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Defects and Molecular Alignment in Monolayers of Thermotropic Liquid Crystals on Water Marc N. G. de Mu1 and J. Adin Mann, Jr.* Department of Chemical Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106-7217 Received January 25, 1995. In Final Form: July 17, 1995@ Using Brewster angle microscopy, smectic disclination defects were observed in monolayers of 4‘-alkyl [l,l’-biphenyl]-4-~arbonitriles (nCB, n = 9,lO) at the air-water interface. Bilayer domains formed on top of the monolayer during collapse induce a splay orientation of the molecules in the surrounding monolayer along their boundary lines. In addition, bend defects are formed when the continuous bilayer breaks up to form a two-dimensional foam on the monolayer. This is the first observation of molecular alignment in monolayers on water along surface structures out ofthe plane ofthe monolayer. The results are significant for the structure of self-assembled films and biological membranes. Thin films of liquid crystalline materials have been widely studied and the existence of macroscopic molecular orientation gradients is well-kn0wn.l The average molecular orientation direction, the director, in the bulk is determined by the boundary conditions forced on the director field at the surface. Alignment of the molecules at the surface causes the director close to the surface to assume a uniform anchoring direction.2 This effect is applied in liquid crystal displays in combination with an electric field to switch the molecular tilt direction between an off and on position. Whether the alignment at the surface is set by the first monolayers, however, has not been resolved, although certain monolayer structures can induce alignment sufficient for the operation of device^.^ Moreover, the effect of mesomorphic phase transitions in the bulk films on the structure at the surface is not known. Direct information on the molecular orientation at a surface or interface has been obtained in studies of freestanding films of smectic liquid crystals by polarized reflection micro~copy,~,~ while X-ray reflectivity measurements and light scattering studies6-8 showed that selforganization of the molecules at the surface into layers occurs in nematic phases. Recently, observations by molecular-resolution microscopes have shown that organized monolayers can also be prepared at solid Moreover, clear evidence that surfaces induce layer formation in adjacent films has been obtained in thin films of 4’-octyl [l,l’-biphenylld-carbonitrile (8CB) at the airwater interface. Using Brewster angle microscopy (BAM),12J3monolayers of this compound were found to collapse reversibly to trilayers and multilayers on com-
* To whom correspondence may be addressed.
Abstract published inAduance ACSAbstracts, August 15,1995. (1)de Gennes, P. G.; Prost, J. The Physics ofLiquid Crystals, 2nd ed.; Clarendon Press: Oxford, 1993. (2) JBrBme, B. Rep. Prog. Phys. 1991,54,391. (3)Albarici, A. Langmuir-Blodgett Films as Alignment Layers for Nematic Liquid Crystal Displays. M.S. thesis, Case Western Reserve University, 1994. (4)Dierker, S.B.; Pindak, R.; Meyer, R. B. Phys. Reu. Lett. 1988,56, 1819. ( 5 ) Langer, S. A.; Sethna, J. P. Phys. Rev. A 1988,34,5035. (6)Bottger, A.; Joosten, J. G. H. Europhys. Lett. 1987,4,1297. (7)Pershan, P. S.J. Phys. Colloq. 1989,50,C7-1. (8) Gierlotka, S.;Lambooy, P.; de Jeu, W. H. Europhys. Lett. 1990, 12, 341. (9) Smith, D. P. E.; Horber, H.; Gerber, C.; Binnig, G. Science 1989, 245,43. (10)Fang, J. Y.; Lu, Z. H.; Ming, G. W.; Ai,Z. M.; Wei, Y.;Stroeve, P. Phys. Rev. A 1992,46,4963. (11) Patrick, D. L.; Beebe, T. P. Langmuir 1994,10,298. (12)HBnon, S.; Meunier, J . Rev. Sci. Instrum. 1991,62, 936. (13)Honig, D.; Mobius, D. J. Phys. Chem. 1991,95,4590. @
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Figure 1. Surface pressure vs average area per molecule A isotherm of lOCB at 20.0 & 0.1 “C.
pression.14J5 The existence of multilayers in.these films was proposed previously on the basis of surface pressure vs area per molecule isotherm measurements and ellipsometry results.16 Recently, we have confirmed conclusively that multilayers are formed by quantitative analysis of the BAM images.17 Contrary to what might be expected based on measurements of the molecular tilt angle by second-harmonic generation,18director gradients were not detected in the 8CB films. We hypothesized before that the monolayer is too disorganized to produce homogeneously aligned regions with a size larger than the resolution limit of our microscope. This argument raised the possibility that larger aligned regions occur in monolayers of 4’-alkyl [l,1’biphenyl]-4-carbonitriles (nCB) with a longer alkyl chain length. Subsequent experiments have shown that this is indeed the case. In the following we will present direct observations of large regions with a homogeneous molecular orientation direction in monolayers of 9CB and lOCB at the air-water interface. In some regions of the isotherm defects exist that appear very similar to the disclinations observed in thicker smectic and nematic films.lJg Moreover, remarkable one-dimensional alignment effects occur along the boundary line of multilayered domains and the monolayer. We interpret alignment here as referring to orientation of the molecules in the monolayer induced by (14)de Mul, M. N. G.; Mann, J. A. Langmuir 1994,10,2311. (15)Friedenberg, M. C.; Fuller, G. G.; Frank, C . W.; Robertson, C. R. Langmuir 1994,10,1251. (16)Xue, J.; Jung, C. S.; Kim, M. W. Phys. Rev. Lett. 1992,69,474. (17)de Mul, M. N. G.; Mann, J. A. To be published. (18)Guyot-Sionnest,P.; Hsiung, H.; Shen,Y. R. Phys. Reu. Lett. 1986, 57,2963. (19)Lavrentovich, 0. D. Liq. Cryst. Today 1992,2,4.
0743-746319512411-3292$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 9,1995 3293
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1
Figure2. (a,top left) Two hairy disclination defects in the monolayer region (lOCB, 0.42 nm2/molecule,recorded during expansion). (b, top right) Trilayer domains linked by strings of constant 8 (9CB, 0.30 nm2/molecule,compression).(c, bottom left) Hairy disclination defects in the centers of two-dimensional foam holes (lOCB, 0.16 nm2/molecule, expansion). (d, bottom right) Molecular alignment at the Plateau borders of a thinning two-dimensional foam (lOCB, 0.35 nm2/molecule, expansion).
the boundary conditions at the monolayer-substrate interface or, as in this case, at the monolayer-bilayer interface. While long-range orientational order in monolayers at the air-water interface is often observed (e.g. ref 20),alignment in the above sense has not been reported before. The surface isotherms of 9CB and lOCB are practically identical; a representative isotherm is shown in Figure 1. Surface pressure vs molecular area data were measured with a Lauda film balance kept at 20.0 f0.1 "C and located
in a class 100 clean r o o d 4 The isotherm is very similar to partial isotherms reported in the literature21and also to the isotherm of 8CB,14-16with the exception that the height of the second plateau region is lower. The shape of the isotherm of 8CB has been explained before as due to the collapse of the monolayer to a trilayer during the first plateau region, followed by collapse of the trilayer to multilayers during the second plateau region. The trilayer and the multilayers are built up from the monolayer directly at the water surface and a variable number of
(20)Moy, V.T.;Keller, D. J.; Gaub,H. E.; McConnell, H. M. J. Phys. Chem. 1986,90,3198.
(21)Daniel, M. F.;Lettington, 0. C.; Small, S. M. Thin Solid Films 1983,99,61;Mol. Cryst. Liq. Cryst. 1983,96,373.
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interdigitated bilayers. Similar structures exist in the 9CB and lOCB films. However, while we determined the equilibrium spreading pressure (ESP) of 8CB to be 5.2 f 0.1 mN/m, the ESP of the present materials was equal to zero within experimental error. Therefore, the observed multilayered structures are in a metastable state. This is evident during data collection because nucleation of three-dimensional crystals occurs frequently, upon which the surface pressure drops to the ESP. The difference in the stability of smectic phases at the surface is also suggested by the bulk mesomorphic phase transition temperatures of the nCB materials. Whereas 8CB changes from the crystalline state to the smectic-A phase a t 21.5 "C, 9CB and lOCB undergo transitions to the smectic-Aphase at 42 and 44 "C, respectively.22However, the monomolecular films at the air-water interface may be stable at different temperatures. By analogy with the large variety of surface phases and domain shapes found in monolayers of fatty acids at different temperature^,^^ the present experimental results provide only a snapshot of the surface phase diagram. Direct observation of the morphology of the surface films is possible by BAM. This technique is based on the minimum of the reflectance of the water surface for p-polarized light incident at Brewster's angle, equal to 53.1" at 20 "C, so that the reflected beam is due to the interfacial film. The instrument we used is based on the Mobius setup13 and was built in our laboratory. Apart from the CCD camera used as the detector and the focusing optics, the system components are analogous to those of an ellipsometer, and it is possible to operate with a polarizer, sample, and analyzer (PSA) arrangement or with a PS setup without the analyzer. The reflected intensity I , relative to the incident intensity Ii depends on the angle of the analyzer with the plane of incidence and can be related back to the dielectric properties of the film. Using a matrix the reflectance R (=IJIi) of the film can be computed theoretically once the dielectric tensor F of the film is known. Assuming that the principal axes of E lie along the molecular axes, R can be related to 4, the average molecular tilt angle with respect to the surface normal, and 8, the angle of the director projected on the surface plane with the light propagation direction.25*26 In this paper, we assumed that the components of F perpendicular to the long molecular axis are both equal to cI, the dielectric constant perpendicular to the optical axis of a thick film. Similarly, the component of F parallel to the long molecular axis was taken as €11. We used values of and cII available in the literature2' for 7CB and neglected the effect of the longer alkyl tails of 9CB and 10CB. These approximations are valid for images obtained in the PS arrangement. However, in the PSA setup, R as a function of the analyzer angle was found to depend strongly on G. In this case the assumption of cylindrical symmetry of the molecules implicit in the use of cI does not work. The resulting complications will be discussed in a later paper. Figure 2 shows PS images of the morphology of 9CB and lOCB films in different regions of the isotherm. After spreading a t 0.60 nm2/moleculethe film is in a condensed (22)As stated by manufacturer. (23)Ruiz-Garcia,J.; Qiu, X.; Tsao, M.-W.;Marshall, G.; Knobler, C. M.; Overbeck, G. A.; Mobius, D. J . Phys. Chem. 1993,97,6955. (24)Azzam, R. M.A.; Bashara, N. M. Ellipsometry and Polarized Light , 1st ed.; North-Holland: Amsterdam, 1992;p 340. (25)Hosoi, IC;Ishikawa, T.; Tomioka, A.; Miyano, K. Jpn. J. Appl. Phys. 1993,322, L135. (26)Overbeck, G. A.; Honig, D.; Mobius, D. Thin Solid Films 1!H4, 242, 213. (27)de Jeu, W. H. Physical Properties ofLiquid Crystalline Materials, 1st ed.; Gordon and Breach Science Publishers: New York, 1980.
Letters
Figure 3. Calculated image corresponding to the defect structure in Figure 4.
monolayer state. The monolayer contains "hairy" disclination defect structures consisting of two dark fans originating from a central singular point (Figure 2a). Upon compression of the monolayer to 0.40 f0.02 nm2/molecule and 4.9 f 0.1 mN/m, the monolayer collapses and circular bilayer domains are formed on top of the monolayer. We find that the molecules in the monolayer align a t the boundary line of the collapsed bilayer domains. The aligned regions can extend as far as the distance between the bilayer domains so that strings of constant 8are formed (Figure 2b). On further compression the bilayer domains coalesce and form a homogeneous bilayer on top of the monolayer. When the film is subsequently expanded, the homogeneous bilayer breaks up and a two-dimensional foam is formed in a process of reverse collapse, in which molecules are returned from the collapsed domains to the monolayer. The remarkable feature of this foam is that a hairy defect is centered in many foam cells (Figure 2c). Further expansion leads to foam thinning until the twodimensional Plateau borders between the foam holes are exceedingly thin. However, as is apparent from the step change in reflectance, molecules in the monolayer on each side of a Plateau border appear to be unaware of the inplane molecular orientation direction 8 of the monolayer on the other side (Figure2d). Evidently the Plateau border has an aligningeffect on the monolayer molecules identical to the alignment mentioned before around the circular bilayer domains. The molecular orientation in the hairy defects can be understood by comparing the measured images to model calculations.26We used Mathematica (WolframResearch) running on a Macintosh to generate computed images of model smectic defects, which can be described by the angle 8 as a function of the position in the surface plane. For simplicity we only included radially symmetric defects. This is justified since the dark fans in the centers of the defects are always oriented along the vertical of the image as in Figure 2c. Only two defect structures remain, a splay structure, where the molecules are pointed radially outward from the singularity, and a bend structure, in which the molecules are aligned perpendicular to the lines extending radially from the defect center. Both the splay and the bend structures have two varieties since 8 and 8 + n are not identical. However, in the PS arrangement the BAM cannot distinguish between the in-plane mo-
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\ \ Figure 4. Model of the molecular arrangement of the monolayer in a two-dimensional foam hole. The director points along the long molecular axis away from the water surface, and the arrows indicate the director projected on the surface plane. Therefore,O is the angle of an arrow with the light propagation direction k. Note that the monolayer has a bend orientation at the central singularity, which shifts to a splay orientation at the edge.
+
lecular orientation angles 8 and 8 n. Therefore, we need to consider only the general splay and bend structures and combinations of these. The computed image corresponding most closely to the BAM image of the defect centered in a two-dimensional foam hole (Figure 2c) is shown in Figure 3. This image corresponds to the in-plane molecular orientations around the defect given in Figure 4. As discussed above, in the PS setup we cannot discriminate between the defect structure shown in this figure and a Structure in which each molecule is rotated by n in the plane of the monolayer. Moreover, we have also observed defect structures in equal amounts which are the mirror image of the present structure. We find that the molecules form a bend orientation immediately around the central singularity, which gradually changes to a splay orientation at the boundary line with the continuousbilayer. Similarly, the monolayer around the bilayer domains nucleated during compression (Figure 2b) can also assume a splay orientation. Finally, the defects present in the monolayer before
nucleation of the collapsed domains can have a splay or bend orientation at the singular point (Figure 2a). Although the bend defect in this figure is similar to the defect in the foam holes, the splay defect appears to have a more complicated nature which we cannot resolve in the PS setup, involving for example a different tilt angle 4 than in the surrounding film. We conclude that the bilayer film on top of the monolayer can induce a splay orientation at its edge, while disclination defects in the monolayer have a bend orientation at their centers if they are formed during foam breakup. During reverse collapse, disclination defects with a mixed splay-bend nature as shown in Figure 4 are centered in the holes of the two-dimensional foam. Therefore, the structure and location of these defects must necessarily be linked to the bilayer breakup mechanism. The defects may have been present before foam formation is initiated. However, we do not consider this likely, because the monolayer is found to have an excess of defects after complete breakup of the foam. These defects rapidly recombine in pairs on expansion to return the monolayer to its initial state, in which the number of disclinations is much lower. Nucleation of a hole in the interdigitated bilayer, which is highly coherent due to strong attractive dipole-dipole forces, must be coupled to the formation of a defect in the monolayer below. Because not all foam holes contain a centered defect, other reverse collapse mechanisms also occur. In conclusion,we have observed that disclination defects exist not only in thick liquid crystalline films but also in monolayers at the air-water interface. The defects can have a bend or splay nature of which the structure may be determined by modeling the monolayer as a smectic film and determining the reflectance as a h c t i o n of the director orientation in the surface plane. In interfacial films of 9CB and lOCB, the molecules can align in a splay orientation along the one-dimensional boundary line between interdigitated bilayer domains and the continuous monolayer. This is the first direct observation of molecular alignment in monolayers on the water surface along surface features out of the monolayer plane. In thin films of other compounds such as fatty acids and phospholipids, similar alignment phenomena may occur on a smaller scale. Therefore, the results presented here are significant for a wide variety of systems including self-assembled films and biological membranes.
Acknowledgment. We acknowledgefinancial support from NASA Lewis Research Center (Grant No. NCC3-266). LA9500540
