J. Phys. Chem. C 2008, 112, 15777–15783
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Structural Features of Mixed Monolayers of Oleanolic Acid and Stearic Acid G. Brezesinski,† D. Vollhardt,*,† K. Iimura,‡ and H. Co¨lfen† Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany, and Department of AdVanced Interdisciplinary Sciences, Graduate School of Engineering, Utsunomiya UniVersity, 7-1-2 Yoto, Utsunomiya 321-8585, Japan ReceiVed: April 23, 2008; ReVised Manuscript ReceiVed: June 19, 2008
The features of oleanolic acid at interfaces and its effect on another component of the epicuticular wax are studied using a binary monolayer system consisting of oleanolic acid (OLA) and stearic acid (SA). A combination of lateral pressure studies, grazing incidence X-ray diffraction (GIXD), and specular X-ray reflectivity (SR) measurements, is proven to be optimal in this respect. In the case of the OLA monolayer, the LS phase exists in the whole accessible pressure region, whereas the SA monolayer develops the phase sequence from the nearest neighbor tilted L2 phase to the next nearest neighbor tilted OV phase and, finally, to the hexagonal LS phase with nontilted molecules with increasing lateral pressure. The cross-section area of 44.1 Å2 · molecule-1 obtained for the pure OLA monolayer from the position of the diffraction peak agrees well with the circular cross-section of the OLA molecular data. The thicknesses of the OLA monolayer determined by GIXD and XR coincide perfectly. The structure data obtained by GIXD and XR experiments, indicating a perpendicular orientation of the OLA molecules in the condensed monolayer phase in the whole lateral pressure region, are consistent with the calculated molecular data of OLA.The structural and phase properties of selected compositions of binary SA/OLA system are studied and result in a schematic phase diagram characterized by large miscibility gaps. The packing properties of both components after addition of minor amounts of the other component are completely different. The amount of OLA which can be integrated into the SA lattice is smaller than 10 mol %, whereas more than 10 mol % of SA can be incorporated into the OLA lattice. The addition of small amounts of OLA to the SA monolayer leads to a change in the phase sequence by disappearance of the OV phase, whereas the addition of small amounts SA to the OLA monolayer changes the packing density but does not change the phase structure. Introduction Oleanolic acid (OLA) is a ubiquitous triterpenoid in plant kingdom, medical herbs, and it is an integral part of the human diet. Triterpenoids have unique and potentially usable biological effects and have been found in a variety of plants and fruits.1 Oleanane triterpenoids belong to the most important triterpenoid structures and, besides the ongoing extraction and isolation of natural material, synthetic derivatives with higher therapeutic potential have been developed.1,2 Oleanolic acid is an important component of the cuticular membrane of plant leaves and fruits. Important functions for the performance of the cuticular membrane, such as protection from diseases, insects, and drought, are related to the composition and molecular arrangement of the epicuticular waxes, composed mainly of alkyl esters, fatty acids, fatty alcohols, and triterpenoids.3,4 Recent pharmacological activities on the basis of oleanolic acid resulted in various therapeutic and cosmetic implications. Oleanolic acid is relatively nontoxic and therapeutically used in various diseases, including anticancer chemotherapies. The occurrence of oleanolic acid as an important component of cuticular membranes suggests the important role of the interfacial ordering in the complex mixture of the epicuticular waxes for the cuticle performance. On the other hand, it is known that the mechanical barrier function of cell membranes in animals is enhanced by
cholesterol.5,6 This behavior has been related to the chemical structure of cholesterol. Interestingly, both cholesterol and OLA have a multicyclic, rigid planar structure and a hydrophilic 3βhydroxyl group. Therefore, the effect of OLA on the fluidity and stability of dipalmitoyl phosphatidylcholine (DPPC) liposomal membrane was studied.7 Although it is a known fact that the occurrence and function of oleanolic acid are associated with the cuticula, so far corresponding studies of the role of OLA and its effect on other components of epicuticular waxes in monolayers are missing. Langmuir monolayers are representative model systems to obtain information of the interfacial ordering of amphiphilic components. Therefore, it has been the objective of this work to provide a first contribution to fill this gap. Long-chain fatty acids are also essential amphiphilic components of the cuticular membrane. SA has been often used for monolayer studies so that their structural and phase characteristics are largely known.8 The present studies focus in two directions: (i) to characterize the structural features of pure OLA monolayers and to compare them with those of SA monolayers, and (ii) to obtain information on the structural and phase properties of a binary monolayer system consisting of OLA and SA. A combination of lateral pressure studies, grazing incidence X-ray diffraction (GIXD) and specular X-ray reflectivity (SR) measurements has proven to be optimal in this respect. Experimental Section
* To whom correspondence should be addressed. † Max Planck Institute of Colloids and Interfaces. ‡ Utsunomiya University.
Materials. Stearic acid (SA) and oleanolic acid (OLA) purchased from Sigma in a nominal g99% purity were used
10.1021/jp803544s CCC: $40.75 2008 American Chemical Society Published on Web 09/11/2008
15778 J. Phys. Chem. C, Vol. 112, No. 40, 2008 without further purification. Ultrapure deionized water with a conductivity of 0.055 µS/cm used for the monolayer experiments was produced by the “Purelab Plus” system (Seral, Germany). The spreading solvent was chloroform (p.a. grade, Baker, Holland). Methods. Lateral Pressure-Area per Molecule Measurements. The lateral pressure-area (π-A) isotherms were measured at different temperatures using a self-made computerinterfaced film balance.9 The Wilhelmy method was used to measure the lateral pressure. Using a roughened glass plate, the accuracy of the lateral pressure was reproducible to (0.1 mN m-1 and the area per molecule to (5 × 10-3 nm2. Equilibrium π-A isotherms were recorded at a compression rate of 0.1 nm2/ (molecule · min). Grazing Incidence X-ray Diffraction. The lateral structures in condensed monolayers of the pure compounds and of selected mixtures at the air/water interface were investigated using grazing incidence X-ray diffraction measurements at the BW1 beamline, HASYLAB, DESY (Hamburg, Germany). The Langmuir film balance was thermostatted (25 °C) and placed into a hermetically closed container filled with helium. The Langmuir trough was equipped with a single movable barrier and a Wilhelmy plate for monitoring the lateral pressure. At BW1, a monochromatic X-ray beam (λ ) 1.304 Å) strikes the water surface at a grazing incidence angle Ri ) 0.85Rc (Rc ) 0.13°) and illuminated roughly 2 × 50 mm2 of the monolayer surface. During a diffraction experiment, the trough was laterally moved to avoid any sample damage by the strong X-ray beam. A linear position-sensitive detector (PSD, OEM-100-M, Braun, Garching, Germany) measured the diffracted signal and was rotated to scan the in-plane Qxy component values of the scattering vector. The vertical channels of the PSD measured the out-of-plane Qz component of the scattering vector between 0.0 and 1.0 Å-1. The diffraction data consisted of Bragg peaks at diagnostic Qxy values. The in-plane lattice repeat distances d of the ordered structures in the monolayer were calculated from the Bragg peak positions: d ) 2π/Qxy. To access the extent of the crystalline order in the monolayer, the in-plane coherence length Lxy, was approximated from the full-width at half-maximum (fwhm) of the Bragg peaks using Lxy ∼ 0.9(2π)/fwhm(Qxy). The diffracted intensity normal to the interface was integrated over the qxy window of the diffraction peak to calculate the corresponding Bragg rod. The thickness of the monolayer was estimated from the fwhm of the Bragg rod using 0.9(2π)/fwhm(Qz). Experimental details are described in the literature.10-15 Specular X-ray ReflectiWity. Specular X-ray reflectivity (XR) experiments were performed using the same liquid-surface diffractometer on the undulator beamline BW1. The experimental setup and evaluation procedure have been described in detail elsewhere.16-18 XR experiments reveal information on the electron-density distribution along the surface normal and may be used to determine the thickness of thin layers. The reflected intensity was measured by a NaI scintillation detector as a function of the vertical incidence angle, Ri, with the geometry Ri ) Rf ) R, where Rf is the vertical exit angle of the reflected X-rays. The vertical scattering vector component Qz ) (4π/λ)sin Rr was measured in a range between 0.01 and 0.85 Å-1. The background scattering from, e.g., the subphase was measured at 2θhor ) 0.7° and subtracted from the signal measured at 2θhor ) 0°.16-18 The reflectivity data were inverted by applying a model-independent approach.19 The obtained electron density profile was interpreted by applying either a box model or assuming a symmetrical electron density distribution.
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Figure 1. π-A isotherms of the pure components (stearic acid (SA) and oleanolic acid (OLA)) and selected compositions of binary mixtures (SA/OLA, 9:1, 7:3, 6:4, 2:8, 9:1) measured at 25 °C.
Molecular Structure. The molecule structure in vacuum was calculated using Cerius2 Version 4.6. (Accelrys) after calculation of charges via the charge equilibration method.20,21 Energy minimization was performed with the “Smart Minimizer” using the default program settings applying the universal force field with additional parameters for dummy atoms.22 Results and Discussion Model studies of the features of OLA at interfaces and its effect on another component of the epicuticular wax have been carried out with the binary monolayer system consisting of oleanolic acid and stearic acid. π-A Isotherms of Binary SA/OLA Mixtures. First fundamental information about the phase behavior of the pure components and selected binary mixtures can be obtained from the π-A isotherms. Figure 1 shows the π-A isotherms of the pure components SA and OLA and selected compositions of binary mixtures (SA/OLA, 9:1, 7:3, 6:4, 2:8, 9:1) measured at 25 °C. The π-A isotherm of the pure SA monolayer has the characteristic kink at ∼25 mN · m-1 known as lateral transition pressure for long-chain fatty acids. The other pure component OLA forms also insoluble monolayers. However the artifactfree recording of the π-A isotherms up to high lateral pressure causes large difficulties because of the extreme stiffness of the compressed monolayers. On compression, the Wilhelmy plate starts to tilt after reaching a critical packing density leading to increasing deviations of the recorded lateral pressure values from the real values. A correct π-A isotherm for OLA monolayers can be measured only up to π ) ∼8 mN · m-1 (see Figure 1). A similar stiffness effect only slightly reduced is observed for the binary mixture containing 10 mol % SA so that in this case correct isotherms are obtained only up to π ) ∼10 mN · m-1. In the case of 20 mol % SA in the OLA monolayer, artifactfree isotherms can already be recorded up to high lateral pressures (π ) ∼27 mN · m-1). In the low lateral pressure region measured, the shape of the π-A isotherms for mixtures with g90 mol % OLA proportion is different from those with a dominant SA proportion. In the case of mixtures with g90 mol % OLA, the lateral pressure starts to increase very slowly at ∼75 Å2 · molecule-1 and attains a value of only ∼1 mN · m-1 at ∼64 Å2 · molecule-1. Looking on the chemical structure of the OLA molecule shown in Figure 2, one realizes that the
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Figure 2. Chemical structure of OLA molecule.
triterpenoid OLA molecule is bulky. Therefore, reliable knowledge of the characteristics of the pure OLA monolayers is only possible on the basis of direct structural information. The shape of the π-A isotherms of the mixtures with a higher percentage of SA (g60 mol %) is similar to that of the pure SA monolayer but shifted to higher area values. The kink of the isotherms indicating the lateral transition pressure does not change but attenuates progressively with higher proportions of OLA. Structural Features of the Monolayers of the Pure Components. Grazing incidence X-ray diffraction was applied to elucidate the two-dimensional symmetry of the molecular inplane structures of the pure and mixed monolayers on the angstrom scale. GIXD is sensitive only to the condensed parts of the monolayer, while the liquid-expanded phase contributes to the background scattering. All diffraction studies were performed on water at 25 °C at different lateral pressures between 10 and 30 mN · m-1. Figure 3 shows the corrected X-ray intensities as a function of the in-plane scattering vector component Qxy (Bragg peaks) for the two pure components (SA and OLA) at 8 mN · m-1. The SA monolayer shows two Bragg peaks corresponding to the nearest neighbor (NN)-tilted L2 phase. The OLA monolayer has only one Bragg peak revealing a LS phase with hexagonal lattice structure. In the case of the OLA monlayer, the LS phase was found in the whole accessible pressure region. The diffraction peak was observed at 1.017 Å-1. This corresponds to a crosssection area of 44.1 Å2 · molecule-1. For comparison, the estimated value of the cross-section area using the OLA molecular data (see Figure 6C and later discussion) and assuming free rotation around the long molecular axis, is 44.2 Å2 · molecule-1. Compression did not distinctly change the peak position. Figure 4 (top) shows selected contour plots of the corrected X-ray diffraction intensities as a function of the in-plane scattering vector component Qxy and the out-of-plane scattering vector component Qz of pure SA obtained at 10, 20, and 30 mN · m-1. SA exhibits the well-known phase sequence L2-OV-LS. At 10 mN · m-1, the diffraction pattern shows two low-order diffraction peaks. The degenerate reflection above the horizon and the nondegenerate reflection at zero Qz are indicative of a condensed phase with a centered rectangular lattice of chains tilted in the direction toward nearest neighbors (NN) along the short axis of the in-plane unit cell (L2 phase). The lattice is distorted from hexagonal packing in NN direction (Qnxy > Qdxy, where Qnxyis the maximum position of the nondegenerate peak). With increasing pressure, the degenerate peak moves to larger Qxy and smaller Qz values indicating a decrease of the tilt angle. After reaching a critical tilt angle, the transition into the NNN-tilted OV phase, presented in Figure 3 (top) for 20
Figure 3. Corrected X-ray intensities versus the in-plane scattering vector component Qxy for pure SA (xOLA ) 0; top) and pure OLA (xOLA ) 1; bottom) obtained at 8 mN · m-1.
mN · m-1, occurs. This phase is characterized by an intensity distribution with two diffraction peaks at nonzero Qz values with Qzn ) 2Qzd. On further compression, a second phase transition into the LS phase occurs (shown for 30 mN · m-1 in Figure 3 (top)). The LS phase exhibits hexagonal lattice symmetry (one Bragg peak), and the chains are nontilted. This transition can be seen in the isotherm (see Figure 1) as a change of the slope indicating a change of the monolayer compressibility. Figure 4 (bottom) shows the corresponding contour plots of a mixture containing 10 mol% OLA. Distinct differences, that will be discussed later, can be seen. Further information about the two-dimensional packing features can be obtained from the thickness of the monolayers. The thickness of the diffracting layer can be estimated using Lz ∼ (0.9(2π))/fwhm(Qz). The SA monolayer is in the nontilted LS phase at high lateral pressure. The hydrocarbon chains are in the all-trans conformation so that a molecule length of 21.7 Å can be expected because the maximum length of a stretched alkyl chain with n CH2 groups amounts to lmax ) (1.26n + 1.5) Å. In the case of the SA monolayer, the measured fwhm(Qz) amounts to 0.26 Å-1 (the vertical instrumental resolution is negligible in this case) giving a thickness of 21.7 Å. The average value of the fwhm(Qz) of the OLA monolayer amounts to 0.38 Å-1 yielding a thickness of 14.9 Å. The thickness of the OLA monolayers was additionally derived from specular X-ray reflectivity data. Figure 5 shows the reflectivity curve together with the fit using a free-form model.19 The electron density profile derived from this approach
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Figure 4. Contour plots of the corrected X-ray intensities as a function of the in-plane and out-of-plane scattering vector components Qxy and Qz for pure SA (top; phase sequence, L2-Ov-LS) obtained at 10, 20, and 30 mN · m-1 (from left to right) and for the SA/OLA 9/1 mixture xOLA ) 0.1 (bottom; phase sequence, L2-LS) obtained at the same lateral pressures. Please note that only the Q-range corresponding to the measurement of the pure SA monolayer is shown and not the Q-range corresponding to the OLA-rich phase.
was described by a one-box model because the molecule does not exhibit regions with distinctly different electron densities. The molecular area determined from the π-A isotherm and the number of electrons of the molecule were used as fixed parameters during the fitting procedure. The thickness of the box amounts to z ) 15.2 ( 0.4 Å with an average electron density of F ) 0.332 electrons · Å-3 (F/Fwater ) 0.993). Thus, the thicknesses of the OLA monolayer determined by GIXD and XR coincide perfectly. The roughness of the layer (2.9 Å) is not larger than the roughness of water.23 That is a further indication for a tight packing of perpendicularly oriented OLA molecules at the water surface. For the discussion of the structural data obtained for the pure OLA monolayers, the OLA molecule structure of minimum energy, calculated using the Cerius2 Version 4.6. (Accelrys), after calculation of charges via the charge equilibration method is informative. Figure 2 shows the chemical structure of the OLA molecule. Figure 6 presents different perspectives of the OLA molecular structure: two different side views (A, B) and a plane view (C). Despite the presence of two hydrophilic groups (COOH and OH) in opposite positions, it is more probable that the COOH group is directed toward the aqueous subphase. The largest extension of the main axis of the OLA molecule with 14.6 Å (see Figure 6B; here, the van der Waals radii of the atoms are not considered) agrees well with the highest value for the thickness of the OLA monolayer derived from the X-ray data. The plane view of the OLA molecule (Figure 6C) displays a diameter of the cylindrical molecule (assuming free rotation) along the main axis of 7.5 Å corresponding to a circular crosssection area of 44.2 Å2 in complete agreement with the crosssection area of 44.1 Å2 · molecule-1 obtained from the position of the diffraction peak at 1.017 Å-1. The molecular data of OLA are consistent with the structure data of the GIXD results, indicating a perpendicular orientation of the OLA molecules in the condensed monolayer phase in the whole lateral pressure region.
Figure 5. Top: XR data (•) with calculated reflectivity (full line) corresponding to the parameter-free model used. Bottom: Electron density profile normalized by the electron density of water, F(z)/Fwater, versus the depth z corresponding to the reflectivity model shown above. The one-box model describing the observed electron profile is shown by dashed lines.
Structural Features of the Mixed SA/OLA Monolayers. In the following, the structural and phase properties of a binary monolayer system consisting of OLA and SA are studied. Figure 7 shows selected 3D 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 for the two pure components and several mixtures of SA and OLA at 20 mN · m-1. Already the addition of 10 mol % OLA to SA leads to the appearance of an additional Bragg peak at small Qxy values typical for the OLA lattice. This is an indication of a phase separation between SA and OLA. The additional Bragg peak increases continuously in intensity with increasing mole fraction of OLA, whereas the Bragg peaks ascribed to SA decrease in intensity. At a mole fraction of xOLA ) 0.9, only one single Bragg peak can be observed. This shows that under these conditions the mixed monolayer forms a homogeneous condensed phase. Obviously, more than 10 mol % of SA can be incorporated into the OLA lattice, while the same amount of OLA added to SA leads already to a phase separation. However, the phase separation is also not complete on this site of the phase diagram. The comparison of the contour plots of pure SA (Figure 3 top) and a mixture containing 10 mol % OLA (Figure 3 bottom) measured at the same lateral pressures reveals clear differences between both systems. The addition of 10 mol % OLA to SA leads not only to the appearance of an additional Bragg peak but also to a change in the phase sequence. As seen in Figure 3, the OV phase does not appear on compression of the mixed monolayer. The tilted L2 phase transforms directly into the nontilted LS phase.
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Figure 6. Different perspectives of the OLA molecular structure: two different side views (A, B) and a plane view (C).
Since the π-A isotherm shows a linear relation between lateral pressure and the molecular area, plotting of 1/cos(t) (t, tilt angle) as a function of the lateral pressure π, under the assumption that the cross-sectional area of the alkyl chains A0 is pressure-independent, allows the determination of the tilting phase transition pressure πt to the nontilted LS phase by extrapolation toward 1/cos(t) ) 1.24 Figure 8 shows such a linear extrapolation for the pure SA monolayer and the mixed layer containing 10 mol % OLA (xOLA ) 0.1). The extrapolated transition pressure of 24.6 mN/m for pure SA agrees well with the inflection point of the π-A isotherm. The tilting transition pressure of the mixture is slightly reduced (23.7 mN · m-1) compared with pure SA. These findings show that a small amount (clearly less than 10 mol %) of OLA has to be incorporated into the SA lattice, leading to a changed phase sequence and a reduction of the tilting transition pressure. The comparison of the Bragg peak position of pure OLA with the corresponding peak positions in the mixtures with 10 and 20 mol % SA shows that also the OLA lattice has been slightly
changed. The diffraction peak at 1.017 Å-1, observed for the pure OLA monolayer, corresponds to a molecular area of 44.1 Å2 · molecule-1. The OLA monolayer exhibits a very low compressibility; therefore, nonsymmetric compression by using a single barrier leads to the tilting of the Wilhelmy plate and to large inaccuracy of the measured pressure values. This is increasingly reduced in the mixed monolayers as the proportion of SA increases. In the mixed monolayer with 10 mol % SA, only one peak indicating a OLA-rich phase is observed. The mean molecular area determined from the peak position of the OLA-rich phase amounts to 42.8 Å2 · molecule-1. This shows that the incorporation of a small amount of SA into the OLA lattice leads to a tighter packing. Only in the mixtures containing 20 mol % SA, in addition to the Bragg peak of the OLA-rich phase, the Bragg peaks of SA appear indicating phase separation. The Bragg peaks have been fitted using a Lorentzian model. Assuming that the monolayer consists of 2D crystallites that are perfect and have a finite size Lxy, this size can be estimated by Lxy ∼ (0.9(2π))/fwhm(Qxy).18 A finite size of the crystalline
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Figure 7. 3D plots of the corrected X-ray intensities versus the in-plane and out-of-plane scattering vector components Qxy and Qz for pure SA (xOLA ) 0) (top left), the mixtures with xOLA ) 0.1 (top right), xOLA ) 0.7 (middle left), xOLA ) 0.8 (middle right), xOLA ) 0.9 (bottom left), and pure OLA (xOLA ) 1) (bottom right) obtained at 20 mN · m-1.
domains gives rise to broadening of the Bragg peaks exceeding the instrumental resolution of 0.008 Å-1. Otherwise, assuming an exponential decay of the correlation as found in liquid crystals, the fwhm corresponds to the correlation length ξ ) 2/fwhm(Qxy).15 In both cases, the measured fwhm has to be corrected by the instrumental resolution:
fwhm ) √(fwhmmeas)2 - (fwhmresol)2 For the pure OLA monolayer, Lxy ≈ 260 Å (ξ ≈ 93 Å) in the LS phase. This value is considerably smaller compared with that in the LS phase of SA (Lxy ≈ 490 Å). The addition of SA to OLA improves not only the packing density (42.8 Å2 · molecule-1 instead of 44.1 Å2 · molecule-1) but also the
correlation length in the layer (Lxy ≈ 350 Å in the mixture compared with Lxy ≈ 260 Å in the pure OLA monolayer). Figure 9 summarizes the results in a schematic phase diagram. The phase diagram is characterized by large miscibility gaps. At low lateral pressures, both compounds form different monolayer phases. Therefore, a limited miscibility can be already expected on the basis of the miscibility selection rules of liquid crystals.25 At high lateral pressures, both compounds form a LS phase, but they are still largely immiscible because the molecular areas of these two compounds, and, therefore, the packing conditions are completely different. The length of the SA molecule of ∼22 Å is larger than that of the OLA molecule with ∼14.6 Å. This is obviously the reason that the
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Figure 8. Tilt angle t (as 1/cos(t)) of pure SA (2) and the 9:1 SA/ OLA mixture (•) vs the lateral pressure. The extrapolation (solid line for the 9:1 SA/OLA mixture and dashed line for pure SA) to 1 yields the tilting transition pressure.
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15783 OV phase, and, finally, to the hexagonal LS phase with increasing lateral pressure. The cross-section area of pure OLA obtained from the position of the diffraction peak at 1.017 Å-1 agrees well with the circular cross-section of 44.2 Å2 · molecule-1 determined from the modeled OLA molecule. The thicknesses of the OLA monolayer determined by GIXD with 14.9 Å and XR with 15.2 ( 0.4 Å coincide reasonably. The structure data of the GIXD and XR results, indicating a perpendicular orientation of the OLA molecules in the condensed monolayer phase in the whole lateral pressure region, are consistent with the calculated molecular data of OLA. Insight has been gained into the structural and phase properties of a binary monolayer system consisting of OLA and SA. A schematic phase diagram, which is characterized by large miscibility gaps, has been constructed on the basis of the structure data obtained for the selected binary SA/OLA mixtures. The packing properties of both components after addition of minor amounts of the other component are completely different. The amount of OLA which can be integrated into the SA lattice is smaller than 10 mol %, whereas more than 10 mol % of SA can be incorporated into the OLA lattice. The addition of small amounts of OLA to the SA monolayer leads to a change in the phase sequence by disappearance of the OV phase, whereas the addition of small amounts of SA to the OLA monolayer improves the packing density from 44.1 to 42.8 Å2 · molecule-1 and the correlation length from ∼260 to ∼350 Å, but does not change the phase structure. References and Notes
Figure 9. Schematic phase diagram of SA/OLA. π is the lateral pressure and xOLA is the mole fraction of OLA. LS1 and LS2 are the SA and OLA rich phases, correspondingly, with a hexagonal lattice structure.
amount of OLA which can be integrated into the SA lattice must be clearly less than 10 mol %, whereas more than 10 mol % of SA can be incorporated into the OLA lattice before the overall system attains an unfavorable energetic state. The lattices of the pure compounds are changed by the addition of the other component. The addition of OLA to SA leads to a change in the phase sequence by disappearance of the OV phase. The addition of SA to OLA changes the packing density but does not change the phase structure. Conclusions Langmuir monolayers are representative model systems to obtain information of the interfacial ordering of oleanolic acid and its interaction with other components of epicuticular waxes. These features have been studied using a binary monolayer system consisting of OLA and stearic acid. A combination of lateral pressure studies, grazing incidence X-ray diffraction and specular X-ray reflectivity measurements, has proven to be optimal in this respect. The structural features of pure OLA monolayers have been characterized and compared with those of SA monolayers. The OLA monlayer exists only in the LS phase in the whole accessible pressure region. The SA monolayer develops the phase sequence from the NN-tilted L2 phase to the NNN-tilted
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