneity of the domains. Furthermore, we proposed that the domains formed at low molecular areas and high surface pressures consist of stacked interdigitated bilayers on top of the trilayer. While this model accounts well for the experimental results, the stability of the stacked interdigitated bilayers could not be explained satisfactorily, and more work is necessary to determine the dynamics of the collapse processes during which the layers are formed. The dynamical phenomena observed during coalescence of stacked interdigitated bilayer domains with a different number of layers are directly significant to other multilayered systems. The well-ordered liquid crystalline film may be a useful model for the interactions and dynamics of multilayers in general. Moreover, the order observed in the 8CB multilayers may be conserved on transfer to a solid surface by Langmuir-Blodgett deposition. Knowledge of the structure of the film on a solid substrate relative to the structure on the water surface may be used to obtain information on the mechanism of alignment of liquid crystals by solid surfaces.
Acknowledgment. We would like to thank Dr. J. B. Lando and Dr. K. D. Singer for helpful discussions on monolayers and experimental optics. This work was supported by NASA Lewis Research Center and by the National ScienceFoundation Center for Advanced Liquid Crystalline Optical Materials. Registry Number Supplied by Author. 4’-Octyl[1,l’biphenyl]-4-~arbonitrile, 52709-84-9.
