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Long Range Tilt Orientational Order in Fatty Acid Ethyl Ester Monolayers G. Weidemann and D. Vollhardt* Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chaussee 5, D-12489 Berlin, Germany Received September 11, 1995. In Final Form: June 17, 1996X The condensed phase of fatty acid ethyl ester monolayers exhibits long range orientational order. The aliphatic chains are only weakly tilted in these monolayers. This has an effect on the textures of the monolayers. The domains of the condensed phase formed during a transition from the fluid phase to a condensed phase with tilted chains do not have a defined substructure. Different types of domain substructures can be observed. A few domains have a segment structure with chains tilted perpendicular to the bisector of the segment. Other domains of the same monolayer have small parts of a deviating orientation embedded in the major part of the domain having an uniform orientation. Using domains with this texture, the anisotropy of the line tension is qualitatively estimated. Jumps of the molecular orientation occur along various dense lattice directions, which corresponds to the observation of up to four main growth directions in the dendritic growth regime of ethyl stearate monolayers. However the large spread of textures is not only introduced by the dendritic growth. This becomes evident from the study of the ethyl palmitate monolayers, where no dendritic growth regime exists. Also in these monolayers various structures occur; some of them are identical to those of the ethyl stearate monolayers.
Introduction New microscopy methods give insight into the tilt orientational order of amphiphilic monolayers at the airwater interface. Orientational order in amphiphilic monolayers was first observed by fluorescence microscopy with polarized laser excitation from the side.1-6 However, this method is restricted to the observation of fluorescence anisotropy of dye probes which are soluble in the condensed phases of amphiphilic monolayers. Therefore the method was applied only to a few amphiphilic monolayers. Recently, Brewster angle microscopy allowed the direct observation of the optical anisotropy of the tilted aliphatic chains of the amphiphiles.7-15 Recent studies showed that the domains of the condensed phases formed during transitions from a fluid to a condensed phase exhibit well defined textures in various monolayers.1-15 The domains observed in fatty acid8 and fatty acid methyl ester3-6,9 monolayers have been described X
Abstract published in Advance ACS Abstracts, August 15, 1996.
(1) Moy, V. T.; Keller, D. J.; Gaub, H. E.; McConnell, H. M. J. Phys. Chem. 1986, 90, 3198-3202. (2) Moy, V. T.; Keller, D. J.; McConnell, H. M. J. Phys. Chem. 1988, 92, 5233-5238. (3) Qiu, X.; Ruiz-Garcia, J.; Stine, K. J.; Knobler, C. M. Phys. Rev. Lett. 1991, 67, 703-706. (4) Qiu, X.; Ruiz-Garcia, J.; Knobler, C. M. Interface Dynamics and Growth. Materials Research Society Symposium Proceedings, Boston, December 2-6, 1991; Elsevier: New York, 1992; Vol. 273, pp 263-270. (5) Qiu, X.; Ruiz Garcia, J.; Knobler, C. M. Prog. Colloid. Polym. Sci. 1992, 89, 197-201. (6) Fischer, B.; Tsao, M. W.; Ruiz-Garcia, J.; Fischer, T. M.; Schwartz, D. K.; Knobler, C. M. J. Phys. Chem. 1994, 98, 7430-7435. (7) Ho¨nig, D.; Overbeck, G. A.; Mo¨bius, D. Adv. Mater. 1992, 4, 419424. (8) Henon, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148-9154. (9) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 242, 213-219. (10) Dierker, S. B.; Pindak, R.; Meyer, R. B. Phys. Rev. Lett. 1988, 56, 1819-1822. (11) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf., A 1993, 6, 187-195. (12) Weidemann, G.; Gehlert, U.; Vollhardt, D. Langmuir 1995, 11, 864-871. (13) Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Colloid Interface Sci. 1995, 174, 392-399. (14) Brezesinski, G.; Scalas, E.; Struth, B.; Mo¨hwald, H.; Bringezu, F.; Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8758-8762. (15) Gehlert, U.; Vollhardt, D. Langmuir, in press.
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according to a similar texture in thin liquid crystal films, i.e. the star textural defect.10 The structures occurring in 1-monoglyceride monolayers have been found to be determined by the geometry of the two-dimensional crystal lattice at the air-water interface.11-15 The orientational order in these monolayers was determined by the highly tilted aliphatic chains. A comparison with the orientational order of a monolayer with only weakly tilted chains is of interest. The monolayers of fatty acid ethyl esters are such monolayers. The object of this work is the characterization of the long range orientational order in these monolayers and the study of the influence of the weak chain tilt on the orientational order. Experimental Section Imaging of the monolayers was performed with a Brewster angle microscope (BAM1 from NFT Go¨ttingen) mounted on a Langmuir film balance from Lauda (FW2). The experimental setup has been described in detail elsewhere.11 Brewster angle microscope (BAM) images are distorted due to the observation at the Brewster angle. To correct the distortion, the video images from the CCD camera were digitized and treated with imageprocessing software. The software was also used to increase the low anisotropy contrast of the monolayers. The lateral resolution of the Brewster angle microscope is about 4 µm. The BAM images presented here are in reality mirror images of the monolayer, since, in the BAM1, the reflected beam is directed by a mirror into the CCD camera. As only a small part of the image is sufficiently sharp and well illuminated, most figures in the present work are parts of the original images. The subphase water used for the experiments was Millipore filtered (pH 5.5). Heptane which is used for spreading was obtained from Merck (UVASOL). Palmitic acid ethyl ester and stearic acid ethyl ester were purchased from Sigma (approximately 99%) and used without further purification. After spreading, the monolayers were compressed at constant rates.
Results and Discussion The surface pressure (π)-area (A) isotherms of ethyl palmitate as well as of ethyl stearate exhibit a plateau region in the isotherm which indicates a phase transition from a fluid to a condensed phase. For ethyl palmitate the plateau region becomes apparent for temperatures exceeding 8 °C, and for ethyl stearate it occurs above © 1996 American Chemical Society
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Figure 1. Surface pressure (π)-area (A) isotherms of ethyl stearate and ethyl palmitate.
28 °C. The tilt angle of the molecules decreases with increasing surface pressure, thus causing a decrease of the optical anisotropy. For an evaluation of the molecular orientation a higher optical anisotropy is favorable. However, for a very low plateau pressure, at decompression, the dissolution of the condensed phase might be hindered. Therefore the ethyl palmitate monolayers were studied at 16 °C and the ethyl stearate monolayers were studied at 35 °C (Figure 1). The isotherms exhibit a second-phase transition to a state with erected chains at about 10.3 mN/m for ethyl palmitate at 16 °C and about 9.0 mN/m for ethyl stearate at 34 °C. Although this phase transition is not well pronounced in the isotherms, it is clearly indicated by the vanishing of the anisotropy contrast in the microscopy images. Only one phase with tilted chains is observed over the whole temperature range where the monolayer undergoes a phase transition from the fluid into a condensed phase. This is evident from the observation of the same textures in the monolayers within this temperature range. The circular-shaped condensed phase domains of ethyl stearate observed under conditions close to equilibrium show a wide spread of textures (Figure 2). In some domains the molecules have an uniform orientation (Figure 2a and b). Other domains show more (Figure 2c and d) or less (Figure 2e and f) irregularly arranged regions of different orientation. Some of the more regular structures occur more frequently. The most well defined structure among the regular structures is a sixfold subdivision of the domain (Figure 3), which reveals a similarity to the star textural defect in thin liquid crystal films.10 These domains are subdivided by sharp straight lines into six segments of different uniform brightness, i.e. different uniform azimuthal chain orientation. A geometric analysis allows the determination of the orientation in the different parts of the domain. The observed optical anisotropy is introduced by the tilted chains, as the erection of the chains is accompanied by a disappearance of the anisotropy contrast. Thus only areas with chains parallel to the plane of incidence reflect the incident p-polarized light without a change of polarization. These areas are, therefore, the brightest areas for parallel polarizers and the darkest for crossed polarizers. For the textures observed in the ethyl stearate monolayers these are the segments at the top and the bottom of the domain (Figure 3 c and d). The plane of incidence is horizontal in the images. Therefore the molecules are tilted perpendicular to the bisector of these segments. In the observed structure two segments separated by a boundary either could have a mirror image relation or could be rotated by about 60° (Figure 4). In the first case the molecules in two opposite segments would have the same
Figure 2. Domains of the condensed phase of ethyl stearate formed during the phase transition from a fluid to a condensed phase, showing various textures. In some domains, the whole domain has an uniform orientation of the chain azimuth. In parts a and b such domains are shown for two different positions of the analyzer. The bar represents 100 µm. Other domains exhibit a more (c and d) or less (e and f) irregular arrangement of areas of uniform orientation. Parts e and f show two of the more frequently occurring types of textures. Parts (c-f) show a smaller part of the original images. The bar given in part c represents again 100 µm.
orientation (Figure 4 a and b). This can be discounted because of the different reflectivity of such segments observed for polarizers which are neither parallel nor crossed. Thus the lattice in adjoining segments is rotated by about 60°. In this case two subtypes of textures are expected, one being the mirror image of the other (Figure 4 c and d). That means the orientation in each segment is changed by 180° with respect to that in the other subtype. However one does not observe the mirror image of the texture; the same orientation occurs in the opposite segment of the domain of the other type (the symmetry operation is an inversion). The experimental results satisfy the symmetry; the two kinds of domains are observed (Figure 3a, b and e, f). Similar textures have also been observed in various monolayers of other amphiphiles such as fatty acids,8 fatty acid methyl esters,1-6,9 and 1-monoglycerides.11-15 However, in contrast to the arrangement observed here for 1-monoglycerides,11-15 fatty acids,8 and the high-temperature phase of fatty acid methyl esters,6,9 the molecules are tilted along the bisector of the segments. Molecules tilted perpendicular to the bisector of a segment have only been observed for liquid crystals,10 but recently for domains in fatty acid methyl ester monolayers a transition to a low-temperature phase with a texture similar to that observed here has been reported.6 Ethyl stearate and
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Figure 4. For two major types of textures, the segments at the top and the bottom of the domain are the brightest for parallel polarizers and the darkest for crossed polarizers. In one, the segment boundaries are mirror lines (a and b), and in the other the lattices of the adjoining segments are rotated by 60° (c and d). The two subcases c and d are mirror images of each other, whereas the two subcases a and b can be regarded as mirror images of each other as well as to be rotated by 60° against each other.
Figure 3. The most regular texture in ethyl stearate monolayers is a subdivision of the domain into six parts of uniform orientation: (a) analyzer angle of about 60°; (b) analyzer angle of about -60°; (c) parallel polarizer and analyzer; (d) crossed polarizer and analyzer. [In half the domains of this type, the arrangement of the segments is as in parts a and b. The other domains of this type are inverted images of those of parts a and b; (e) analyzer angle of about 60°; (f) analyzer angle of about -60°. The bar represents 100 µm.
ethyl palmitate domains show, however, such texture in a high-temperature phase without herringbone order. The nature of this phase was recently determined by synchrotron X-ray diffraction16 for ethyl palmitate at 16 °C. The alkyl chains are tilted by 13° toward their next nearest neighbors at 4 mN/m. The lattice is only slightly distorted due to the weak tilt of the chains.16 Two different approaches exist to describe segment structures. The textures in fatty acid and fatty acid methyl ester monolayers have been described by a continuum elastic theory. This theory minimizes the energy for the total change of molecular orientation within a domain.17 Such a change of orientation can occur as jumps of orientation by n‚60° at defect lines and as a continuous change of orientation. The textures in ethyl stearate and ethyl palmitate monolayers do not show any continuous changes of the orientation. Therefore we only consider the defect lines to analyze the textures. The defect lines are expected to run along dense lattice rows of the twodimensional lattice.12 The knowledge of the lattice allows us to determine the lattice rows corresponding to the defect lines. In the observed sixfold texture the defect lines are along two next nearest neighbor directions. These are the [31]- and [3h 1]-directions of the rectangular lattice (16) Weidemann, G.; et al. To be published. (17) Fischer, T. M.; Bruinsma, R. F.; Knobler, C. M. Phys. Rev. E 1994, 50, 413-428.
(taking a as the short axis). The angle of 61° between these directions is in good agreement with the observation of six segments. It is well understood that a well defined texture occurs in condensed phase domains. However, only less than 5% of the observed domains show this texture and less than 1% are without any deviation from the idealized structure. Nevertheless, it is more difficult to understand the less ordered textures which occur much more frequently. To attain an understanding of these deviations, attention is now focused on the crystal lattice at the airwater interface. The pressure necessary for the erection of chains is in fatty acid ethyl ester monolayers much lower than that in other monolayers of simple single-chain compounds. This in conjunction with the low anisotropy contrast observed indicates that the aliphatic chains are only weakly tilted. A recent X-ray diffraction study16 gives a polar tilt angle of 13°. Chains of a free rotator phase which are only slightly tilted should give rise to a lattice which deviates only weakly from a hexagonal lattice. In such a lattice, jumps of the orientation are expected to occur along different dense lattice rows of the original hexagonal lattice and not only along two of them as found for the 1-monoglycerides.11-14 In the case of the 1-monoglycerides the chains are found to be highly tilted. For the rectangular lattice of the 1-monoglycerides, a coincidence of the point lattices is achieved only for the [hk]and the corresponding [h h k]- direction. Jumps of the orientation have been observed only along the dense lattice rows next to the tilt direction for the 1-monoglycerides. These are the two next nearest neighbor directions [31] and [3 h 1] (again short axis a). Evidently for this substance, jumps of the orientation seem to be favorable only if they are of the splay type. In the lattice of the fatty acid ethyl ester monolayers with weakly tilted chains, the molecular orientation should jump not only along various lattice rows but also by various angles. This is exactly what has been observed and explains the occurrence of the different types of textures. For ethyl stearate a dendritic growth regime can be observed at higher compression rates. The contour of the domains is known from earlier fluorescence microscopy studies.18 The dendrite observed there was similar to that
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Figure 5. For ethyl stearate a dendritic growth regime exists at higher rates of compression. The inner structure of the dendrites reflects also the small polar tilt angle of the molecules. More than one main growth direction occurs. (a) Four main growth directions can be clearly identified in two of the domains of uniform orientation. (b) In some domains most arms have a different orientation. (c and d) In most domains, at least two or three adjoining dendrite arms have the same azimuthal chain orientation. The bars represent 100 µm.
in Figure 5a and d. Now it was found that the orientation in such dendrites need not to be uniform (Figure 5d). A variety of different structures can be observed due to the weak chain tilt for ethyl stearate. Four main growth directions have been identified unequivocally within single domains of uniform orientation (Figure 5a). In some domains, the molecules have different orientations in the domain arms (Figure 5b). In most domains the chains have the same azimuthal orientation in two or more adjoining dendrite arms (Figure 5c and d). Evidently the small tilt angle allows no selection of one single direction as the main growth direction. A different behavior is observed for the highly tilted 1-monoglycerides. There the main growth direction has been identified as one of the nearest neighbor directionssthe azimuthal chain tilt direction.15 From the observation of more than one main growth direction, one could be induced to conclude that the varying textures in the domains are introduced by dendritic growth. This cannot be fully excluded, as dendritic growth might occur at all compression rates in the initial step of domain growth.15 As even large domains (500 µm diameter) become nearly circular within less than 1 min (completely circular within less than 5 min), the dendritic shape would not be observed, since the small domains in this initial step would become circular even faster. A study of ethyl palmitate shows that the observed structures arise not only from the dendritic growth. In ethyl palmitate no dendritic growth regime exists. Only at extremely high compression rates (0.5 nm2 molecule-1 min-1) does a domain wall instability occur. However the growth is no longer of a dendritic type but fractal-like (Figure 6a). The absence of a dendritic growth regime might be caused by a lower crystallinity of the condensed phase connected with a lower anisotropy of the line tension, (18) Knobler, C. M. Science 1990, 249, 870-874. Knobler, C. M.; Stine, K.; Moore, B. G. In Dynamics and Patterns in Complex Fluids; Onuki, A., Kawasaki, K., Eds.; Springer Proceedings in Physics Vol. 52; Springer: Berlin, Heidelberg, 1990; pp 130-140.
Figure 6. For ethyl palmitate, no dendritic growth can be observed. However, at extremely high rates of compression (0.5 nm2 molecule-1 min-1), fractal-like growth occurs (a). In these structures, the orientation of the molecules is far from equilibrium and the orientation changes within a few seconds: (b) t ) 0 s; (c) t ) 0.5 s; (d) t ) 1.0 s; (e) t ) 1.5 s; (f) t ) 2.0 s. The bar represents 100 µm.
resulting in an extremely fast transformation into a more compact shape (less than 1 min for large structures) combined with transformations inside the domains (Figure 6b-f). These fast transformations might by themselves prevent the occurrence of the dendritic growth regime, as a dendrite would be forced to become circular in a much shorter time. In ethyl palmitate domains a spread of the observed textures is still present (Figure 7) even if most domains are of one type (Figure 8) and the more irregular types of textures occur more frequently at higher rates of compression. In this most frequent structure four small parts of uniform orientation are embedded in the major part of the domain, which is of uniform orientation. Domains of uniform azimuthal chain orientation have not been observed. The sixfold structures observed in the ethyl palmitate monolayers occur more frequently than those in the ethyl stearate monolayers and are of the same type (Figure 9). The observation of different stable textures in ethyl palmitate monolayers is quite interesting with respect to the knowledge that transitions into equilibrium are possible and very fast, as in Figure 6c-f. This allows the assumption that all the observed textures are in equilibrium or close to equilibrium. Thus, the small parts of uniform orientation in the most frequent domain type are expected to reorient if they are not in equilibrium. The absence of domains with a single uniform orientation shows that these small domain parts minimize the energy of the domains. This can be understood if the line tension
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Figure 9. The sixfold domains occur in the ethyl palmitate monolayers slightly more frequently than in the ethyl stearate monolayers and are of the same type. Parts a and b show the domain for 60° and -60°. The bar represents 100 µm.
Figure 7. In ethyl palmitate monolayers, again various more (a) or less (b-d) irregular textures occur. The bar represents 100 µm.
Figure 8. Most domains in ethyl palmitate monolayers have the same structure: (a) analyzer angle of about 60°; (b) analyzer angle of about -60°; (c) parallel polarizers; (d) crossed polarizers. The bar represents 100 µm.
line energy, is of no advantage. There is a large spread in the size of these inclusions. For the different observed sizes of the inclusions, the energy of the defect lines is, obviously, nearly the same as the energy arising from the reduced line tension. On the other hand, it is not clear if these small areas of deviating orientation are formed to reduce the line energy of the domain or if they are only stabilized by the anisotropic line tension and originate from another process. The geometry of these domains is of special interest. Two of these small areas which are opposite to each other have the same orientation. This means the line tension is the same or nearly the same for a domain part rotated by 180° with respect to the boundary. The absence of opposite inclusions with orientations rotated by 180° with respect to each other indicates a different energy of the defect lines in this case. An analysis of the brightness in the domain for crossed polarizers shows that, for domains aligned as in Figure 8, the major part of the domain is arranged parallel to the plane of incidence. The small inclusions are formed at positions of the domain where the chain orientation would be (almost) perpendicular to the domain perimeter without these inclusions. Thus, the line tension would have maxima for these parts of the boundary and minima for the top and the bottom of the domain where the chains are tilted parallel to the perimeter. This is in agreement with the observed molecular orientation in the sixfold domains. There the molecules are tilted almost parallel to the perimeter for the whole domain. The line tension Λ(φ) depends on the intersection angle φ of the tangent at the point at the perimeter and the chain tilt azimuth at this point. As the line tension is a periodic function with a period of 360°, it can be expanded into a Fourier series.
Λ ) a0 +
∑ai cos(iφ) + ∑bi sin(iφ)
is anisotropic. Obviously these small inclusions are formed in order to avoid the occurrence of parts of the domain boundary with a high line tension. The coupling of the texture inside the domain and the anisotropy of the line tension were discussed by Rudnick and Bruinsma.19 In the case of ethyl esters two energies have to be compared, the energy of the domain boundary and the energy of the defect lines. The energy cost for the defect line inside the domain at which the orientation jumps seems to be of the same order as the decrease in line energy which results from the lower line tension due to the other molecular orientation related to the domain boundary. The small size of these embedded areas does however show that a further growth of these areas, which would be accompanied by an increase in defect
A rectangular lattice of chains tilted with respect to the water surface has a mirror symmetry parallel to the tilt direction. Thus all bi are zero, since the sine function is not symmetrical with respect to zero. In most cases only the first two terms of the Fourier series are kept.8,20 From our observation we conclude that Λ has minima for φ ) 0° and φ ) 180° and maxima for φ ) 90° and φ ) 270°. Therefore a0 and a2 cos(2φ) are the dominant expressions. However, the domain shape is always circular. The circular shape shows that, for a large φ-interval (especially in the irregular domains), the energy cost of an elongation of the domain boundary due to a deviation from a circle is much higher than the effect of a lower line tension caused by the deviating course of the domain boundary. An effect of the coefficient a1 on the domain structure has not been observed, but it is probably not zero.
(19) Rudnick, J.; Bruinsma, R. Phys. Rev. Lett. 1995, 74, 2491-2494.
(20) Langer, S. A.; Sethna, J. P. Phys. Rev. A 1986, 34, 5035-5046.
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have either a higher or a lower contrast than the lower three segments for a definite position of the analyzer. Such domains have indeed been observed (Figure 7b), confirming the geometric analysis of the domains. The absence of domains with small inclusions in ethyl stearate monolayers and the occurrence of domains with a uniform orientation over the whole domain are not necessarily caused by a deviating line tension. This behavior can also be caused by a higher energy of the defect lines due to the higher crystallinity of this substance, indicated by the dendritic growth. This higher crystallinity could also hinder a reorientation of domain parts, preventing the formation of these inclusions.
Figure 10. In the most frequently occurring domain type in ethyl palmitate monolayers, the orientations of the molecules in the opposite small inclusions are not rotated by 180° from each other as in part a or b. The opposite parts have the same orientation as in parts c and d.
To explain the observed domain structure, attention has to be focused on details. As the line tension has minima for chains tilted parallel to the perimeter, the chains should be tilted almost parallel to the perimeter in the small inclusions. In this case the line tension is the same for chains rotated by 180°, as concluded above. Now the question remains why the defect line energy is different if the domain parts would be rotated by 180° with respect to each other. To find the reason for that behavior, the structure of such (nonoccurring) textures has to be analyzed. In Figure 10a and b two of the four possible arrangements of this kind are shown; the other two are mirror images of these structures. One finds that the orientation jumps at some of the short lines by 120°, in the same way as in the sixfold textures which do not occur in reality (Figure 4a and b). This could mean that the energy of lines with a 60° jump is lower, and then one can explain why the textures in Figure 10a and b are not formed. In a structure where the orientation in the opposite inclusions is the same, only 60° jumps occur (Figure 10c and d). The lines at the top and the bottom of the domain are of the same type as in the sixfold structures of the ethyl ester monolayers studied here. Since the chains are tilted toward the next nearest neighbors, these lines are the next nearest neighbor directions ([31] and [3h 1]) for the lattice of the major domain part as well as for the lattice of the inclusion. The lines at the sides of the domain run along the two nearest neighbor directions ([11] or [1 h 1]). At these boundaries the jump of orientation is of the splay type comparable to that in monolayers of 1-monoglycerides, long chain fatty acids, and methyl esters. Two types of domains with inclusions must occur which are mirror images of each other (Figure 10c and d). The orientations in the different parts in one of these domains correspond either to the three upper segments of one of the sixfold domains or to the three lower segments of such a domain (compare Figure 4c and d with Figure 10c and d). In one type of the sixfold domains the three upper segments have a low contrast and the three lower segments have a high contrast (Figure 9 and Figure 3e and f), and in the other type the arrangement is the inverse. Thus domains with a higher and a lower contrast are expected to appear, as the three upper segments of a sixfold domain
Conclusions New textures have been found in long chain fatty acid ethyl ester monolayers. Most domains have quite irregular textures in ethyl stearate monolayers, whereas in ethyl palmitate monolayers the textures in most domains are of the same type. Apart from these major types, many irregular textures can be observed. In both ethyl stearate and ethyl palmitate monolayers, a minority of domains have a very well defined substructure, a sixfold subdivision of the domain. In each of the six segments the domains have a different uniform orientation. Similar textures have been observed in previous studies of fatty acids,8 fatty acid methyl esters,3-6,9 and 1-monoglycerides,11-15 but only in the low-temperature phase of fatty acid methyl ester monolayers do the chains have the same orientation in the segments. The molecules are tilted perpendicular to the bisector of each segment, comparable to the star textural defect in thin liquid crystal films.10 Evidently the energy of the textures differs from that in the previously studied monolayers. The energy of the defect lines and the anisotropy of the line tension determine the energy of the textures. The defect lines run along the next nearest neighbor directions. As the chains are tilted toward the next nearest neighbors for fatty acid ethyl esters, the orientation of the tilt azimuth with respect to the next nearest neighbor directions is different from that in 1-monoglycerid14 and fatty acid monolayers.8 Another preferred orientation of the molecules with respect to the domain boundary is revealed by an analysis of the texture occurring most frequently in the ethyl palmitate monolayers. There, small inclusions are formed to avoid a course of the domain boundary with chains tilted perpendicular to the boundary. This indicates two minima of the line tension for φ ) 0° and φ ) 180° and two maxima for φ ) 90° and φ ) 270°. Nevertheless the anisotropy of the line tension is rather small, as the domains prefer a circular shape over a large φ-interval. The analysis shows, furthermore, that defect lines at which the orientation jumps by 60° are favored. Only 60° jumps and no 120° jumps are observed in domains with a well defined substructure (i.e. the sixfold domains and the domains with inclusions). The most interesting finding was the occurrence of a variety of very irregular textures with boundaries along various dense lattice rows. The lattice is close to a hexagonal lattice, as expected for a free rotator phase with slightly tilted chains. Such a small deviation from the hexagonal lattice explains why the energy for boundaries along the different dense lattice is almost the same. Acknowledgment. Financial assistance from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. LA950755X
