Langmuir Monolayers with Fluorinated Groups in ... - ACS Publications

Langmuir 2001, 17, 4581-4592

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Langmuir Monolayers with Fluorinated Groups in the Hydrophilic Head: 2. Morphology and Molecular Structure of Trifluoroethyl Behenate and Ethyl Behenate Monolayers J. G. Petrov,* G. Brezesinski, N. Krasteva, and H. Mo¨hwald Max-Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, D-14476 Golm/Potsdam, Germany Received December 21, 2000. In Final Form: April 6, 2001 Morphology and molecular structure of Langmuir monolayers of trifluoroethyl behenate (TFEB) and ethyl behenate (EB) were investigated via Brewster angle microscopy and grazing incidence X-ray diffraction (GIXD) at the air-water interface. When spread at 60 Å2/molecule, both substances form islands of a condensed phase. At smaller areas, they organize in “archipelago structures” with compact sections and 2D suspensions of microcrystals floating in streams on the free water surface. The microcrystals that are close to the “coastlines” adhere to the compact monolayer. The latter contains dark areas (“lakes”) that are being closed under compression. At expansion, the compact monolayers disintegrate to the same archipelagos but the microcrystals are now located mainly along the coastlines. Such mechanism of growth and disintegration of the apparently compact TFEB and EB films suggests a polycrystalline structure. Different friction at the microcrystal boundaries could be responsible for different compressional moduli of the TFEB and EB monolayers. The presence of voids in the films could be the cause of the compressionexpansion hysteresis of the dynamic π/A isotherms of TFEB and EB and the larger mean molecular areas of TFEB. The GIXD part of the study shows that the TFEB islands and microcrystals consist of closely packed vertically oriented molecules occupying 18.9 Å2 of the water surface. The islands of EB are also densely packed, but their molecules are tilted at 17.6 ( 0.3° toward next-nearest neighbors. Compression does not change the GIXD molecular area of the TFEB monolayer. It decreases the tilt angle of the EB molecules causing transition from the tilted L2′ phase to the upright S or CS phase. Above 12.3 ( 1.2 mN/m, the EB molecules occupy Axy ) 18.8 Å2. At high surface pressure, TFEB and EB monolayers have the same structure with centered rectangular unit cells and the same lattice parameters. The vertical hydrocarbon chains form a “herringbone” arrangement. Analysis of literature data for other Langmuir monolayers and phospholipid bilayers suggests that the structural difference at low surface pressure could be due to the increased hydrophobicity of the trifluoroethyl group as compared with the ethyl ester group. On the basis of some previous GIXD results for monolayers of fatty acids and their methyl and ethyl esters, we speculate that the -CH2CF3 group could be oriented toward air even in the solid condensed S or CS phase. Such a “hook” conformation would explain the negative surface potential found for the TFEB film. This scenario is supported by a molecular model of the TFEB monolayer that will be presented in part 3 of this study.

I. Introduction The fluorinated amphiphilic compounds decrease surface energy of liquids and solids and diminish surface friction to a significantly greater extent than their nonfluorinated analogues.1-4 In several related papers, we have studied the effect of fluorination of the hydrophilic head of long-chain nonionic amphiphiles on the behavior of their Langmuir monolayers. Mechanical properties and stability, morphology and structure, and electrostatic potential and ellipsometric thickness of trifluoroethyl behenate films were examined. It was found that the surface potential of this monolayer is negative in contrast to the positive ∆V values observed for monolayers of * Corresponding author. Permanent address: Institute of Biophysics of the Bulgarian Academy of Sciences, 1 Acad. G. Bonchev Str., Block 21, 1113 Sofia, Bulgaria. (1) Fluorinated Surfactants: Synthesis, Properties, Applications; Kiss, E., Ed.; Marcel Dekker: New York, 1994. (2) Hare, E. F.; Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1954, 58, 236. Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1957, 61, 1046. Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1962, 62, 740. (3) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Luethi, R.; Howald, L.; Guenterodt, H.-J.; Fudjihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (4) Fox, H. W. J. Phys. Chem. 1957, 61, 1058. Barnett, M. K.; Zisman, W. A. J. Phys. Chem. 1963, 67, 1534. Barnett, M. K.; Jarvis, N. L.; Zisman, W. A. J. Phys. Chem. 1964, 68, 3520. Elbert, R.; Folda, T.; Ringsdorf, H. J. Am. Chem. Soc. 1984, 106, 7687. Vogel, V.; Mo¨bius, D. J. Colloid Interface Sci. 1988, 126, 408.

insoluble ethyl esters and other uncharged amphiphiles with nonfluorinated heads.5 Trifluoroethyl behenate (TFEB) forms more unstable films with higher compressibility (lower compressional modulus) than the nonfluorinated ethyl behenate (EB).6 Both esters exhibit compression-expansion hysteresis of the surface pressurearea isotherms, but the hysteresis loop of TFEB is shifted to larger molecular areas. The difference between the limiting areas obtained via extrapolation of the lowpressure sections of the isotherms to zero surface pressure cannot be explained by the Connoly volumes (the volume of an isolated headgroup accessible for water molecules) of the trifluoroethyl and ethyl ester heads estimated from molecular models.6 At high surface pressure, the TFEB molecules occupy a larger mean area6 than EB although the same hydrocarbon chain should give rise to the same molecular packing of the two films.7 The continuously recorded compression surface pressure-area isotherm of TFEB does not show clear phase transitions,6 whereas the EB isotherm contains three sections of different slope suggesting two phase transitions. Adam8 reports only one kink in the equilibrium (5) Petrov, J. G.; Mo¨hwald, H. J. Phys. Chem. 1996, 100, 18458. (6) Petrov, J. G.; Polymeropoulos, E. E.; Mo¨hwald, H. Langmuir 2000, 16, 7411. (7) Peterson, I. R.; Brzezinski, V.; Kenn, R. M.; Steitz, R. Langmuir 1992, 8, 2995. (8) Adam, N. K. Proc. R. Soc. London 1922, A101, 452.

10.1021/la0017835 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/27/2001

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(relaxed) compression isotherm of EB at 10.5 mN/m. Stenhagen’s9 phase diagram of ethyl behenate also predicts only one phase transition, but located at 2.5 mN/ m. All transition surface pressures in our EB compression isotherms exceed these literature values. The expansion isotherms of TFEB and EB monolayers are rather similar but differ from the compression isotherms. They exhibit linear low-pressure parts, transition sections of varying slope ending with kinks, and steep high-pressure regions. The kinks imply that second-order phase transitions occur in both monolayers. The different number and location of the slope changes of the isotherms at compression and expansion, the discrepancy of the existing data for EB, and the ambiguous identification of the monolayer phases based only on π/A data require in situ study of the structure of TFEB and EB films. In this paper, we investigate morphology and molecular structure of TFEB and EB monolayers at the air-water interface using Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD). The results obtained show that TFEB forms islands of closely packed vertically oriented molecules even at zero surface pressure. Compression of the monolayer expels the voids between the islands and within them but does not change the molecular area in the compact film sections. Similar islands are formed when EB is spread, but they consist of closely packed molecules tilted toward nextnearest neighbors. Monolayer compression expels the voids and decreases the tilt angle causing transition to a phase with densely packed upright molecules. At high surface pressure, TFEB and EB monolayers have the same structure and lattice parameters. The structural difference at low surface pressure seems to result from different hydrophilicity of the headgroups because of substitution of the CH3 by a CF3 group. Existence of residual voids in TFEB and EB monolayers qualitatively explains the π/A compression-expansion hysteresis and the different limiting area per molecule found in part 1 of this investigation.6 II. Materials and Experimental Methods Materials. Ethyl behenate with 99% purity was purchased from Sigma and used as received. The synthesis and characterization of trifluoroethyl behenate was described in our previous publication.5 Both substances were dissolved in chloroform to a concentration of 1 mM. Addition of 2% v/v absolute ethanol of p.a. grade of purity facilitated dissolution of the trifluoroethyl ester. (The same surface pressure-area isotherms were obtained substituting ethanol with acetone. This coincidence excludes the effect of solvates on monolayer properties, because they are formed with ethanol but not with acetone.) Aliquots of these solutions were spread on the water surface and left several minutes for evaporation of the solvent. Milli-Q Millipore water was used as a liquid substrate. All experiments were performed at 20 °C. Compression and expansion of the monolayers were performed at 0.8 Å2/molecule min. Surface Pressure-Area Isotherms. The compressionexpansion π/A isotherms of TFEB and EB monolayers in Figure 1 are reproduced from part 1 of this study6 to show the conditions at which the BAM images (solid circles) and X-ray diffraction data (arrows) were collected. They also illustrate the structural changes that could be expected from the π/A curves. The isotherms were continuously recorded at the same velocity used in the BAM and GIXD experiments. Compression was followed up to 18 mN/m for TFEB and to 33 mN/m for EB. These values corresponding to 75% of the collapse pressures (23.7 and 44.3 mN/m, respectively) were chosen to avoid formation of a bulk phase during compression. (9) Stenhagen, E. In Determination of Organic Structures by Physical Methods; Braude, E. A., Nachod, F. C., Eds.; Academic Press: New York, 1955; p 335.

Petrov et al.

Figure 1. Compression-expansion π/A isotherms of TFEB and EB monolayers. Solid circles and letters show the location of the analyzed BAM images, and the arrows indicate the coordinates of the GIXD experiments. Brewster Angle Microscopy. The morphology of the monolayers was studied using the Brewster angle microscope BAM-2 (NFT, Go¨ttingen). The optical system and the Langmuir film balance (Teflon trough and Wilhelmy dynamometer with a strip of filter paper) are mounted on an antivibration table. A p-polarized beam of a diode laser is directed to the pure water surface at the Brewster angle to give zero reflection. Spreading the amphiphilic substance and varying its density with the movable barrier change optical properties of the air-water interface so that the monolayer can be visualized through detection of the reflected light. An analyzer in the reflected beam enables optical anisotropy of the film to be distinguished. Monolayer compression or expansion is recorded on a videotape, and the frames are grabbed and digitized by a computer supplying images with dimensions 560 × 420 µm. Knowing the velocity of the movable barrier and the time of compression or expansion, one can relate each image to a particular molecular area. Grazing Incidence X-ray Diffraction. The monolayer structure was studied on a molecular level using the liquid surface diffractometer at the undulator beam line BW1, HASYLAB, DESY, Hamburg, Germany.10-12 A hydrophilic borosilicate slab under the Langmuir film damped the capillary waves induced by mechanical vibrations. The horizontal fan beam from the synchrotron was monochromated and deflected downward by a beryllium crystal in transmission (Laue) diffraction configuration. (10) Als-Nielsen, J.; Jaquemain, D.; Kjaer, K.; Lahav, M.; Leveiller, F.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (11) Kjaer, K. Physica B 1994, 198, 100. (12) DeWolf, C.; Brezesinski, G.; Weidemann, G.; Mo¨hwald, H.; Kjaer, K.; Howes, P. B. J. Phys. Chem. B 1998, 102, 3238.

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The horizontal distribution of the diffracted X-rays was determined by scanning a Soller collimator. A linear position-sensitive detector registered the vertical distribution. The horizontal resolution function was very close to a Gaussian with 0.008 Å-1 full width at half-maximum (fwhm). The accumulated positionresolved counts were corrected for polarization, effective area, and powder averaging (Lorentz factor). The Yoneda-Vineyard peak was used to determine the detector position. Model peaks taken to be Lorentzian for the in-plane and Gaussian for the out-of-plane directions were fitted to the corrected intensities. A single first-order peak implies a hexagonal molecular arrangement, and two first-order peaks suggest a centered rectangular (distorted hexagonal) unit cell. The position of the two peaks along Qz allows determination of the azimuths of molecular tilt and lattice distortion.13,14 The in-plane components of the scattering vectors of the h,k-peaks Qhk xy give the corresponding lattice spacings in the horizontal plane dh,k:

2π dhk ) hk Qxy

(1)

The values of dhk give the parameters of the monolayer lattice in the plane of the water surface. The positional correlation length ζ can be estimated from the fwhm of each in-plane peak assuming an exponential decay of the order14

σ)

2 fwhm(Qhk xy )

(2)

The out-of-plane vector components Qhk z yield information about the angle θ of the molecular tilt with respect to the surface normal and about tilt direction.14 The centered rectangular unit cell with molecular tilt to the nearest neighbors (NN) exhibits one nondegenerate in-plane peak Qnxy at Qnz ) 0 and one 2-fold degenerate peak Qdxy at Qdz > 0. The tilt angle θ is given by the relationship

(x

θ ) arctan

Qdz

)

(3)

(Qdxy)2 - (Qnxy/2)2

For a tilted phase whose molecules point toward next-nearest neighbors (NNN), both Qnxy and Qdxy are located at nonzero Qz values having a ratio Qnz /Qdz ) 2:1. In this case, the tilt angle can be calculated from the formula

Qnz θ ) arctan n Qxy

(4)

The molecular tilt is defined by its magnitude η ) sin θ and azimuth β. The orientation of the planes of the zigzag bonds of the hydrocarbon chains determines the distortion of the monolayer lattice.14 This distortion also has a magnitude,

ξ)

ll2 - ls2 ll2 + ls2

(5)

where ll and ls are the long and short axes of the ellipse passing through the six nearest neighbors of a given molecule and an azimuth ω, that coincides with the long axes. The above parameters give the signed distortion D,

D ) ξ cos 2(ω - β)

(6)

enabling two causes of lattice deformation to be distinguished, molecular tilt and arrangement of the zigzag planes of the chains.14 Landau theory shows that if the lattice distortion is (13) Dutta, P. In Phase Transitions in Surface Films 2; Taub, H., Ed.; Plenum Press: New York, 1991; p 183. (14) Kaganer, V. M.; Moehwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779.

due only to molecular tilt the plot of D versus η2 should be linear and should have zero intercept. A nonzero intercept implies that the ordering of the zigzag planes of the hydrocarbon chains in a “herringbone structure” also plays a definite role.14,24 The area per molecule in the horizontal plane Axy giving the scale of the monolayer lattice can be calculated from the lattice spacings dhk. For a hexagonal lattice, d02 ) d11 ) d. For a centered rectangular lattice, Axy is given by15

Axy )

2d022d11

x4d022 - d112

(7)

The area per molecule A0 in the plane perpendicular to the hydrocarbon chains is

A0 ) Axy cos θ

(8)

III. Results Morphology of TFEB and EB Monolayers on Compression and Expansion. When spread at an initial molecular area of 60 Å2, TFEB and EB form islands of a 2D condensed phase. At smaller areas, both monolayers attain “archipelago structures” shown in Figures 2a and 3a. Suspensions of 2D particles with average dimensions of 4-5 µm float in the nonreflecting dark areas of both monolayers. The compact parts of the TFEB archipelago are optically homogeneous. The compact parts of the EB monolayer exhibit a mosaic structure composed of domains with different lengths but almost the same width of 3-5 µm. Large single crystals or their aggregates are sometimes incorporated in the mosaics of the EB film. The compact parts of both monolayers contain closed dark areas (“lakes”); they were found in the TFEB film even at 6.5 mN/m (cf. Figure 2b and Figure 1). Unfortunately, BAM 2 cannot directly prove their disappearance under compression, because identification of monolayer defects depends on the probability for their location in the laser beam spot on the water surface. A scanning version of the instrument would be very useful for this purpose. During compression of the monolayers, the 2D suspension forms streams as seen in Figure 3a. The suspension particles of the stream adhere to the “coastlines” thus increasing the area of the compact monolayer. The area of the lakes decreases via a similar particle attachment mechanism. Intermediate states of the closing of the lakes can sometimes be seen as “gray lakes” covered by a concentrated 2D suspension (Figure 2b). Thus, homogenization of the TFEB and EB monolayers via sintering of the archipelagos and closing of lakes occurs with the same elementary step, attachment of 2D particles to the coastlines. Expansion of TFEB and EB monolayers follows the inverse sequence of events. At some stage, the compact monolayers disintegrate and the archipelago structures are restored (Figures 2f and 3f). However, much less suspension particles are now seen in the nonreflecting dark areas. They are located in the narrow cracks and lakes close to the coastlines (Figure 2f). This location implies detachment of 2D particles. Such mechanism of growth and disintegration of the apparently compact TFEB and EB films suggests that they retain a polycrystalline structure even at high surface pressure. Bright 10-20 µm objects are found in some of the expansion images of both monolayers (Figures 2e and 3e). They reflect light much stronger than the surrounding condensed monolayer and most probably represent 3D (15) Kenn, R. M.; Boehm, C.; Bibo, A. M.; Peterson, I. R.; Moehwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092.

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Figure 2. BAM images of trifluoroethyl behenate monolayers obtained at continuous compression (a-c) and expansion (d-f). The bar represents 100 µm.

crystals resulting from monolayer collapse. Because compression has been stopped at 18 mN/m for TFEB and 33 mN/m for EB (cf. Figure 1), their appearance in Figures 2e and 3e means that the actual collapse starts below these values. Comparison of the optical behavior of TFEB and EB monolayers displays several important differences. The intensity of the light reflected from the compact TFEB monolayer does not change upon rotation of the analyzer. Such isotropic light reflection implies the absence of inplane anisotropy. The same behavior is found both for the condensed parts of the archipelago structure and for the highly compressed film before collapse. No change of this isotropy is observed during expansion of the TFEB monolayer. Therefore, the variation of the molecular area during compression and expansion does not cause any “tilting transition” in TFEB film in contrast to the conclusion suggested by the π/A isotherms in Figure 1. At low surface pressure, the intensity of the light reflected from the mosaic structure of the EB monolayer changes upon rotation of the analyzer thus manifesting an optical anisotropy. The large circular domains or their aggregates sometimes appearing in the archipelago

structure are also anisotropic. When the film is compressed, the contrast of the mosaic drops down at 19.7 Å2, at a surface pressure of ∼13 mN/m, but a small anisotropy remains existing even at 19.1 Å2 (33 mN/m) (Figure 3c). An opposite change of the optical response of the EB monolayer occurs during expansion; the sharpness of the mosaic is restored at 19.4 Å2 (∼14 mN/m). These values agree with the changes of the slope between the low- and high-pressure sections of the compression and expansion isotherms of EB shown in Figure 1. Molecular Structure of TFEB and EB Monolayers. Figure 4 presents the contour plots of the corrected X-ray intensities for TFEB and EB monolayers at 3 mN/m as a function of the Qxy and Qz components of the scattering vector. The occurrence of two diffraction peaks is characteristic for the centered rectangular molecular arrangement. For the TFEB film, both peaks are located at zero Qz indicating a vertical orientation of the molecules. Under the same conditions, the EB film exhibits two Bragg peaks at nonzero Qz values in a ratio 2:1. Such a peak distribution suggests NNN molecular tilt. The positions of the Qxy and Qz maxima at different surface pressures are given in Table 1. It shows that


Langmuir, Vol. 17, No. 15, 2001 4585

Figure 3. BAM images of ethyl behenate monolayers taken at continuous compression (a-c) and expansion (d-f). The bar represents 100 µm.

increasing of π up to 12 mN/m does not change the positions of the maxima and thus the molecular arrangement of TFEB monolayer. For the EB film, the positions vary at 01 compression, but the ratio Q02 z /Qz remains 2:1 from 1 to hk 10 mN/m. The larger Qz corresponds to larger Qhk xy , indicating NN distortion of the lattice, that is characteristic for the NNN tilted L2′ phase.14,15 Thus, our GIXD data show that the low-pressure linear section of the EB isotherm corresponds to the L2′ phase. The unit cell parameters of TFEB and EB monolayers at different surface pressures are presented in Table 2. For TFEB, they remain the same from 0.5 to 12 mN/m but change under compression for the EB film. At 3 mN/m, a TFEB molecule occupies an area of 18.9 Å2, whereas an EB molecule with the same hydrocarbon chain fills 19.5 Å2 of the water surface. This difference is due to a tilt of the EB molecules at 15° from the surface normal. Both the area per molecule in the plane of the water surface Axy and the tilt angle θ decrease with increasing of π. The first dependence is shown in Figure 5. It shows that Axy linearly decreases with increasing of π up to 10 mN/m (R ) 0.9844, SD ) 0.0548) and does not change above 22 mN/m. The extrapolated low- and high-pressure parts

intersect at 13.4 mN/m that is exactly the value of πtr obtained in ref 6 via similar extrapolation of the linear regions below and above the transition section of the compression π/A isotherm. The Axy/π data between 10 and 22 mN/m could represent an intermediate linear section, but this conclusion would be rather speculative because the Axy values above 18 mN/m differ within the limits of the experimental error (0.1 Å2. Equation 8 implies a linear dependence of 1/cos θ on surface pressure if Axy is proportional to π (see Figure 5) and the molecular cross-sectional area A0 remains constant under compression. Figure 6 shows this relationship. It has only two linear branches whose extrapolations intersect at 12.3 ( 1.2 mN/m. This is the surface pressure at which the hydrocarbon chains become vertical (1/cos θ ) 1); above this value, the EB film exists in the S or CS phase. The extrapolation to π ) 0 gives a maximum tilt angle of 17.6° ( 0.3°. Because this dependence is more reliable (R ) 0.9941, SD ) 0.0018), one could conclude that between 0 and 31 mN/m the EB monolayer undergoes only one L2′-S phase transition. Figure 7 shows that the signed distortion D of the EB monolayer lattice decreases linearly with increasing of

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Petrov et al. Table 2. Lattice Parameters of TFEB and EB Monolayers at Different Surface Pressuresa monolayer

π [mN/m]

a [Å]

Axy A0 b)c R β)γ θ [Å] [deg] [deg] [deg] [Å2] [Å2]

TFEB TFEB TFEB TFEB EB EB EB EB EB EB EB EB

0.5 (25 Å2) 3 8 12 1 3 6 8 10 18 22 31

5.04 5.04 5.04 5.03 5.03 5.02 5.02 5.02 5.02 5.03 5.02 5.01

4.52 4.52 4.52 4.52 4.66 4.63 4.60 4.58 4.54 4.52 4.51 4.50

a

112.3 112.3 112.3 112.3 114.7 114.3 113.8 113.4 112.9 112.3 112.4 112.4

123.9 123.8 123.8 123.8 122.7 122.9 123.1 123.3 123.5 123.8 123.8 123.8

0 0 0 0 17 15 13 11 7 0 0 0

18.9 18.9 18.9 18.9 19.7 19.5 19.4 19.2 19.0 18.9 18.8 18.8

18.9 18.9 18.9 18.9 18.9 18.9 18.8 18.9 18.8 18.9 18.8 18.8

Designation is given in the text.

Figure 4. Contour plots of the corrected X-ray intensities as a function of the in-plane Qxy and out-of-plane Qz components of the scattering vector for trifluoroethyl behenate and ethyl behenate monolayers. The measurements were performed at 3 mN/m. Table 1. In-Plane, Qxy, and Out-of-Plane, Qz, Components of the Scattering Vector for the Two Characteristic {11} and {02} Diffraction Peaks Registered in TFEB and EB Monolayers at the Air-Water Interface monolayer

π [mN/m]

Qxy{11} [Å-1]

Qz{11} [Å-1]

Qxy{02} [Å-1]

Qz{02} [Å-1]

TFEB TFEB TFEB TFEB EB EB EB EB EB EB EB EB

0.5 (25 Å2) 3 8 12 1 3 6 8 10 18 22 31

1.501 1.502 1.502 1.503 1.485 1.489 1.493 1.496 1.502 1.504 1.507 1.509

0 0 0 0 0.242 0.219 0.193 0.160 0.108 0 0 0

1.673 1.673 1.673 1.674 1.603 1.616 1.630 1.643 1.660 1.675 1.677 1.679

0 0 0 0 0.484 0.438 0.386 0.320 0.216 0 0 0

sin2 θ. Such proportionality implies that the molecular tilt is the reason for the deviation of the monolayer lattice from hexagonal symmetry. However, the nonzero intercept D0 ) 0.147 at sin2 θ ) 0 indicates an additional contribution to D because of the herringbone ordering of the zigzag planes of the chains.14,24 This source of distortion is preserved also in the upright (S or CS) phase thus causing the small optical anisotropy detected in the BAM images at high surface pressure. At high surface pressures, the TFEB and EB monolayers exhibit identical lattice structures. The parameters of the corresponding unit cells agree very well with those of the S phase found in heneicosanoic acid monolayers at temperatures above 8 °C (a ) 5.03 Å, b ) c ) 4.53 Å).13,16 However, the CS-phase parameters (a ) 5.01 Å, b ) c ) 4.49 Å)10 are also very close to our data especially for the (16) Lin, B.; Shih, M. C.; Bohanon, T. M.; Ice, G. E.; Dutta, P. Phys. Rev. Lett. 1990, 65, 191.

Figure 5. Plot of the GIXD area per molecule Axy versus π for the ethyl behenate monolayer.

Figure 6. Plot of 1/cos θ versus π. Extrapolation to 1/cos θ ) 1 gives πtr ) 12.3 ( 1.2 mN/m at which the tilt angle becomes zero. The extrapolated value at π ) 0 yields the maximum tilt angle θmax ) 17.6° ( 0.3°.

EB film at 31 mN/m, so one cannot clearly specify the high-pressure phase of EB as S or CS. Such a difficulty has already been reported for other Langmuir monolayers.17 On the other hand, this unit cell is a fingerprint of the herringbone packing in a face centered orthorhombic subcell that is common for long hydrocarbon chains in bulk organic crystals.18 At high pressure, the fwhm of all peaks is resolution limited, that is, ζ > 250 Å. Only at low (17) Bommarito, G. M.; Foster, W. J.; Pershan, P. S.; Schlossman, M. L. J. Chem. Phys. 1996, 105, 5265. (18) Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultants Bureau: New York, 1961.


Langmuir, Vol. 17, No. 15, 2001 4587

Here, φ accounts for the specific effect of the hydrophilic heads, P characterizes the contribution of the hydrocarbon chains, and t is the Celsius temperature. Equations 9 and 10 predict that if the ethyl behenate monolayer exhibits a L2′-LS transition at 20 °C, πtr should be located at 10.8 mN/m. The latter value coincides surprisingly well with the equilibrium transition pressure of 10.5 mN/m found by Adam.8 Figure 1 shows that it is close to the beginning of the transition section in our dynamic compression isotherm (11.5 mN/m) but considerably below the limits of the transition section (13.0-16.5 mN/m) at expansion.6 For the L2′-S transition, the theory of equivalent states7 and Lundquist’s20 dynamic expansion isotherms for films of even ethyl esters give Figure 7. Signed lattice distortion D of the ethyl behenate monolayer versus square of the tilt magnitude sin2 θ. The linear dependence implies that the molecular tilt causes the deviation from the hexagonal symmetry, but the nonzero intercept indicates additional contribution due to a herringbone ordering of the zigzag planes of the chains. Table 3. Comparison of the Calculated and Experimental Maxima of the Diffraction Peaks of the TFEB Monolayer at 8 mN/m h

k

1 0 1 2 0 2

1 2 2 0 3 1

Qhk, calcd

2.087 2.495 2.510 2.631

Qhk, observed 1.502 1.673 2.086 2.495

h

k

Qhk, calcd

Qhk, observed

1 2 0 2 1

3 2 4 3 4

2.802 3.004 3.346 3.539 3.581

2.802 3.004 3.348 3.542 3.573

2.634

surface pressures is the fwhm of the (02) peak of EB slightly larger than the resolution. The structure of the TFEB monolayer at 8 mN/m was additionally investigated at larger Qxy values. Figure 8 shows the lower and higher order peaks obtained. The seven second-order peaks imply a very well ordered crystalline structure and probably a CS phase. In Table 3, we compare the positions of the experimental peaks with those calculated from the positions of the {11} and {02} peaks. The occurrence of peaks with an odd sum of the h,k-values that are “forbidden” for a centered rectangular unit cell indicates a herringbone packing of the TFEB molecules.19 This structure is typical for the CS and S untilted phases. The large steps of the in-plane scattering angle are the reason the {20} and {03} reflections could not be resolved. The first attempts to find higher order peaks also for the EB monolayer failed on this stage of the study because of beam damage during the long scans. Phase Transitions Estimated from the Theory of Equivalent States. Peterson et al.7 formulated a theory of the equivalent states that relates the surface pressure of a second-order phase transition in Langmuir monolayers to the length λ of the hydrocarbon chain:

πtr ) φ + λP

(9)

Utilizing Lundquist’s20 dynamic expansion isotherms for monolayers of even ethyl esters, the authors of ref 7 specified the parameters of eq 9 for the L2′-LS transition:

φ ) 11.4 - 0.67t

P ) 0.24t

(19) Dutta, P. Colloids Surf., A 2000, 171, 59. (20) Lundquist, M. Chem. Scr. 1971, 1, 197.

(10)

φ ) 22 + 0.44t

P ) 4.0 × 109 mN/m

(11)

From eqs 9 and 11, one obtains that at 20 °C the EB monolayer should undergo a L2′-S transition at 20 mN/ m. This value coincides very well with the kink (19.5 mN/ m) in our dynamic expansion isotherm of EB suggesting that it corresponds to the L2′-S transformation. Tables 3 and 4 of ref 7 show that the value of the parameter φ, specifically accounting for the contribution of the hydrophilic heads to πtr, decreases with decreasing of headgroup hydrophilicity. For the L2′-S transition in fatty acid monolayers, φ ) 30 mN/m, whereas for the ethyl ester films with the same hydrocarbon chain, φ ) 22 mN/ m. Because the TFEB head is less hydrophilic than the EB one,6 one would expect a lower value of φ and πtr for TFEB. This prediction is in qualitative agreement with the expansion branches of the TFEB and EB isotherms in Figure 1 but contradicts our BAM and GIXD data. They unambiguously show that the TFEB monolayer does not undergo any phase transitions between 0.5 and 18 mN/m. Integrity of TFEB and EB Monolayers. After spreading, TFEB forms domains of closely packed vertically oriented molecules occupying Axy ) 18.9 Å2 of the water surface. This value does not change under compression whereas the mean area per molecule A plotted in the π/A isotherm decreases with increasing of the surface pressure, Figure 9a. Therefore, the change of A is due to closer packing of the domains, expelling of the voids between them, and closing of the lakes in the compact parts of the film (cf. Figure 2a-c). Similar 2D sintering takes place during compression of the EB monolayer, but in the low-pressure L2′ phase it is accompanied by a decrease of the tilt angle that reduces the molecular area Axy in the compact monolayer parts (Figure 9b). Comparing the compression π/A isotherms with the GIXD π/Axy data in Figure 9 shows that the difference A - Axy is significantly larger for TFEB than for EB film. Table 4 presents this difference on a relative scale comparing (A - Axy)/A as a characteristic of the fraction of the mean molecular area due to the presence of voids in the monolayer. It can be seen that at the same pressure and temperature this quantity has larger values for TFEB than for EB film. Both TFEB and EB expansion π/A isotherms, although closer to the π/Axy dependencies, are also shifted to larger areas with respect to them. These shifts show that even when compressed to 75% of the collapse pressure TFEB and EB monolayers contain voids. The mean area per molecule in such composite films Acmp could be presented as

Acmp ) XcAc + XvAv

(12)

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Petrov et al.

Figure 8. Low- and high-order diffraction peaks of a trifluoroethyl behenate monolayer at 8 mN/m. The seven peaks indicate a highly ordered crystalline structure. Table 4. Comparison of the Ratio (A - Axy)/A in Percent Taken as a Measure of the Partial Area of the Voids in TFEB and EB Monolayers at Different Surface Pressures TFEB π [mN/m]

A [Å2]

Axy [Å2]

0.5 (25 Å2) 8.0 18.0

25.5 21.2 19.9

18.9 18.9 18.9

EB

(A - Axy)/A [%] 25.9 10.8 5.0

A [Å2]

Axy [Å2]

(A - Axy)/A [%]

23.5 20.3 19.5

19.7 19.2 18.9

16.2 5.4 3.1

corresponding monolayer states. Rearranging eq 12 and substituting Ac by Axy, one obtains

Acmp ) Axy + (Av - Axy)Xv

(13)

When determining equilibrium isotherms of fluid monolayers, we measure the thermodynamic surface pressure π ) γ0 - γ, where γ0 is the surface tension of the pure water and γ stands for the same characteristics of the water surface covered by the monolayer. For a composite film, γ ) γcmp is a mean quantity depending on the partial areas occupied by the condensed monolayer, fc ) Sc/S, and the voids, fv ) Sv/S, as well as on the specific surface free energies γc and γv of the corresponding parts of the air-water interface:

γcmp ) fcγc + fvγv

(14)

S is the total area of the composite monolayer, and Sc and Sv are the areas of the condensed film and the voids, respectively. For such monolayers,

πcmp ) γ0 - γcmp

(15)

that together with eq 14 gives Figure 9. Comparison of the dependencies surface pressuremean area per molecule A, determined by the Langmuir-Adam balance (open circles), and surface pressure-molecular area in the condensed phase Axy obtained from the GIXD experiments (solid squares). The location of the π/Axy isotherms at smaller areas shows the presence of voids in the TFEB and EB films.

Ac is the area per molecule in the condensed phase (L2′, S, or CS), and Av is the molecular area in the voids. Xc and Xv are the molar fractions of the amphiphiles in the

πcmp ) γ0 - γC - fV(γV - γC)

(16)

Equations 13 and 16 represent the surface pressuremolecular area isotherm of a heterogeneous monolayer. It deals with equilibrium distribution of the amphiphilic substance on the water surface and does not include kinetic effects. Different mass exchange rates between monolayer parts that could be substantial at continuous compression or expansion will complicate the situation because nonequilibrium local values of Ai, Xi, γi, and fi should be introduced.


IV. Discussion Monolayer Structure and Compression-Expansion Hysteresis of the π/A Isotherms. Our BAM and GIXD data suggest that the presence of voids in TFEB and EB monolayers could be the reason for the compression-expansion hysteresis of the dynamic π/A isotherms in Figure 1. As Table 4 shows, the partial area of the voids (A - Axy)/Axy in the TFEB film is larger which explains the larger hysteresis and mean molecular areas for this system. The lower density of suspension particles in the dark areas at expansion as compared to compression (Figures 2 and 3) implies that their detachment from the compact TFEB and EB monolayers overcomes a higher energy barrier than their attachment during compression. This difference would relate the compression-expansion hysteresis of the surface pressure-area isotherms to kinetic reasons. Such an explanation seems plausible if we recall that Adam’s “equilibrium” π/A isotherm of EB does not show any hysteresis. Monolayer Structure and Compressibility of the TFEB and EB Films. Expelling of the voids in the archipelago structure and closing of the lakes in the compact TFEB and EB monolayers under compression imply internal rearrangements and mobility of their building elements (molecules or microcrystals) relative to each other. This mobility seems to be maintained at high surface pressure and to be different for TFEB and EB films. Such a difference could explain the 2.5-fold lower compressional modulus of TFEB at 18 mN/m as compared to the EB. At this surface pressure, both monolayers have almost the same mean area per molecule, and the same GIXD molecular area, and their molecules are arranged in the same closely packed untilted S or CS phase (see Tables 2 and 4). Despite that, the compressional modulus Cs-1 ) -A(dπ/dA) ) 250 mN/m for TFEB and Cs-1 ) 650 mN/m for EB. If the TFEB and EB films are polycrystalline, as our BAM data suggest, the different mobility of their elements might arise from different friction at the domain boundaries. The friction between the TFEB chains might be smaller because they possess larger parallel dipoles than the EB chains due to the stronger polarization caused by the fluorinated head (see the maps of molecular electrostatic potentials presented in ref 6). The friction between the TFEB heads should be also smaller than the friction between the ethyl ester heads because fluorination of grafted hydrocarbon chains strongly reduces surface friction.1,3 Sometimes, the compressibility or compressional modulus has been evaluated using GIXD molecular area, Cs-1 ) -Axy(dπ/dAxy).15,17 Such an approach ignores the more complicated physical meaning of the surface pressure of a composite film (see eq 16) relating πcmp which characterizes the whole monolayer to Axy that is the molecular area in its condensed part only (cf. eq 13). In this sense, the calculation of Cs-1 ) -Acmp(dπcmp/dAcmp) as done in ref 6 is more unambiguous and could better serve for comparison of the mechanical properties of TFEB and EB monolayers. However, any thermodynamic interpretation of the Cs-1 jumps as indicating phase transition should be avoided. Comparison of the Phase Transitions Deduced from π/A Isotherms with the GIXD and BAM Data for EB and TFEB Monolayers. Adam8 was probably the first who reported a static isotherm of ethyl stearate (monolayer has been left to relax to a constant surface pressure for each point of the isotherm). He found that it

Langmuir, Vol. 17, No. 15, 2001 4589

does not exhibit compression-expansion hysteresis and is identical within the experimental error with the isotherms of ethyl palmitate and ethyl behenate. This “typical curve of a condensed ethyl ester monolayer”8 consists of two linear sections ascribed by him to liquid condensed and solid condensed states of the monolayer. Stenhagen9 published a phase diagram of ethyl behenate monolayers distinguishing one liquid condensed L2 phase with maximum tilt angle of 25°-27° and three solid condensed phases with upright molecules: superliquid LS, solid S, and condensed solid CS. According to his diagram, at 20 °C one should observe an L2-CS phase transition at 2.5 mN/m. The equilibrium πtr value of Adam8 is just slightly below the beginning of the transition section (11.5-16.5 mN/m) following the linear low-pressure part of our dynamic compression isotherm of ethyl behenate.6 However, we observe also a kink at 28.0 mN/m and collapse at significantly higher surface pressure (44.3 mN/m). This difference could be due to dynamic effects in our measurements, which are known to increase collapse and transition surface pressures of Langmuir films. The proximity of the kink in our isotherm to the “equilibrium” collapse surface pressure found by Adam (27.5 mN/m) suggests that the kink indicates a transition to a metastable phase that is absent in the static isotherm. Lundquist20 composed phase diagrams of ethyl ester monolayers with 15-20 carbon atoms in the acidic chain employing dynamic expansion isotherms. She found the same untilted LS, S, and CS phases as Stenhagen9 but two tilted phases L2′ and L2′′ with maximum tilt angles of 25°-27° and 23°-24°, respectively. Her diagram for ethyl eicosanoate (the closest even homologue to ethyl behenate) shows that at 20 °C one should observe L2′LS, LS-LS′, and LS′-S′ phase transitions at 12.5, 26, and 31 mN/m, respectively. Because LS′ and S′ are metastable phases, one should expect only one L2′-LS transition in equilibrium isotherms that is in qualitative agreement with Adam’s findings. Bibo, Knobler, and Peterson21 also found three changes of the slope in the dynamic expansion isotherm of ethyl eicosanoate, but located at 5.5, 12.5, and 17.0 mN/m. They assumed the existence of two tilted phases, L2* and S*, instead of the L2′ phase, and interpreted the slope changes in their isotherm as L2-L2*, L2*-S*, and S*-S phase transitions. These results can be reconciled with Adam’s equilibrium isotherm if the L2* and S* phases are also metastable. Previous in situ X-ray diffraction studies of Langmuir monolayers quantitatively characterized the above phases.13-27 It was found that in the LS phase the monolayer molecules freely rotate around their long axes so that their effective cross-sections are circular and they form a hexagonal lattice with small positional order. In the S phase, this rotation is partially impeded to an oscillation around the long axes, the lattice unit cell is distorted to centered rectangular, and the positional order is of quasi-long-range (the correlation function is ∼r-ζ). The CS phase is a 2D crystal with a centered rectangular unit cell and long-range positional order (∼e-r/ζ). The partially or entirely “frozen” hydrocarbon chains in the S and CS phases have elliptical cross sections forming a herringbone packing in which the ellipses are almost perpendicular to each other. The chains in the L2 and L2′′ phases are also closely packed in a centered rectangular lattice but tilted toward nearest neighbors. The L2′, L2*, and S* phases have closely packed chains arranged in a (21) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591.

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Petrov et al.

centered rectangular lattice with tilt to the next-nearest neighbors. The S* phase has higher positional order than the L2* phase.22,23 Our GIXD data for EB show two diffraction peaks appearing at both low and high surface pressure. Their occurrence excludes the existence of an LS phase because its hexagonal lattice would exhibit only one in-plane peak. Therefore, the L2′-LS transition that could be anticipated from the compression EB isotherm and the theory of equivalent states (see above) obviously does not take place at 20 °C. At low surface pressure, the EB film exists in the NNN tilted L2′ phase but not in the NN tilted L2 phase predicted by Stenhagen’s phase diagram.9 The maximum tilt angle at zero compression is 17.6° ( 0.3° that is considerably below Stenhagen’s9 and Lundquist’s20 values of θmax ) 25°-27°. Such a difference seems to result from the nonhomogeneity of the EB monolayer. In refs 9 and 20, the maximum tilt angle was calculated using the formula

cos θmax )

Aπ⊥ Aπ)0

(17)

where A is the mean molecular area determined in the Langmuir trough. Aπ⊥ corresponds to the lowest surface pressure at which the tilt angle becomes zero, and Aπ)0 is obtained via extrapolation of the π/A dependence of the tilted phase to zero surface pressure. However, if the values of Axy from Table 2 are substituted in eq 17 one finds θmax ) 17.3° in excellent agreement with our GIXD value. The unit cell parameters of the high-pressure phase of EB and TFEB (Table 2) agree with the parameters of both S and CS phases, so we could not definitely decide which one corresponds to the actual monolayer state. The cross section of a hydrocarbon chain for the CS phase is typically around 18.2 Å2, whereas that for the S phase is near 19.2 Å2. For EB, we find A0 ) 18.8 Å2 that is between these values. On the other side, the highly ordered crystal structure of TFEB with the same cross-sectional area of 18.9 Å2 gives seven second-order peaks (Figure 8) thus suggesting a CS phase. The optical anisotropy of the mosaic structure of the EB monolayer detected by BAM at low surface pressure implies that this film consists of TO domains (domains of equal tilt order). Such domains are characteristic for the tilted S* phase registered in BAM observations of eicosanol and docosanoic acid monolayers.23 The decrease of the contrast of the TO domains at compression is typical for the S*-S phase transition.23 The weak anisotropy remaining in the high-pressure phase could result from a herringbone arrangement of the chains in the S phase. Thus, our BAM data suggest that the EB monolayer undergoes L2*-S* and S*-S transitions under continuous compression and the opposite phase sequence under continuous expansion. The plot of the GIXD molecular area Axy versus π in Figure 5 also might be interpreted as indicating two transitions, at 10.7 mN/m (19.0 Å2) and 22.3 mN/m (18.8 Å2). However, the more reliable data for the tilt angle in Figure 6 show continuous linear decrease of 1/cos θ with increasing of surface pressure and only one tilting transition. The single linear dependence of the signed lattice distortion D on sin2 θ down to θ ) 0° also disagrees with the subdivision of the L2′ phase in two tilted L2* and S* phases (Figure 7). Additional arguments (22) Knobler, Ch. M.; Desai, R. C. Annu. Rev. Phys. Chem. 1992, 43, 207. (23) Overbeck, G. A. Ph.D. Thesis, Georg-August-Universitaet Goettingen, Goettingen, Germany, 1993; p 59.

against such subdivision were found in the theoretical symmetry analysis.14,24 The GIXD and BAM data for TFEB monolayers unambiguously show that even at zero surface pressure this substance forms domains of closely packed upright molecules. The molecular area of 18.9 Å2 and the vertical orientation of the chains remain unchanged under compression (Figure 9a). The dense packing and vertical molecular orientation of the TFEB film at low surface pressure represent the main difference from the EB monolayer. At high surface pressure, the unit cell parameters of TFEB and EB films are indistinguishable. The second-order peaks in Figure 8 imply that the TFEB film has crystalline structure with high positional order. Its “sintering” under compression expels the voids so that the changes of the slope in the π/A isotherm manifest accidental jumps of its density. Thus, the interpretation of these changes as phase transitions is a striking example of an erroneous conclusion based only on the analysis of the π/A isotherms. Hydrophilicity of the Headgroup and Orientation of the Hydrocarbon Chains in Closely Packed EB and TFEB Monolayers. The maps of molecular lipophilicity of single EB and TFEB molecules in vacuo presented in ref 6 showed that substitution of the ester CH3 by a CF3 group causes a significant decrease of hydrophilicity of the polar head.6 This implies a different degree of hydration of EB and TFEB heads that might play an important role for the structure of the two monolayers. GIXD comparison25 of palmitic acid, methyl palmitate, and ethyl palmitate monolayers in the condensed state showed systematic structural changes with decreasing hydrophilicity of the headgroup at constant molecular area. The tilt angle at zero compression decreases from 33° for palmitic acid to 18° for methyl palmitate and 16° for ethyl palmitate. The tilt azimuth is oriented to nearest neighbors in palmitic acid and methyl palmitate films but to next-nearest neighbors in the ethyl palmitate monolayer. Thus, decreasing the hydrophilicity of the head via substitution of the OH in the COOH group through OCH3 and OCH2CH3 causes a decrease of the tilt angle and changes the direction of the tilt azimuth from NN to NNN. Shih et al.26 commented on the determining role of the hydrophilic headgroups for the low-pressure NN and NNN tilted phases and the significance of the hydrocarbon chains for the structure of the monolayer at high surface pressure. A “swiveling transition” changing the tilt direction from NN to NNN has been found for the isothermal compression of monolayers of behenic acid,15 arachidic acid,27 and heneicosanoic acid.26,28 An isobaric increase of the temperature at 35 mN/m also causes an NN-NNN transition in heneicosanoic acid28 at practically constant and almost zero tilt angle of 4° ( 3° implying constant molecular area. Both the isothermal compression and the isobaric increase of temperature at constant molecular area decrease hydration of the headgroups. This (24) Kaganer, V. M.; Peterson, I. R.; Kenn, R. M.; Shih, M. C.; Durbin, M.; Dutta, P. J. Chem. Phys. 1995, 102, 9412. (25) Weidemann, G.; Brezesinski, G. M.; Vollhardt, D.; Bringezu, F.; de Meijere, K.; Moehwald, H. J. Phys. Chem. 1998, 102, 148. (26) Shih, M. C.; Durbin, M. K.; Malik, A.; Zschack, P.; Dutta, P. J. Chem. Phys. 1994, 101, 9132. (27) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippman-Krayer, P.; Moehwald, H. J. Phys. Chem. 1989, 93, 3200. (28) Bohanon, T. M.; Lin, B.; Shih, M. C.; Ice, G. E.; Dutta, P. Phys. Rev. B 1990, 41, 4846.


fact provokes the conclusion that less hydrated heads orient the tilted hydrocarbon chains toward next-nearest neighbors. Similar observations have been reported for phospholipid model membranes. In an X-ray diffraction study, Levine29 observed dependence of the tilt angle in phosphatidylcholine (PC) bilayers on relative humidity and thus hydration of the headgroup. At 2% water content, the DPPC chains were vertically oriented, but for 20% water a limiting angle of 28° was found. Tardieu et al.30 found a similar increase of the tilt angle from 17° to 33° in the same system when increasing the molar fraction of water in it. Hui31 determined 16° for hydrated oriented PC multilayers but no tilt in dehydrated single bilayers. Vertical molecular orientation appears to be typical for the fully hydrated gel phases of phosphatidylethanolamine (PE).32,33 Such orientation could be rationalized in the framework of the above picture if one recalls the lower hydration number of the PE head compared to the PC head.34 At low temperatures and low pH values, phosphatidyl glycerol (PG) bilayers exhibit untilted molecular packing similar to that of PE. However, at neutral pH and a high salt concentration they have a tilt of 30°-35° as in the case of PC.35,36 This change seems also to result from stronger hydration of the charged PG head at neutral pH; the electrostatic repulsion has been screened by the high ionic strength. Smith, Sirota, Safinya, and Clark37 performed a thorough study of the above structural changes under welldefined conditions. Freely suspended stacked DMPC bilayers were well oriented with respect to the X-ray beam giving more unambiguous structural information. Temperature and vapor pressure were monitored and molecular tilt angle, tilt azimuth, and layer spacing were determined as a function of the chemical potential of water. It was found that decreasing the water chemical potential changes the tilt direction of the chains from NN to NNN causing a small decrease of the tilt angle from 30° to 26°. Monolayers of long-chain alcohols would present an exception if their OH group were more hydrophilic than the COOH head of the fatty acids. At zero surface pressure, heneicosanol molecules are tilted in the NNN direction (∼20°), whereas heneicosanoic acid tilts in the NN direction (∼30°).26 This is opposite to the above examples. However, if the COOH head was more hydrophilic both the magnitude of the tilt angle and tilt azimuth would fit in our scenario. On the other hand, one could speculate that not the hydration of the head as a total determines the structure of the tilted phase but the asymmetry of the headgroup hydration. Such asymmetry could be expected for the headgroups of the fatty acids (OdC-OH), their methyl esters (OdC-OCH3), ethyl esters (OdC-OC2H5), phospholipids, and so forth, whereas hydration of the -OH group of the alcohols should be symmetric. Summarizing the above analysis of the monolayer and bilayer structure, one might conclude that hydration of (29) Levine, Y. K. Prog. Biophys. Mol. Biol. 1972, 24, 3; 1973, 3, 279. (30) Tardieu, A.; Luzatti, V.; Reman, F. C. J. Mol. Biol. 1973, 75, 711. (31) Hui, S. W. Chem. Phys. Lipids 1976, 16, 9. (32) McIntosh, T. J. Biophys. J. 1980, 29, 237. (33) Tenchov, B.; Kojnova, R.; Rappolt, M.; Rapp, G. Biochim. Biophys. Acta 1999, 1417, 183. (34) Nagle, J. F.; Wiener, M. C. Biochim. Biophys. Acta 1988, 942, 1. (35) Watt, A.; Harlos, K.; Marsh, D. Biochim. Biophys. Acta 1981, 645, 91. (36) Blaurock, A. E.; Mintosh, T. J. Biochemistry 1986, 25, 299. (37) Smith, G. S.; Sirota, E. B.; Safinya, C. R.; Clark, N. A. Phys. Rev. Lett. 1988, 60, 813.

Langmuir, Vol. 17, No. 15, 2001 4591

the polar heads affects the tilt angle and orientation of the closely packed hydrocarbon chains. A lower degree of hydration of the head decreases the tilt angle and changes the tilt direction from NN to NNN. In this context, one could speculate that the decreased hydrophilicity and weaker hydration of the trifluoroethyl ester group cause the vertical orientation of the TFEB chains at low surface pressure. Under the same conditions, the EB molecules having more hydrophilic heads are tilted toward nextnearest neighbors. Conformation of the Headgroups in Condensed EB and TFEB Monolayers. In part 1 of this study, we commented on the possibility that the shift of the TFEB hysteresis loop to larger molecular areas (cf. Figure 1) might be due to different conformations of the heads in EB and TFEB monolayers. Our present GIXD data show that in the upright high-pressure phase TFEB and EB molecules occupy the same area of 18.9 ( 0.1 Å2. This result suggests the same headgroup conformation or no effect of the latter on molecular area even at the close packing of the upright chains. Longer alkyl chains in monolayers of fatty acid esters should orient toward air because they are hydrophobic. Adam38 found such an arrangement in ester monolayers containing four or more alkyl C-atoms. Adam’s work38 and a study of Alexander and Schulman39 suggested that the C2H5 groups in condensed ethyl ester films are accommodated below the acidic chains. This model explains the surface potential-area isotherm of ethyl stearate rather well,39 but some later structural studies commented below show that other headgroup conformations might be also possible. The change of the tilt azimuth in the sequence palmitic acid (NN)-methyl palmitate (NN)-ethyl palmitate (NNN) could be due to orientation of the ethyl group toward the air-water interface according to its energetic preference. The change of the tilt azimuth helps satisfy the requirement for minimum molecular area at a given surface pressure and temperature. In this context, it is rather interesting that the positional correlation length of the high-pressure untilted phase of the above monolayers decreases in the same sequence at the same area per molecule (20.0 ( 0.2) Å2. For palmitic acid ζ > 200 Å, for methyl palmitate it is 180 Å, and for ethyl palmitate ζ ) 125 Å. Such a decreasing order of the untilted phase following the sequence fatty acid > methyl ester > ethyl ester could be easily understood if the ethyl groups look for a place at the water surface and orient to the nextnearest neighbors to minimize the lattice distortion of the closely packed hydrocarbon chains. If they orient underneath the acidic chains, as supposed by Alexander and Schulman,39 at high surface pressure ethyl ester films should have the same positional order as fatty acid and methyl ester films, because the C2H5 groups have the same cross section as the acidic chains and the van der Waals attraction between the “extended” ethyl ester molecules should be stronger. This conclusion is supported by the investigation of Shih et al.26 where the pressure variation of the lattice parameters in the plane normal to the molecules and in the water surface was studied. The authors assumed that the chains determine the parameters in the plane normal to the molecules and the headgroups are responsible for those in the water surface. After the swiveling transition in the monolayer of heneicosanoic acid, the distances in (38) Adam, N. K. Proc. R. Soc. London 1929, A126, 366. (39) Alexander, A. E.; Schulman, J. H. Proc. R. Soc. London, Ser. A 1937, 161, 115.

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the plane normal to the chains remained constant but those in the horizontal plane varied with increasing of the surface pressure. This suggests that compression of the L2′ phase where the hydrocarbon chains are closely packed and NNN tilted forces the headgroups to reorient from the NN to the NNN direction while the chains retain the same close packing and orientation. After the tilting L2′-S transition at which the tilt angle becomes zero, the distances in both planes remain constant thus implying that the heads maintain their NNN orientation also in the upright phase with closely packed chains (cf. Figures 4 and 5 in ref 26). If such headgroup reorientation occurs under compression of fatty acid monolayers, it would be much more probable for ethyl ester films because of the energetic preference of the ethyl chain to be located at (or closer to) the air-water interface. Our GIXD analysis established NNN orientation of the tilted acidic chains of the EB monolayer, and there is no reason to believe that the ethyl groups of the heads have a different orientation at high surface pressure. One could expect that after the L2′-S transition the ethyl groups would have NNN orientation because this position minimizes the molecular area. Our GIXD value of Axy ) 18.8 Å2 for the high-pressure upright phase considerably exceeds the molecular cross section of

Petrov et al.

18.2 Å2 found via X-ray diffraction in bulk crystals of EB. In bulk crystals, the ethyl groups are located underneath the acidic chains40 enabling a close packing of the hydrocarbon chains with A0 ) 18.2 Å2. This intriguing difference supports a “hook” conformation of the EB molecules with vertical closely packed chains and NNN oriented headgroups. A similar structure seems to be formed in the TFEB monolayer even at low surface pressure because of the higher hydrophobicity of the CH2CF3 group that is equivalent to a longer alkyl chain. This scenario will be further discussed in part 3 of this study where a TRIPOS molecular model of an ensemble of 36 TFEB or EB molecules with a hook headgroup conformation will be presented. It gives the same equilibrium molecular area of about 20 Å2 for both TFEB and EB films but negative surface potential for TFEB and positive ∆V for EB films in agreement with the experimental results.5 Acknowledgment. J. G. Petrov thanks Professor B. Tenchov for the useful discussions and literature sources concerning the structure of phospholipid bilayers. LA0017835 (40) Mathieson, M. A.; Welsh, H. K. Acta Crystallogr. 1965, 18, 953.

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