J . Phys. Chem. 1993,97, 6955-6957
6955
Splay Stripe Textures in Langmuir Monolayers Jaime Ruiz-Garcia,' Xia Qiu,* Mei-Wei Tsao, Gary Marshall, and Charles M. Knobler' Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024
Gernot A. Overbeck and Dietmar M6bius Max- Planck- Institut f i r biophysikalische Chemie, Postfach 2841, 0-3400 Gattingen, Germany Received: May 10, 1993 Stripe textures in the liquid condensed phaseof Langmuir monolayers have been observed by polarized fluorescence microscopy and Brewster angle microscopy. The textures, which were found in myristic and pentadecanoic acids, are associated with regular variations in the molecular tilt azimuth. The stripes are similar to those observed in hexatic phases of chiral smectic liquid crystals. They are present in monolayers of achiral molecules because the air/water interface breaks the head-tail symmetry.
A complex phase diagram for simple amphiphiles adsorbed at the air/water interface was inferred from early measurements of surface pressurs-area isotherms of insolublelong-chain fatty acids and Recent diffraction studies of such Langmuir monolayers3s4 have confirmed the existence of a number of condensed monolayer phases and revealed their structures. It is now apparent that these phases bear a close resemblanceto known smectic liquid crystalline phases.5 This correspondence is supported by the observation6 in monolayers of certain point defects (star defects) that had been discoveredin freely suspended films of thermotropic liquid crystals.' Star defects in films and monolayers are associated with hexatic phases that contain rodlike molecules tilted with respect to z, the normal to the plane of the film or surface. The projection of the molecular axis on the x,y plane is c, whose components are cos 4, sin 4, where 4 is the azimuthal angle. A star defect is composed of a central vortex surrounded by pie-shaped regions in which c is constant or slowly varying. The regions are separated by arms at which there is a 2 ~ / jump 6 in 4. Such defects are evident in the texture of the liquid crystal, Le., in the variation of the reflectivity of polarized light. They are seen in fluorescence microscopy of monolayers when the excitation is polarized. A stripe texture associated with variations in the tilt azimuth has also been observed in freely suspended films of a chiral liquid crystal.8 The stripes are a periodic variation of the director c; 4 varies slowly within each stripe and changes discontinuously between stripes. Theoretical analyses8v9have attributed the stripe texture to the presence in the free energy of terms such as V X c and V-c that are allowed in chiral films. SelingerlO has recently pointed out that the lack of head-tail symmetry in monolayers allows stripe textures in achiral systems. His argument has been employed by Maclennan and Seul," who attribute the presence of stripes in freely suspended films of an achiral liquid crystal to a surface phase, which has the symmetry of a monolayer. Qiu et a1.12 reported the observation of stripe textures in Langmuir monolayersof pentadecanoic acid (PDA) by polarized fluorescence microscopy. The monolayer was made visible by the addition of a probe; its influence on the defects was assumed to be unimportant, but this could not be proved. We demonstate here that the texture is not induced by the probe, and we examine indetail thenatureof thedefects and their relation to thestructure of the monolayer phases. To whom correspondence should be addressed. t Present address: Instituto de Fisica "Manuel Sandoval Vallarta",
Universidad Autonoma de San Luis Potosi, San Luis Potosi, S.L.P., 78000 Mexico. t Present address: Department of Physics, California State University, Long Beach, CA 90840.
Figure l a is an image of the stripe texture in a monolayer of PDA obtained with polarized fluorescencemicroscopy. The probe was NBD-hexadecylamine at a concentration of 1%; details of the experimental technique are given in ref 6. Stripes related to modulations in the monolayer density have been observed in monolayers13 by fluorescence microscopy with unpolarized excitation, in which case the contrast arises from variations in the probe concentration. In the experiments described here, the fluorescence is uniform when the exciting light is unpolarized, and the texture therefore reflects the orientation of the probe. The intensity of the fluorescence is proportional to f cos A4 gcoszA4, where A4 is the difference in azimuthal angle between the transition moment of the probe and the plane of incidence of the laser beam and f and g are functions of the angles at which the probe and the exciting light are tilted with respect to surface normal. An image of myristic acid (MYA) obtained under similar conditions at the Max Planck Institute by Brewster angle microscopy (BAM)*4J5 is shown in Figure lb. No probe is required for this method; the contrast is produced by the optical anisotropy of the monolayer. It is therefore evident that the stripe texture is intrinsic and is not induced by presence of a probe. The results obtained in G6ttingen by BAM on PDA and MYA agree fully with those obtained for these substances at UCLA by fluorescence microscopy. The stripe textures develop spontaneously in PDA and MYA monolayers at low temperatures. Most of the measurements have been initiated below the gas-liquid expanded-liquid condensed triple points (17 OC in PDA and 5 OC in MYA16) at densities of the order of 25 A2 molecule-', but the textures have also been observed at temperatures as high as 12 OC in MYA. When the monolayer is first prepared, significant amounts of the LE phase are present even under conditions for which it should be metastable. The stripes tend to be aligned perpendicular to LE-LC interfaces (Figure 2a) and grow primarily by diffusion; coalescences of LC domains are relatively rare. Some stripes are connected to spiral defects, Figure 2b. Such structures are more common in MYA than in PDA. In a study of MYA in which 56 spirals were observed, 29 were right-handed and 27 left-handed. The spirals had from 1 to 10 arms. Onethird had six arms, and half of the total was divided roughly equally between 4-, 5-, and 7-arm spirals. The widths of the stripes vary from 20 to 80 fim, probably as a result of the different sizes of LE domains from which they originate. There are, however, large regions in which the widths are highly regular, differing by only a few percent. With time, stripes reach several centimeters in length; their orientation is
0022-3654/93/2097-6955$04.00/00 1993 American Chemical Society
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6956 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
Letters
Figure 1. Images of fatty acid monolayers. (a, top) Polarized fluorescence image of PDA at 8 "C obtained with 1% NBD hexadecylamine as probe. The figure is a montage of 12 strips, each of which is the width of the laser beam. (b, bottom) BAM image of MYA at 6.3 "C.
on compressionand expansion in response to the changes in width. Right-hand spirals and left-hand spirals rotate in opposite directions. The stripe pattern becomes more difficult to observe as the monolayer is compressed. This loss of contrast in both the fluorescence and BAM images presumably arises because the molecules become more nearly vertical. The stripe pattern reappears when the pressure is reduced. The tilted LC phases of fatty acid monolayers have been identified with the smectic I, F, and L liquid crystal phase^.^ Selingerlo has proposed a model Hamiltonian to describe the spatial variations in the tilt and bond directions in these hexatic monolayer phases: Figure 2. Fluorescence images showing preferential orientation of PDA stripes with respect to an LE interface and a spiral defect in MYA. The bars represent 100 pm.
not determined by the trough geometry. When the monolayer is compressedisothermally,the stripe width increases; the process is reversible. Changes in width are accommodated by a loss or gain of stripes. A stripe near the edge of a large domain may be lost by a continuous narrowing, or it may be pinched off by a point defect that moves laterally with the compression. Stripes may be generated in a similar fashion when the pressure is reduced and the stripe width decreases. Spirals connected to stripes rotate
A, cos[6(~-6)]V.c- A, sin[6(4-8)]V X c)
In this expression, V(4-8)is the tilt-bond interaction potential, 8 is the bond direction angle, and K6 and K1 are the elasticconstants for variations in the bond and tilt directions. In the F and the I phases, 4-6 is locked at 0 and 30°, respectively; it has an intermediate value in the L phase, which is chiral. The AS term favors splay and the AB term favors bend. It is present only in the L phase, where the chiral order parameter sin 6(4-8) is nonzero. Selinger has shown that, for appropriate values of the constants, this Hamiltonian leads to a variety of stripe textures.
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 6957
Letters
k t t. K
4
\
t
A
Figure 3. Schematic drawing of (a, top) splay and (b, bottom) bend stripes and the fluorescence patterns that they would produce. The orientation of the laser beam is indicated by the heavy arrows. The intensity patterns show the variations that would be observed with the laser excitation parallel and perpendicular to the stripes.
The fluorescence intensity profile within a stripe can be determined by image analysis. Plots of the intensity relative to its maximum value within the stripe, III,,,, against the fractional distance across the stripe are superimposable. Thus, the stripe texture corresponds to a regular variation in the tilt azimuth of the NBD fluorophore. We cannot determine from the fluorescence experiments the precise variation of the director that constitutes the stripe texture orientation, but some information can be deduced from the stripe pattern and how it changes with the orientation of the laser beam (Figure 3). If the beam is normal to the stripe direction, splay in the tilt azimuth will produce a change from light to dark (or dark to light) across the stripe as Atp increases (or decreases). If the beam is parallel to the stripe, splay stripes will appear bright (or dark). The intensity will vary symmetrically about their centers, but the modulation will be much smaller than that found in the normal orientation, because the changes in cos Atp are smaller. For bend stripes, the intensity will be maximum (or minimum) at the center of the stripe when the beam is normal to the stripe direction. There will be little difference between the intensities observed with the beam in the two parallel directions. The observed intensity variations are consistent only with splay. While the relation between the tilt azimuth of the probe and that of the fatty acid is not known, it is reasonable to assume that they are nearly parallel. The regular 6-fold patterns of fluorescencein the star defects of esters with chain lengthscomparable to MYA and PDA show that the tilt azimuth of the probe has a fixed relation to that of ester tails. From the symmetry of the fluorescence with respect to the beam direction, the stars could either be bend defects in which the probe tilt is perpendicular to that of the ester or splay defects in which the tilt of the probe and
ester are parallel. The splay organization of the probe that we have found in PDA and MYA cannot correspond to bend stripes with the probe tilt perpendicular to that of the chains. If this were the case, the intensity variation across a stripe would be symmetric about its center when the excitation was perpendicular to the stripe direction. Thus, we concludethat the stripes represent a periodic variation in splay, a result consistent with an analysis of the BAM images." The preference for splay rather than bend is supported by estimatesof the ratio of the splay to bend constants. If the dynamic properties of an hexatic phase are modeled as those of a heavily dislocated solid,18the ratio of the splay and bend elastic constants, Ks/KB, can be related to t1 and 511, the positional correlation lengths perpendicular and parallel to c: = (tL/[11)2. Diffraction studies of monolayers of simple amphiphiles show that 411 > tl. For example, Kenn et al.3 have obtained the positional correlation lengths for the L2 and L2' phases of docosanoicacid as a function of surface pressure and temperature. These phases are the equivalent of the smectic I and F phases, respectively. For the L2 phase, Ks/KB = 0.1 and for L2' the ratio is about 0.4. Thus, splay is energetically more favorable than bend. We do not know with certainty which of the tilted phases of PDA and MYA is being examined in our experiments, but the presence of the spiral defects suggests that it is chiral. Bend stripes in a chiral phase such as smectic L may exhibit an alternation in chirality.10 Maclennan and Seul' believe that they have found such a chiral decoration of stripes in a film of a nonchiral hexatic. If such decorated bend stripes existed in the monolayers, they would be detectable as a light-dark alternation in fluorescence;this is not observed. No variation is expected in splay stripes of alternating chirality because the chiral order parameter does not couple to splay. It is evident, however, that alternating chirality is not present in some of the patterns we observe because adjacent stripes are connected to the arms of a single spiral. Acknowledgment. This work was supported by the National Science Foundation, the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Bundesministerium fur Forschung und Technologie ( 0 3 M48O5). References and Notes (1) Stenhagen, E. In Determination of Organic Structures by Physical Methods; Braude, E. A., Nachod, F. C., Eds.; Academic Press: New York, 1955; p 325. (2) Lundquist, M. Chem. Scr. 1971, 17, 5 . (3) Kenn, R. M.; Bohm, C.; Bibo, A. M.; Peterson, I. R.; Mohwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092. (4) Knobler, C. M.; Desai, R. C . Annu. Rev. Phys. Chem. 1992,43,207. ( 5 ) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591.
(6) Qiu, X.;Ruiz-Garica, J.; Stine, K. J.; Knobler, C. M.; Selinger, J. V. Phys. Rev. Lett. 1991, 67, 703. (7) Dierker, S. B.; Pindak, R.; Meyer, R. B. Phys. Rev. Lett. 1986, 56, 1819. ( 8 ) Langer, S. A.; Sethna, J. P. Phys. Rev. A 1986, 34, 5035. (9) Hinshaw, Jr., G. A.; Petschek, R. G. Phys. Rev. A 1989,39,5914. (10) Selinger, J. V. Mater. Res. Soc. Symp. Ser. 1992, 248, 29. (11) Maclennan, J.; Seul, M. Phys. Rev. Lett. 1992, 69, 2082. (12) Qiu, X.; Ruiz-Garcia, J.; Knobler, C. M. Mater. Res. SOC.Symp. Ser. 1992, 237, 263. (13) Seul, M.; Sammon, M. J. Phys. Rev. Lett. 1991, 64, 1903. (14) Honig, D.; Overbeck, G. A.; Mobius, D. Adv. Mater. 1992.6.419. (15) Hhon, S.; Meunier, J. Rev. Sei. Znstrum. 1991, 62, 936. (16) Harkins, W.D.; Boyd, E. J. Phys. Chem. 1941, 45,20. (17) Honig, D.; Overbeck, G. A.; Mobius, D. To be published. (18) Ostlund, S.; Toner, J.; Zipelius, A. Ann. Phys. 1982, 144, 345.
