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Langmuir 1996, 12, 4892-4896
Chiral Discrimination in Monolayers of Monoglycerides U. Gehlert,* D. Vollhardt, G. Brezesinski, and H. Mo¨hwald Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chaussee 5, D-12489 Berlin, Germany Received April 19, 1996X The influence of chirality on the structure of high- and low-temperature phases of monoglyceride monolayers has been studied by Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GID). While there is no effect of the chiral center of monopalmitoylglycerol on the monolayer structure at 20 °C, an influence of chirality has been found for monostearoylglycerol at 5 °C. At low temperature the latter compound has an oblique lattice of the molecule chains at all investigated surface pressures, while the racemic mixture exhibits a rectangular structure. Additionally, it can clearly be visualized by BAM that the racemic mixture exhibits a phase transition between 6 and 12 mN/m at 5 °C characterized by a change of the tilt azimuth. By using GID, this transition could be identified as a change of the azimuthal tilt angle from a nearest neighbor (NN) direction to a next nearest neighbor direction (NNN).
Introduction A strong motivation for studying chiral discrimination within monolayers is to obtain information about chiralitydependent interactions between molecules in a twodimensional layer at the air/water interface.1-8 The first observation of chiral discrimination in monolayers was made by McConnell using fluorescence microscopy.9,10 The monolayer has the advantage of being well defined, providing a large variability in density and, because of that, a variety of two-dimensional monolayer phases. The domain and lattice structures of monoglyceride monolayers have been studied by Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GID) and were described in a previous work.11 In the present paper, the attention is focused on the influence of chirality on the domain morphology and monolayer phase behavior. Monoglycerides have a glycerol backbone linked by an ester group to an aliphatic hydrocarbon chain. The aliphatic chain situated at one end of the glycerol backbone gives rise to an optically active carbon atom at the C-2 position. It has been shown that the phase behavior and the domain structure can mainly be characterized by the linkage group between the aliphatic chain and the glycerol head group.12 An exciting feature of monoglyceride monolayers is the substructure of seven segments within the condensed phase domains observed by Brewster angle microscopy X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131. (2) Rose, P. C.; Harvey, N. G.; Arnett, E. M. Adv. Phys. Org. Chem. 1993, 28, 45. (3) Andelmann, A. J. J. Am. Chem. Soc. 1989, 111, 6536. (4) Rietz, R.; Brezesinski, G.; Mo¨hwald, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 9, 1394. (5) Gehlert, U.; Weidemann, G.; Vollhardt, D.; Bringezu, F.; Struth, B.; Scalas, E.; Brezesinski, G.; Mo¨hwald, H. Ann. Rep. HASYLAB DESY 1994, 421. (6) Vollhardt, D.; Emrich, G.; Melzer, V.; Weidemann, G.; Gehlert, U. Proceedings Moriond 94 Workshop “Short and Long Chains at Interfaces”; Frontı´eres: France, 1995; Vol. M85, p 149. (7) Gericke, A.; Hu¨hnerfuss, H. Langmuir 1994, 10, 3782. (8) Eckhardt, C. J.; Peachey, N. M.; Swanson, D. R.; Takacs, J. M.; Khan, M. A.; Gong, X.; Kim, J.-H.; Wang, J.; Uphaus, R. A. Nature 1993, 362, 614. (9) Weis, R. M.; McConnell, H. M. Nature 1984, 310, 47. (10) McConnell, H. M. Annu. Rev. Phys. Chem. 1991, 42, 171. (11) Brezesinski, G.; Scalas, E.; Struth, B.; Bringezu, F.; Mo¨hwald, H.; Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8758-8762. (12) Gehlert, U.; Vollhardt, D. Prog. Colloid Polym. Sci. 1994, 97, 302.
S0743-7463(96)00382-4 CCC: $12.00
(BAM).13-16 This well defined long-range molecular orientational order offers the possibility to visualize not only changes of the tilt angle but also a transition of the tilt azimuth within tilted phases. Experimental Section The monoglycerides (Sigma, purity approximately 99 mol %) were dissolved in heptane/ethanol (9:1) (Merck p.a. grade). The subphase water was purified by a Milli-Q system and had a resistivity of 18.2 MΩ cm. A Brewster angle microscope from NFT, Go¨ttingen, mounted on a Langmuir film balance from LAUDA was used to observe the microscopic structures of the monolayer. The microscope is sensitive to changes of the refractive index resulting from differences in thickness and density and to optical anisotropy caused by differences in the molecular orientation of the monolayer in the micrometer range. The Brewster angle microscope has been described in detail previously.17,18 The reflected light passes through a lens to a CCD (charge coupled device) camera, and the resulting video signal is fed to a video system. Optical anisotropy of the monolayer is detected by an analyzer in the reflected beam path. The grazing incidence X-ray diffraction (GID)19,20 is sensitive to the periodicity of the chain arrangement in the angstrom range. Experiments were performed using the liquid-surface diffractometer on the undulator beamline BW 1 at HASYLAB, DESY, Hamburg, Germany. A monochromatic synchrotron beam enters the surface at grazing incidence. The diffracted radiation was detected by a linear position-sensitive detector as a function of the vertical scattering angle Rf. For Ri , Rf (Ri ) angle of incidence) the in-plane component of the scattering vector is given by
Qxy = (4π/λ) sin(2θhor/2)
(1)
and the out-of-plane component by
Qz = (2π/λ) sin(Rf)
(2)
hk From the in-plane component Qxy of the scattering vector at
(13) Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Colloid Interface Sci. 1995, 174, 392. (14) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf., A 1993, 76, 187. (15) Weidemann, G.; Gehlert, U.; Vollhardt, D. Langmuir 1995, 11, 864. (16) Gehlert, U.; Siegel, S.; Vollhardt, D. Prog. Colloid Polym. Sci. 1993, 93, 247. (17) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (18) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (19) Kenn, R. M.; Bo¨hm, C.; Bibo, A. M.; Peterson, I. R.; Mo¨hwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092. (20) Als-Nielsen, J.; Mo¨hwald, H. In Handbook on Synchrotron Radiation; Ebashi, S., Koch, M., Rubenstein, E., Eds.; North-Holland: Amsterdam, Oxford, New York, Tokyo, 1994; Vol. 4, pp 1-53.
© 1996 American Chemical Society
Chiral Discrimination in Monolayers of Monoglycerides
Langmuir, Vol. 12, No. 20, 1996 4893
Figure 1. Monolayer of 3-Monopalmitoyl-sn-glycerol at 20 °C. Surface pressure (π)-area (A) isotherm and (a-c) BAM images of the monolayer at the points designated on the π-A isotherm. The bar represents 100 µm. maximum intensity one gets information on the periodic structure of the monolayer parallel to the surface:
dhk )
2π Qhk xy
(3)
Information about the tilt direction Ψ and the polar tilt angle t is available from the peak positions of the Bragg rods by the out-of-plane component Qz of the scattering vector at maximum intensity. For an oblique lattice one has a set of three equations,
Qzmax ) Qxymax cos Ψhk tan t
(4)
to determine the polar tilt angle t and the azimuthal tilt angles Ψhk, corresponding to the three appearing diffraction in-plane peaks.
Results High Temperature Phase Behavior. For 3-monopalmitoyl-sn-glycerol, the pure R-enantiomer, a surface pressure (π)-area (A) isotherm at 20 °C is shown in Figure 1. During the first-order phase transition in the plateau region of the π-A isotherm one observes two-dimensional domains (Figure 1a), circular or cardioid shaped under equilibrium conditions. The sevenfold substructure of the circular domains can be seen by positioning an analyzer in the reflected beam path. The optical anisotropy is due to regions of different molecular orientation. The polar tilt angle within a monolayer domain has a defined value, whereas the chain tilt azimuth is changed at each segment
boundary. The analysis of the BAM experiments shows that the aliphatic chains are tilted radially along the bisector of the segment boundaries.13 During the first-order phase transition the domains grow at the expense of the surrounding phase of lower density. The BAM images reveal no further phase transitions during monolayer compression (Figure 1b). The steep increase of the surface pressure in the isotherm is coupled to the deformation of the domains, resulting in a dense packing of the domains at higher surface pressure (Figure 1c). The domain substructure with straight segment boundaries remains from the beginning of the domain formation up to the point of irreversible collapse.14 If the π-A isotherm and BAM measurements of the pure enantiomer are compared to those of the racemic mixture, it can be seen that the results are completely identical. GID has been applied to 3-monopalmitoyl-sn-glycerol monolayers. Figure 2 shows contour plots of the corrected intensities as a function of the in-plane scattering vector component Qxy and the out-of-plane scattering vector component Qz at surface pressures of 6 and 15 mN/m, respectively. Two maxima occur, one for Qz ) 0 Å-1 and one for Qz > 0 Å-1, indicating a centered rectangular unit cell at both surface pressures. The chains are tilted to a nearest neighbor direction of the centered rectangular lattice with the tilt azimuth parallel to the a axis. At 15 mN/m the position of the two-fold degenerate peak moves to lower Qz, corresponding to a decrease of the tilt angle. The lattice parameters obtained from the peak positions for the pure
4894 Langmuir, Vol. 12, No. 20, 1996
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Figure 2. Contour plots of the corrected intensity for a 3-monopalmitoyl-sn-glycerol monolayer vs the in-plane and outof-plane scattering vector components Qxy and Qz at 6 and 15 mN/m at 20 °C. Table 1. Lattice Dimensions a, b, and γ, Unit Cell Area per Molecule Axy, Tilt Angle with Respect to the Normal, t, Chain Cross Section A0, and Tilt Azimuth, TA (NN, Nearest Neighbor Direction; NNN, Next Nearest Neighbor Direction, i, Intermediate Direction), for Different Surface Pressures at 20 °C π (mN/m)
a (Å)
b (Å)
γ (deg) Axy (Å2) t (deg) A0 (Å2)
TA
6 15
(a) 3-Monopalmitoyl-sn-glycerol 5.694 8.566 90 24.4 34.9 5.475 8.543 90 23.4 31.2
20.0 20.0
NN NN
6 15 30 44
(b) 1-Monopalmitoyl-rac-glycerol 5.678 8.578 90 24.4 34.7 5.458 8.543 90 23.3 31.0 5.224 8.474 90 22.1 26.2 5.072 8.423 90 21.4 21.9
20.0 20.0 19.9 19.8
NN NN NN NN
enantiomer and the racemic monolayer (Table 1) are the same within experimental error.11 The chain packing of the racemic mixture has been discussed in detail in a previous work, and the results can also be applied to the pure enantiomer. In agreement with BAM, X-ray diffraction shows no difference between enantiomer and racemate, indicating that the chirality has no influence on the monolayer structure.11,21 Low-Temperature Phase Behavior. When the temperature is decreased, the monolayer becomes more condensed and the plateau region of the π-A isotherms shifts to lower surface pressures. The same effect can be achieved by increasing the chain length of the amphiphilic molecule. For 3-monopalmitoyl-sn-glycerol the same isotherms as for 3-monostearoyl-sn-glycerol can be observed at approximately 12 °C lower temperature.13,14 The π-A isotherms of 3-monostearoyl-sn-glycerol are fully condensed at temperatures below 30 °C. When the monolayers of 3-monostearoyl-sn-glycerol are investigated at 5 °C, an influence of the chiral center at the second carbon atom of the glycerol backbone on the low-temperature phase behavior becomes apparent. π-A isotherms have been measured after spreading at 35 °C, compressing the monolayer into the two-phase coexistence region (0.35 nm2/molecule), and cooling down (21) Weidemann, G.; Gehlert, U.; Vollhardt, D.; Struth, B.; Scalas, E.; Brezesinski, G.; Mo¨hwald, H. Ann. Rep. HASYLAB DESY 1994, 419.
Figure 3. Surface pressure (π)-area (A) isotherms of (a) 1-monostearoyl-rac-glycerol and (b) 3-monostearoyl-sn-glycerol at 5 °C.
to 5 °C. In the case of the racemic monolayer the isotherms indicate a phase transition by a small kink at 10 mN/m (Figure 3a, arrow). On the other hand, the pure enantiomer shows a continuous increase of the surface pressure on compression (Figure 3b). The morphology of the monolayers prepared as described above has been observed by BAM upon compression at 5 °C. The sevenfold substructure of the condensed phase domains remains preserved as the temperature is decreased. In the case of the enantiomer, the domains at all compressed states are again characterized by segments of different brightness with sharp straight boundaries, corresponding to the morphology of the high-temperature phase behavior. In the case of the racemic mixture, it is weakly pronounced by the isotherms but clearly indicated by BAM that the molecules of the racemic monolayer change orientation. The images in Figure 4 visualize the same domain of the racemic monolayer at 6 and 12 mN/m. While straight segment boundaries can be observed at 6 mN/m, the lines are irregular at 12 mN/m. It can be seen that areas within domains change brightness. For example, the darkest segment gets larger at the expense of the adjoining gray segments. The racemic mixture exhibits a phase transition characterized by a reorientation of the molecules. For the pure enantiomer there is no indication of a change of orientation during compression, neither from BAM observations nor in the π-A isotherm. These results have been supported by the investigation of the low-temperature lattice structure by GID. The contour plots of the corrected intensities for the pure enantiomer and the racemic mixture as functions of the in-plane and out-of-plane scattering vector components
Chiral Discrimination in Monolayers of Monoglycerides
Figure 4. BAM image of the 1-monostearoyl-rac-glycerol monolayer at 5 °C: (a) π ) 6 mN/m; (b) π ) 12 mN/m.
Qxy and Qz at different surface pressures are given in Figure 5. The enantiomer (Figure 5a) exhibits three inplane diffraction peaks, indicating a phase with an oblique lattice of the molecule chains at the pressures investigated. On the other hand, in the racemic mixture one observes two maxima (Figure 5b). This means that the lattice structure is rectangular. Additionally between 6 and 12 mN/m there is a transition from a phase where the molecules are tilted toward nearest neighbors (NN) to a phase with tilt toward next nearest neighbors (NNN). This is deduced from the observation that two peaks appear for the low-pressure phase: one with a maximum for Qz ) 0 Å-1 and the other one with a maximum for Qz > 0 Å-1, as expected for NN. For the high-pressure phase, both peaks have a maximum for Qz > 0 Å-1, indicating NNN. The values of the lattice parameters for the enantiomer are given in Table 2a, and those for the racemic mixture, in Table 2b. Conclusions These results suggest that the temperature influences the chiral discrimination within monolayers of monoglycerides. Whereas there is no indication of a chiral discrimination for the monolayer of monopalmitoylglycerol at 20 °C, an influence of chirality was detected at 5 °C for the monolayer of monostearoylglycerol. Additionally it can clearly be observed by BAM that the racemic mixture exhibits a phase transition connected with a change of the tilt azimuth between 6 and 12 mN/m at 5 °C. This phase transition is identified as a transition from a phase with NN tilt direction to a phase with NNN tilt direction by GID. The pure enantiomer has an oblique structure at all pressures investigated.
Langmuir, Vol. 12, No. 20, 1996 4895
Figure 5. Contour plots of the corrected intensity vs the inplane and out-of-plane scattering vector components Qxy and Qz at different surface pressures at 5 °C for (a) 3-monostearoylsn-glycerol and (b) 1-monostearoyl-rac-glycerol. Table 2. Symbols are as in Table 1 at 5 °C π (mN/m) a (Å)
b (Å) γ (deg) Axy (Å2) t (deg) A0 (Å2)
TA
1 6 12 25
5.156 5.107 5.058 5.039
(a) 3-Monostearoyl-sn-glycerol 5.222 114.0 24.6 38.2 5.187 115.2 24.0 35.6 5.161 116.8 23.3 33.8 5.071 118.8 22.4 30.6
19.3 19.4 19.3 19.3
i i i i
1 6 12 25 40
(b) 1-Monostearoyl-rac-glycerol 5.661 8.702 90 24.6 38.4 5.533 8.673 90 24.0 36.1 5.042 9.261 90 23.3 34.3 5.039 8.925 90 22.5 30.9 5.030 8.625 90 21.7 27.4
19.3 19.4 19.3 19.3 19.3
NN NN NNN NNN NNN
We were able to investigate two phenomena simultaneously: (i) the temperature-dependent influence of chirality and (ii) a phase transition based on a change of the tilt azimuth within the tilted phases. Related to the phase diagram for fatty acid monolayers,22,23 the phase transition at 5 °C can be described as a transition within tilted mesophases from L2 to L2′. For further investigations it will now be possible to determine a phase transition line, as well as a phase diagram, only by BAM investigations. This is important because GID experiments are very time consuming and the phase diagram of the monolayer cannot be investigated with X-rays as completely as in isotherm or microscopic studies. The effect of temperature on the influence of chirality can be explained as follows: At room temperature the (22) Kaganer, V. M.; Peterson, I. R.; Kenn, R. M.; Shih, M. C.; Durbin, M.; Dutta, P. J. Chem. Phys. 1995, 102, 9412. (23) Rivie`re, S.; He´non, S.; Meunier, J.; Schwartz, D. K.; Tsao, M.W.; Knobler, C.M. J. Chem. Phys. 1994, 101, 10045.
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lattice of the pure enantiomer has a centered rectangular structure. This lattice symmetry seems to be higher than the enantiomer molecular structure allows. It can be assumed that the thermal motion of the head groups prevents chiral interactions. This is confirmed by the chain cross section obtained by GID, which is near the value of 0.2 nm2/molecule, characteristic for a free rotator phase of alkanes.24 When the temperature is decreased, the thermal motion of the head groups is reduced. The chain cross section at 5 °C has a value of 0.193 nm2/ molecule. For fatty acids this cross section corresponds to that of the hindered rotator phases L2, L2′, and S.19,22 It can be concluded that the head groups of the molecules have a more dense packing, causing a more specific interaction than at higher temperatures. As the chiral (24) Kenn, R. M. Ph.D. Thesis, University of Mainz, 1994.
Gehlert et al.
carbon atom is situated in the head group, chiral interactions can occur, explaining the influence of chirality on the low-temperature phase behavior. Acknowledgment. The authors gratefully acknowledge B. Struth (University of Mainz), F. Bringezu, R. Rietz (University of Halle), and G. Weidemann (MPI fu¨r Kolloidund Grenzfla¨chenforschung) for contributions to the X-ray experiments. We also thank K. Kjaer for help with setting up the X-ray experiments. We are indebted to support by Prof. Findenegg (Technische Universita¨t, Berlin), the Deutsche Forschungsgemeinschaft, the Bundesministerium fu¨r Forschung und Technolgie, and the Fond der Chemischen Industrie. LA960382F
