12-(Hydroxy)stearoyl ... - ACS Publications

8704

J. Phys. Chem. B 2000, 104, 8704-8711

Mixed Stearoyl-rac-glycerol/12-(Hydroxy)stearoyl-rac-glycerol Monolayers on the Air/Water Interface: Brewster Angle Microscopy and Grazing Incidence X-ray Diffraction Investigation N. Krasteva, D. Vollhardt,* and G. Brezesinski Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany ReceiVed: April 18, 2000; In Final Form: June 22, 2000

Mixed monolayers of 1-monostearoyl-rac-glycerol (StGl) and 1-(12-hydroxy)monostearoyl-rac-glycerol (12OHStGl) were studied by means of pressure-area (π-A) isotherms, Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD). Thermodynamic analysis of the π-A isotherms of the mixed films showed strong deviations from the ideal behavior at all compositions. Negative deviations from ideality are observed in the monolayers with low 12OH-StGl content. The 12OH-StGl molecules are homogeneously distributed in the StGl matrix. The inclusion of 12OH-StGl molecules decreases the pressure at which the NN f NNN tilt transition occurs. A tilted phase with an oblique lattice and chain tilt in an intermediate (I) direction is observed in the mixed monolayers with small xOH values. With increasing mole fraction of the 12OH-StGl component positive deviations from ideal behavior are observed. The averaged interactions between the like molecules of the pure substances dominate, and the mixed monolayer tends to phase separate. Continuous change in the shape of the condensed phase domains during their growth also suggests partial phase separation in the mixed monolayers with high content of 12OH-StGl. Two condensed phases, StGlrich and 12OH-StGl-rich, with different lattices coexist in the mixed StGl/12OH-StGl monolayers. Partial phase separation in these films might be due to the different lattice symmetry of the condensed phases of StGl and 12OH-StGl monolayers. The formation of the hydrogen bonds between the aliphatic chains of the molecules is supposed to stabilize the 12OH-StGl-rich phase. The composition-pressure phase diagram for the mixed StGl/12OH-StGl monolayers is constructed according to the data obtained by the π-A isotherms, BAM and GIXD.

Introduction In the multicomponent monolayers of insoluble amphiphiles spread on liquid interfaces, the two-dimensional miscibility of the components is a problem of tremendous importance. In general, mixing ability of two components is affected by the steric compatibility of the amphiphilic molecules1-6 and their lateral interactions.7 Miscibility is shown to depend on the similarity of the film states of the pure component monolayers as well.8,9 Conclusions about the mixing process of two pure monolayers can be made comparing surface pressure-area isotherms of the mixed and the pure films.8,10-14 However, in some cases it is difficult to distinguish the phase separation from the ideal mixing only on the basis of this macroscopic technique. With the development of the highly sensitive imaging methods such as the Brewster angle microscopy (BAM)15-18 and the synchrotron grazing incidence X-ray diffraction (GIXD)19-21 it became possible to characterize directly the 2D phases in the Langmuir monolayers. In particular, mixtures of fatty acids with their esters18,22 or with fatty alcohols23,24 have been used to trace the motion of the phase boundaries and to identify the existing phases. Langmuir monolayers of monoglycerides are good candidates for systematic structure and texture studies. Therefore, extensive studies on the structure and morphology of the monolayers of monoglycerides with different chain length have been performed.25-30 The comparison of 1-monostearoyl-rac-glycerol (StGl) and

1-(12-hydroxy)monostearoyl-rac-glycerol (12OH-StGl) produced proof for the remarkable differences in the monolayer features of both monoglycerols caused only by the presence of one OH group in 12 position of the alkyl chain. The monolayers of both amphiphiles differ considerably not only in the π-A isotherms but also in the lattice structure and the domain morphology. Whereas the π-A isotherms of the StGl monolayers show the “plateau” of the two-phase coexistence region only above 30 °C,25 those of the 12OH-StGl monolayers have the “plateau” at π > 0 mN/m even at temperatures less than 5 °C.29 The domain morphology of StGl and 12OH-StGl is completely different. StGl condensed phase domains are subdivided into segments with different reflectivity. Sharp segment boundaries meet in the center or on the periphery of the domain. The inner structure of the StGl domains remains visible even at the monolayer collapse.25 On the other hand, the condensed phase domains of the 12OH-StGl have an irregularly curved shape and look rather crystal like.29,30 Changes in reflectivity inside of one domain are not observed. Striking differences exist also in the lattice structure of both components. The condensed phase of the StGl monolayer is characterized by a rectangular lattice and alkyl chain tilt in symmetry direction.27,30 The condensed phase of the pure 12OHStGl has an oblique chain lattice and chain tilt in the nonsymmetrical lattice direction. The area per molecule in a closely packed condensed state is about the same as in the case of StGl monolayer.30

10.1021/jp001479k CCC: $19.00 © 2000 American Chemical Society Published on Web 08/22/2000

StGl/12OH-StGl Monolayers at the Air/Water Interface

J. Phys. Chem. B, Vol. 104, No. 36, 2000 8705

Figure 1. π-A isotherms of the pure StGl (1) and 12OH-StGl (8) and of the mixed StGl/12OH-StGl monolayers with different mole fraction xOH of 12OH-StGl: 2-0.04, 3-0.14, 4-0.33, 5-0.5, 6-0.67, 7-0.9. The inset shows the transition pressure derived from the kink in the isotherms as a function of xOH.

The objective of the present work is to obtain information on the two-dimensional (2D) mixing behavior of the two monoglycerides StGl and 12OH-StGl, which differ only slightly in the chemical structure of the alkyl chain. Experimental Section 1-Monostearoyl-rac-glycerol StGl (purity >99%) was obtained from Sigma and used without further purification. 1-(12Hydroxy)monostearoyl-rac-glycerol (12OH-StGl) was synthesized by direct enzymatic monoacylation of glycerol with 12hydroxystearic acid in the presence of phenylboronic acid in n-hexane. The product was purified by thin-layer chromatography, liquid chromatography and crystallization from methanol (purity g99%). The monolayer-forming substances were dissolved in a 9:1 (V:V) mixture of n-heptane (for spectroscopy, Merck) and ethanol (p.a., Merck) to stock concentrations of 1 mM and kept refrigerated. Solutions with different mole fractions xOH of 12OH-StGl were prepared by mixing certain volumes of the StGl and 12OH-StGl stock solutions. Aliquots of 40 µL of the obtained mixtures were then spread on the water surface by means of micrometric syringe. The solvent was allowed to evaporate for about 15 min before monolayer compression. Utrapure Milli-Q-filtered water (Millipore Co.) with a specific resistance 18.2 MΩ cm was used as a subphase. Pressure-area (π-A) isotherms of the mixed monolayers are collected by means of an automated Langmuir trough with a surface area of approximately 300 cm2. The trough was equipped with a Wilhelmy balance for surface pressure determination and a temperature control system. The experiments were conducted at a constant temperature of 25 °C. All isotherms were obtained with a compression rate of 1.5 Å2/(molecule min). A Brewster angle microscope (BAM-2, NFT, Go¨ttingen), mounted on the Langmuir trough, was used to observe morphol-

Figure 2. BAM images of the mixed films with mole fraction of 12OHStGl xOH ) 0.04. The bar represents 100 µm: (top) 0 mN/m; (bottom) 40 mN/m.

ogy of the monolayers.15 The p-polarized beam of the diode laser was directed to the pure air/water interface at Brewster angle, giving a minimum surface reflectivity. Apparent reflectivity from the interface is observed, when the monolayer is spread onto the water surface. Optical anisotropy caused by the differences in molecular orientation in the monolayer in a micrometer range is visualized by introducing of an analyzer in the reflected beam path. The reflected light was detected by means of a CCD camera and recorded on videotape. BAM images were obtained on continuous monolayer compression simultaneously with the π-A isotherm. The film morphological features were monitored with a spatial resolution of about 3 µm. Grazing incidence X-ray diffraction experiments were performed at the liquid-surface difractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, Germany. The monochromatic X-ray beam was adjusted to strike the surface at an angle of incidence Ri ) 0.85Rc, where Rc ) 0.14° is the

8706 J. Phys. Chem. B, Vol. 104, No. 36, 2000

Krasteva et al.

Figure 3. BAM images of the mixed films with xOH ) 0.33. The bar represents 100 µm: (top) 0 mN/m; (bottom) 30 mN/m.

critical angle for total external reflection. The diffracted intensity was monitored by a linear position sensitive detector (PSD, OED 100-M, Braun, Garching, Germany). The in-plane divergence of the diffracted beam was restricted by a Soller collimator in front of the detector. According to the geometry of diffraction,19-21 the scattering vector Q can be written in terms of an in-plane component Qxy and an out-of plane component Qz

Qxy )

2π cos2 Ri + cos2 Rf - 2 cos Ri cos Rf cos 2θ (1) λx Qz )

2π (sin Ri + sin Rf) λ

(2)

where Rf and 2θ are the vertical and the horizontal scattering angle, respectively. The obtained peak intensities were leastsquares fitted as a Lorentzian parallel to the water surface and as a Gaussian normal to it. The lattice repeat distances dhk are

Figure 4. BAM images of the mixed films with xOH ) 0.67. The bar represents 100 µm: (top) 10 mN/m; (middle) 13 mN/m; (bottom) 16 mN/m.



Figure 6. Measured mean molecular area Amix for the mixed StGl/ 12OH-StGl monolayers as a function of the monolayer composition at different surface pressures. The dotted lines illustrate the additivity rule.

Figure 5. BAM images of the mixed films with xOH ) 0.90. The bar represents 100 µm: (top) 12 mN/m; (bottom) 20 mN/m.

determined from the in-plane peak positions according to

dhk )

2π Qhk xy

(3)

The polar tilt angle t and the tilt azimuth ψhk of the alkyl chains are calculated using the relation between the positions of the maximum of the scattering vector components Qz and Qxy hk Qhk z ) Qxy cos ψhk tan t

(4)

Results and Discussion 1. Surface Pressure-Area Isotherms and BAM Study. π-A isotherms for six mixtures with mole fraction of 12OHStGl xOH ) 0.04, 0.14, 0.33, 0.5, 0.67, and 0.90 were measured at 25 °C. They are presented in Figure 1 together with the isotherms of the pure StGl and 12OH-StGl. The shape of the isotherms of the binary monolayers depends on the monolayer

composition. The isotherms of the mixtures with xOH e 0.04 reveal condensed phase formation after spreading similar to the pure StGl monolayer. The isotherms of the mixtures with xOH > 0.04 show a plateau at a pressure of about 13 mN/m. The width of the plateau decreases with decreasing mole fraction of 12OH-StGl and only a kink is observed in the case of the 0.96:0.04 StGl/12OH-StGl mixture. The plateau pressure remains nearly constant for the mixtures with 0.33 e xOH e 0.67 and diminishes with increasing the mole fraction of the hydroxy compound (see the inset in Figure 1). Representative BAM images of mixed films, taken during continuous compression of the monolayers, are shown in Figures 2-5. The morphology of the mixed StGl/12OH-StGl monolayers depends on the monolayer composition. Condensed phase clusters are seen at zero pressure in mixed monolayers with xOH e 0.50 (Figure 2 and Figure 3). At xOH e 0.04 their morphology resembles that of the pure StGl monolayer (Figure 2). The clusters reduce in size and become more uniformly distributed on the water surface with increasing content of 12OH-StGl (Figure 3). They come closer to each other during the compression until the compact condensed phase is formed. The optical anisotropy, observed within the clusters of mixed monolayers with low xOH (Figure 2), disappears as the 12OHStGl mole fraction reaches 0.33 (Figure 3). BAM does not identify condensed phase domains upon spreading of mixed films with xOH g 0.67. The first domains are observed at about 10 mN/m, well below the plateau pressure. They are circular with a segment structure at the beginning of the formation (Figure 4a). As the surface pressure increases the domains obtain a starlike form and the observed optical anisotropy diminishes (Figure 4b,c). In the mixed monolayer with xOH ) 0.90 domain formation starts at the beginning of the plateau in the isotherm. Irregularly shaped domains without defined inner structure are observed in this case (Figure 5). The comparison between the measured mean molecular areas


Krasteva et al. given by

∆Gex )

∫0π(Amix - (1 - xOH)A - xOHAOH) dπ

(5)

The energy of interaction between the molecules ∆h is calculated according to31

∆h )

Figure 7. Calculated ∆Gex and ∆h values for the mixed StGl/12OHStGl monolayers as a function of the monolayer composition. The dotted line illustrates the ideal behavior.

Amix and the calculated molecular areas Aid ) (1 - xOH)A + xOHAOH in the mixed monolayer, assuming ideal mixing behavior, is presented in Figure 6. Here A and AOH are the molecular areas in the pure StGl and 12OH-StGl monolayers, respectively. It is seen that the mixed monolayers with xOH less than about 0.2 have mean molecular areas Amix smaller than Aid at all pressures; i.e., they are more compressed than the pure monolayers. At higher values of xOH the mixed monolayers are more expanded than in the pure state, as the values of Amix are greater than Aid. The observed deviations from ideality decrease with increasing surface pressure. The evaluation of the excess free energy of mixing ∆Gex is applied as a criterion for the miscibility of the pure components in the film.8,10,11 The molar excess free energy of mixing ∆Gex for a process of isothermal mixing of two pure components is

∆Gex z((1 -

xOH)x2OH

+ (1 - xOH)2xOH)

(6)

where the coordination number z equals 6 when the molecules pack in a two-dimensional hexagonal lattice and there is no great disparity in size between the components.32 In the case of ideal mixing or complete phase separation both ∆Gex and ∆h should have zero values at any mixture composition.7,8,10,11 The dependencies of the ∆Gex and ∆h on composition for the mixed StGl/12OH-StGl monolayers at two surface pressures are presented in Figure 7. Strong deviations from zero in all mixtures are observed. The variations at low mole fractions of 12OH-StGl are negative, while they become positive at higher xOH. This suggests that the miscibility of the two pure components at the given temperature and compositions is nonideal in the whole composition range.8,12,13,33 The obtained negative deviations of ∆Gex and Amix for the mixtures with xOH < 0.20 suggest that the attractive interactions between the unlike StGl and 12OH-StGl molecules are stronger than those between the like molecules of the pure species. The two substances are miscible and the molecules of 12OH-StGl are homogeneously distributed in the lattice of the StGl condensed phase. With increasing mole fraction of 12OH-StGl the excess free energy of mixing becomes positive. Now the increasing portion of 12OH-StGl molecules disturbs the StGl lattice owing to the steric effect of the OH substitution in the 12 position so that the averaged attraction between the like molecules of the two pure components prevails and the mixed monolayer tends to phase separate. Coexistence of two condensed phases, an StGlrich and a 12OH-StGl-rich phase, is expected at high mole fractions of the hydroxy component. The constant value of the plateau pressure in the isotherms and the continuous change in the shape of the condensed phase domains during their growth also suggest that phase separation occurs in the mixed monolayers with higher content of 12OH-StGl. 2. Grazing Incidence X-ray Diffraction. GIXD data confirm the conclusions about the miscibility of the StGl and 12OH-

Figure 8. Contour plots of the diffracted intensity as a function of the in-plane component Qxy and the out-of-plane component Qz of the scattering vector for the mixed monolayer with xOH ) 0.17.



TABLE 1: Lattice Parameters a, b, and c, Tilt Angles t, and Tilt Azimuths ψ for Mixed StGl/12OH-StGl Monolayers with Low xOH Near the NN f NNN Transition xOH

π, mN/m

a, Å

b, Å

c, Å

ψ, deg

t, deg

0.01

35 43 30 35 42 30 40 46 30 35 38 43

5.11 4.85 5.21 4.94 4.88 5.22 4.90 4.90 5.25 4.93 4.91 4.89

4.94 4.94 4.99 4.99 5.01 4.99 4.97 4.98 5.01 5.03 5.02 5.01

4.94 4.94 4.99 5.14 5.01 4.99 5.07 4.98 5.01 5.14 5.08 5.01

NN NNN NN 3 NNN NN 7 NNN NN 8 18 NNN

23 18 26 24 21 26 23 20 27 23 22 21

0.04 0.10 0.17

StGl made on the basis of isotherm and BAM studies. They show that the condensed phase structure of the mixed monolayers with low and high content of 12OH-StGl is different. The contour plots of the corrected intensity for mixed films with xOH ) 0.17 are shown in Figure 8 as an example. Two in-plane diffraction peaks, a nondegenerate (02) reflex at Qz ) 0 Å-1 and a degenerate (11)+(-11) reflex at Qz > 0 Å-1, are seen at pressures below 35 mN/m in the mixed monolayers with xOH < 0.67. This indicates a centered rectangular lattice with a chain tilt in the nearest neighbor (NN) direction. At higher pressure (π g 40 mN/m) two in-plane reflexes are seen as well, both located at Qz > 0 Å-1. The chain lattice is centered rectangular with chain tilt in next-nearest neighbor (NNN) direction. At surface pressures between 35 and 40 mN/m a change in the tilt azimuth occurs, indicating a phase transition from NN to NNN phase. The transition occurs at a well-defined pressure in the pure StGl monolayers and in the mixed film with xOH ) 0.01. In the mixed monolayers with 0.01 < xOH e 0.17 a new phase with an oblique chain lattice and a chain tilt in an intermediate (I) direction appears between the NN and NNN phases. The NN f NNN phase transition splits into two successive NN f I and I f NNN transitions. With increasing the mole fraction of the 12OH-StGl the intermediate state becomes more extended. Such an intermediate state has also been observed in fatty acid monolayers,34,35 and in mixed fatty acid/fatty acid esters monolayers.22 The lattice parameters a, b, and c, calculated from the obtained dhk values, are shown in Table 1 together with the tilt angles t and tilt azimuth angles ψ. The variation of the a, b, and c values with the pressure for the mixed monolayer with xOH ) 0.17 is presented in Figure 9a as an example. In the NN phase region the lattice distances gradually decrease with increasing surface pressure. At the transition pressure from NN to I phase the three lattice distances change jumpwise. With further increase in the surface pressure the a, b, and c values decrease steadily again. The transition from I to NNN phase is characterized by a rather continuous change in the lattice distances. The dependence of the tilt angle and tilt azimuth angle with the lateral pressure is presented in Figure 9b. The tilt angle changes abruptly at the NN f I transition, while the I f NNN transition is connected with a continuous decrease of the tilt. The observed abrupt change in the lattice distances and the tilt at the NN f I transition allow us to assume that it is a firstorder transition. The I f NNN is most likely a second-order transition, as revealed by the continuous change of the condensed phase lattice parameters at the transition point. At high content of 12OH-StGl five in-plane reflections are seen in the contour plots, Figure 10. This proves the coexistence

Figure 9. Condensed phase lattice parameters of the mixed StGl/12OHStGl monolayer with mole fraction xOH: ) 0.17: (a) lattice distances (9) a, (b) b, and (2) c; (b) tilt (9) and tilt azimuth (b) angles.

of two condensed phases with different lattice symmetrysone phase with an oblique lattice and another one with a rectangular lattice. Previous investigations have shown that pure StGl film exhibits two diffraction peaks corresponding to a centered rectangular lattice,27 while the 12OH-StGl shows three peaks and oblique lattice.30 Thus, the two condensed phases in the mixed monolayers could be described as StGl-rich and 12OHStGl-rich phases. The StGl-rich phase forms a centered rectangular lattice, while the 12OH-StGl-rich phase exhibits an oblique lattice. The calculated lattice distances and the tilt and


Krasteva et al.

Figure 10. Contour plots of the diffracted intensity as a function of the in-plane component Qxy and the out-of-plane component Qz of the scattering vector for the mixed monolayer with mole fraction of 12OH-StGl xOH ) 0.50.

TABLE 2: Lattice Parameters a, b, and c, Tilt Angles t, and Tilt Azimuths ψ for Mixed StGl/12OH-StGl Monolayers with xOH ) 0.5 near the NN f NNN Transition π, mN/m

phase

a, Å

b, Å

c, Å

ψ, deg

t, deg

20

StGl-rich 12OH-StGl rich StGl-rich 12OH-StGl rich

5.22 4.98 4.92 4.91

5.02 4.99 5.07 4.99

5.02 5.47 5.07 5.41

NN 21 NNN 18

28 27 23 25

30

tilt azimuth angles for the two phases are presented in Table 2. The NN f NNN transition without passing through the I state is observed for the StGl-rich phase in the pressure range 2730 mN/m. The transition pressure of the mixed system is much lower than that of the pure StGl, indicating that an appreciable amount of the hydroxy compound is included in the StGl matrix. During the pressure increase, phase transitions are not seen in the oblique phase. The pressure-composition diagram, constructed according to our data, is shown in Figure 11. One can distinguish three parts in the phase diagram. At low mole fractions of 12OHStGl 0 e xOH e 0.12 the two components are completely miscible. The mixed system is characterized by an appearance of the new tilted phase between the NN and the NNN phases known for the pure StGl. At intermediate compositions 0.2 < xOH < 0.8, partial phase separation takes place. Below the transition pressure the NN phase of the StGl-rich domains should coexist with the liquidexpanded phase of the 12OH-StGl-rich mixture. Indeed, the condensed domains are observed in the mixed monolayer with xOH ) 0.33 at low pressures (Figure 3). At a mole fraction xOH ) 0.67 these domains seem to be too small at low pressures to be observed, and therefore they appear in the BAM measurements only at higher pressures (Figure 4) but still well below the plateau region in the isotherm. At high-pressure two condensed phases with different symmetry coexist in the mixed monolayers. It is interesting that the NN f NNN phase transition, observed at about 50 mN/m in the pure StGl monolayer, shifts to lower pressure values on addition of the 12OH-StGl. The shift increases with increasing mole fraction of the hydroxy compound, and the NN f NNN transition occurs at about 30 mN/m in the monolayer with xOH ) 0.33. A similar effect is observed in the mixed fatty acid/fatty acid esters monolayers.35 At 0.8 < xOH < 1 the two components are expected to be miscible, and the molecules of the StGl to be spread within the

Figure 11. Pressure-composition diagram for the mixed StGl/12OHStGl monolayers: ([) transition pressure determined from the isotherms; (b) NNN phase; (2) intermediate phase I; (9) NN phase; (0) coexistence of NN and oblique phases; (O) coexistence of NNN and oblique phases. The points (b), (2), (9), (0), and (O) are determined from the GIXD measurements.

12OH-StGl domains, without changing the structure of the condensed phase. The miscibility in this composition range is deduced from the continuous increase in the slope of the plateau in the isotherms during compression (Figure 1, curve 7). Indeed, the expanded-condensed phase transition for a pure monolayer has to occur at a constant pressure; that is, the plateau of the isotherm should be ideally flat. In contrast, in a mixture there exists a substantial region of phase coexistences between the liquidus and the solidus lines in the phase diagram. This will lead to a smearing of the plateau or even to a complete disappearance of some fine details in the isotherm. Conclusions Negative values of excess free energy of mixing are obtained for the monolayers with low 12OH-StGl content. This reveals stronger interactions between the unlike molecules of the two species in the monolayer and complete miscibility of the two components. The molecules of the hydroxy compound are homogeneously distributed in the StGl matrix. This does not

StGl/12OH-StGl Monolayers at the Air/Water Interface change the condensed phase morphology and alkyl chain lattice constants but reduces the pressure at which the NN f NNN tilting transition occurs. A phase with an oblique lattice and chain tilt in the intermediate direction appears between the NN and NNN phases in the mixtures with low xOH. The pressure range where this intermediate oblique phase exists increases with increasing mole fraction of 12OH-StGl. On further increase in the mole fraction of 12OH-StGl in the mixed films, the values of the excess free energy of mixing and the excess molecular area become positive, indicating that the affinity between the like molecules of the pure substances dominate and the mixed film tend to phase separate. Two condensed phases coexist in the mixed monolayerssthe StGlrich phase with rectangular alkyl chain lattice and the 12OHStGl-rich one with oblique alkyl chain lattice. The composition-pressure diagram of the mixed system reveals the existence of two regions of miscibility at mole fractions of the 12OH-StGl 0 < xOH < 0.12 and 0.8 < xOH < 1, separated by a miscibility gap. As the StGl and 12OH-StGl molecules have the same hydrophilic headgroups their mixing behavior is mainly governed by the interactions in the hydrophobic region of the film. The observed partial phase separation in the mixed monolayers might be a result of different packing symmetries of the pure StGl and 12OH-StGl films. Additional stabilization of the 12OH-StGl-rich phase due to the formation of hydrogen bonds between the hydroxy groups in the alkyl chains might also occur. Acknowledgment. Financial assistance from the Deutsche Forschungsgemeinschaft (Sfb 312) and the Fonds der Chemischen Industrie is acknowledged. We thank Dr. Lang, Institute of Biochemistry and Biotechnology, Braunschweig, for the preparation of the 1-(12-hydroxy)monostearoyl-rac-glycerol, and Dr. Weidemann for the assistance in performing GIXD analysis. References and Notes (1) Ries, H. E., Jr.; Swift, H. J. Colloid Interface Sci. 1974, 64, 111. (2) Ries, H. E., Jr.; Swift, H. Colloids Surf. 1989, 40, 145. (3) Korner, D.; Benita, S.; Albrecht, G.; Baszkin, A. Colloids Surf. B 1994, 3, 101. (4) Zaitsev, S. Yu.; Zubov, V. P.; Mo¨bius, D. Colloids Surf. A 1995, 94, 74. (5) Angelova, A.; Van der Auweraer, M.; Ionov, R.; Vollhardt, D.; De Schryver, F. C. Langmuir 1995, 11, 3167. (6) Kasselouri, A.; Coleman, A. W.; Baszkin, A. J. Colloid Interface Sci. 1996, 180, 384.

J. Phys. Chem. B, Vol. 104, No. 36, 2000 8711 (7) Lucassen-Reynders, E. H. J. Colloid Interface Sci. 1973, 42 (3), 554. (8) Pagano, R. E.; Gershfeld, N. L. J. Phys. Chem. 1972, 76 (9), 1238. (9) Do¨rfler, H. D. AdV. Colloid Interface Sci. 1990, 31, 1. (10) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interface; Interscience: New York, 1966. (11) Gaines, G. L., Jr. J. Colloid Interface Sci. 1966, 21, 315. (12) Costin, I. S.; Barnes, G. T. J. Colloid Interface Sci. 1975, 51 (1), 106. (13) Costin, I. S.; Barnes, G. T. J. Colloid Interface Sci. 1975, 51 (1), 122. (14) Bacon, K. J.; Barnes, G. T. J. Colloid Interface Sci. 1978, 67 (1), 70. (15) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (16) Fischer, B.; Teer, E.; Knobler, C. M. J. Chem. Phys. 1995, 103, 2365. (17) Mo¨bius, D. Curr. Opin. Colloid Interface Sci. 1996, 1, 250. (18) Teer, E.; Knobler, C. M.; Lautz, C.; Wurlitzer, S.; Kildae, J.; Fischer, T. M. J. Chem. Phys. 1997, 105, 1913. (19) Als-Nielsen, J.; Mo¨hwald, H. In Handbook of Synchrotron Radiation; Ebashi, S., Koch, M., Rubenstein, E., Eds.; Elsevier: Amsterdam, 1991; Vol. 4, pp 1-53. (20) Als-Nielsen, J.; Jacquermain, D.; Kjaer, K.; Lahav, M.; Leveiller, F.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (21) De Wolf, C.; Bringezu, F.; Brezesinski, G.; Mo¨hwald, H.; Howes, P.; Kjaer, K. Physica B 1998, 248, 199. (22) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591. (23) Shih, M. S.; Durbin, M. K.; Malik, A.; Zschack, P.; Dutta, P. J. Chem. Phys. 1994, 101 (10), 9132. (24) Durbin, M. K.; Shih, M. S.; Malik, A.; Zschack, P.; Dutta, P. Colloids Surf. A 1995, 102, 173. (25) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf. A 1993, 76, 187. (26) Weidemann, G.; Gehlert, U.; Vollhardt, D. Langmuir 1995, 11, 864. (27) Gehlert, U.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. Langmuir 1996, 12, 4892. (28) Brezesinski, G.; Scalas, E.; Struth, B.; Mo¨hwald, H.; Bringezu, F.; Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8758. (29) Vollhardt, D. Unpublished results. (30) Weidemann, G.; Vollhardt, D.; Lang, S.; Bringezu, F.; DeWolf, C. DESY Annu. Rep. 1996, 459. (31) Joos, P.; Demel, R. A. Biochim. Biophys. Acta 1969, 183, 447. (32) Quickenden, T. I.; Tan, G. K. J. Colloid Interface Sci. 1972, 48, 382. (33) Aveyard, R.; Binks, B. P.; Cross, A. W.; Fletcher, P. D. I. Colloids Surf. A 1995, 98, 83. (34) Durbin, M. K.; Malik, A.; Richter, A. G.; Ghaskadvi, R.; Gog, T.; Dutta, P. J. Chem. Phys. 1997, 106, 8216. (35) Peterson, I. R.; Ken, R. M.; Goudot, A.; Fontaine, F.; Rondelez, R.; Bowman, W. G.; Kjaer, K. Phys. ReV. E 1996, 53 (1), 667.

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