Phase Transition in Monolayers of Straight Chain and 2-Methyl

BAM images provides information on the phase behavior and the first-order main phase transition of the monolayers. Striking differences in the depende...
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Langmuir 2002, 18, 6571-6577

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Phase Transition in Monolayers of Straight Chain and 2-Methyl Branched Alcohols at the Air-Water Interface D. Vollhardt,*,† G. Emrich,† S. Siegel,† and R. Rudert‡ Max-Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany, and Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12200 Berlin, Germany Received February 14, 2002 Monolayers of straight chain and 2-methyl branched chain alcohols with alkyl chain lengths of C10-C18 are experimentally studied by a conventional film balance technique combined with a Brewster angle microscope (BAM). The comparison of the surface pressure (π)-area (A) isotherms with the corresponding BAM images provides information on the phase behavior and the first-order main phase transition of the monolayers. Striking differences in the dependence of the phase transition pressure on temperature of the straight chain and 2-methyl-substituted alcohols are correlated with differences in the molecular ordering. The general conditions for the main phase transition in the corresponding homologous alcohols can be derived. The effect of alkyl chain length and 2-methyl substitution on the general textural features of the condensed phase domains is determined under equilibrium and nonequilibrium conditions. n-Alcohol monolayers form defined and well-shaped condensed phase domains, often with inner texture in equilibrium. The long-range orientational order is strongly reduced in the condensed phase of 2-methyl-alcohols. Therefore, in the two-phase coexistence region of 2-methyl-alcohol monolayers only irregularly shaped domains without any inner structure are formed, which cannot be observed at the medium alkyl chain length C14 because of the low contrast. Model calculations of the two-dimensional lattice structure of the racemic 2-methylhexadecanol on the basis of the pg space group are performed and correspond well with the reduced ordering concluded from the experiments.

Introduction The continuous interest in Langmuir monolayers is mainly motivated by the fact that amphiphilic monolayers are representative of two-dimensional model systems. New sensitive techniques have provided new insights into the microscopic and submicroscopic structure of monolayer phases. In recent years, surface spectroscopic techniques,1 such as sum-frequency spectroscopy,2 Fourier transform infrared spectroscopy,3 surface plasmon spectroscopy,4 Raman scattering,5 and linear and nonlinear optical techniques, especially Brewster angle microscopy,6 fluorescence microscopy,7 ellipsometry, and second-harmonic generation8 but also X-ray and neutron diffraction9,10 or reflectivity,11 have been developed for studying the features of monolayers. Information on ordering and structure features of Langmuir monolayers has been effectively progressed by coupling the results of Brewster * Corresponding author. † Max-Planck Institute of Colloids and Interfaces. ‡ Federal Institute for Materials Research and Testing. (1) Bain, C. D.; Greene, P. R. Curr. Opin. Colloid Interface Sci. 2001, 6, 313. (2) Bell, G. R.; Li, Z. X.; Bain, C. D.; Fischer, P.; Duffy, D. C. J. Phys. Chem. B 1998, 102, 9461. (3) Blaudez, D.; Buffetau, T.; Desbat, B.; Turlet, J. M. Curr. Opin. Colloid Interface Sci. 1999, 4, 265. (4) Knoll, W. Curr. Opin. Colloid Interface Sci. 1999, 4, 273. (5) Beattie, D. A.; Haydock, S.; Bain, C. A. Vib. Spectrosc. 2000, 24, 109. (6) Meunier, J. Colloids Surf., A 2000, 171, 33. (7) Lo¨sche, M.; Mo¨hwald, H. Rev. Sci. Instrum. 2000, 55, 1968. (8) Mo¨ller, G.; Schradre, S.; Motschmann, H.; Prescher, D. Langmuir 2000, 16, 4594. (9) Als-Nielsen, J.; Mo¨hwald, H. In Handbook on Synchrotron Radiation; Ebashi, S., Koch, M., Rubenstein, E., Eds.; New York, 1994; Vol. 4, pp 1-53. (10) Penfold, J. Curr. Sci. 2000, 78, 1458. (11) Gilchrist, V. A.; Lu, J. R.; Garrett, P.; Penfold, J. Langmuir 1999, 15, 250.

angle microscopy (BAM) and X-ray diffraction at grazing incidence (GIXD) with thermodynamic data.12-14 Although a large number of monolayer-forming amphiphiles has been synthesized, simple long-chain fatty acids, their methyl- and ethylesters, and long-chain alcohols have been investigated most extensively and are often used as model substances.15-17 The differences between the three homologous series are significant. Modifications of the chemical structure of the amphiphile, such as branching of the alkyl chains, the degree of unsaturation along the chains, and functionalization in dependence on the location of the substitution have found continuous interest. Some special features of the homologous alcohols can be expected since the diameter of the uncharged headgroup (-OH) is smaller than that of the alkyl chain. For example, the polar tilt of the alkyl chains is small and is determined by the packing of the alkyl chains.18 It is interesting to note that the number of monolayer phases is smaller than that of the fatty acids. There are at least three reasons for our interest in a further study of medium- and long-chain alcohols: (i) Medium-chain alcohols are the main surface active impurities of soluble sodium alkyl sulfates and have a dominant effect on their surface properties. (ii) The lattice structure has been mostly studied for long-chain alcohols.18 At chain lengths equal to or larger than C16, condensed phase domains are (12) Vollhardt, D. Adv. Colloid Interface Sci. 1996, 64, 143. (13) Vollhardt, D. Adv. Colloid Interface Sci. 1999, 79, 19. (14) Vollhardt, D. Adv. Colloid Interface Sci. 2000, 86, 103. (15) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (16) Mingotaud, A.-F.; Mingotaud, C.; Patterson, L. K. Handbook of Monolayers; Academic Press: San Diego, CA, 1993; Vol. 1. (17) Foster, W. J.; Shih, M. C.; Pershan, P. S. Mater. Res. Soc. Proc. 1995, 375, 187. (18) Kaganer, V. M.; Brezesinski, G.; Mo¨hwald, H.; Howes, P. B.; Kjaer, K. Phys. Rev. E 1999, 59, 2141.

10.1021/la0201671 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/26/2002

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formed already at π ≈ 0 mN m-1, that is, already after spreading. Therefore, the textures of the long-chain alcohols are determined by the spreading conditions and the prehistory.19 (iii) Long-chain alcohols are good candidates for studying the effect of chain branching as the larger diameter of the alkyl chain dominates the lattice structure. In the present paper, we focus on Langmuir monolayers of straight and 2-methyl branched chain alcohols with alkyl chain lengths of C10-C18 by combination of the results of the π-A isotherms and BAM. Comparison of the temperature dependence of the phase transition pressure for the two homologous series should provide information on the effect of the 2-methyl substitution on the phase behavior and molecular ordering in the condensed phase. The reduced ordering expected for the 2-methyl substitution of the alkyl chain should be corroborated by the morphological characterization of the condensed monolayer phases and model calculations of the two-dimensional lattice of the racemic 2-methyl-hexadecanol.

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Figure 1. π-A compression isotherms of n-tetradecanol monolayers at different temperatures.

Experimental Section The experimental setup used consists of a computer-interfaced film balance combined with a Brewster angle microscope (BAM 2; NFT Go¨ttingen, Germany).12 The surface pressure-area (πA) isotherms were measured with the Wilhelmy method using a roughened glass plate. The surface pressures are reproducible to (0.1 mN m-1, and the areas per molecule to (0.005 nm2. The film balance and microscope are sheltered in a cabinet in order to avoid excessive disturbances by convection and contamination by impurities. The microscope is equipped with a special scanning technique for providing sharp images. Images of the monolayer morphology were stored using a system that included video camera, recorder, monitor, and image processing unit. The application of a green laser (Uniphase, San Jose, CA) allows a lateral resolution of the BAM of approximately 3 µm. After evaporation of the spreading solvent, the molecules remaining at the air-water interface were continuously compressed and expanded at rates of 0.002-1.35 nm2 molecule-1 min-1 depending on the amphiphilic alcohol measured. The n-alcohols and the spreading solvent n-heptane were obtained from Merck or Sigma (Germany) and used as received. The samples had a nominal purity of 99+%. The same grade of purity was achieved for 2-methyl-alcohols prepared by malon ester synthesis and analyzed by gas-liquid chromatography. The subphase water was Millipore filtered, and 10-3 M spreading solutions were used.

Results and Discussion For a homologous series, the stability of the monolayer and the shape of the π-A isotherms depend particularly on both chain length and temperature.15,16 Therefore, the π-A isotherms were recorded at different temperatures. As examples, the π-A isotherms of n-tetradecanol (Figure 1) and n-dodecanol (Figure 2), measured in the temperature interval from 10 to 30 °C, are presented. The monolayers of n-tetradecanol are stable, and the π-A curves are equilibrium isotherms. This was corroborated easily by the fact that the shapes remain unaltered by varying the compression rate. At temperatures T > 20 °C, the π-A isotherms show a plateau region representing the two-phase coexistence region for the main phase transition from the fluid to condensed phase. The kink of the main phase transition point, πt, can be clearly seen (Figure 1). At T e 20 °C, the temperature is low enough that the coexisting phases exist already at π ≈ 0 mN m-1. (19) Gutberlet, T.; Vollhardt, D. J. Colloid Interface Sci. 1995, 173, 429.

Figure 2. π-A isotherms of spread n-dodecanol monolayers at different temperatures.

n-Dodecanol monolayers have well-developed plateau regions over the whole measured temperature region, indicating a corresponding two-phase coexistence. However, at T g 20 °C dissolution of monolayer material into the aqueous subsolution increasingly affects the isotherms with increasing temperature. It could be estimated that the mass loss due to dissolution did not exceed 15% for the most tightly compressed monolayers at the highest temperature. Therefore, these isotherms were linearly corrected in the direction of larger area values per molecule. The plateau regions in the π-A isotherms of the oddnumbered straight chain alcohols n-undecanol and ntridecanol were also found in the accessible temperature and surface pressure range. The plateau regions of the other alcohols with both larger and smaller carbon numbers were found to be inaccessible in the measurable region.20,21 The π-A isotherms of the longer chain alcohols (C g 14) show a characteristic kink in the steep surface pressure increase which can be attributed to a phase transition in (20) Lawrie, G. A.; Barnes, G. T. J. Colloid Interface Sci. 1994, 162, 36. (21) Berge, B.; Konovalov, O.; Lajzerowicz, J.; Renault, A.; Rieu, J. P.; Vallade, M.; Als-Nielsen, J.; Gru¨bel, G.; Legrand, J. F. Phys. Rev. Lett. 1994, 73, 1652.

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Figure 3. π-A isotherms of 2-methyl-hexadecanol monolayers at different temperatures.

a solid phase.12,20 The monolayers of the longer chain alcohols collapse usually at π g 50 mN m-1. The effect of methyl branching of the alkyl chain on the monolayer properties has been studied by using the evennumbered 2-methyl-dodecanol, 2-methyl-tetradecanol, 2-methyl-hexadecanol, and 2-methyl-octadecanol. Figure 3 shows the π-A isotherms of 2-methyl-hexadecanol monolayers at different temperatures. This example demonstrates that the π-A isotherms of 2-methyl substituted alcohol monolayers can also have the plateau region indicating the main phase transition from the fluid to the condensed phase. However, differences as compared to the isotherms of straight chain alcohols are obvious. It is clearly seen that the plateau of the main phase transition has a strong bias. The 2-methyl substitution of the alkyl chain shifts the measurable region for the two-phase coexistence region to higher chain length. Correspondingly, 2-methyl-tetradecanol and 2-methyl-hexadecanol monolayers have the two-phase coexistence region in the accessible temperature and surface pressure region. The monolayers of 2-methyl branched chain alcohols collapse at approximately 40 mN m-1; that is, they are less stable than those of the straight chain alcohols. A characteristic feature of the amphiphilic monolayers is the dependence of the phase transition pressure πt on temperature T. Figure 4 shows the πt-T relationship of the homologous straight chain alcohols (C10-C14), and correspondingly, Figure 5 shows that of the 2-methylsubstituted alcohols (2m-C14 and 2m-C16). All alcohols show a linear increase of the phase transition pressure with temperature. For the straight chain alcohols of different alkyl chain lengths, the straight lines of the πt-T diagram have the same slope, fitted by linear regression to be 0.97 mN m-1 per 1 °C. The translation of the parallels corresponds to 11.3 °C per one CH2 difference in the chain length. The single point for n-decanol, which was adopted from the results of Berge et al.22 using their excess drop method, fits in well and allows an extension of the diagram in the direction of shorter and more soluble n-alcohols. For other homologous series of amphiphiles, especially for fatty acids, fatty esters, and phospholipids, the temperature dependence of two-dimensional (2D) phase transitions has been systematically investigated.17,23-25 (22) Bonosi, F.; Renault, A.; Berge, B. Langmuir 1996, 12, 784. (23) Peterson, I. R. Langmuir 1992, 8, 2995.

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Figure 4. Dependence of the phase transition pressure πt on temperature T of the homologous straight chain C10-C14 alcohols.

Figure 5. Dependence of the phase transition pressure πt on temperature T of the 2-methyl-substituted alcohols (2m-C14 and 2m-C16).

A phase diagram of generalized character has been discussed on the basis of fatty acid monolayers.23,20 Similar systematic studies of the homologous alcohol series have not been performed. However, recent studies suggest that the generalized phase diagram of the fatty acid monolayers cannot be transferred to the homologous alcohol series. Though the monolayers of long-chain alcohols are not studied as much as those of fatty acids, there are some results which indicate a smaller number of monolayer phases for alcohol monolayers. Careful GIXD studies of n-octadecanol monolayers at different surface pressures and temperatures have revealed that no more than three phases can exist, namely, two phases with nontilted molecules (a hexagonal LS phase and a distortedhexagonal (centered rectangular) S phase) and a centered rectangular lattice with a chain tilt in the next-nearestneighbor (NNN) direction.26,27 The GIXD studies per(24) Overbeck, G.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 242, 213. (25) Albrecht, O.; Gruler, H.; Sackmann, E. J. Phys. France 1978, 39, 301. (26) Brezesinski, G.; Kaganer, V. M.; Mo¨hwald, H.; Howes, P. B. J. Chem. Phys. 1998, 109, 2006.

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formed with dodecanol monolayers28 and weakly soluble n-decanol monolayers indicated only one condensed phase.21 To compensate for the desorption of weakly soluble alcohols into the aqueous subphase, a small droplet of the alcohol was deposited at the surface as a permanent source of monolayer-forming material. For such densely packed n-decanol monolayers, a hexagonal lattice structure was obtained at low temperatures of 1-15 °C. The comparison of the dependence of the phase transition pressure πt on temperature T of straight chain and 2-methyl substituted alcohol monolayers reveals striking differences in the slope of the straight lines for both homologous series. The πt-T diagram of 2-methylsubstituted alcohols (Figure 5) shows a bigger slope of the straight lines fitted to be 2.16 mN m-1 per 1 °C. The translation of the parallels is also different from that of the n-alcohols and corresponds to 9.8 °C per one CH2 difference in the chain length. The large shift in the phase properties by the substitution of a methyl group in the 2-position is obviously the result of differences in the 2D aggregation process. In Langmuir monolayers, the aggregation of the amphiphilic molecules to a highly ordered condensed phase results in an entropy decrease (-∆S) of the system.29 As simultaneously in the nonaggregated fluidlike phase the number of possible states of molecules decreases, a compensating entropy increase in this phase is improbable. Therefore, it has been proposed that the term T∆S represents a quantity for characterizing the degree of ordering, that is, the crystallinity within the condensed phase. The ordering of the condensed phase seems to be mainly affected by van der Waals interactions of the alkyl chains. A methyl group substituted in the 2-position of long-chain alcohols enlarges the distance between the neighbored alkyl chains and thus reduces their interactions and ordering. The lower ordering in the 2-methyl-alcohol monolayers should reduce the entropy decrease at the 2D aggregation compared with that for the straight chain alcohols. The much larger increase in the phase transition pressure with temperature found for the 2-methyl-alcohols indicates that correspondingly the degree of ordering is also much more reduced with an increase of temperature than in the case of straight chain alcohols. Similar effects were obtained by substitution or branching of the alkyl chain in previous papers.30,31 It was found that the larger the branching entity and/or the more centrally located the branching in the alkyl chain, the more reduced the stability of fatty acid monolayers and the more enlarged the molecular area. The conclusion on the extension of the 2D lattice by the 2-methyl substitution can be supported by model calculations of the two-dimensional crystal structure of 2-methylhexadecanol. With the help of the program HARDPACK,32 different predictions for a two-dimensional crystal structure were made. The molecular structure was modeled by force field methods. Partial atomic charges were calculated by the method of Gasteiger.33 In the following crystal structure calculations, only conformational changes in the headgroup region were allowed. It was assumed that the (27) Brezesinski, G.; Kaganer, V. M.; Mo¨hwald, H.; Howes, P. B.; Kjaer, K. Phys. Rev. E 1999, 59, 2141. (28) Vollhardt, D.; Brezesinski, G.; Siegel, S.; Emrich, G. J. Phys. Chem. B 2001, 105, 12061. (29) Vollhardt, D.; Fainerman, V. B.; Siegel, S. J. Phys. Chem. B 2000, 104, 4115. (30) Matuo, H.; Cadenhead, D. A. Colloids Surf. 1989, 41, 287. (31) Menger, F. C.; Wood, M. G.; Richardson, S.; Zhou, Q.; Elrington, A. R.; Sherrod, M. J. J. Am. Chem. Soc. 1988, 110, 6797. (32) Rudert, R. Acta Crystallogr., Sect. A (Supplement) 1996, 52, C-94. (33) Gasteiger, J.; Marsili, M. Tetrahedron 1980, 36, 3219.

Vollhardt et al. Table 1. Cell Constants of the Lowest Energy Crystal Structures of 2-Methyl-hexadecanol Predicteda space group E [kJ/mol] p1 p1 pg pg a

-39.52 -39.21 -37.94 -36.96

a [Å]

b [Å] γ [deg] A [Å2] γ [deg]

4.62 4.66 10.89 9.79

5.51 5.84 4.60 5.38

103.6 101.6 90 90

24.7 26.7 25.1 26.3

42.0 45.8 45.3 42.3

E, potential energy; A, area per molecule; γ, polar tilt angle.

Figure 6. Predicted lowest energy pg crystal structure of 2-methyl-hexadecanol, E ) -37.9 kJ/mol. Filled sphere, oxygen; shaded sphere, carbon. Above: molecules, viewing direction parallel to the plane; below: headgroups, viewing direction perpendicular to the plane.

two-dimensional space group of the crystal structure is one of the most common space groups, p1, pg, or p2, with only one molecule in the asymmetric unit. Crystals with the space group pg always contain a racemic mixture, whereas those with p1 and p2 contain only one enantiomer. The program created a few dozen possible crystal structures for each space group and optimized them with respect to minimum potential energy. The potential energy of a crystal structure was defined as the sum of all intermolecular atom-atom potentials, consisting of van der Waals, repulsion, and electrostatic potentials (DREIDING model34). For the rotatable parts of the headgroup, intramolecular potentials were also taken into account. Thermal motion and interaction with water molecules were ignored. The crystal structures of space group p1 had the lowest energy, followed by pg and, by a large distance, p2. Table 1 lists the cell constants of the lowest energy p1 and pg structures for 2-methyl-hexadecanol. The high cross section areas A g 25 Å2/molecule and the high polar tilt angle of the molecules correspond well with the reduced ordering concluded from the πt-T diagram. In the present work, only the racemic 2-methyl-hexadecanol was avail(34) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III J. Phys. Chem. 1990, 94, 8897.

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Figure 7. BAM images of the condensed phase domains in n-tetradecanol monolayers at 25 °C. All micrographs have the same scale bar. (a,b) Equilibrium domains formed in the two-phase coexistence region of the π-A isotherms; compression rate v ) 0.02 nm2 molecule-1 min-1. (c,d) Nonequilibrium structures formed at fast compression: v ) 1.2 nm2 molecule-1 min-1.

Figure 8. BAM images of the condensed phase domains in n-dodecanol monolayers. (a) Typical nonequilibrium shapes. T ) 9 °C; π ) 12 mN m-1; A ) 0.23 nm2 molecule-1; v ) 0.03 nm2 molecule-1 min-1. (b) Compact domains of nearly circular shape obtained after a short compression stop following compression at v ) 0.006 nm2 molecule-1 min-1. T ) 6 °C; π ) 6.8 mN m-1; A ) 0.25 nm2 molecule-1.

able for the experimental studies. The calculations of the pg structures are representative for the two-dimensional space group of racemic substances. The predicted lowest energy pg crystal structure of 2-methyl-hexadecanol monolayers is shown in Figure 6. BAM studies provide information on morphology and growth kinetics of the condensed monolayer phases in the micrometer/millimeter scale. The more or less inclined nonhorizontal plateau regions of the π-A isotherms and their temperature dependence indicate a first-order main phase transition which is visualized by coupling with BAM experiments.

Starting at the main phase transition point, condensed phase domains were formed in the plateau region at further compression of the monolayers. Representative examples are presented in Figure 7 for n-tetradecanol monolayers and in Figure 8 for n-dodecanol monolayers. The first domains appear at the beginning of the plateau region and grow at further compression to the final size.35 The equilibrium domains of n-tetradecanol monolayers are circular; see Figure 7a,b. Higher compression rates lead to nonequilibrium domains characterized by dendritic (35) Siegel, S.; Vollhardt, D. Colloids Surf., A 1996, 116, 195.

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Figure 9. BAM image of condensed phase domains of 2-methyloctadecanol monolayers. T ) 40 °C; π ) 4.8 mN m-1; A ) 0.31 nm2 molecule-1; v ) 0.006 nm2 molecule-1 min-1. (a) General image; (b) well-developed single domains.

growth.12 In the case of n-tetradecanol, a very high compression rate larger than 1.1 nm2 molecule-1 min-1 is necessary to obtain noncircular domains; see Figure 7c,d. After the compression is stopped, the domains relax in approximately 10 s to their circular equilibrium shape. This behavior suggests a high line tension of the condensed phase domains.36 Finally, at further compression up to the kink in the steep part of the π-A isotherm at 12.4 mN m-1 the surface is completely covered with the condensed phase. Then the tilt of the alkyl chains decreases to zero (the weak inner domain texture disappears) and a hexagonal phase is formed.35,37 A different domain morphology is formed in the plateau region of n-dodecanol monolayers. Here, already at low compression rates typical nonequilibrium domains with fingering shapes are formed (Figure 8a). A stop of the compression within the plateau region or a very low compression rate leads to the transformation to compact but irregular domain shapes (Figure 8b). Nevertheless, striking differences remain as compared to the circular (36) Siegel, S.; Vollhardt, D. Thin Solid Films 1996, 284-285, 424. (37) Dutta, P. In Phase Transitions in Surface Films 2; Taub, H., et al., Eds.; Plenum Press: New York, 1991; p 183.

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equilibrium domains of the homologous n-tetradecanol monolayers. It is interesting to realize that after a compression stop different relaxation steps of the domains can coexist next to each other; that is, fingered domains exist apart from compact domains, suggesting progressive domain formation in the preceding compression step. Condensed phase patterns of 2-methyl-alcohol monolayers were observed for alkyl chain lengths of 16 and 18 C-atoms, whereas the BAM images of 2-methyl-tetradecanol monolayers do not show any long-range orientational order. Evidently, there is almost no contrast between the condensed and fluid phases in the BAM images of 2-methyl-tetradecanol, so in the two-phase coexistence region the monolayer appears only slightly more inhomogeneous than in the fluid phase. The contrast improves continuously with the alkyl chain length. The best patterns are formed by 2-methyl-octadecanol monolayers although the main phase transition can be expected only at lowest pressure and temperatures of g40 °C, as can be concluded from the translation per one CH2 group of the π-T diagram. Figure 9 shows that in 2-methyl-octadecanol monolayers irregularly shaped domains without any inner structure are formed. These irregular domain textures remain unchanged with time so that the irregularity expresses mainly the loss of a long-range ordering. Beyond the coexistence region, a further increase in the surface pressure leads finally to a nearly isotropic, dense condensed monolayer. The irregularly shaped domains indicate a low line tension of the condensed phase. These BAM results point clearly to a loss of long-range tilt orientational order by 2-methyl substitution in medium- and long-chain alcohols. There is obviously a correlation between the absence of long-range tilt orientational order and the disordered packing of alkyl chains in the condensed phase. Recent GIXD studies performed on monolayers of various 2-methyl-substituted amphiphiles such as 2-monopalmitoyl-glycerol, 2-hexadecanol, and 2-hydroxypalmitic acid revealed diffraction patterns that also indicate a disordered packing of alkyl chains.38 The detailed analysis of the results has shown that the disordering is due to a superposition of lattices with varying tilt azimuth of alkyl chains, whereas no indication of a variation of the polar tilt angle was found. Conclusions Systematic information on phase behavior and firstorder main phase transition of the monolayers of straight chain and 2-methyl branched chain alcohols with alkyl chain lengths of C10-C18 can be obtained on the basis of a combination of π-A isotherms with BAM in the accessible temperature region. The 2-methyl substitution of the alkyl chain shifts the measurable region for the two-phase coexistence to higher alkyl chain length. Striking differences in the dependence of the phase transition pressure on temperature of the straight chain and 2-methyl-substituted alcohols can be correlated with differences in the molecular ordering in the condensed phase. The general conditions for the main phase transition in the corresponding homologous straight chain and 2-methyl branched chain alcohols can be derived. The alkyl chain length and 2-methyl substitution affect in different ways the general textural features of the condensed phase domains. The effect of alkyl chain length and 2-methyl substitution on the general textural features of the condensed phase domains is determined under equilibrium and nonequilibrium conditions. n-Alcohol (38) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; Mo¨hwald, H. Langmuir 1998, 14, 6485.

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monolayers form well-shaped condensed phase domains in the equilibrium state which are circular and have an inner texture in tetradecanol monolayers. The alkyl chain length largely determines the stability of the dendritic or fingered nonequilibrium textures. The domains of the 2-methyl-alcohol monolayers are irregularly shaped and do not have any inner structure. This loss or strong reduction of the long-range orientational order is obviously correlated with a less ordered packing of alkyl chains in the condensed phase. Recent studies of some 2-methylsubstituted amphiphiles suggest a superposition of lattices with varying tilt azimuth of the alkyl chains. Model

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calculations of the two-dimensional lattice structure of the racemic 2-methyl-hexadecanol on the basis of the pg space group performed by using the program HARDPACK correspond well with the reduced ordering concluded from the experiments. Acknowledgment. Financial assistance from the Fonds der Chemischen Industrie is gratefully acknowledged. LA0201671