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Disorder in Langmuir Monolayers: 2. Relation between Disordered Alkyl Chain Packing and the Loss of Long-Range Tilt Orientational Order G. Weidemann, G. Brezesinski, D. Vollhardt,* C. DeWolf, and H. Mo¨hwald Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chaussee 5, 12489 Berlin, Germany Received August 5, 1998. In Final Form: January 21, 1999 A loss of long-range tilt orientational order in 1,2-hexadecandiol monolayers is observed by Brewster angle microscopy. It coincides with a coexistence of two different lattices in the monolayer over a large surface pressure range detected by grazing incidence X-ray diffraction. In one lattice the alkyl chains are tilted toward nearest neighbors and in the other toward next-nearest neighbors. In monolayers of 2-hydroxypalmitic acid, 2-palmitoylglycerol, and 2-hexadecanol a condensed phase with short-range tilt orientational order is formed. The diffraction patterns show a superposition of the peaks from lattices which differ with respect to the tilt azimuth of the alkyl chains. The behavior of 1,2-hexadecandiol monolayers is related to that of 2-hydroxypalmitic acid, 2-palmitoylglycerol, and 2-hexadecanol. In monolayers of 2-hydroxypalmitic acid/palmitic acid mixtures a crossover from one behavior to the other is observed. The loss of order in Langmuir monolayers is initiated by a misfit of alkyl chains and headgroups. Chains can only adapt to headgroups which are additionally too large perpendicular to the tilt direction by a distortion of their packing. If the size of the headgroup perpendicular to the tilt direction exceeds a particular value, a disordered packing results. If substances with smaller headgroups are added to such monolayers, initially the spacing perpendicular to the chain tilt direction is optimized and then second the polar tilt angle is reduced.
Introduction The order in Langmuir monolayers has been addressed in many recent papers.1-14 The monolayers were studied both with respect to the packing of the molecules and with respect to the textures in the monolayers. The molecular packing in Langmuir monolayers was studied with grazing incidence diffraction (GID), while the textures were studied with polarized fluorescence microscopy and * Corresponding author. (1) 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. (2) Dutta, P. Phase Transition in Surface Films 2; New York, 1991; pp 183-200. (3) Peterson, I. R.; Brzesinski, V.; Kenn, R. M.; Steitz, R. Langmuir 1992, 8, 2995-3002. (4) Kaganer, V. M.; Peterson, I. R.; Kenn, R. M.; Shih, M. C.; Durbin, M.; Dutta, P. J. Chem. Phys. 1995, 102, 9412-9422. (5) Brezesinski, G.; Scalas E.; Struth, B.; Mo¨hwald, H.; Bringezu, F.; Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8758-8762. (6) Scalas, E.; Brezesinski, G.; Mo¨hwald, H.; Kaganer, V. M.; Bouwman, W. G.; Kjaer, K. Thin Solid Films 1996, 284/285, 56-61. (7) Weidemann, G.; Brezesinski, G.; Bringezu, F.; de Meijere, K.; Vollhardt, D.; Mo¨hwald H. J. Phys. Chem. 1998, 102, 148-153. (8) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 224, 213-219. (9) Henon, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148-9154. (10) a) Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Colloid Interface Sci. 1995, 174, 392-399. (b) Weidemann, G.; Gehlert, U.; Vollhardt, D. Langmuir 1995, 11, 864-871. (11) Qiu, X.; Ruiz-Garcia, J.; Stine, K. J.; Knobler, C. M.; Selinger, J. V. Phys. Rev. Lett. 1991, 67, 703-706. (12) Weidemann, G.; Vollhardt, D. Langmuir 1996, 12, 5114-5119. (13) (a) Schwartz, D. K.; Ruiz-Garcia, J.; Qiu, X.; Selinger, J. V.; Knobler, C. M. Physica A 1994, 204, 606-615. (b) Ruiz-Garcia, J.; Qiu, X.; Tsao, M.-W.; Marshall, G.; Knobler, C. M.; Overbeck, G. A.; Mo¨bius, D. J. Phys. Chem. 1993, 97, 6955-6957. (c) Schwartz, D. K.; Tsao, M.-W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 8258-8261. (d) Rivie´re, S.; Meunier, Phys. Rev. Lett. 1995, 74, 2495-2498. (14) (a) Fischer, B.; Tsao, W.-M.; Ruiz-Garcia, J.; Fischer, Th. M.; Schwartz, D. K.; Knobler, C. M. Thin Solid Films 1996, 284/285, 110114. (b) Fischer, B.; Tsao, M.-W.; Ruiz-Garcia, J.; Fischer, T. M.; Schwartz, D. K.; Knobler, C. M. J. Phys. Chem. 1994, 98, 7430-7435.
Figure 1. π-A isotherm of a 1,2-hexadecandiol monolayer at 27 °C.
Brewster angle microscopy (BAM). Langmuir monolayers of most substances possess long-range tilt orientational order (up to several 100 µm)8-14 while the positional order extends only over a few 100 Å.1-7 However, for 1-hexadecylglycerol monolayers the length of the tilt orientational order was observed to fall below the resolution limit of the microscope during compression.15 Monolayers of 1,2hexadecandiol show the same behavior. To investigate the origin of such loss of orientational order, these monolayers are studied both with BAM and GID. Other substances such as 2-hydroxypalmitic acid, 2-palmitoylglycerol, and 2-hexadecanol were studied and found to show no tilt orientational order at the resolution limit of our microscope. The question arises whether the absence of long-range tilt orientational order is correlated to the disorder on the molecular level. GID measurements showed that the packing of these molecules is disordered in monolayers and that this is caused by headgroups which (15) Gehlert, U.; Vollhardt, D. Prog. Colloid Polym. Sci. 1994, 97, 302-306.
10.1021/la980981h CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
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Figure 2. BAM images of a 1,2-hexadecandiol monolayer at 27 °C upon compression at 3 mN/m (a), 3.5 mN/m (b), 4 mN/m (c), 9 mN/m (d), 20 mN/m (e), and 42 mN/m (f). The bar represents 100 µm.
are too large to allow an adaptation of the chains to the headgroup lattice.16 To test this finding, 2-hydroxypalmitic acid was mixed with palmitic acid to improve the packing. The consequences both for the packing and for the tilt orientational order are discussed. Finally, 1,2-hexadecandiol was mixed with 1-hexadecanol to probe whether the loss of tilt orientational order observed for this substance can also be ascribed to a misfit of headgroups and alkyl chains. (16) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; Mo¨hwald H. Langmuir 1998, 14, 6485-6492.
Experimental Section Grazing Incidence Diffraction. Grazing incidence X-ray diffraction was performed using the liquid-surface diffractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, Germany.17-19 A monochromatic beam with a wavelength of 1.481 (17) Als-Nielsen, J.; Mo¨hwald, H. in “Handbook on Synchrotron Radiation”, Vol. 4, Ed. Ebashi, S., Koch, M., Rubenstein, E. Amsterdam, Oxford, New York, Tokyo, 1994 pp 1-53. (18) Kjaer, K. Experimental Stations at HASYLAB. January 1994, Jan, 88-89. (19) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Lahav, M.; Leiveiller, F. Leiserowitz, L. Phys. Rep. 1994, 246, 251-321.
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Figure 3. Diffraction patterns of a 1,2-hexadecandiol monolayer at 26 °C upon compression at different surface pressures (indicated). Å was adjusted to strike the surface with an angle of incidence Ri ≈ 0.85Rc (Rc ≈ 0.14° is the critical angle for total reflection). A linear position sensitive detector (PSD) (OED-100-M, Braun Garching, Germany) was used to monitor the diffracted intensity as a function of the vertical scattering angle Rf. A Soller collimator in front of the PSD provides a resolution of 0.09° for the in-plane scattering angle 2θ. According to the geometry of diffraction20 the scattering vector Q can be written in terms of an in-plane component Qxy and an out-of plane component Qz, where
Qxy )
2π cos2 Ri + cos2 Rf - 2 cos Ri cos Rf cos 2θ λx
(1)
2π (sin Ri + sin Rf) λ
(2)
and
Qz )
Whereas the in-plane component provides information about the lattice spacings, the out-of-plane scattering component can provide information about the polar tilt angle t and the tilt azimuth Ψ of the alkyl chains. These two parameters can be obtained from the scattering vector components according to the cylinder model:21 hk Qhk z ) Qx cos Ψhk tan t
(3)
Brewster Angle Microscopy. For the observation of the monolayers a Brewster angle microscope (BAM1 from NFT, Go¨ttingen) was mounted on a Langmuir film balance (FW2 from Lauda). The lateral resolution of the BAM1 is about 4 µm. An image processing software was used to optimize the contrast and to correct the BAM images for the distortion due to the observation at the Brewster angle. In the present work only the well-illuminated part (500 × 500 µm2) of the original images was used. (20) Kjaer, K. Physica B 1994, 198, 100-109. (21) Als-Nielsen, J.; Kjaer, K. In Phase Transitions in Soft Condensed Matter; Rist, T., Sherrington, D., Eds.; NATO ASI Series B; 1989; New York, Vol. 211, pp 113-138.
Materials. Palmitic acid and 1-hexadecanol were purchased from Sigma, Deisenhofen (approximately 99%); DL-2-hydroxypalmitic acid was obtained from Avocado Research Chemicals, Heysham, England (approximately 98%). These substances were used without further purification. DL-1,2-hexadecandiol was purchased from Fluka, Neu-Ulm (>90%) and distilled with a 50 cm long Fischer-Spaltrohr column (Fischer) in a vacuum (10-210-3 mbar). The purity was proven to be >98% by HPLC. Heptane (UVASOL) and ethanol (p.A. grade), which are used to prepare spreading solutions (9:1), were obtained from Merck, Darmstadt. The subphase water used for the experiments was purified by a Millipore desktop (Millipore, Eschborn). For the investigation of 2-hydroxypalmitic acid monolayers and the mixed monolayers containing this substance the pH of the subphase water was adjusted to 2 using HCl (Titrisol) from Merck, Darmstadt.
Results and Discussion In 1,2-hexadecandiol monolayers, the condensed phase is formed from a fluid phase by a first-order phase transition indicated by a plateau region in the π-A isotherm at T g 26 °C (Figure 1). The domains initially formed at 27 °C are subdivided into regions of different reflectivity (Figure 2a). The optical anisotropy of the tilted alkyl chains gives rise to a reflectivity which depends on the molecular orientation with respect to the plane of incidence. The regions within the domains differ, consequently, with respect to the molecular orientation. All domains have a characteristic notch. Starting from this notch, in some domains regions without visible optical anisotropy are formed (disordered region at the left side of the domain in Figure 2b). The visible optical anisotropy vanishes when the size of the regions of uniform orientation falls below the resolution limit of the microscope. This indicates that the tilt orientational order becomes short range. Upon further compression, the regions without long-range tilt orientational order grow at the expense of the regions with long-range tilt orientational order (Figure 2c). The anisotropic regions start to disap-
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Table 1. Peak Positions of 1,2-Hexadecandiol Monolayers (Qxy, In-Plane Scattering Vector Component; Qz, Out-of-Plane Component of the Scattering Vector)a 11 + 11 h
02
02 (NN)*
DL-1,2-hexadecanediol, 26 °C
Qxy [Å-1]
Qz [Å-1]
Qxy [Å-1]
Qz [Å-1]
Qxy [Å-1]
7 mN/m 11 mN/m 15 mN/m 22 mN/m 30 mN/m 43 mN/m
1.360 1.437 1.443 1.450 1.474 1.491
0.63 0.32 0.31 0.27 0.17 0
1.455 1.356 1.374 1.404 1.455
0 0.66 0.61 0.53 0.34
1.457 1.460 1.465
a At some pressures additionally the Q value of a lattice with xy NN-tilted chains can be observed.
pear at the end of the plateau region of the isotherm (Figure 2d). However, optical anisotropy evolves again upon further compression at about 20 mN/m (Figure 2e). This anisotropy does not disappear until the alkyl chains become upright at about 42 mN/m, corresponding to the kink in the isotherm (Figure 2f). This is quite similar to what has been reported for monolayers of 1-hexadecylglycerol.15 To investigate the reason for this behavior, GID measurements were performed at 26 °C. Two peaks of a lattice with alkyl chains tilted toward nearest neighbors (NN)4 are observed in the diffraction pattern of a 1,2hexadecandiol monolayer at 7 mN/m (Figure 3). The diffraction pattern at 11 mN/m clearly shows three peaks which cannot be attributed to an oblique lattice of tilted alkyl chains. According to the cylinder model ( eq 321), no tilt azimuth can give rise to three peaks with such maximum positions with respect to Qz. Two of the peaks rather belong to a lattice of alkyl chains tilted toward next-nearest neighbors (NNN). The additional peak with Qz ) 0 can be identified as the (02)-peak of a lattice with NN-tilted chains (Table 1). This peak also appears in the diffraction patterns at 15 and 22 mN/m in addition to the peaks of the NNN-tilted lattice. At 30 mN/m, it is difficult to decide whether it is still present. The degenerated peak of the lattice with NN-tilted chains is broader and too weak to be clearly observed. At 43 mN/m the chains stand upright in agreement with the BAM observations. The GID results suggest that the loss of the long-range tilt orientational order is related to the coexistence of two different lattices in the monolayer over a large surface pressure range. If attention is focused on the shape of the peaks, one notices that the degenerate (11),(11 h )-peak (i.e., the peak with Qz > 0 for NN-tilted chains and the peak with the smaller Qz value for NNN-tilted chains) is asymmetric which can be attributed to a variation of the tilt azimuth Ψ. A deviation from Ψ ) 0° (NN) or Ψ ) 90° (NNN) forces the (11)- and the (11h )-peak to shift in opposite directions, resulting in an asymmetric peak.16 Analyzing the lattice parameters of the 1,2-hexadecandiol monolayers, one finds a high cross-sectional area of alkyl chains A0. Even in the state with upright chains, it amounts to 20.5 Å2 (Table 2). Evidently, the headgroup is too large to allow the chains to achieve a dense packing. Then, the addition of molecules with a smaller headgroup should allow the monolayer to cross over to a more ordered arrangement. Such a molecule is 1-hexadecanol. The plateau pressure of the isotherm is decreased by the addition of 1-hexadecanol to 1,2-hexadecandiol. Thus, condensed material is formed during spreading at 27 °C in the mixed monolayers. To compensate for this effect, the 1,2-hexadecandiol/1-hexadecanol mixtures were studied at 33 °C (Figure 4). The 1,2-hexadecandiol shows the same behavior as that at 27 °C: first domains with long-
Table 2. Lattice Parameters of 1,2-Hexadecandiol Monolayers (a,b, Lattice Spacings; Axy, In-Plane Area per Molecule; t, Polar Tilt Angle; TA, Tilt Azimuth; A0, Cross-Sectional Area of Alkyl Chains; bNN, Lattice Spacing of the Lattice with NN-Tilted Chains Additionally Observed at Some Surface Pressures) DL-1,2-hexadecanediol, 26 °C a [Å] b [Å] Axy [Å] t [deg] 7 mN/m 11 mN/m 15 mN/m 22 mN/m 30 mN/m 43 mN/m
5.468 4.959 4.951 4.952 4.901 4.866
8.637 9.267 9.146 8.950 8.637 8.428
23.6 23.0 22.6 22.2 21.2 20.5
29 26 24 21 13 0
TA NN NNN NNN NNN NNN
A0 [Å2] bNN [Å] 20.7 20.7 20.7 20.7 20.6 20.5
8.625 8.607 8.578
Figure 4. π-A isotherms of 1,2-hexadecandiol/1-hexadecanol mixed monolayers at 33 °C. The mixing ratios are indicated.
range tilt orientational order are formed (Figure 5a) and upon further compression the tilt orientational order becomes short-range (Figure 5b). In a monolayer containing 10 mol % 1-hexadecanol the notch in the domains, in which the formation of material with short-range tilt orientational order starts, is absent (Figure 5c). Most domains are subdivided into seven segments of different molecular orientation. Such an arrangement was first observed in monolayers of 1-palmitoylglycerol.10 The segment angle was found to correlate to a particular lattice angle.5 This lattice angle should increase with a decrease in the polar tilt angle.5,10 Consequently, a crossover to six segments is expected with decreasing polar tilt angle. The addition of 1-hexadecanol reduces the average headgroup size and thus the polar tilt angle. This is confirmed by the observation of about 70% 6-fold subdivided domains for a 1-hexadecanol content of 20 mol % (Figure 5e). The loss of long-range orientational order upon further compression occurs only at the edge of the domains in monolayers with a 1-hexadecanol content of 10 mol % (Figure 5d). A sudden reorientation in the domains indicates a transition from a phase with NN-tilted chains to a phase with NNN-tilted chains. In monolayers with a 1-hexadecanol content of 20 mol %, only the sudden reorientation is observed (Figure 5f), indicating that the tilt orientational order remains long-range upon compression. After the influence of a reduction of the average headgroup size was studied, an increase of headgroup size is now of interest. This was done by use of 2-hydroxypalmitic acid, a molecule with a terminal carbonyl group. This compound shows a plateau region in the π-A isotherm corresponding to a first-order phase transition. Brewster angle microscopy, however, shows no conspicuous formation of domains of the condensed phase (Figure 6a). Evidently, there is almost no contrast between the condensed and fluid phases in the BAM. The monolayer appears only slightly more inhomogeneous than in the fluid phase.
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Figure 5. BAM images of a 1,2-hexadecandiol/1-hexadecanol monolayer at 33 °C: Pure 1,2-hexadecandiol monolayers at 10 mN/m (a) and 14 mN/m (b); monolayer containing 10 mol % 1-hexadecanol at 6 mN/m (c) and 9 mN/m (d); monolayer containing 20 mol % 1-hexadecanol at 4 mN/m (e) and 7 mN/m (f). The bar represents 100 µm.
In a previous paper 2-hydroxypalmitic acid was studied by GID.16 The diffracted intensity indicated a disordered packing of alkyl chains. The diffracted intensity was found to result from the superposition of the peaks of lattices with the same polar tilt angle but different tilt azimuth. Obviously, there is a correlation between the disordered packing of alkyl chains and the absence of long-range tilt orientational order. It is conceivable that the variation of the tilt azimuth with respect to the lattice directions results in a variation of the tilt azimuth with respect to macroscopic directions. This is confirmed by the coinci-
dence of both phenomena in monolayers of other substances. A variation of the tilt azimuth with respect to the lattice directions was also observed for 2-palmitoylglycerol and 2-hexadecanol.16 In monolayers of both substances domains without long-range orientational order are formed (Figure 6b,c). Again, the contrast between the fluid and the condensed phase is very low in the BAM images. The domains appear as a diffuse pattern. These observations raise two questions: (I) Is it also possible in this system to cross over from disorder to order by decreasing the average headgroup size?
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Figure 6. BAM images of the phase coexistence region of monolayers without visible optical anisotropy: 2-hydroxypalmitic acid (a), 2-palmitoyl-glycerol (b), 2-hexadecanol (c). The bar represents 100 µm.
Figure 7. π-A isotherms of 2-hydroxypalmitic acid/palmitic acid mixed monolayers at 24 °C, pH 2. The mixing ratios are indicated.
(II) Do we observe a behavior similar to that of 1,2hexadecandiol as an intermediate step? To answer these questions, again, a mixture of substances with large and small headgroups is studied. 2-Hydroxypalmitic acid and palmitic acid were chosen owing to their miscibility. The mixed monolayers were studied at 24 °C (Figure 7). The plateau pressure of the π-A isotherms decreases as a consequence of the addition of the substance with a smaller headgroup (i.e., palmitic acid). Even for a monolayer containing 60 mol % palmitic acid, the plateau is still present in the isotherm.
A mixed monolayer containing 20 mol % palmitic acid still shows only a diffuse pattern of domains without longrange tilt orientational order (Figure 8a). Monolayers containing 40 mol % palmitic acid resemble the images of 1,2-hexadecandiol (Figure 8b). Domains with regions in which the tilt orientational order extends over several 100 µm coexist with regions in which the parts of homogeneous orientation are smaller than the resolution limit of the microscope. When the content of palmitic acid is increased to 50 mol %, only domains with long-range tilt orientational order are initially formed (Figure 8c). However, the long-range tilt orientational order is lost upon compression (Figure 8d), similar to the mixed monolayers of 90 mol % 1,2-hexadecandiol and 10 mol % 1-hexadecanol. In monolayers containing 60 mol % palmitic acid, the long-range tilt orientational order remains present until the chains become upright (Figure 8e,f). A sudden reorientation, characteristic for a transition from a phase with NN-tilted chains to a phase with NNN-tilted chains is observed (Figure 8f). Altogether, we observe by BAM a transition from disorder to order and the behavior of 1,2-hexadecandiol layers appears to be an intermediate step in this transition. What happens during the transition from disorder to order on the molecular level? This question was addressed using GID measurements on mixed monolayers of 2-hydroxypalmitic acid and palmitic acid. In a pure 2-hydroxypalmitic acid monolayer the diffracted intensity is distributed along a characteristic curve (Figures 9 and
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Figure 8. BAM images of 2-hydroxypalmitic acid/palmitic acid mixed monolayers at 24 °C, pH 2: Monolayers containing 20 mol % palmitic acid at 11 mN/m (a); 40 mol % palmitic acid at 7 mN/m (b); 50 mol % palmitic acid at 5 mN/m (c) and 7 mN/m (d); 60 mol % palmitic acid at 3 mN/m (e) and 12 mN/m (f). The bar represents 100 µm.
10). This curve can be interpreted as the superposition of peaks from various chain lattices with the same polar tilt angle but different polar tilt azimuths16 (Table 3). The intensity distribution found in the experimental curve arises when the tilt azimuth varies over the whole range from a nearest neighbor to a next-nearest neighbor direction. In monolayers containing 20 mol % palmitic acid the diffracted intensity is still distributed along a curve (Figures 9 and 10), in agreement with the absence of the long-range tilt orientational order. However, upon compression the chains become upright at about 38 mN/
Table 3. Peak Positions Which Can Be Determined from the Intensity Distribution of 2-Hydroxypalmitic Acid Monolayers Are the Qxy Values for NN-Tilted Chains and the Qz, and Qxy Values for NNN-Tilted Chains (the Latter No As Precisely As Usual) DL-2-hydroxypalmitic acid, pH 2, 24 °C 18 mN/m 25 mN/m 35 mN/m 45 mN/m
02 (NN)*
02 (NNN)*
Qxy [Å-1]
Qxy [Å-1]
Qz [Å-1]
1.450 1.454 1.461 1.471
1.35 1.38 1.40 1.43
0.57 0.52 0.47 0.37
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Figure 9. Diffraction patterns of 2-hydroxypalmitic acid/palmitic acid mixed monolayers (100/0 and 80/20) at 24 °C, pH 2, and different surface pressures (indicated). Table 4. Peak Positions Which Can Be Determined from the Intensity Distribution of Mixed Monolayers Containing 80 mol % 2-Hydroxypalmitic Acid and 20 mol % Palmitic Acid Are the Qxy Values for NN-Tilted Chains and the Qz and Qxy Values for NNN-Tilted Chains (the Latter No As Precisely As Usual) DL-2-hydroxypalmitic acid/palmitic acid: 8/2, pH 2, 24 °C 18 mN/m 25 mN/m 35 mN/m 45 mN/m
02 (NN)* Qxy [Å-1] 1.458 1.466 1.476 1.487
02 (NNN)*
Qz [Å-1]
Qxy [Å-1]
Qz [Å-1]
1.37 1.41 1.45
0.57 0.52 0.37
0
For 45 mN/m a single peak at Qz ) 0 Å-1 according to a lattice with vertical chains is observed. a
m. This allows the determination of the cross-sectional area of the alkyl chains at 45 mN/m which still amounts to 20.6 Å2 (Table 4), larger than that observed for 1,2hexadecandiol. After the palmitic acid content is increased to 40 mol %, sharp peaks are observed, even though the diffracted intensity was quite low at 10 mN/m, resulting in very noisy peaks (Figures 9 and 10, Tables 5 and 9). The diffraction pattern at 18 mN/m resembles that of 1,2-
hexadecandiol at 11 mN/m. Again a coexistence of two lattices with NN- and NNN-tilted chains is observed. For a monolayer with 60 mol % palmitic acid the long-range tilt orientational order is preserved upon compression. In agreement with this observation no indication of a disordered packing of alkyl chains is observed by GID (Figures 9 and 10, Tables 6 and 10). We can summarize: monolayers which show a similar behavior with respect to the long-range orientational order give rise to a similar diffraction pattern. The transition from a disordered to an ordered packing of alkyl chains can be studied quantitatively by an analysis of the diffraction pattern. In a previous work it was shown that, in the case of the distributed intensity, information about the packing of the molecules is still available.16 The position of the intensity maximum in the xy-direction at Qz ) 0 Å-1 corresponds to the Qxy value of the (02)-peak of a lattice with NN-tilted chains (Tables 3 and 4). The diffraction curve ends in a sharp edge toward low Qxy, which corresponds to the (02)-peak of a lattice with NNNtilted chains. Consequently, the b-axis of the lattice can be determined for NN- and NNN-tilt (the latter not as precisely as usual) (Tables 7,8). Furthermore, the polar
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Figure 10. Diffraction patterns of 2-hydroxypalmitic acid/palmitic acid mixed monolayers (60/40 and 40/60) at 24 °C, pH 2, and different surface pressures (indicated). Table 5. Peak Positions of Mixed Monolayers Containing 60 mol % 2-Hydroxypalmitic Acid and 40 mol % Palmitic Acid (Qxy, In-Plane Scattering Vector Component; Qz, Out-of-Plane Component of the Scattering Vector) DL-2-hydroxypalmitic acid/palmitic acid: 6/4, pH 2, 24 °C 10 mN/m 18 mN/m 25 mN/m 35 mN/m
02 Qxy [Å-1]
Qz [Å-1]
1.461 1.387 1.431 1.490
0 0.55 0.41 0
11 + 11 h Qxy Qz [Å-1] [Å-1] 1.362 1.450 1.467
0.62 0.30 0.25
02 (NN)* Qxy [Å-1] 1.467 1.473
a At some pressures additionally the Q value of a lattice with xy NN-tilted chains can be observed.
tilt angle can be estimated from the Qz-position of the (02)-peak of the lattice with NNN-tilted chains. The values are also given in Tables 7 and 8. A large headgroup was expected to be the reason for the disorder.16 Chains usually adapt to large headgroups by increasing the tilt angle. However, for 2-hydroxypalmitic acid monolayers the polar tilt angles are quite small. On the other hand, the spacing perpendicular to the tilt direction (bNN ) 8.666 Å) is much larger than that expected for alkyl chains packed hexagonal in the plane perpen-
Table 6. Peak Positions of Mixed Monolayers Containing 40 mol % 2-Hydroxypalmitic Acid and 60 mol % Palmitic Acid (Qxy, In-Plane Scattering Vector Component; Qz, Out-of-Plane Component of the Scattering Vector) DL-2-hydroxypalmitic acid/palmitic acid: 4/6, pH 2, 24 °C 10 mN/m 18 mN/m 25 mN/m 35 mN/m
11 + 11 h
02 Qxy
[Å-1]
1.474 1.423 1.475 1.498
Qz
[Å-1]
0 0.46 0.23 0
Qxy [Å-1]
Qz [Å-1]
1.393 1.467 1.488
0.55 0.24 ∼
dicular to their axis. This indicates that the alkyl chain packing must be distorted to accommodate the large headgroups.16 Obviously, there exists a limit for this distortion. In a previous paper a bNN value of 8.6 Å was found to be the limit up to which an adaptation is possible.16 The bNN values decrease continuously with increasing content of the substance with a smaller headgroup (palmitic acid), whereas the polar tilt angle remains almost constant (Table 11). First, the packing perpendicular to the polar tilt direction is optimized and then the polar tilt angle is decreased. The cross-sectional area of alkyl chains is also decreased, however, not as drastically as bNN. On the whole, a disordered packing of alkyl chains results if
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Table 7. Lattice Parameters Which Can Be Determined from the Intensity Distribution of 2-Hydroxypalmitic Acid Monolayers Are the b-Spacings for Chains Tilted toward Nearest (bNN) and Next-Nearest Neighbors (bNNN) as Well as the Polar Tilt Angle (t) DL-2-hydroxypalmitic acid, pH 2, 24 °C
b (NN)* [Å]
b (NNN)* [Å]
t [deg]
8.666 8.643 8.601 8.543
9.45 9.11 8.98 8.79
23 21 19 15
18 mN/m 25 mN/m 35 mN/m 45 mN/m
Table 8. Lattice Parameters Which Can Be Determined from the Intensity Distribution of Mixed Monolayers Containing 80 mol % 2-Hydroxypalmitic Acid and 20 mol % Palmitic Acid Are the b-Spacings for Chains Tilted toward Nearest (bNN) and Next-Nearest Neighbors (bNNN) as Well as the Polar Tilt Angle (t) DL-2-hydroxypalmitic acid/palmitic acid: 8/2, pH 2, 24 °C
b (NN)* [Å]
b (NNN)* [Å]
t [deg]
8.618 8.572 8.514 8.451
9.17 8.91 8.67 8.451
23 20 14 0
18 mN/m 25 mN/m 35 mN/m 45 mN/m
Table 9. Lattice Parameters Which Can Be Determined from the Intensity Distribution of Mixed Monolayers Containing 60 mol % 2-Hydroxypalmitic Acid and 40 mol % Palmitic Acid DL-2-hydroxypalmitic acid/palmitic acid: 6/4, pH 2, 24 °C 10 mN/m 18 mN/m 25 mN/m 35 mN/m
a [Å]
b [Å]
Axy [Å2]
t [deg]
5.466 4.934 4.906 4.869
8.601 9.060 8.781 8.434
23.5 22.4 21.5 20.5
28 22 16 0
TA
A0 [Å2]
NN 20.7 NNN 20.7 NNN 20.7 20.5
the headgroup extension exceeds a certain value (about 8.6 Å) perpendicular to the tilt direction. Conclusions The absence of long-range tilt orientational order observed in monolayers of different substances is correlated to a disordered packing of alkyl chains. The diffracted intensity can be explained by the superposition of the peaks of lattices with the same polar tilt angle but different tilt azimuths. This behavior results if the alkyl chains are not able to adapt their packing to the lattice of the headgroup. Chains can only adapt to headgroups which are additionally too large perpendicular to the tilt direction by a distortion of their packing. If the size of the headgroup exceeds a particular value perpendicular to the tilt direction, a disordered packing results. Mixing such substances with substances with a smaller headgroup allows an improvement of the packing of the alkyl chains. First, the spacing perpendicular to the chain tilt direction is reduced whereas the polar tilt angle remains almost constant. A decrease of the polar tilt angle is observed when the chains have achieved an ordered packing.
Table 10. Lattice Parameters Which Can Be Determined from the Intensity Distribution of Mixed Monolayers Containing 40 mol % 2-Hydroxypalmitic Acid and 60 mol % Palmitic Acid (a,b, Lattice Spacings; Axy, In-Plane Area per Molecule; t, Polar Tilt Angle; TA, Tilt Azimuth; A0, Cross-Sectional Area of Alkyl Chains) DL-2-hydroxypalmitic acid/palmitic acid: 6/4, pH 2, 24 °C 10 mN/m 18 mN/m 25 mN/m 35 mN/m
a [Å]
b [Å]
5.315 4.898 4.862 4.843
8.525 8.831 8.520 8.389
Axy t [Å2] [deg] 22.7 21.6 20.7 20.3
25 18 9 0
TA
A0 [Å2]
NN NNN NNN
20.5 20.6 20.5 20.3
Table 11. Variation of the Characteristic Lattice Parameters as a Function of the Composition of 2-hydroxypalmitic Acid/Palmitic Acid Mixed Monolayers (a, b - Lattice Spacings; AXy- in Plane Area Per Molecule; t - Polar Tilt Angle; TA - Tilt Azimuth; Ao - Cross Sectional Area of Alkyl Chains) A0 [Å2] 2-hydroxypalmitic palmitic bNN [Å] tNN [deg] (upright acid acid (18 mN/m) (18 mN/m) chains) 100% 80% 60% 40%
0% 20% 40% 60%
8.666 8.618 8.566 8.525*
23 23 22 18
>20.6 20.6 20.5 20.3
The extent of loss of order can vary. First, a loss of long-range tilt orientational order is observed upon compression, coinciding with the coexistence of lattices with NN- and NNN-tilted chains over a large surface pressure range (>10 mN/m). The degenerate peaks of the rectangular lattice are clearly asymmetric, indicating additionally a distribution of the tilt azimuth around the two most probable values 0° (NN) and 90° (NNN). In monolayers with a less dense packing of chains the tilt azimuth is distributed over the whole range between a NN and a NNN direction. In such monolayers the tilt orientational order is already absent in the condensed phase formed from the fluid phase in the first-order transition indicated by the plateau region in the π-A isotherms. The contrast between condensed and fluid phases is very weak. The stepwise loss of order was confirmed by the study of 2-hydroxypalmitic acid/palmitic acid mixtures. Pure 2-hydroxypalmitic acid monolayers show a complete absence of long-range tilt orientational order whereas monolayers containing 40 mol % palmitic acid behave like 1,2-hexadecandiol monolayers, which show a loss of long-range tilt orientational order upon compression. Acknowledgment. Financial assistance from the Deutsche Forschungsgemeinschaft (Sfb 312) and the Fonds der Chemischen Industrie is gratefully acknowledged. The authors are grateful to Dr. G. Czichocki for the purification of 1,2-hexadecandiol. We also thank K. Kjaer for help with setting up the X-ray experiment. LA980981H
