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J. Phys. Chem. 1996, 100, 3126-3130
Structural Changes of Monolayers at the Air/Water Interface Contacted with n-Alkanes G. Brezesinski,† M. Thoma,‡ B. Struth,‡ and H. Mo1 hwald*,†,‡ Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chaussee 5, 12489 Berlin, Germany, and UniVersita¨ t Mainz, Institut fu¨ r Physikalische Chemie, Welder Weg 11, 55099 Mainz, Germany ReceiVed: July 25, 1995; In Final Form: October 26, 1995X
Monolayers of the phospholipids 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1,2-dipalmitoylphosphatidylethanolamine (DPPE) at the air/water interface in contact with hexadecane and dodecane are studied by Synchrotron X-ray diffraction. The results show the existence of ordered phases with a reduction of the tilt angle of the aliphatic tails compared to films at the air/water interface. For DPPC, where due to the large head group, a nontilted phase does not exist at all at the air/water interface, insertion of oil yields a phase with zero tilt angle. In addition a phase sequence is found where with increasing pressure the tilt azimuth changes from next-nearest neighbor to nearest neighbor orientation. For DPPE the oil can be completely squeezed out of the film. For the two different alkanes the effects differ quantitatively as regards the reduction of tilt angle or projected area. They differ qualitatively, as for dodecane, in contrast to hexadecane, only untilted phases and a hexagonal lattice are formed.
Introduction Monolayers at the oil/water interface are an interesting model system to investigate the structure of microemulsions at the molecular level.1 However, one problem with these studies is that one has to design techniques that probe the interface with beams penetrating one of the bulk phases or one has to prepare an extremely thin bulk phase. We have taken the last approach, where a monolayer at the air/water interface is in contact with a nonwetting oil. In this case an ultrathin oil film is formed, and one can measure equilibrium isotherms distinguished from those at the oil/water as well as the air/water interface.2,3 We have concentrated on studies with phospholipids because these amphiphiles are insoluble in oil as well as water, and hence amphiphile densities and phases can be varied in a defined way. Due to the thinness of the film, it can be studied by grazing incidence X-ray diffraction, and this is the first structural study on such a mixed oil/lipid film. The questions to be answered then are: To what extent are phase sequences and local symmetry affected by the presence of oil? Are the structural changes lipid and/or oil specific? To answer this we compare two phospholipids with different head groups and two alkanes as oil with different chain lengths. Experimental Section Materials. The racemic phospholipids 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1,2-dipalmitoylphosphatidylethanolamine (DPPE) (Sigma, Taufkirchen) were obtained 98% pure and spread from a chloroform/methanol (10:1) solution. nDodecane and n-hexadecane (Sigma) were 99% pure and additionally purified via a silica gel and an aluminum oxide column. Film Balance. The Teflon film balance with modified glass barriers, integrated into a fluorescence microscope, has been described previously.4 The glass barrier is moved on two partly hydrophobized rails of Duran 50 glass. The aqueous phase is in contact with the vertical, hydrophilic surface of the side rims. To prepare a film in contact with excess oil, we filled the side †
Max-Planck-Institut fu¨r Kolloid-und Grenzfla¨chenforschung. Universita¨t Mainz. X Abstract published in AdVance ACS Abstracts, January 15, 1996. ‡
0022-3654/96/20100-3126$12.00/0
Figure 1. Sectional drawing of airtight trough with oil in contact at total rim.
rims of the trough with oil after spreading the lipid/chloroform/ methanol solution (Figure 1). This way the area of macroscopic oil which covers the water and the length of the three-phase line, oil/water/air, is reproducible.3 Pressure-area isotherms were measured with a Wilhelmy filter paper. For these experiments it is crucial to seal the trough to avoid evaporation of the oil. The dimensions of the sealed, He-filled, and thermostated X-ray trough are 28 cm long × 14 cm wide. After spreading the lipid solution, adding the oil, sealing the trough, and flushing the trough with He, we waited at least 1 h before compressing the monolayer and starting the X-ray experiments. The exchange of air by He does not affect the isotherms and also not the structure measured by X-ray diffraction. Filling the trough with He was found to reduce the background in X-ray scattering. Therefore, we prefer this procedure. Still we prefer to refer to the air/water interface, because the data are expected to be largely independent of the gas if it is inert. Synchrotron X-ray Diffraction. Synchrotron grazing incidence X-ray diffraction (GID) experiments were performed at 20 °C using the liquid-surface diffractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, Germany.5 The Synchrotron beam was made monochromatic by Bragg reflection by a beryllium (002) crystal and was adjusted to strike the surface at an incident angle Ri ) 0.85Rc, where Rc is the critical angle for total external reflection. An elevator places the sample at the correct height. The diffracted radiation was detected by a linear position-sensitive detector (PSD) (OED-100-M, Braun, Garching, Germany), as a function of the vertical scattering angle Rf. The vertical full width at half-maximum (fwhm) was 0.005 Å-1. A Soller collimator in front of the PSD provided resolution of 0.009 Å-1 of Qxy. Qxy is the in-plane component (Qxy ) (4π/λ) sin(2θxy/2)) and Qz ) (2π/λ) sin(Rf) the out-of-plane component of the scattering © 1996 American Chemical Society
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J. Phys. Chem., Vol. 100, No. 8, 1996 3127
Figure 3. Contour plots of the corrected X-ray intensities as a function of the in-plane component Qxy and the out-of-plane component Qz of the scattering vector Q of DPPE monolayers in contact with dodecane (left) and hexadecane (right) at different lateral pressures (indicated).
Figure 2. Surface pressure π as a function of molecular area per lipid for a monolayer of DPPC (above) and DPPE (below) on a pure water surface and at the air/water interface in contact with hexadecane (C16) and dodecane (C12). The points A-J indicate the surface pressures at which diffraction data have been taken.
vector Q, where λ ) 1.338 Å is the X-ray wavelength and 2θxy is the horizontal scattering angle. The accumulated positionresolved counts were corrected for polarization, effective area, and Lorentz factor. The intensities were least-squares fitted to model peaks which were arbitrarily taken as the product of a Lorentzian parallel to the water surface with a Gaussian normal to it.6 From the in-plane diffraction data, it is possible to obtain the lattice spacings and from these the lattice parameters. The scattered intensity recorded in channels along the PSD, but integrated over Qxy intervals, produces Bragg rod profiles. The tilt angle t of the molecular long axis with respect to the normal and the lateral tilt direction were calculated from these intensity profiles.7,8 The positional correlation length ξ was calculated from the fwhm (corrected for resolution effects) of each Qxy scan peak assuming exponential decay of positional correlations with increasing separation within the layer, typical for liquid crystals: ξ ) 2/fwhm(Qxy).9,10 The fwhm of the Bragg rod yields an estimation of the length of the coherent scatter through the Scherrer11 formula L ≈ (0.9)2π/fwhm. Results Figure 2 compares pressure-area isotherms of DPPC and DPPE in contact with different oils with those at the pure water surface at 20 °C. One clearly observes the transition from a liquid expanded (LE) to a condensed phase for both lipids in contact with dodecane, whereas the contact with hexadecane prevents the existence of a LE phase at this temperature. It was previously shown that the LE phase consists of a homogeneous mixture of lipid and alkane.4,12 This explains the smooth onset in the transition region and is also obvious comparing the molecular areas with those for the air-water
interface. By quantitative analysis of fluorescence microscopic images, it was shown that the oil may partition also in the condensed phase.4 Comparing the isotherms, one observes that in case of DPPC with a bulky head group the increase of the area per lipid indicates that the oil is incorporated in the lipid monolayer also at very high lateral pressures, whereas from the molecular area in the DPPE film a squeezing out of the oil is assumed. Figure 3 shows contour plots of the corrected X-ray intensities as a function of the in-plane scattering vector component Qxy and the out-of-plane scattering vector component Qz at different surface pressures π of a racemic DPPE monolayer in contact with different alkanes. In the case of the DPPE monolayer in contact with dodecane (Figure 3, left) only one diffraction peak occurs at all pressures investigated. The maximum position of this peak shifts to slightly higher Qxy values with increasing pressure. The corresponding Bragg rod has its maximum intensity at the horizon (Qz ) 0 Å-1). For DPPE in contact with hexadecane (Figure 3, right), on the other hand, one observes a dependence of the contour plots on pressure. At higher lateral pressures only one diffraction peak with maximum intensity at Qxy ≈ 1.52 Å-1 and Qz ) 0 Å-1 can be observed. At lower pressure (Π ) 5 mN/m) the contour plot has a very large extension in Qz. The Bragg rod shows a shoulder corresponding to the overlap of two peaks, one with the maximum at Qz ) 0 Å-1 and the other one at Qz > 0 Å-1. Racemic DPPC in contact with dodecane shows the same behavior as DPPE/dodecane. There is only one diffraction peak with maximum intensity at Qz ) 0 Å-1. The Qxy position (1.50 Å-1) is also very similar to that of the DPPE/dodecane system. Figure 4 shows contour plots in Qxy, Qz space obtained with a DPPC monolayer in contact with hexadecane on increasing the lateral pressure from top to bottom. At the lowest lateral pressure of 7 mN/m one realizes two diffraction peaks, one at Qxy ) 1.49 Å-1, Qz ) 0.18 Å-1 and one at Qxy ) 1.47 Å-1, Qz ) 0.36 Å-1. The third very sharp maximum just above the horizon (at Qz ≈ 0.01 Å-1) is the Yoneda-Vineyard peak,13 which was used for calibration of the PSD. This peak does not contain any structural information. Increasing the pressure (10 mN/m), not only does the maxima shift to lower Qz and
3128 J. Phys. Chem., Vol. 100, No. 8, 1996
Brezesinski et al.
Figure 5. Qz resolved Bragg rod profiles integrated over the whole Qxy interval of the Bragg peaks at different lateral pressures (indicated). The first two from below Bragg rods were fitted with a superposition of two Gaussian curves. In the right lower corner the Bragg rods at 7 mN/m integrated over small Qxy intervals are presented.
TABLE 1: Dimensions a and b of a Centered Rectangular Unit Cell, Projected Area per Chain Axy, Tilt Angle t, Tilt Azimuth TA, and Cross Section Area A0 at Different Surface Pressures π Figure 4. Contour plots of the corrected X-ray intensities as a function of the in-plane component Qxy and the out-of-plane component Qz of the scattering vector Q of DPPC monolayers in contact with hexadecane at different lateral pressures (indicated).
higher Qxy but the contour plot changes drastically. There is one diffraction peak at Qz ) 0 Å-1 and Qxy ) 1.5 Å-1 and a second, unresolved peak is indicated from a bending of the contour with respect to the horizontal. On further increasing the lateral pressure, all peaks merge into one with Qz ) 0 Å-1 and Qxy ≈ 1.51 Å-1. A more quantitative analysis of the diffraction profiles is given in Figure 5, showing the intensity as a function of Qz for a narrow Qxy interval. It is important to note from the inset that the peak at low pressure can be deconvoluted into two peaks with maxima in Qz differing by a factor of 2. This is different at higher lateral pressures. At 10 and 20 mN/m the deconvolution yields one peak with Qzmax ) 0 Å-1 and one with Qzmax > 0 Å-1. At still higher pressure there is only one peak with Qz ) 0 Å-1. The lattice parameters deduced from the diffraction data are summarized in Table 1. The two peaks with Qzmax > 0 Å-1 observed for DPPC with hexadecane contact at 7 mN/m can be ascribed to a centered rectangular lattice with next-nearest neighbor (NNN) tilt. Increasing the lateral pressure the tilt azimuth changes into a nearest neighbor (NN) direction, and the tilt angle is reduced as well as the projected area Axy. Inspecting Table 1, one realizes that the tilt angle at 10 mN/m amounts to 10° and that at 20 mN/m to 11°. Although there is
sample DPPC + hexadecane
DPPC + dodecane DPPE + hexadecane DPPE + dodecane
π (mN/m)
a (Å)
b (Å)
7 10 20 38 45 8 20 40 5 20 40 5 20 40
4.842 4.880 4.876 4.796 4.794 4.850 4.840 4.821 4.854 4.789 4.779 4.827 4.805 4.783
8.531 8.378 8.378 8.307 8.303 8.400 8.383 8.350 8.355 8.295 8.277 8.361 8.323 8.284
Axy t A0 (Å2) (deg) (Å2) 20.7 20.4 20.4 19.9 19.9 20.4 20.3 20.1 20.3 19.9 19.8 20.2 20.0 19.8
14 10 11 0 0 0 0 0 7 0 0 0 0 0
20.1 20.1 20.1 19.9 19.9 20.4 20.3 20.1 20.1 19.9 19.8 20.2 20.0 19.8
TA NNN NN NN
NN
within experimental error no difference in tilt angle, these data are inconsistent since from a comparison of data at 7 and at 10 mN/m, respectively, we would have expected a tilt angle near 0° at 20 mN/m. The reason for this discrepancy can be explained by the experimental setup. After 1 h of equilibration with oil, the DPPC monolayer was compressed to a molecular area of 65 Å2 to get a lateral pressure of 20 mN/m. Then the pressure was increased, and only after the measurements at higher pressure, the monolayer was expanded to measure the lateral pressures of 10 and 7 mN/m (fully expanded monolayer). Therefore, the higher tilt angle at 20 mN/m could be an indication of an incomplete equilibration after 1 h. At still higher pressures the tails are aligned vertically, the lattice becomes hexagonal, and the projected area assumes values
Monolayers at the Air/Water Interface
Figure 6. Projected area per chain Axy as derived from in-plane diffraction data as function of surface pressure π for DPPC (b) and DPPE (9) on a pure water surface and in contact with both hexadecane (2 ) DPPC and 1 ) DPPE) and dodecane(× ) DPPC and [ ) DPPE).
Figure 7. Tilt angle t as a function of pressure for DPPC (b) and DPPE (9) on a pure water surface and in contact with hexadecane (2 ) DPPC and 1 ) DPPE) as well as with dodecane (× ) DPPC and [ ) DPPE).
characteristic for alkane cross sections in the rotator phases (Axy ) (20.0 ( 0.2) Å2). The latter structure is also observed for DPPE in contact with hexadecane at high pressures and for DPPC and DPPE in contact with dodecane at all pressures. Discussion Figure 6 compares the projected areas as a function of pressure for the two types of phospholipid monolayers in the absence or presence of contact with hexadecane. DPPC monolayers at the air/water interface were previously studied at 15 °C, where the transition from a liquid expanded to a condensed phase (LC) occurs between 2 and 3 mN/m. However, all available data for the LC phase of DPPC indicate that there is no structural change between 15 and 20 °C. The projected area per molecule amounts to approximately 45 Å2 at higher pressures. These large values are generally ascribed to the large phosphocholine head group limiting the molecular area.14,15 In the case of contact with oil we still observe an ordered arrangement of aliphatic tails, but with lower Axy and with tilt angles approaching 0° at higher lateral pressures. This proves that hexadecane and also dodecane penetrate the phospholipid lattice to become part of it. The alkane is also ordered in order to observe diffraction. Figure 7 compares the tilt angle t as a function of lateral pressure for the two lipids on a water surface with that in contact with the two different alkanes. One clearly observes the reduction of tilt angle due to the oil contact.
J. Phys. Chem., Vol. 100, No. 8, 1996 3129 It was previously shown by quantitative analysis of fluorescence micrographs with monolayers at the oil/water interface that the area per DPPC molecule in the ordered phase decreases continuously with increasing pressure from about 80 Å2 (at 10 mN/m) to about 50 Å2 (at 50 mN/m) irrespective of temperature.12 Since the isotherms of the ordered phase (LC) at the air/water interface in contact with oil and at the oil/water interface are superimposable, we can deduce a similar composition of oil and lipid also in the present case. By X-ray diffraction we measured a projected area near 20 Å2/chain at high lateral pressures. Since upon compression the isotherm varies from about 100 Å2/lipid to 60-55 Å2/lipid and the lipid contributes two chains, we can deduce that the alkane content varies from 3 molecules/lipid to about 1 molecule/lipid. As the variation occurs continuously, we can exclude any compound formation. On the other hand, a model considering the arrangement of alkane and lipid has to account for the fact that the DPPC head requires an area of 45 Å2; the two tails of the lipid, 40 Å2. Hence for a dense packing (55 Å2) with one alkane per two phospholipids, the free space below one alkane (20 Å2) has to be shared by the heads of at least two phospholipid molecules. This is possible, but it requires that the distribution of both components is not random. The insertion of alkanes also decouples the head group, and this is why X-ray structure and domain structure of chiral resolved DPPC monolayers do not indicate influences of chirality, in contrast to findings at the air/water interface.5,16 For DPPE monolayers the data in Figures 5 and 6 do not prove (nor disprove) alkane insertion. For this system at the air/water interface the smaller head group allows for projected areas near 20 Å2/chain, and these are observed at high pressures by X-ray diffraction.17 The fact that these small values are achieved at lower pressures in the case of contact with oil then suggests that these monolayers behave as if they experienced a higher pressure. Fluorescence micrographs for DPPE at the hexadecane/water interface again revealed oil insertion into the ordered phase at low pressure since the area per lipid molecule amounts to about 65 Å2 at 10 mN/m.3,4 It steadily decreases with pressure toward 40 Å2 (at 40 mN/m), indicating the squeezing out of oil. For DPPE in contact with dodecane we can deduce similar behavior from the similarity of the isotherm part corresponding to the ordered phase. The fact that the comparison of the arrangement with that at the air/water interface shows a drastic reduction of the tilt angle can be understood in the following way: Insertion of alkane reduces head group repulsion and hence increases cohesive attraction, as found, for example, on going from fatty acids to fatty salts.18,19 Since the area at the interface not covered by lipid molecules is covered by alkane molecules, this would yield a repulsive contribution, which in addition has to be compensated by the decreased head group repulsion. On the other hand, a model for the DPPE head group ordering was deduced from the simulations of the Bragg rod profiles.17,20 The head groups are interlinked by a hydrogen-bonding network. Therefore, at high lateral pressures and close head group contacts the oil is squeezed out to establish this hydrogen-bonding network. For DPPC such a hydrogen-bonded complex between the head groups does not exist because the nitrogen atom is shielded by the bulky trimethyl group. We also note that a phase with NNN tilt as observed here for DPPC in contact with hexadecane has not been proven for any phospholipid, and for fatty acids the reverse sequence NN f NNN is observed on increasing the lateral pressure.21 This indicates that the two tilt directions are very close in energy and slight variations of head group interactions may thus affect the tilt direction.
3130 J. Phys. Chem., Vol. 100, No. 8, 1996 It is also interesting to compare the structural data obtained at high pressures (30-45 mN/m), where the monolayer exists predominantly as one ordered phase. Comparing the isotherms in the absence and presence of alkane indicates that the alkane is inserted into the DPPC monolayer, but not into the DPPE monolayer. Accordingly, we could measure qualitative changes of lattice parameters, e.g. the transition from a rectangular to a hexagonal lattice for the DPPC system in contact with oil. This also holds for the positional correlation lengths ξ deduced from the line widths in the Qxy direction. For DPPE near 40 mN/m they correspond to 210 Å in the absence of oil and to 190 and 210 Å in the case of contact with hexadecane and dodecane, respectively. On a pure water surface DPPC exhibits an oblique structure at 45 mN/m with ξ values between 50 and 90 Å for the different directions. In contact with oil a hexagonal arrangement of upright molecules was observed, and the positional correlation lengths amount to 170 Å (with hexadecane) and 190 Å (with dodecane). Irrespective of an error margin near 10% of these values, these data show clearly that oil insertion into DPPC causes an increased lateral order. Probably this is due to a removal of the distortion caused by the large choline head group with films at the air/water interface. The similar ξ values for DPPE in the presence and absence of oil could be an indication for a squeezing out of the oil at high pressures. Oil insertion is also expected to cause a change in the widths of the Bragg rods, which correspond roughly to the length L of the coherently scattering unit. For DPPE near 40 mN/m we derive L ) 22.6 ( 0.5 Å irrespective of the presence or absence of oil, which is in reasonable agreement with the length lt of the aliphatic tail in the all-trans conformation, lt ) 17 × 1.25 Å ) 21.3 Å, measured from the terminal carbon of a C16 chain to the C1 carbon of the glycerol backbone. The difference may be accounted for by the fact that also the glycerol backbones are ordered and contribute to diffraction. Comparing L for the two types of oils in contact with DPPC, we observe L ) 21.7 Å (at 45 mN/m for hexadecane) and 20.2 Å (at 40 mN/m for dodecane). The latter value being smaller indicates that dodecane inserted into the film does not enable uniform structure across the entire tail length. This would be expected in view of its shorter length, but a more elaborate analysis of this is needed to conclude on a probable arrangement within the lipid matrix. We should also state that the conclusion from the isotherms that the structures at the air/water/ oil contact and oil/water interfaces are equal can be correct only to a zeroth order approximation. In both cases the monolayer is in contact with excess bulk oil; however, in the latter case the interface is toward an oil film, whereas for the former case ellipsometric and X-ray reflectivity data have shown that the oil is inserted, not on top of the film.22 Conclusions We have shown in this work that if the projected area is determined by a large head group (DPPC), alkanes can be incorporated into ordered lipid arrays, thus changing tail orientation, lattice structure, and phase sequence. For the shorter chain alkane (dodecane) a smaller concentration in the ordered phase with exclusively vertical tail arrangement is found. The
Brezesinski et al. longer alkane (hexadecane) allows for tilted phases, but with reduced tilt angle compared to the bare air/water interface. For the lipid with a small head group (DPPE) the oil penetrating can be squeezed out on compression with changing the structure of the alkane lattice. In this case the influence of the oil can be understood as reducing head group repulsion and thus increasing the attractive lateral interactions. Finally we should also note that changes due to contact with oil are reversible, and we therefore have demonstrated a new way to manipulate monolayer structure. The contact is not limited to liquid alkanes but also to that with gaseous alkanes, and we hope to be able to demonstrate in the near future that also films on solid support can be manipulated similarly. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft and the Bundesministerium fu¨r Forschung and Technologie (BMFT). We thank HASYLAB at DESY, Hamburg, Germany, for beam time and for providing excellent facilities and support. The collaboration with K. Kjaer and W. G. Bouwman is gratefully acknowledged. References and Notes (1) Kahlweit, M.; Strey, R.; Haase, T.; Kunieda, H.; Schmeling, T.; Faulhaber, B.; Borkovec, M.; Eicke, H.-F.; Busse, G.; Eggers, F.; Funck, T.; Richmann, H.; Margid, L.; So¨dermann, O.; Stilbs, P.; Winkler, J.; Dittrich, A.; Jahn, W. J. Colloid Interface Sci. 1987, 118, 436-453. (2) Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I. Progr. Colloid Polym. Sci. 1990, 81, 36-40. Lu, J. R.; Thomas, R. K.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Sokolowski, A.; Penfold, J. J. Phys. Chem. 1992, 96, 10971-10978. (3) Thoma, M.; Pfohl, T.; Mo¨hwald, H. Langmuir 1995, 11, 28812888. (4) Thoma, M.; Mo¨hwald, H. J. Colloid Interface Sci. 1994, 162, 340349. (5) Brezesinski, G.; Dietrich, A.; Struth, B.; Bo¨hm, C.; Bouwman, W. G.; Kjaer, K.; Mo¨hwald, H. Chem. Phys. Lipids 1995, 76, 145-157. (6) Durbin, M. K.; Malik, A.; Ghaskadvi, R.; Shih, M. C.; Zschack, P.; Dutta, P. J. Phys. Chem 1994, 98, 1753-1755. (7) Als-Nielsen, J.; Jaquemain, D.; Kjaer, K.; Lahav, M.; Leveiller, F.; Leiserowitz, L. Phys. Rep. 1994, 246, 251-321. (8) Kjaer, K. Physica B 1994, 198, 100-109. (9) Leadbetter, A. J. In Thermotropic Liquid Crystals; Gray, G. W., Ed. Critical Reports on Applied Chemistry; Wiley: Chichester, New York, 1987; Vol. 22, pp 1-27. (10) Helm, C. A.; Mo¨hwald, H.; Kjaer, K.; Als-Nielsen, J. Biophys. J. 1987, 52, 381-390. (11) Guinier,A. X-ray Diffraction in Crystals, Imperfect Crystals and Amorphous Bodies; W. H. Freeman and Co.; San Francisco, London, 1968. (12) Thoma, M.; Mo¨hwald, H. Colloid Surf. 1995, 95, 193-200. (13) Vineyard, G. H. Phys. ReV. B 1982, 26, 4146-4159. (14) Helm, C. A.; Mo¨hwald, H.; Kjaer, K.; Als-Nielsen, J. Europhys. Lett. 1987, 4, 697-703. (15) Vaknin, D.; Kjaer, K.; Als-Nielsen, J.; Lo¨sche, M. Biophys. J. 1991, 59, 1325-1332. (16) Weis, R. M.; McConnell, H. M. Nature 1984, 310, 47 -49. (17) Bo¨hm, C.; Mo¨hwald, H.; Leiserowitz, L.; Als-Nielsen, J.; Kjaer, K. Biophys. J. 1993, 64, 553-559. (18) Kjaer, K.; Als-Nielsen, J.; Kenn, R. M.; Bo¨hm, C.; TippmannKrayer, P.; Helm, C. A.; Mo¨hwald, H.; Leveiller, F.; Jaquemain, D.; Lahav, M.; Leiserowitz, L.; Deutsch, M. In Surface X-ray and Neutron Scattering; Zabel, H., Robinson, I. K., Eds.; Springer: Berlin, Heidelberg, New York, 1992; pp 143-146. (19) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippmann-Krayer, P.; Mo¨hwald, H. J. Phys. Chem. 1989, 93, 3200-3206. (20) Bo¨hm, C. Ph.D. Thesis, University of Mainz, 1993. (21) 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, 2092-2097. (22) Pfohl, T.; Thoma, M.; Riegler, H.; Baltes, H.; Schwendler, M.; Helm, C. A.; Mo¨hwald, H. To be published.
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