Effect of Hydroxyl Group Position and System Parameters on the

The characteristic features of hydroxystearic acid monolayers OH-substituted in the mid position of the alkyl chain deviate considerably from those of...
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Langmuir 2004, 20, 7670-7677

Effect of Hydroxyl Group Position and System Parameters on the Features of Hydroxystearic Acid Monolayers D. Vollhardt,*,† S. Siegel,† and D. A. Cadenhead‡ Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany, and Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000 Received March 12, 2004. In Final Form: June 21, 2004 The characteristic features of hydroxystearic acid monolayers OH-substituted in the mid position of the alkyl chain deviate considerably from those of the usual nonsubstituted stearic acid. The phase behavior, domain morphology, and two-dimensional lattice structure of 9-, 11-, and 12-hydroxystearic acids are studied, using π-A isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD), to obtain detailed information on the effect of the exact position of the OH-substitution. The π-A isotherms of all three hydroxyoctadecanoic acids have an extended flat plateau region, the extension of which only slightly decreases with the increase of temperature. At the same temperature, the extension of the plateau region increases and the plateau pressure decreases from 9-hydroxyoctadecanoic acid to 12-hydroxyoctadecanoic acid. The absolute -∆H and -∆S values for the phase transition increase slightly from 9-hydroxyoctadecanoic acid to 12- hydroxyoctadecanoic acid and indicate differences in the ordering of the condensed phase under consideration of the special reorientation mechanism of these bipolar amphiphiles at the fluid/condensed phase transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region is specific for the position of the OH-substitution of the alkyl chain, just as the lattice structures of the condensed monolayer phase. 11-hydroxyoctadecanoic acid monolayers form centered rectangular lattices with the chain tilt toward the NNN (next nearest neighbor) direction, and 12-hydroxyoctadecanoic acid monolayers have an oblique lattice over the entire pressure range. A special feature of 9-hydroxystearic acid monolayers is the phase transition between two condensed phases observed in the π-A isotherm of 5 °C at ∼18 mN/m, where the centered rectangular lattice shows a NNN/NN transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region, just as the lattice structures of the condensed monolayer phase, reveal the high specifity of the monolayer feature of the bipolar hydroxystearic acids OH-substituted in the mid position.

Introduction Membrane lipids, in which hydroxyl (OH) groups are positioned in the lipophilic part as secondary polar groups, are constituents of the structural architecture of biological membranes and, thus, abundant in biological systems.1 The membrane characteristics are modified in different ways by the presence of a secondary polar OH group in lipids when the OH group is substituted at different positions of the alkyl chain.2 Therefore, already, early studies have focused on the use of Langmuir monolayers of hydroxyfatty acids as model systems to obtain some information on their special features. Bergstro¨m et al. synthesized all 17 isomeric hydroxystearic acids with the objective to obtain more knowledge on the effect of the functionalization in dependence on the location of the OHsubstitution.3 As they never published results of the surface properties of this interesting isomeric series, there are only results of monolayer studies with selected isomeric hydroxyfatty acids so far. In previous studies,4-6 Cadenhead et al. found a remarkable dependence of the twodimensional phase properties on the position of the OH * Corresponding author. † Max Planck Institute of Colloids and Interfaces. ‡ State University of New York at Buffalo. (1) Dowing, D. T. Rev. Pure Appl. Chem. 1961, 11, 196. (2) Pascher, I. Biochim. Biophys. Acta 1976, 455, 433. (3) Bergstro¨m, S.; Aulin-Erdtman, G.; Rolander, B.; Stenhagen, E.; O ¨ stling, S. Acta Chem. Scand. 1952, 6, 1157. (4) Kellner, B. M.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452. (5) Kellner, B. M.; Cadenhead D. A. Chem. Phys. Lipids 1979, 23, 41. (6) Matuo, H.; Rice, D. K.; Balthasar, D. M.; Cadenhead, D. A. Chem. Phys. Lipids 1982, 30, 367.

group in the alkyl chain on the basis of surface pressurearea (π-A) isotherms of hydroxypalmitic acids. They found large differences in the π-A isotherms between fatty acids OH-substituted in the 2-position and those where the OHsubstitution is in the mid position of the alkyl chain. Some tailored and selected bipolar amphiphiles of interest for membrane lipids were investigated to obtain information about molecules with different polar substituents in the hydrocarbon chain and their intermolecular interactions in monolayers.7-10 More recent studies11,12 have shown that, in the case of OH-substitution in the 2-position, a loss of ordering in the condensed monolayer phase was observed, due to a misfit of the alkyl chain and the headgroup enlarged by the neighboring OH group in the 2-position. On the other hand, Brewster angle microscopy results obtained with 9-hydroxypalmitic acid monolayers revealed the formation of well-shaped condensed phase domains and two-dimensional lattice structures in the condensed phase state.13 Fluorescence microscopy studies of 12-hydroxystearic acid monolayers indicated also (7) Menger, F. M.; Richardson, S. D.; Wood, M. G.; Sherrod, M. J. Langmuir 1989, 5, 833. (8) Huda, M. S.; Fujio, K.; Uzu, Y. Bull. Chem. Soc. Jpn. 1996, 69, 3387. (9) Jacobi, S.; Chi, L. F.; Plate, M.; Overs, M.; Scha¨fer, H. J.; Fuchs, H. Thin Solid Films 1998, 327, 180. (10) Overs, M.; Fix, M.; Jacobi. S.; Chi, L. F.; Sieber, M.; Scha¨fer, H. J.; Fuchs, H.; Galla, H. J. Langmuir 2000, 16, 1141. (11) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; Mo¨hwald, H. Langmuir 1998, 14, 6485. (12) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; DeWolf, C.; Mo¨hwald, H. Langmuir 1999, 15, 2901. (13) Siegel, S.; Vollhardt, D.; Cadenhead, D. A. Prog. Colloid Polym. Sci. 2002, 121, 67.

10.1021/la049345b CCC: $27.50 © 2004 American Chemical Society Published on Web 08/07/2004

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regular condensed phase domains.14 This suggest that, at OH-substitution of fatty acids in different mid positions of the alkyl chain, structured condensed monolayer phases may be expected. The objective of the present paper is to obtain some knowledge on how the monolayer features of stearic acid are affected if the OH-substitution in the mid positions of the alkyl chain is slightly changed. For this purpose, the phase behavior, morphological texture, and lattice structure of 9-, 11-, and 12-hydroxystearic acid monolayers are studied at different temperatures using surface pressure-area (π-A) isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD). Experimental Section 9- and 11-Hydroxystearic acids obtained from Nu-Check Prep Inc., Elysian, MN, were recrystallized several times from heptane prior to use. 12-Hydroxystearic acid purchased from Sigma in a nominal 99.9% purity was used without further purification. Ultrapure deionized water produced by “Purelab Plus” (Seral, Germany), with the conductivity of 0.055 µS/cm, was used to prepare the pH 3 subphase water adjusted by the addition of 0.1 M HCl. The monolayer materials were dissolved in a 9:1 (v/v) mixture of n-heptane (for spectroscopy, Merck) and ethanol (p.a., Merck) and spread onto ultrapure water. The surface pressure-area (π-A) isotherms were measured at different temperatures using a self-made computer-interfaced film balance. The trough was equipped with a Wilhelmy balance for the surface pressure determination and a temperature control system. The surface pressure was measured with a reproducibility of (0.1 mN/m using the Wilhelmy method and a roughened glass plate. Imaging of the monolayers was performed with a Brewster angle microscope (BAM1+, NFT, Go¨ttingen, Germany) mounted on the film balance. The light source of the Brewster angle microscope was a He-Ne laser (10 mW). The lateral resolution of the BAM1+ was ∼4 µm. An image-processing software was used to correct the distortion of the digitized images, as the BAM images are distorted due to the observation at the Brewster angle. More detailed information on the experimental setup and the BAM method is given elsewhere; see, for example, refs 15 and 16. The grazing incidence X-ray diffraction (GIXD) experiments were performed using the liquid-surface diffractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, Germany. After the monolayer was spread, the trough container was closed, purged with He gas, and brought to the desired temperature. The scattered X-ray intensity was detected by a position-sensitive detector (PSD) equipped with a Soller collimator which provides a resolution for the horizontal scattering angle (2θxy) of ∼0.01 Å-1. Each sample consists of two-dimensional (2D) crystallites that are randomly oriented vis-a`-vis the normal to the aqueous surface and, thus, represents a 2D powder. The measurements were performed by scanning the horizontal scattering factor Qxy ≈ (4π/λ) sin θxy, where 2θxy is the angle between the incident and diffracted beams projected onto the horizontal plane. The scattering vector Q ) kf - ki consists of an in-plane component (Qxy) and the out-of-plane component Qz ≈ (2π/λ) sin Rf, where λ is the X-ray wavelength (λ ) 1.30 Å).17,18 The GIXD data are represented as a 2D intensity, I(Qxy, Qz), as a function of Qxy and Qz. The diffracted intensities were corrected for polarization, effective area, and Lorentz factor. The Qxy positions of the Bragg peaks yield the lattice-repeat distances d ) 2π/Qxy which can be indexed by two indices (h and k) to yield the lattice parameters of the unit cell area (Axy) which is defined by Axy ) ab sin γ. The polar tilt angle (t) of the long molecule axis and the tilt azimuth (ψxy) are calculated from the positions of the hk Qz maxima according to Qhk z ) Qxy cos ψhk tan t. The cross(14) Asgharian, B.; Cadenhead, D. A. Langmuir 2000, 16, 677. (15) Vollhardt, D. Adv. Colloid Interface Sci. 1996, 64, 143. (16) Meunier, J. Colloids Surf., A 2000, 171, 33. (17) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (18) Kjaer, K. Physica B 1994, 198, 100.

Figure 1. Surface pressure-area isotherms of 9-hydroxyoctadecanoic acid monolayers at different temperatures.

Figure 2. Surface pressure-area isotherms of 11-hydroxyoctadecanoic acid monolayers at different temperatures.

Figure 3. Surface pressure-area isotherms of 12-hydroxyoctadecanoic acid monolayers at different temperatures. sectional area of the alkyl chain (A0) is related to the unit cell area (Axy, area per molecule parallel to the interface) and the tilt angle (t) by A0 ) Axy cos t.

Results and Discussion The experimental π-A isotherms of the monolayers of n-hydroxyoctadecanoic acid (n ) 9, 11, or 12) measured at various temperatures and subphase pH 3 are shown in Figures 1-3. At a small pH of the subphase, the hydroxyfatty acid monolayers are nearly undissociated. The isotherms of all three hydroxyoctadecanoic acids are characterized by an extended flat (over a large area range) plateau region, the extension of which only slightly decreases as the temperature increases. Consequently, for all measured temperatures, the π-A isotherms have the main phase transition between the fluid phase and the condensed phase at π > 0 in the accessible region. The

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Figure 4. Comparison of the surface pressure-area isotherms of 9 (black)-, 11 (red)-, and 12 (blue)-hydroxyoctadecanoic acid monolayers at T ) 20 °C.

surface pressure of the plateau increases less than it does in the case of usual amphiphiles as the temperature increases. The kink point at A ) Ac is characteristic for the onset of the first-order phase transition. Correspondingly, at A > Ac, the monolayers exist in the fluid (gaseous, LE) state. Theoretical calculations of the π-A isotherms have been performed to explain the strong effect of the position of the OH-substitution on the thermodynamic properties of the monolayers. The calculations provide reasonable agreement between the theoretical predictions and the experimental π-A isotherms.19 The OH-substitution in the mid position of the alkyl chain is obviously responsible for the special characteristics of the π-A isotherms deviating from those of typical amphiphilic monolayers. This conclusion is corroborated by a comparative study of 1-(12-hydroxy)stearoyl-racglycerol monolayers which show a similar flat and extended plateau region over a large area range, although the unsubstituted 1-stearoyl-rac-glycerol monolayers behave as usual amphiphilic monolayers.20 Despite the general similarities, the comparison of the π-A isotherms of 9 (Figure 1)-, 11 (Figure 2)-, and 12 (Figure 3)hydroxyoctadecanoic acid monolayers reveals clear differences. At the same temperature, the extension of the plateau region increases from 9-hydroxyoctadecanoic acid to 12-hydroxyoctadecanoic acid, whereas the plateau pressure decreases. This is clearly seen in Figure 4, which shows the π-A isotherms of 9-, 11-, and 12-hydroxyoctadecanoic acid monolayers at a selected temperature of 20 °C. The π-A isotherms point to a special feature of 9-hydroxyoctadecanoic acid monolayers that is well-visible at 5 °C (Figure 5). In addition to the main phase transition (bottom arrow in Figure 5), a kink at ∼18 mN/m indicates a phase transition between two condensed phases (top arrow in Figure 5). This interesting second phase transition at π ) 18 mN/m between two condensed phases has been satisfactorily described by a theoretical model which assumes two-dimensional compressibility of the condensed monolayer.19,21 The GIXD results discussed later provide detailed information on this phase transition. It is interesting to consider the temperature dependence of the phase transition pressure (πt) of the octadecanoic acids OH-substituted in the mid position (Figure 6). Linear πt (T) relations exist for all three n-hydroxyoctadecanoic (19) Vollhardt, D.; Fainerman, V. B. J. Phys. Chem. B 2004, 108, 297. (20) Vollhardt, D.; Weidemann, G.; Lang, S. J. Phys. Chem. B 2004, 108, 3781. (21) Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. B 2003, 107, 3098.

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Figure 5. Surface pressure-area isotherm of a 9-hydroxyoctadecanoic acid monolayer at T ) 5 °C. The arrows indicate two phase transition points.

Figure 6. Temperature dependence of the main phase transition pressure (πt) of 9-, 11-, and 12-hydroxyoctadecanoic acid monolayers.

monolayers. The dπt/dT values are very similar: 0.23 mN/m per K for 9-hydroxyoctadecanoic acid, 0.20 mN/m per K for 11-hydroxyoctadecanoic acid, and 0.20 mN/m per K for 12-hydroxyoctadecanoic acid. In comparison to usual amphiphiles with one alkyl chain (with dπt/dT ∼ 1 mN/m per K), the dπt/dT slopes of the 9-, 11- and 12hydroxyoctadecanoic acid monolayers are very small and only very slightly dependent on the position of the OHsubstitution. The two-dimensional Clapeyron equation representing a one-component approximation can be used for calculating the enthalpy change (∆H) of the phase transition

∆H ) (Ac - Ae)T

dπt dT

(1)

where Ae is the molecular area at the onset of the phase transition at the surface pressure πt and Ac is the area of the condensed phase. Figure 7 shows that ∆H depends linearly on the temperature for all three hydroxyoctadecanoic acid monolayers OH-substituted in the mid position. Recently, a new theoretical model has been introduced that accounts for the presence of solvent at the surface. It was shown that the absolute enthalpy values calculated

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Figure 7. Temperature dependence of the enthalpy change for the main phase transition of 9-, 11-, and 12-hydroxyoctadecanoic acid monolayers.

Figure 8. Temperature dependence of the entropy change for the main phase transition of 9-, 11-, and 12-hydroxyoctadecanoic acid monolayers.

from this two-dimensional solution model are ∼2 times lower than those obtained using the Clapeyron equation. This fact should not affect the differences obtained in the general thermodynamic comparison of the three OHsubstituted hydroxyoctadecanoic acid monolayers.22 The temperature dependence of the entropy change (∆S) for the phase transition presented in Figure 8 resembles that of ∆H. ∆S is given by the expression ∆H/T. Negative ∆H and ∆S values are obtained according to the exothermic nature of the main phase transition at compression of amphiphilic monolayers and an increase in the ordering of the system. Now, a comparison of the absolute (22) Vollhardt, D.; Fainerman, V. B. J. Phys. Chem. B 2002, 106, 12000.

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∆H and ∆S values are of interest. For all three octadecanoic acids OH-substituted in the mid position, the absolute values of -∆H and -∆S increase as the temperature decreases, indicating that the ordering of the condensed phase increases as the temperature decreases. However, the absolute values of -∆H and -∆S are different for the three OH-substituted octadecanoic acids and increase from 9-hydroxyoctadecanoic acid to 12- hydroxyoctadecanoic acid. As already discussed in previous papers, such longchain fatty acids OH-substituted in the mid position can be considered to be bipolar amphiphiles that in the very expanded state lie flat on the surface as both polar groups contact the aqueous subphase. At the main phase transition, the weaker polar OH group is forced out of the water interface and well-ordered 2D lattice structures stabilized by hydrogen bonding between the OH groups are formed. Differences in the absolute -∆S values suggest differences in the ordering of the condensed phase with the consequence of higher ordering for the higher absolute -∆S values. Accordingly, from the comparison of the thermodynamic data for the main phase transition, it may be interpreted that the ordering of the condensed monolayer phase increases from 9-hydroxyoctadecanoic acid to 12hydroxyoctadecanoic acid. The BAM studies provide information on the morphological features of the condensed phase domains formed in the two-phase coexistence region. Now, it is interesting to note in which way the small changes of the OHsubstitution in the mid position of the alkyl chain affects the morphology of the condensed phase domains. Representative examples for the domains of 9-hydroxyoctadecanoic acid grown in the two-phase coexistence region at 5, 20, and 25 °C are presented in Figure 9. It is seen that the domain shape shows striking differences at different temperatures but at all temperatures a center exists. However, the homogeneous reflectivity of the domains indicates the absence of an inner texture. At low temperatures (5 °C), compact domains are formed which have at the two opposite irregular edges in each case two extensions. At higher temperatures (20 and 25 °C), fourarm structures with 2-fold symmetry are developed. The domains grow rather irregular with more or less developed sidearms. At 20 °C, two small acute angles and two large obtuse angles between the main arms are formed in opposite directions. With increasing temperature (25 °C), these angles approach each other and become ∼90°. The domain morphology of 9-hydroxyoctadecanoic acid monolayers resembles that of the lower chain homologue 9-hydroxypalmitic acid, the shape of which is more regular and reaches with the development of two additional arms at 25 °C 6-fold symmetry.13 The morphology of 11-hydroxyoctadecanoic acid domains is very different. Figure 10 shows characteristic domain shapes formed in the two-phase coexistence region at different temperatures (5, 15, 20, 25, and 30 °C). Whereas at the lower temperatures (5-15 °C) the domains look rather needlelike, their shape resembles lancets with increasing thickness in the mid position at higher temperatures (g20 °C). Also, in the case of OH-substitution in the 11-position, homogeneous reflectivity points to the absence of inner textures. The domain growth begins, just as in the case of 9- and 12-hydroxyoctadecanoic acids, at the onset of the phase transition (A g Ac). The number and size of the domains increase at further compression. Figure 11 shows three typical steps of domain growth at compression of an 11-hydroxyoctadecanoic acid monolayer within the two-phase coexistence region at T ) 30 °C. The domain shape is preserved during the growth at the phase transition within the plateau region.

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Figure 9. Representative condensed phase domains of 9-hydroxyoctadecanoic acid monolayers at different temperatures. Image size: 750 × 750 µm2.

Finally, it is seen from the BAM images of the 12-hydroxyoctadecanoic acid domains that small changes in the position of the OH-substitution change their main characteristics. Figure 12 shows typical domain textures of 12-hydroxystearic acid monolayers measured at 10, 20, and 30 °C. Again, the domains of 12-hydroxyoctadecanoic acid are homogeneously reflecting, but, in the case of 12OH-substitution, they develop several arms with the tendency to form curvatures, especially in the medium temperature region, and grow rather irregularly with differences in the growth direction. Studies of 1-(12hydroxy)stearoyl-rac-glycerol monolayers have shown that similar phase and structural features are observed with an amphiphile of another homologous series having features completely different to those of fatty acids.20 The similarity of the π-A isotherms and the domain morphology demonstrates the dominating effect of the alkyl chain

Figure 10. Representative condensed phase domains of 11hydroxyoctadecanoic acid monolayers at different temperatures. Image size: 750 × 750 µm2.

substitution by a hydroxyl group in the mid position on the monolayer properties. The GIXD data confirm the conclusions on the effect of small changes in the OH-position in the alkyl chain on the structure of the condensed monolayer phase. According to earlier work, the microscopic textural features of the condensed monolayer phases are related to the twodimensional lattice structure.23-25 The contour plots of (23) Brezesinski, G.; Scalas, E.; Struth, B.; Mo¨hwald, H.; Bringezu, F.; Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8758. (24) Melzer, V.; Vollhardt, D.; Weidemann, G.; Brezesinski, G.; Wagner, R.; Mo¨hwald, H. Phys. Rev. E 1998, 57, 901.

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Figure 11. Domain growth at slow compression of an 11hydroxyoctadecanoic acid monolayer within the two-phase coexistence region at T ) 30 °C. Image size: 750 × 750 µm2.

the corrected diffraction intensities as a function of the in-plane (Qxy) and out-of-plane (Qz) components of the scattering vectors reveal considerable differences between 9-, 11-, and 12-hydroxyoctadecanoic acid monolayers (Figures 13-15). The structure data calculated for different surface pressures of the three OH-substituted octadecanoic acids are listed in Table 1, wherein a, b, and γ are the unit cell parameters, Axy is the in-plane molecule area, t is the polar tilt angle, td is the tilt direction, ψa is the angle between the azimuthal tilt direction and the a-axis, and A0 is the cross-sectional area of the alkyl chain. The contour plots of 9-hydroxyoctadecanoic acid are of special interest because the kink in the π-A isotherm at 5 °C indicates a phase transition of two condensed phases at ∼18 mN/m (Figure 13). The two reflexes of the contour (25) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; DeWolf, C.; Mo¨hwald, H. Langmuir 1999, 15, 2901.

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Figure 12. Representative condensed phase domains of 12hydroxyoctadecanoic acid monolayers at different temperatures. Image size: 750 × 750 µm2.

plots indicate a centered rectangular lattice, but according to their position in the lower surface pressure region at π ) 10 mN/m (both reflexes with Qz > 0), the molecular tilt is in the NNN (next nearest neighbor) direction, whereas, at higher surface pressures (π ) 20 mN/m), the molecules are tilted toward the NN (nearest neighbor) direction (Qz ) 0 and Qz > 0). This change of the lattice structure, which occurs at ∼18 mN/m, corroborates the phase transition between the two condensed phases. The tilt of the alkyl chains decreases as the surface pressure increases. Finally, the molecules are not tilted at high surface pressures (π ) 25 mN/m). The GIXD studies demonstrate that, in agreement with the π-A isotherms, 11- and 12OH-substituted octadecanoic acids show no phase transition between condensed monolayer phases. The contour plots of 11-hydroxyoctadecanoic acid show two reflexes with Qz > 0 (Figure 14) characteristic of a centered rectangular lattice with the tilt of the alkyl chains toward the NNN direction. The

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Figure 13. Contour plots of the corrected diffraction intensities as a function of the in-plane (Qxy) and out-of-plane (Qz) components of the scattering vectors of a 9-hydroxyoctadecanoic acid monolayer at 5 °C. Left, π ) 10 mN/m; right, π ) 20 mN/m.

Figure 14. Contour plots of the corrected diffraction intensities as a function of the in-plane (Qxy) and out-of-plane (Qz) components of the scattering vectors of an 11-hydroxyoctadecanoic acid monolayer at 5 °C. Left, π ) 10 mN/m; below, π ) 20 mN/m.

Figure 15. Contour plots of the corrected diffraction intensities as a function of the in-plane (Qxy) and out-of-plane (Qz) components of the scattering vectors of a 12-hydroxyoctadecanoic acid monolayer at 5 °C. Left, π ) 10 mN/m; below, π ) 20 mN/m.

polar tilt of the alkyl chains is somewhat larger than that in the case of the 9OH. Considerable differences compared to the lattice structures of 9- and 11-hydroxyoctadecanoic acids are seen in the contour plots of 12-hydroxyoctadecanoic acid (Figure 15). The three reflexes (Qz > 0) indicate an oblique lattice of the alkyl chains over the entire surface pressure range. Despite the OH-substitution of the alkyl chain in the mid position, the cross-sectional area of the alkyl chain (A0) is not increased compared to that of amphiphiles with nonsubstituted alkyl chains, as shown by the A0 data of Table 1. Conclusions The characteristic features of n-hydroxyoctadecanoic acids are essentially affected if the OH-substitution in the mid position of the alkyl chain is only slightly changed. This can be demonstrated by studying the phase behavior,

morphological texture, and lattice structure of 9-, 11-, and 12-hydroxyoctadecanoic acid monolayers at different temperatures using π-A isotherms and BAM and GIXD techniques. The OH-substitution in the mid position of the alkyl chain is obviously responsible for the common special characteristics of the π-A isotherms deviating from those of typical amphiphilic monolayers. An extended flat plateau region, the extension of which only slightly decreases with the increase of temperature, is typical for the π-A isotherms of all three hydroxyoctadecanoic acids. Despite the general similarities, there are clear differences in the π-A isotherms. At the same temperature, the extension of the plateau region increases from 9-hydroxyoctadecanoic acid to 12-hydroxyoctadecanoic acid, whereas the plateau pressure decreases. The absolute -∆H and -∆S values for the phase transition are different for the three OH-substituted octadecanoic acids and increase

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Table 1. Lattice Structure Data of 9-Hydroxyoctadecanoic Acid, 11-Hydroxyoctadecanoic Acid, and 12-Hydroxyoctadecanoic Acid Monolayersa 9-Hydroxyoctadecanoic Acid conditions

π (mN/m)

a (Å)

b (Å)

γ (deg)

Axy (Å2)

t (deg)

td

A0 (Å2)

pH 3, 5 °C

10 20 25

4.79 4.63 4.62

4.89 4.82 4.81

120.7 122.6 122.6

20.7 19.6 19.5

12.6 4.8 0

NNN NN

20.2 19.5

conditions

π (mN/m)

a (Å)

b (Å)

γ (deg)

Axy (Å2)

t (deg)

td

A0 (Å2)

pH 3, 5 °C

10 20

4.95 4.69

4.99 4.85

120.5 122.1

21.4 19.9

21.5 9.3

NNN NNN

19.9 19.7

conditions

π (mN/m)

a (Å)

b (Å)

γ (deg)

Axy (Å2)

t (deg)

Ψa

A0 (Å2)

pH 3, 5 °C

6 10 20

4.61 4.60 4.40

4.99 4.99 5.01

112.3 112.3 114.0

21.3 21.2 20.2

20.3 19.5 5.5

28 25 30

20.0 20.0 20.0

11-Hydroxyoctadecanoic Acid

12-Hydroxyoctadecanoic Acid

a π, surface pressure; a, b, and γ, lattice constants; A , molecular area; t, polar tilt angle; A , cross-sectional area of the alkyl chain; td, xy 0 tilt direction; ψa, angle between the azimuthal tilt direction and the a-axis.

slightly from 9-hydroxyoctadecanoic acid to 12- hydroxyoctadecanoic acid. The differences in the absolute -∆S values suggest differences in the ordering of the condensed phase under consideration of the special reorientation mechanism of these bipolar amphiphiles at the fluid/ condensed phase transition. The results of the BAM and GIXD studies provide information on the structural features of the monolayers of 9-, 11-, and 12-hydroxyoctadecanoic acids. Specific domain textures, more or less temperature dependent and homogeneously reflecting, are formed by each of these isomeric amphiphiles, for instance, four-arm structures growing from a center at 9-hydroxyoctadecanoic acid, needles or lancetlike domains at 11-hydroxyoctadecanoic acid, and curved needlelike structures at 12-hydroxyoctadecanoic acid.

The lattice structures of the n-hydroxyoctadecanoic acids are also affected by the position of the OHsubstitution of the alkyl chain. In the case of 11- and 12hydroxyoctadecanoic acids, the lattice type of the condensed monolayer phase is unchanged over the accessible surface pressure range. 11-Hydroxyoctadecanoic acid monolayers have a centered rectangular lattice with the chain tilt toward the NNN direction; 12-hydroxyoctadecanoic acid monolayers have an oblique lattice. A phase transition between two condensed phases is a special feature of 9-hydroxyoctadecanoic acid monolayers. According to the kink in the π-A isotherm of 5 °C at ∼18 mN/m, the centered rectangular lattice shows a NNN/ NN transition. LA049345B