Dipolar Interactions and Miscibility in Binary Langmuir Monolayers

Langmuir 2009, 25, 3659-3666

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Dipolar Interactions and Miscibility in Binary Langmuir Monolayers with Opposite Dipole Moments of the Hydrophilic Heads Jordan G. Petrov,*,† Tonya D. Andreeva,† and Helmuth Moehwald‡ Institute of Biophysics of the Bulgarian Academy of Sciences; 1 Acad. G. BoncheV Str., 1113 Sofia, Bulgaria and Max-Planck Institute of Colloids and Interfaces; D-14476 Golm/Potsdam, Germany ReceiVed December 16, 2008. ReVised Manuscript ReceiVed January 10, 2009 We investigate unusual binary Langmuir monolayers with the same long CH3(CH2)21 hydrocarbon chains and fluorinated -O-CH2CF3 (FEE) versus nonfluorinated -O-CH2CH3 (EE) hydrophilic heads, whose opposite dipoles assist miscibility, in contrast to the equally oriented polar head dipoles of almost all natural or synthetic amphiphiles that minister to phase separation. Although two-component bulk micelles, lipid bilayers, and monolayers with fluorinated and nonfluorinated chains, which also have opposite dipoles, often show phase separation, we find complete miscibility and nonideality of the FEE-EE mixtures demonstrated via deviation of the composition dependencies of the mean molecular area at fixed surface pressure from the additivity rule. The composition dependencies of the excess molecular areas exhibit minima and maxima which show specific structural changes at particular compositions. They originate from the dipolar and steric interactions between the polar heads, because the interactions between the same chains of FEE and EE do not vary. The π/A isotherms and the π/XFEE phase diagram reveal that mixtures with molar fractions XFEE g 0.3 exist in an upright solid phase even in uncompressed state. This result is confirmed by the compressibility values and via Brewster angle microscopy, which does not show optical anisotropy at XFEE g 0.3. Comparison of the collapse and phase-transition molecular areas with literature data suggests that the upright architecture corresponds to LS-phase or S-phase with more defects as the S-phase in the pure monolayers. The mixtures with XFEE < 0.3 exist in tilted L2′ phase at low surface pressures. Their mean molecular areas are smaller than the corresponding values in the EE film, which manifests reduction of the tilt of the EE chains with increasing FEE content. We ascribe the chain erection to partial dehydration of the EE heads caused by dipolar attraction between the EE and FEE heads. The excess free energy of mixing ∆Gexcπ is positive but much smaller than the negative total free energy of mixing ∆G mixπ showing a spontaneous miscibility at all compositions due to an entropy increase. The analysis of the conflict between the ∆Gmixπ minimum at molar fraction XFEE ) 0.5 and the minimum and negative value of the excess molecular area Aπ,exc at XFEE ) 0.8 shows that the Aπ,exc/XFEE minimum has not an electrostatic but a short-range structural origin.

Introduction Binary Langmuir monolayers are often used to investigate the link between miscibility and molecular interactions of their components. The goal is to model and better understand the function-structure relationship of biomembranes,1-3 behavior of the lung surfactant film that stabilizes the alveoli,4 chiral separation of left- and right-handed enantiomers at interfaces,5 or to develop biosensors,6 and new drug delivering carriers as complex liposomes,7 and microcapsules.8 A specific area of these studies deals with binary monolayers containing fluorinated amphiphiles which become increasingly popular because of their biomedical,9 environmental,10 and industrial applications.11 The mixing behavior of Langmuir films depends on the van der Waals attraction between their hydrocarbon chains and the elec* Corresponding author E-mail: [email protected]. † Institute of Biophysics of the Bulgarian Academy of Sciences. ‡ Max-Planck Institute of Colloids and Interfaces. (1) Vanounou, S.; Parola, A. H.; Fishov, I. Mol. Microbiol. 2003, 49, 1067. (2) Heimburg, T.; Angerstein, B.; Marsh, D. Biophys. J. 1999, 76, 2575. (3) Sennato, S.; Bordi, F.; Cametti, C.; Coluzza, C.; Desideri, A.; Rufini, S. J. Phys. Chem. B 2005, 109, 15950. (4) Discher, B. M.; Schief, W. R.; Vogel, V.; Hall, S. B. Biophys. J. 1999, 77, 2051. (5) Kuzmenko, I.; Rapapport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leserowitz, L. Chem. ReV. 2001, 101, 1659. (6) Broniatowski, M.; Obidowicz, K; Romeu, N. V.; Broniatowska, E.; Dynarowicz-Łatka, P J. Colloid Interface Sci. 2007, 313, 600. (7) (a) Ries, J. G. Drug Targ. 1994, 2, 455. (b) Kraft, M. P.; Riess, J. G. Biochimie 1998, 80, 489. (8) Sukhorukov, G. B.; Moehwald, H. TRENDS Biotech. 2007, 25, 93. (9) Gerber, F.; Kraft, M. P.; Vandamme, T. F.; Goldmann, M.; Fontaine, P. Angew. Chem., Int. Ed. 2005, 44, 2749. (10) Rontu, N.; Vaida, V. J. Phys. Chem. C 2007, 111, 9975. (11) Fluorinated Surfactants: Synthesis, Properties, Applications; Kissa, E., Ed.; Marcel Dekker: New York, 1994.

trostatic repulsion or attraction between the dipoles or charges of their polar or charged headgroups. A theoretical estimate showed that more than 40% of the surface pressure at the initial spreading of phosphatidylcholine vesicles on water is due to the dipole-dipole repulsion between the zwiterionic heads.12 The dipolar repulsion determines the size and shape of the liquid domains in cholesterolphospholipid mixtures near the miscibility critical points,13 and plays a crucial role in the polymorphism of solid monolayers.14 Since almost all polar heads of the natural and synthetic amphiphiles have equally oriented normal dipole moment components in Langmuir or adsorption films (“+” toward air, “-” toward water),15 the head-head dipolar interactions in their mixtures are repulsive and favor phase separation of the components. In this context, the binary monolayers of docosyl ethyl ether (EE), C21H43CH2OCH2CH3, and docosyl trifluoroethyl ether (FEE), C21H43CH2OCH2CF3 studied in the present work, are extremely unusual. They have the same CH3(CH2)21 hydrocarbon chains, which experience the same dispersive attraction and dipolar and steric repulsion between the dissimilar and the same molecules. Their simple polar heads, -O-CH2CH3 versus -O-CH2CF3, produce positive µ⊥ ) + 235 mD, versus negative µ⊥ ) -120 mD normal components of the effective molecular dipole moment,16so that the dipolar attraction between the EE and FEE heads would assist the miscibility in the EE-FEE binary Langmuir films. However, numerous studies of bulk micelles containing amphiphiles with fluorocarbon and hydrocarbon chains, which also (12) Panaiotov, I.; Ivanova, Tz.; Sanfeld, A. AdV. Colloid Interface Sci. 1992, 40, 147. (13) Kellner, S. L.; McConnel, H. M. Phys. ReV. Lett. 1999, 82, 1602. (14) Roan, J.-R.; Kawakatsu, T. J. Phys. Soc. Jpn. 2001, 70, 738. (15) Haydon, D. A.; Hladky, S. B. Q. ReV. Biophys. 1972, 5, 187. (16) Petrov, J. G.; Andreeva, T. D.; Kurth, D. G.; Moehwald, H J. Phys. Chem. B 2005, 109, 14102.

10.1021/la804136j CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

3660 Langmuir, Vol. 25, No. 6, 2009 have opposite molecular dipole moments caused by the δ-(F)3-Cδ+ and δ-(F2C)-(CH2)δ+ dipoles, report phase separation for neutral or equally charged head-groups, and partial or complete miscibility for oppositely charged heads.17-22 Phase separation of insoluble fluorinated and nonfluorinated amphiphiles was observed also in Langmuir films.23,24 The miscibility in binary monolayers of hydrocarbon and perfluorinated or partially fluorinated undissociated carboxylic acids was found to vary from phase separation to complete miscibility depending on the length of the perfluorinated chain or degree of chain fluorination.25,26 Similar differences were observed in two-component monolayers of perfluorinated and partially fluorinated carboxylic acids and alcohols mixed with dipalmitoylphosphatidylcholine or a phosphatidylcholine with a single partially fluorinated chain.27-31 The above examples show that the interactions between the fluorinated and nonfluorinated chains in such molecular aggregates are complex so that one cannot a priori state how the dipolar interactions between the fluorinated and nonfluorinated heads would affect the miscibility of the binary EEFEE monolayers. Our previous investigation32 of the EE and FEE monolayers via grazing incidence X-ray diffraction (GIXD) showed that EE forms a crystal L2′ phase of tilted closely packed molecules after spreading at π ≈ 0, whereas the uncompressed FEE film organizes in a crystal S-phase with closely packed upright molecules.33 Above 10 mN/m, the EE monolayer assumes the same phase state and molecular area as the FEE film. These individual features suggest that the binary monolayers might separate in tilted and upright solid phases at low surface pressures, but form an isomorphic molecular mixture of closely packed upright molecules at high surface pressures. We studied the mixing behavior of the above couple of amphiphiles recording the surface pressure-mean molecular area isotherms π/A at different compositions, and analyzing the dependencies of the characteristic parameters extracted from them on the molar fraction XFEE of the fluorinated component. Brewster angle microscopy (BAM) was applied to check the optical anisotropy and the phase state of the binary films and to visualize the changes of their morphology under compression.

Materials and Methods Synthesis, purification, and characterization of EE and FEE were described previously.16 Different volumes of 1 × 10-3 M solutions of EE and FEE in chloroform (J. T. Baker) were mixed prior to the experiment and spread on Mili-Q Millipore water at initial area of ca. 70 Å2/molecule. After 5 min left for evaporation of the solvent they were compressed at a constant velocity of 2.2 Å2/molecule · min. The water substrate was kept at 20.0 ( 0.1 °C via temperature control system. (17) Mysels, K. L. J. Colloid Interface Sci. 1978, 66, 331. (18) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (19) Funasaki, N.; Hada, S. J. Phys. Chem. 1983, 87, 342. (20) Kunitake, T.; Tawaki, S.; Nakashima, N. Bull. Chem. Soc. Jpn. 1983, 56, 3235. (21) Mukerjee, P. Colloids Surf. A 1994, 84, 1. (22) Guo, W.; Guzman, E. K.; Heaven, S. D.; Li, Z.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 2368. (23) Elbert, R.; Lashewsky, A.; Ringsdorf, H. J. Am. Chem. Soc. 1985, 107, 4134. (24) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (25) Shibata, O.; Yamamoto, S. K.; Lee, S.; Sugihara, G. J. Colloid Interface Sci. 1996, 184, 201. (26) Lehmler, H.-J.; Bummer, P. M. J. Colloid Interface Sci. 2002, 249, 381. (27) Lehmler, H.-J.; Jay, M.; Bummer, P. M. Biophys. Biochim. Acta 2004, 164, 141. (28) Nakahara, H.; Nakamura, S.; Kawasaki, H.; Shibata, O. Colloids Surf. B 2005, 41, 285. (29) Arora, M.; Bummer, P.; Lehmler, H.-J. Langmuir 2003, 19, 8843. (30) Shibata, O.; Kraft, M. K. Langmuir 2000, 16, 10281. (31) Nakamura, S.; Nakahara, H.; Kraft, M. P.; Shibata, O. Langmuir 2007, 23, 12634. (32) Petrov, J. G.; Brezesinski, G.; Andreeva, T. D.; Moehwald, H. J. Phys. Chem. B 2004, 108, 16154. (33) Kaganer, V. M.; Moehwald, H.; Dutta, P. ReV. Modern Phys. 1999, 71, 779.

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Figure 1. Surface pressure/mean molecular area isotherms π/A at different molar fraction of the fluorinated compound XFEE. The abscissa of the isotherms for 0.1 e XFEE should be read as (A - 20*XFEE) Å2/molecule. The arrows indicate the transition surface pressure πt, at which the tilted-to-upright phase transition ends, the instability surface pressure πi, at which dπ/dA of the steep part of the isotherm start decreasing, and the collapse surface pressure πc, that is followed by a fast formation of 3D bulk phase.

A computerized Langmuir balance with Teflon trough and Wilhelmy dynamometric system was used to record the π/A isotherms with sensitivity of 0.2 mN/m and 0.1 Å2/molecule. The morphology and the optical anisotropy of the films were studied by a Brewster angle microscope BAM-2 (NFT, Goettingen, Germany) as previously described.32

Results and Discussion Surface Pressure-Mean Molecular Area Isotherms. Figure 1 presents the π/A isotherms at different molar fractions XFEE of the fluorinated ether. In order to avoid overlapping each curve for XFEE g 0.1 is shifted by 2 Å2/molecule from the previous one, so that the corresponding abscissa should be read as (A - 20*XFEE) Å2/molecule. The isotherms for XFEE ) 0.1 and 0.2 resemble the one of the pure EE; they have shallow linear parts at low surface pressure, characteristic for the tilted L2′ phase of the EE film, and a steep high pressure linear part corresponding to the upright S phase.32 The π/A isotherms of the monolayers with XFEE g 0.3 look like the one of the FEE monolayer, which was shown to exist in the S phase.32 The surface pressure of the kink πt, at which the tilted-to-upright phase transition ends (see the arrows), decreases with increasing XFEE. The high pressure linear parts of the isotherms retain constant slopes up to πi, where the monolayer becomes instable, and the derivative dπ/dA begins to decrease. Further compression results in a surface pressure maximum πc, followed by fast transformation of the 2D monolayer in the 3D bulk phase of collapsed material. Some isotherms, as the one of the pure EE film, do not show such maxima and πc is determined from the intersection of the linearly extrapolated adjacent parts of the π/A curve. Since the instability pressure πi is determined in the same way for all compositions, the πi/XFEE dependence is more self-consistent than the πc/XFEE one. Dependence of the Collapse, Instability, and Phase Transition Parameters on Monolayer Composition. If the collapse process is quasi-static, then the monolayer and the 3D phase of the collapse material are in equilibrium and πc ≡ πi. In such cases, one can strictly apply the 2D version of the Gibbs phase rule to the πc/XFEE dependence,34,35 which states that the existence of the two phases on the water surface reduces the degrees of freedom to zero and causes an independence of πc on the composition of the binary monolayers. And vice versa, a variation (34) Crisp, D. J. Surface Chemistry, (Supplement to Research); Butterworths: London, 1949. (35) Gaines, G. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966, p 281.

Binary Langmuir Monolayers

Figure 2. Nonequilibrium surface pressure/composition phase diagram delineating the ranges of the tilted L2′, upright LS or S and 3D phase of the binary films under the experimental conditions (a). Composition dependencies of the collapse Ac and instability Ai mean molecular areas (b).

of the quasi-static collapse pressure with composition manifests a molecular miscibility of the components.36 When the monolayer collapse is nonquasi static, the values of πc and πi differ from each other and the πc/XFEE or πi/XFEE dependencies are commonly interpreted as variation of the molecular interaction between the components.37,38 We recorded the π/A isotherms of the individual substances at compression velocities Vcmp between 3 and 36 cm2/min (1 to 10 Å2/molecule · min) and found that the values of πi and πc varied with Vcmp although the isotherms overlapped within 0.2 - 0.5 Å2 up to the instability points (see the Supporting Information). In spite of that we used the πc/XFEE, πi/XFEE and the πt/XFEE data averaged from 3-5 isotherms recorded at Vcmp ) 2.2 Å2/molecule.min to construct the nonequilibrium surface pressure/film composition phase diagram shown in Figure 2a. The area below the πt/XFEE dependence corresponds to binary films in the tilted L2′- phase, the one between πt/XFEE and πi/XFEE presents monolayers in a solid phase with upright chains, and the area above the πc/XFEE dependence displays the 3D bulk phase of the collapsed material. Linear extrapolation of the πt/ XFEE dependence (R2 ) 0.99995) intersects the ordinate at XFEE ) 0.33. Hence, we conclude that an increase of the FEE content to 33% removes the tilted L2′ phase characteristic for the pure EE monolayer at low surface pressures, so that all binary films with 0.33 e XFEE e 1.0 are organized in an upright solid phase at 0 < π < πi. Figure 2b shows that the collapse Ac and instability Ai molecular areas of the binary films, except those for XFEE ) 0.8, lie between 19.1 and 19.5 Å2. These values slightly exceed the range 18.9-19.2 Å2 characteristic for the upright LS phase of long chain compounds with small polar heads, but definitely stand outside the ranges 18.5-18.8 Å2 and 18.1-18.4 Å2, representative (36) Quasi-static πt/XFEE dependences cannot be used to judge about miscibility of binary monolayers, because the number of phases and degrees of freedom do not change at the end of the tilted-to-upright second order phase transition. (37) Costin, I. S.; Barnes, G. T. J. Colloid Interface Sci. 1975, 51, 106. (38) Dynarowicz-Łatka, P.; Kita, K AdV. Colloid Interface Sci. 1999, 79, 1.

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for S, respectively CS phases.39,40 The range of the transition areas At ) 20.0 ( 0.1 Å2 of the films with XFEE e 0.2 also better agrees with the literature data 19.1-20.0 Å2 for the LS phase, than with those for the S-phase 18.8-19.1 Å2 or the CS phase 18.4-18.6 Å2.39 On the other side, the Ac and Ai molecular areas of all binary films exceed the cross-section 18.56 Å2 of all-trans long-chain alkanes in 3D crystals, denoted by the dash line.41 These relationships suggest that the upright solid phase of the binary EE-FEE films might have either a LS phase structure or a S-phase structure with more gauche defects in the chains or mismatched headgroup conformations as the individual films. The more self-consistent πi/XFEE and Ai/XFEE dependencies show maxima and minima at XFEE ) 0.2 and XFEE ) 0.8 which correlate with each other. Following the common understanding,37,38 one would interpret the increase of πi up to XFEE ) 0.2 as enhanced attraction, that could be ascribed to an increasing number of opposite headgroup dipoles in the mixtures. However, the parallel expansion of the film demonstrated by the increase of Ai questions such an explanation. Similar disagreement is observed at XFEE ) 0.8, where the weak but real minimum of πi would indicate reduced EE-FEE attraction, which contradicts the minimum of Ai demonstrating strong EE-FEE attraction. In this context, one should recall that the variation of the composition of the binary films changes not only the lateral molecular interactions but also the average affinity of the hydrophilic heads toward water via replacement of their CH3 by CF3 terminals. Compressibility Moduli of the Binary Mixtures. It is usual practice39,42-46 to determine the compressibility CS ) -(dA/ dπ)/A, or compressibility modulus CS-1 ) -A(dπ/dA) of the monolayers as a function of the molecular area or surface pressure, and to interpret the discontinuities in these dependencies as indication of phase transitions. Figure 3 shows CS-1/A dependencies for the binary films with XFEE ) 0.1, 0.3, 0.5, and 0.7, which are representative also for the adjacent molar fractions. The CS-1/A dependence for XFEE ) 0.1 has a shallow part between 22 and 20 Å2, which indicates continuous tilted-to-upright phase transition. At 20 Å2, the compressibility modulus abruptly goes up, reaches a maximum and steeply decreases due to the fast collapse of the film. The general trend of the CS-1/A dependencies for the binary films with XFEE g 0.3 is similar, and the values at the maxima CS,max-1 ≈ 1500 ( 50 mN/m are almost independent of XFEE, but none of them exhibits the shallow low pressure part. Table 1 shows a CS-1/XFEE dependence at fixed surface pressure of 20 mN/m from the steep parts of the isotherms. The CS-1 values are averaged from 3-5 isotherms and their standard deviation is also presented. They all, as well as the values of CS,max-1, lie within the range of 600-2000 mN/m specified by Harkins45 for the LS phase and the range 1000-2000 mN/m ascribed by Davis and Riedal46 to solid condensed monolayers. This evidence supports the above conclusion about the phase state of the upright architecture of the binary films. Morphology and Optical Anisotropy of the Binary Monolayers. The first and the second columns of Figure 4 show BAM images of monolayers at 0.2-0.3 mN/m and 25.0 Å2, and (39) Lundquist, M. Chem. Scripta 1971, 1, 197. (40) Kenn, R. M.; Bo¨hm, C.; Bibo, A. M.; Peterson, I. R.; Moehwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092. (41) KitaigorodskyA. I. Molecular Crystals and Molecules;Academic Press: New York and London 19734862. (42) Albreht, O.; Gruler, H.; Sackmann, E. J. Colloid Interface Sci. 1981, 79, 319–338. (43) Hirshfeld, C. L.; Seul, M. J. Phys. (Paris) 1990, 51, 1537. (44) Petrov, J. G.; Polymeropoulos, E. E.; Moehwald, H. Langmuir 2000, 16, 7411. (45) HarkinsW. D. The Physical Chemistry of Surface Films; Reinhold Publishing Corporation: New York, 1952; pp 106-109. (46) Davis, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1961; p 265.

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Figure 3. Dependencies of the compressibility modulus CS-1 ) -A(dπ/dA) on the mean molecular area A for binary films with XFEE ) 0.1, 0.3, 0.5, and 0.7. The film with XFEE ) 0.1 exhibits a shallow part between 22 and 20 Å2, which demonstrates the tilted-to-upright phase transition, that is not observed for the films with XFEE g 0.3. Table 1. Compressibility Modulus CS-1 ) -A(dπ/dA) at 20 mN/m as a Function of the Molar Fraction XFEE of the Fluorinated Ether XFEE

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

CS-1mN/m ( CS-1

1260 170

1160 180

1140 110

950 210

1240 140

1160 120

1160 140

1400 220

830 150

1080 220

1150 200

the third column presents the same films at 20 mN/m. The images in the first column were recorded at crossed polarizer and analyzer; those in the second column were taken at parallel polarizer and analyzer. The first row (a, a′, a′′) corresponds to the EE film, the second, third, and the forth rows represent mixtures with XFEE ) 0.3 (b, b′, b′′), XFEE ) 0.5 (c, c′, c′′), and XFEE ) 0.7 (d, d′, d′′), respectively, and the last row (e, e′, e′′) stands for the FEE monolayer. All uncompressed monolayers have bright compact parts which reflect light, and black spots of a nonreflecting water surface covered by a gaseous monolayer. Images a and a′ show that the same part of the uncompressed EE film transforms their appearance from black to white when the analyzer is rotated, thus demonstrating the optical anisotropy of the tilted closely packed chains of the L2′ phase. In contrast to the EE film, the local brightness of the binary monolayers with XFEE g 0.3 does not change upon rotation of the analyzer (cf. images b, c, d with b′, c′, d′). Therefore, these monolayers are optically isotropic and consist of upright molecules even in uncompressed state. This observation proves the conclusion based on Figure 2a that about 30% FEE content eliminates the L2′ phase in the binary films at all surface pressures above π ) 0 and below πi. The third column in Figure 4 illustrates the change of the morphology of the individual and binary films under compression up to 20 mN/m. Although their general appearance is homogeneous, the binary monolayers occasionally exhibit small black spots of water surface uncovered by compact monolayer (see the arrows). Their part of the total film area cannot be strictly determined because they are visible only when fall in the spot of the laser beam. However, if the images b′′, c′′, and d′′ are considered as representative snapshots, the area of the defects is just 0.2-0.3% of the total image area.

Dependence of the Mean and Excess Molecular Areas at Fixed Surface Pressure on the Composition of the Binary Films. The collapse Ac and instability Ai molecular areas in Figure 2b correspond to πc and πi values which vary with XFEE, whereas the correct thermodynamic analyses of the miscibility and ideal or nonideal character of the binary monolayers operates with composition dependencies of the mean molecular area at constant surface pressure Aπ/XFEE.35,37,38 This procedure compares the measured values of Aπ with those calculated via the additivity rule Aadd,ist ) XEEAEE + XFEEAFEE where the calculated mean molecular area Aadd,ist is obtained with AEE and AFEE values from the isotherms. Since the linear Aadd,ist/XFEE dependence is satisfied for complete ideal miscibility or complete phase separation of the components any deviation of the Aπ/XFEE data from this straight line would demonstrate miscibility and a nonideal character of the mixture. Figure 5a-d presents such Aπ/XFEE data for π ) 2, 6, 15, and 30 mN/m. The values of 2 and 6 mN/m are below, and those of 15 and 30 mN/m are above the surface pressure πt ) 10.4 mN/m of the end of the L2′-S phase transition in the pure EE monolayer. At 15 and 30 mN/m, all pure and binary monolayers are in an upright solid phase, whereas at 2 and 6 mN/m only those with XFEE g 0.3 have upright architecture (see Figure 2a). It can be seen that all Aπ/XFEE dependencies deviate from the linear Aadd,ist/ XFEE dependencies presented by the upper straight lines at most compositions. This difference demonstrates miscibility of the binary films and a nonideal character of their mixtures at almost all compositions. The points at XFEE ) 0.9 at 2 and 6 mN/m and those at XFEE ) 0.1, and 0.7 at 15 and 30 mN/m, which match or are very close to the corresponding Aadd,ist values show ideal miscibility or complete separation of the components at these compositions.

Binary Langmuir Monolayers

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Figure 4. BAM images visualizing the morphology and the optical anisotropy of uncompressed monolayers at 25.0 Å2 and 0.2-0.3 mN/m. They represent EE (a, a′, a′′), FEE (e, e′, e′′), XFEE ) 0.3 (b, b′, b′′), XFEE ) 0.5 (c, c′, c′′), XFEE ) 0.7 (d, d′, d′′). The first column corresponds to crossed polarizer and analyzer, the second one - to parallel polarizer and analyzer. The third column shows the same monolayers compressed to 20 mN/m; the arrows point to defects in the films that can be quantified by the 100 µm marker.

The additive dependencies Aadd,ist/XFEE based on the isotherms (the upper straight lines), substantially differ from the additive dependencies Aadd,GIXD/XFEE (the lower straight lines), calculated with the molecular areas AEE and AFEE obtained from the GIXD measurements presented by closed symbols.32 Since GIXD registers only the all-trans chains this disagreement points to a significant amount of defects in the mixed films. The fact that all measured Aπ values exceed the cross-section 18.6 Å2 (see the dash lines) of the closely packed all-trans long hydrocarbons in 3D crystals41 leads to the same conclusion. The relative contribution of these defects (Aadd,ist - Aadd,GIXD)/Aadd,ist at 15 mN/m varies from 2.8 to 1.8% for XFEE ) 0.3 - 0.7. This value is by an order of magnitude larger than the one of the empty

places in Figure 4 so that the latter seem to be negligible as compared to the gauche kinks in the chains or mismatched headgroup conformations. At 2 and 6 mN/m the values of Aπ for the films with XFEE ) 0.1 and 0.2 are smaller than the molecular areas in the pure EE monolayer. This difference indicates reduction of the tilt of the EE molecules in the mixtures. The mixtures with XFEE g 0.3, which exist in an upright solid phase at these surface pressures, are also denser as those in the ideal mixtures with the same composition. This difference varies nonmonotonously with XFEE and approaches the corresponding Aadd,GIXD values at XFEE ) 0.2 and 0.8. The above observations manifest the contraction of both the tilted and upright architecture of the mixtures that should be

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Figure 5. Composition dependencies of the mean molecular area Aπ at fixed surface pressures, averaged from 3-5 π/A isotherms. The values at 2 and 6 mN/m are below the L2′ - S phase transition in the EE-monolayer, those of 15 and 30 mN/m are above the L2′ - S phase transition in the EE-monolayer. The upper straight lines represent the additive Aadd,ist/XFEE dependence calculated with values of AEE and AFEE from the isotherms, the lower straight lines show the additive Aadd,GIXD/XFEE dependence calculated with values of AEE and AFEE determined via GIXD.32 The cross-section of all-trans long chain alkanes in 3D crystals is denoted by the horizontal dashed lines.41

related to the dipolar interaction between the hydrophilic heads, because the van der Waals attraction between the chains remains constant at all compositions. Most of the Aπ values at 15 and 30 mN/m exceed the straight lines showing expansion of the films at such compositions. The only exception is observed at XFEE ) 0.8 where the values of Aπ are located far below the Aadd,ist/XFEE dependence demonstrating significant contraction even at surface pressures where the EE and FEE molecules are closely packed.32 The contraction of this mixture brings the value of Aπ(XFEE ) 0.8) from the isotherm in coincidence with the GIXD value Aadd,GIXD(XFEE ) 0.8), and at 30 mN/m even below it, but still above the all-trans crosssection of the long-chain alkanes in 3D crystals (dashed lines). Since the alkanes do not have polar heads as the EE and FEE molecules one could speculate that this difference points to a determining role of the hydrated EE and FEE heads for the packing density of this mixture. The deviations of the measured mean molecular areas Aπ from the additivity rule, which characterize the interactions in the mixtures, are quantified in Figure 6 by the plots of the composition dependencies of the excess mean molecular area Aπ,exc ) Aπ Aadd,ist. The complex trend of the Aπ,exc/XFEE data with minima and maxima at particular compositions manifests specific structural changes in the EE-FEE mixtures. They result from the variation with XFEE of the dipolar interactions between the heads, because the interactions between the same CH3(CH2)21 hydrocarbon chains of EE and FEE do not change with composition. Such structural specificity points to discrete changes of the headgroup hydration and/or conformation, which dictate the structural repulsion between the heads in the films. Comparison of the Aπ,exc/XFEE dependencies at 2, 6, 15, and 30 mN/m shows largest negative values at 2 mN/m. The Aπ,exc values at 6 mN/m are negative but smaller and those at 15 and 30 mN/m are positive or close to zero. The Aπ,exc/XFEE dependencies at 15 and 30 mN/m practically coincide due to the steric repulsion in the almost incompressible solid phase. These observations suggest that the electrostatic dipolar interactions

Figure 6. Composition dependence of the excess mean molecular area Aπ,exc ) (A - Aadd,ist)π at fixed surface pressures, below the L2′ - S phase transition in the EE-monolayer (a) and above the L2′ - S phase transition in the EE-monolayer (b).

between the heads dominate at low surface pressures whereas the steric head-head repulsion plays a major role at high surface pressure. The minimum and the negative Aπ,exc value at XFEE ) 0.8 do not change above 6 mN/m which suggests that a limiting contraction of the upright solid phase was achieved at this composition. Excess and Total Free Energy of Mixing at Different Binary Film Composition. Since the π/A isotherms recorded at different compression velocity overlap up to the instability pressure πi we

Binary Langmuir Monolayers

Langmuir, Vol. 25, No. 6, 2009 3665

Figure 7. Composition dependencies of the excess free energy of mixing ∆Gexcπ (a), and the total free energy of mixing ∆Gmixπ (b) at fixed surface pressures. The much larger negative values of ∆Gmixπ as compared to the positive values of ∆Gexcπ show spontaneous miscibility of EE and FEE at all compositions caused by a significant increase of the entropy.

have applied the thermodynamic analysis of Goodrich47 at surface pressures of 6, 15, and 30 mN/m, which are below all πi values. We calculated the excess free energy of mixing at fixed surface pressure ∆Gexcπ that measures the difference between the total free energy of mixing ∆Gmixπ, and the free energy of ideal mixing or phase separation of the components ∆Gid-mixπ ) XEEGEEπ + XFEEGFEEπ. Since the excess free energy of mixing is related to the excess mean molecular area Aπ,exc,

(

∂(∆Gπexc) ∂π

)

T,Xi

) NAAπexc

(1)

one can determine ∆Gexcπ through integration of the π/A isotherms of the mixed and individual films:

∆Gπexc ) NA

∫0π Aπexcdπ ) NA(∫0π Adπ - XEE∫0π AEEdπ π XFEE∫0 AFEEdπ) (2)

Here, A is the mean molecular area and XEE and XFEE are the molar fractions of the components in the mixed film, AEE and AFEE are the molecular areas in the individual EE and FEE monolayers, and NA stands for the Avogadro number. Taking into account that ∆Gid-mixπ represents the entropy change term responsible for the formation of an ideal mixture the expression of the total free energy of mixing reads: π π ∆Gmix ) ∆Gπexc + ∆Gid-mix ) ∆Gπexc + RT(XEEln XEE + XFEEln XFEE) (3)

Figure 7a shows the ∆Gexcπ/XFEE dependencies at 6, 15, and 30 mN/m. The only negative ∆Gexc value is the one for XFEE ) 0.1 (47) Goodrich, F. C. Proceedings of the Second International Congress of Surface ActiVity; Butterworths: London, 1957; p 85.

Figure 8. A virtual isomorphic structure of the equimollar mixture of FEE (hatched circle) and EE (open circle) in upright solid phase. The maximum dissimilar nearest neighbors attracting each other, and the minimum similar EE-EE and FEE-FEE nearest neighbors which exhibit dipolar repulsion minimize the value of ∆Gmixπ. The orthorhombic lattice of the pure EE and FEE monolayers at high surface pressure32 is adopted also for the mixtures for simplicity.

at 6 mN/m, where this mixture exists in the L2′ phase.48 All other ∆Gexc values correspond to mixtures with upright architecture and their positive values point to dominating repulsive interactions which favor the phase separation of their components. However, even the highest positive values of ∆Gexcπ are at least 50× smaller than the negative values of the total free energy of mixing ∆Gmixπ presented in Figure 7b. Such a relationship shows that all mixtures of EE and FEE are more stable than the binary monolayers with separated components, and that the spontaneous mixing is due to an increase of the entropy. The minimum of the ∆Gmixπ/XFEE dependence at XFEE ) 0.5 indicates that the equimolar mixture of EE and FEE is thermodynamically most favorable, which disagrees with the location of the minimum and the negative value of Aπ,exc at XFEE ) 0.8 shown in Figure 6b. A virtual isomorphic structure of the upright solid phase of the equimolar mixture is presented in Figure 8. It has maximum number of dissimilar FEE-EE nearest neighbors, whose heads attract each other, and minimum number of similar EE-EE or FEE-FEE nearest neighbors, exhibiting dipolar head-head repulsion. The electrostatic interaction energy of this structure would cancel yielding a minimum of ∆Gmix if the average normal component of the molecular dipole moment µ⊥,avg becomes zero at XFEE ) 0.5. Assuming additivity, and substituting the previously determined16 effective values of µ⊥,EE ) +235 mD and µ⊥,FEE ) -120 mD in the additivity rule, µ⊥,avg ) µ⊥,EEXEE + µ⊥,FEEXFEE, we find that µ⊥,avg ) 0 at XFEE ) 0.66. However, this prediction conflicts the experimental composition dependence16 of the average surface potential ∆Vavg/XFEE, which shows negative deviation from additivity and canceling of µn,avg ) ∆VavgAavgε0 ) 0 at XFEE ) 0.40. Hence, the minimum of the Aπ,exc/XFEE dependence at XFEE ) 0.80 has not an electrostatic, but a short-range structural origin that could be revealed in a future study via in situ X-ray diffraction at grazing incidence. (48) The positive ∆Gexcπ values for XFEE g 0.2 at 6 mN/m might result from the choice of the lower limit of integration π ) 0, which eliminates the initial part of the integrals at large molecular areas and non-zero surface pressure. Their consideration sometimes reverses the sign of ∆Gexcπ but does not significantly change its magnitude.37

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Possible Mechanism of the Erection of the Chains in the Mixed Binary Monolayers. In our previous GIXD study32 of the individual EE and FEE monolayers, we have related the low pressure tilted L2′ phase of the EE film to the stronger hydration and larger volume of its polar head whose CH3 terminal is less hydrophobic than the CF3 terminal of the FEE head. This explanation was based on previous publications of different authors who linked the tilted DPPC and untilted DPPE chains, and the dependence of the tilt on the water content in DPPC suspensions, to different hydration of phospholipid heads.49-52 The tilt of the DPPC chains was removed by addition of La3+ which binds to and partially dehydrates the PC heads.49 A similar effect was caused by specific adsorption of Cd2+ at arachidic acid Langmuir monolayers,53,54 as well as for myristic acid, where the adsorption of Cd2+ transformed the liquid expanded film on water to Cd-myristate monolayer with long-range crystallinity.55 The specific adsorption of phospholipase A2 which recognizes but does not cleave the heads of the D-enantiomer of DPPC, also caused heads-dehydration and erection of the DPPC chains.56 These examples show that monolayers whose headgroups are weaker hydrated have less tilted hydrocarbon chains. Substitution of the EE by FEE heads, which increases the head-head attraction in the mixtures, might partially dehydrate the EE heads and cause erection of the EE chains.

Conclusions I. The composition dependencies of the mean and excess molecular areas Aπ/XFEE and Aπ,exc/XFEE deviate from the additivity rule demonstrating molecular miscibility and a nonideal character of the EE-FEE mixtures. Their minima and maxima indicate specific structural changes at particular compositions which could be only due to dipolar and steric interactions between the headgroups, because the interactions between the same CH3(CH2)21 hydrocarbon chains of EE and FEE do not change with composition. The decrease of the depth of the minima, and the reversal of the sign of the excess molecular area Aπ,exc from negative to positive with increasing π show that the dipolar interactions play determining role at low surface pressure, whereas the steric repulsion becomes more important at high surface pressure. (49) McIntosh, T. J. Biophys. J. 1980, 29, 237. (50) Levine, Y. K. Prog. Biophys. Mol. Biol. 1972, 24, 3. ibid. 1976, 3 279. (51) Tardieu, A.; Luzatti, V.; Reman, F. C. J. Mol. Biol. 1973, 75, 711. (52) Hui, S. W. Chem. Phys. Lipids 1976, 16, 9. (53) Leveveller, F.; Jacquemain, D.; Lahav, M.; Leiserowitz, L.; Deutch, M.; Kjaer, K.; Als-Nielsen, J. Science 1991, 252, 1532. (54) Leveveller, F.; Boehm, C.; Jacquemain, D.; Moehwald, H.; Leiserowitz, L.; Kjaer, K.; Als-Nielsen, J. Langmuir 1994, 10, 819. (55) Boehm, C.; Leveveller, F.; Jacquemain, D.; Moehwald, H.; Leiserowitz, L.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I. Langmuir 1994, 10, 830. (56) Dahmen-Levinson, U.; Brezesinski, G.; Moehwald, H. Thin Solid Films 1998, 327-329, 616.

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II. The π/A isotherms and the π/XFEE phase diagram show that mixtures with molar fraction XFEE g 0.3 exist in an upright solid phase even in uncompressed state. This result is confirmed by the compressibility data and Brewster angle microscopy, which shows that the mixtures with XFEE g 0.3 are optically isotropic at π ≈ 0. Comparison of the mean molecular areas at collapse Ac, and at the end of the tilted-upright phase transition At, with literature data suggests that the upright solid phase is either LS or S, but with more defects as the S-phase of the pure EE and FEE monolayers. The mixtures with XFEE < 0.3 are organized in tilted L2′ phase at low surface pressures. Their mean molecular areas Aπ are smaller than the molecular areas in the EE monolayer at the same π-value, which points to reduction of the tilt of the EE chains with increasing FEE content up to 30%. We ascribe this reduction to partial dehydration of the EE heads caused by dipolar attraction of the EE and FEE heads. III. The additive Aadd/XFEE dependence calculated with AEE and AFEE molecular areas from the π/A isotherms significantly exceeds the one calculated with molecular areas of EE and FEE determined previously via GIXD.32 Since GIXD registers only all-trans chains this difference points to a significant amount of gauche kinks in the chains or mismatched head-groups conformations of the mixed films. IV. The excess free energy of mixing ∆Gexcπ at 6, 15, and 30 mN/m is positive but much smaller than the corresponding negative total free energy of mixing ∆Gmixπ showing that the EE-FEE monolayers spontaneously form mixtures at all compositions due to increase of the entropy term. The minimum negative value of ∆Gmixπ at XFEE ) 0.5, indicating that the equimolar EE-FEE mixture is thermodynamically most favorable, conflicts with the Aπ,exc/XFEE minimum and the negative Aπ,exc value at XFEE ) 0.8. Assuming that the dipolar interactions and their contribution to ∆Gmixπ cancel when the average normal dipole moment µ⊥,avg ) 0, we find that this occurs at XFEE ) 0.40. Hence, the minimum of the experimental Aπ,exc/XFEE dependence has not an electrostatic but a short-range structural origin. Acknowledgment. The authors appreciate the financial support of project NT-1-03 by the National Science Fund of the Bulgarian Ministry of Education and Science. T.D.A. and J.G.P. are grateful to the Max-Planck Society of Germany for the short-term stipends which enabled performing a part of the study at the Max-Planck Institute of Colloids and Interafces in Golm/Potsdam, Germany. Supporting Information Available: Surface pressure-molecular area isotherms π/A of the individual EE and FEE monolayers at different compression velocities Vcmp between 3 and 36 cm2/min, respectively, 1 and 10 A2/molecule.min show variation of the collapse surface pressures with Vcmp although the π/A curves overlap within 0.2-0.5 Å2 up to the instability points. This material is available free of charge via the Internet at http://pubs.acs.org. LA804136J