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Langmuir 1996,11, 864-871
Inner Structure of Condensed Phase Domains in Monolayers at the Air-Water Interface G. Weidemann, U. Gehlert, and D. Vollhardt" Max-Planck-Institutf i r Kolloid- und Grenzflachenforschung, Rudower Chaussee 5, 12489 Berlin, Germany Received June 1, 1994. In Final Form: October 6, 1994@ The inner structure of condensed phase domains in monolayers at the air-water interface is studied by Brewster angle microscopy. Condensedphase domains of 1-monoglyceridesexhibit a 7-fold substructure. Seven segments of different molecular orientation meet at a center. The attention is focused on the orientational relations of the partial lattices separated by a sharp boundary for explaining the observed segment number. The boundary lines ofthe segments are considered to be along dense lattice rows. Thus a defined change of the molecular orientation occurs at a segment boundary. The interdependence of the lattice parameters and the number of segments is described based on a centered orthorhombic lattice. The geometrical analysis of the inner domain structure shows that the chains are tilted along the bisector of each segment and that the segment boundaries are along next nearest neighbor directions.
Introduction Ordering processes in two dimensions have been of considerable interest in recent monolayer research. Information on the structure of monolayers has become available by the development of new techniques. Longrange orientational order in monolayers was found by synchrotron X-ray in~estigation.'-~Optical anisotropy within monolayers at the air-water interface due to long range orientational order has been observed by Brewster angle microscopy (BAMI4-I5 and also with fluorescence microscopy illuminating the probe with polarized laser light from the side.16-19 BAM avoids the use of probe molecules, thus excluding the effect of trace components on the monolayer properties. Optical anisotropy induced by regions of different molecular orientations of alkyl chains within a monolayer have been the subject of recent investigation. These regions are often separated by sharp, straight bounda r i e ~ . ~ - ' ~AJstructure ~J~ with six segments was observed
* Abstract published inAduanceACSAbstracts, January 1,1995.
(1)Kjaer, K.; As-Nielsen, J.; Helm, C. A.;Tippmann-Krayer, P.; Mohwald, H. J . Phys. Chem. 1989,93,3200-3206. (2)Dutta, P. In Phase Transitions in Surface Films 2;Taub, H., et al., Eds.; New York, 1991;pp 183-200. (3)Kenn, R. M.;Bohm, C.; Bibo, A. M.; Peterson, I. R.; Mohwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991,95,2092-2097. (4)Henon, S.; Meunier, J. Rev. Sci. Instrum. 1991,62, 936-939. (5)Honig, D.; Mobius, D. J . Phys. Chem. 1991,95,4590-4592. (6)Honig, D.; Overbeck, G. A.; Mobius, D.Adu. Mater. 1991,4,419424. (7)Henon, S.; Meunier, J. Thin Solid Films 1992,210/211,121123. (8)Overbeck, G. A.;Honig, D.; Mobius, D. Langmuir 1993,9,555560. (9)Honig, D.; Mobius, D. Thin Solid Films 1992,2101211, 64-68. (10)Vollhardt, D.; Gehlert, U.; Siegel, S. CoZZoid Surf. A 1993,76, 187-195. (11)Gehlert, U.;Vollhardt, D. Prog. Colloid Polym. Sci, in press. (12)Gehlert, U.;Siegel, S.; Vollhardt, D. Prog. Colloid Polym. Sci. 1993,93,247. (13)Henon, S.;Meunier, J. J . Chem. Phys. 1993,98,9148-9154. (14)Overbeck, G. A.;Honig, D.; Wolthaus, L.; Gnade, M.; Mobius, D.Submitted to Thin Solid Films. (15)Siegel, S.; Honig, D.; Vollhardt, D.; Mobius, D. J.Phys. Chem. 1992,96,8157-8160. (16)Moy, V.T.; Keller, D. J.; Gaub, H. E.; McConnell, H. M. J.Phys. Chem. 1986,90,3198-3202. (17)Moy, V. T.; Keller, D. J.;McConnell, H. M. J.Phys. Chem. 1988, 92,5233-5238. (18)Xia Qiu; Ruiz-Garcia, J.; Stine, K. J.; Knobler, C. M. Phys. Rev. Lett. 1991,67,703-706. (19)Xia Qiu; Ruiz-Garcia, J.; Knobler, C. M. Mdter. Res. SOC.Symp. Proc. 1992,237,263-270.
0743-746319512411-0864$09.00/0
by Knobler et al.18J9in methyl esters of fatty acids. They propose that this pattern could be described analogously to the star defect in thin films of liquid crystals.z0~21 An alternative approach to explain domain substructures deals with twinning. For example a multiple twin observed in an iron particlezzexhibits a substructure quite similar to that observed in the fatty acid methyl ester monolayers. In several amphiphilic monolayers the difference in chain tilt azimuth of two regions separated by sharp straight lines seems to have a defined value. These lines are thus lattice rows which give rise to a minimum of free energy for such a boundary. In crystals, a defined change in lattice orientation can be realized by twinning. Using fluorescence microscopy, McConnell et al. regarded the three-armed structures observed in DPPCmonolayers being due to t w i ~ ~ n i n g 'and ~ J showed ~ , ~ ~ that the three subdomains in the initial state of domain growth have different tilt dire~ti0ns.l~ Studying the fusion of phospholipid domains, Florsheimer and Mohwald concluded that two domains fuse only when they are brought into contact along special lattice lines.24 They assumed a triangular lattice resulting from hydrocarbon chains which are hexagonal dense-packed and derived contact angles which should be preferentially realized. In both cases, the considerations are not very detailed. Recently, domains of 1-monoglycerides which show a well-defined arrangement of seven segments meeting at a center have been described by Vollhardt et al.IO To explain the 7-fold domain substructure of l-monoglycerides, we focus the attention on the orientational relations of the partial lattices separated by a segment boundary. Obviously, hexagonal or triangular lattices cannot be discussed any longer. The subject of the present work is to develop a model for the inner structure of condensed phase domains in monolayers in agreement with the observed 7-fold domain substructure. The organization ofthe paper is as follows: in the first part of the paper, different structures arising (20)Selinger, J. V.; Nelson, D. R. Phys. Rev. A 1989,A39, 31353147. (21)Dierker, S.B.;Pindak, R.; Meyer, R. B. Phys. Rev. Lett. 1988, 56,1819-1822. (22)Herley, P. J.; Jones, W. Nunostruct. Muter. 1993,2,553-562. (23)McConnell, H. M. Annu. Rev. Phys. Chem. 1991,42,171-195. (24)Florsheimer, M.; Mohwald, H. Thin Solid Films 1990,189,379387.
0 1995 American Chemical Society
Langmuir, Vol. 11, No. 3, 1995 865
Inner Structure of Condensed Phase Domains a
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Figure 1. Orthorhombic unit cell of the hexagonal lattice. One basis vector (E)is dong a nearest neighbor direction and the other basis vector (b) is along a next nearest neighbor direction. The axis ratio a l b is 11J3. from twinning in a centered orthorhombic lattice are discussed. A relation between the tilt angle and the segment number and angle realized is derived assuming chains which are hexagonal close-packed, vertically to their axes. Finally, experimental results on the inner structure of condensed phase domains of 1-monoglycerides visualized by BAM are presented and discussed according to the geometrical considerations.
Experimental Section In the experimental setup, a Brewster angle microscope (BAM) from NFT Gottingen was mounted on a Langmuir film balance from Lauda. Experimental details of the film balance and of the BAM are described elsewhere.6J0 For viewing and image storage, the BAM was combined with a CCD camera and a video system (video recorder, video printer, and video monitor). The images obtained are distorted in a horizontal direction as the angle of incidence is about 53". Therefore, circular structures at the airwater interface become elliptical in the BAM images. A digital image processing software was used to correct the distortion due to the observation at the Brewster angle. The spatial resolution of the method is about 4 pm. 1-Monopalmitoyl-rac-glycerol was obtained from Sigma (purity approximately 99 mol %). The monolayers were spread from a M solution of distilled heptanelethanol(9:l by volume).The subphase water was Millipore filtered. After the spreadingliquid was evaporated, the monolayer was and 3.5 x compressed continuously at rates of 2.3 x nm2 min-l per molecule. Geometrical Considerations. In the 7-fold substructure of 1-monoglcyeridedomains, the difference in chain tilt azimuth at a segment boundary seems to have a definedvalue. Obviously, the partial lattices separated by a boundary have a certain orientational relation comparable to twins. Twinning is a lattice defect occurring frequently in three-dimensional crystals.2s The different partial lattices can be transformed into each other by an additional symmetry element, such as an axis or a mirror plane. Twin planes in three-dimensional crystals, i.e. mirror planes transforming the twinned lattices into each other, are planes with a high loading density.25 The loading density is the number of molecules per unit length. Analogously in two dimensions the lines separating two partial lattices of a multiple twin are lattice rows with a high loading density. In order to obtain information from that fact the lattice has to be analyzed. Aliphatic chains are nearly close-packed,perpendicular to the molecule axes, in condensed phases. If the carbon chains can freely rotate around the axes, the lattice is exactly hexagonal. The lattice parallel to the water surface results from an elongation of this hexagonal lattice if chains are tilted with respect to the water surface. A centered orthorhombic lattice is formed for chains tilted toward a main direction of the lattice. Such directions are the two basic vectors of the orthorhombic unit cell of the hexagonal lattice (Figure 1). In the following, these two cases are discussed. Thereforethe lattice directions are described according to the notation of the orthorhombic lattice. (25)Look for instance: Kleber,W. An Introduction to Crystallography, 1st ed.; Verlag Technik Berlin, 1970; p 102. Kieber, W. Einfhhmng in die Kristallopraphie, 16th ed.; Verlag Technik Berlin, 1983, 1985; p 74.
Figure 2. Each lattice point in a hexagonal lattice has six nearest neighbors and sixnext nearest neighbors (a). The three nearest neighbor directions as well as the three next nearest neighbor directions form an equilateraltriangle. In the triangle of the nearest neighbor directions the bisectrices are next nearest neighbor directions (b) whereas in the triangle of the next nearest neighbor directions the bisectrices are along nearest neighbor directions (c). a
Figure 3. The lattice at the air-water interface is orthorhombic if the aliphatic chains are tilted toward one of the basis vectors of the orthorhombic unit cell of the hexagonal lattice. In the orthorhombic lattice the triangles of both nearest neighbor directions and next nearest neighbor directions are no longer equilateral rather isosceles. Part a shows these triangles for chains tilted toward nearest neighbors. Part b shows the triangles for chains tilted toward next nearest neighbors. In a hexagonal lattice each chain has six nearest and six next nearest neighbo_rs (Figure 2a). The three densest lattice rows [lo], [ll], and [ l l ] (nearest neighbor directions2 as well as the three next densest lattice rows [Oll, [311, and [311 (next nearest neighbor directions) form equilateral triangles. The bisectrices are next nearest neighbor directions (Figure 2b) in the triangle formed by the nearest neighbor directions,whereas the bisectrices are nearest neighbor directions (Figure 2c) in the triangle formed by the next nearest neighbor directions. In a centered orthorhombiclattice resulting from the distortion ofthe hexagonal lattice, these triangles are no longer equilateral but rather isosceles. A centered orthorhombic lattice is formed if chains are tilted along a nearest or next nearest neighbor direction (a- and b-axis in Figure 1). For these two cases, the triangles of nearest and next nearest neighbor directions are shownin Figure 3. In bothcases oflattice disto-rtion, the distance of lattice points is the same for the [ l l l - and [Ill-directions and differs from that along the 1101-direction. Analogously the [311and the [31]-directions have the same point distance, whereas that along the [Oil-direction is different. A similar relation exists for the intersection angles of these dense lattice rows. The
Weidemann et al.
866 Langmuir, Vol. 11, No. 3, 1995 Table 1. Change of the Intersection Angles of Different Nearest Neighbor Directions and Next Nearest Neighbor Directions due to the Chain Tilt tilt toward tilt toward nearest next nearest intersection angle of neighbors, deg neighbors, deg [lo]- and [Ill-directions or 60 [lo]- and [ill-directions [111-and [Ill-directions > 60
