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Langmuir Monolayers with Fluorinated Groups in the Hydrophilic Head. 1. Comparison of Trifluoroethyl Behenate and Ethyl Behenate Monolayers: Molecular...
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Langmuir Monolayers with Fluorinated Groups in the Hydrophilic Head. 1. Comparison of Trifluoroethyl Behenate and Ethyl Behenate Monolayers: Molecular Models, Mechanical Properties, Stability J. G. Petrov,*,† E. E. Polymeropoulos,‡ and H. Mo¨hwald† Max-Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, D-14476 Golm/Potsdam, Germany, and ASTA Medica AG, Weismu¨ lerstr. 45, 60314 Frankfurt, Germany Received November 30, 1999. In Final Form: May 6, 2000 In a series of three related papers we compare mechanical properties and stability, morphology and structure, and electrostatic potential and ellipsometric thickness of trifluoroethyl behenate (TFEB) and ethyl behenate (EB) Langmuir monolayers. The aim of these papers is to study the effect of fluorination of a methyl group in the hydrophilic head on monolayer properties and structure. In the present Part 1 we show that trifluoroethyl ester forms significantly more unstable films with higher compressibility (lower compressional modulus) than the unsubstituted ethyl ester. Both TFEB and EB surface pressurearea isotherms show compression-expansion hysteresis, but this hysteresis is larger for the fluorinated ester. The surface pressure-area loop of TFEB is shifted to larger molecular areas as compared to EB and gives larger limiting molecular areas at zero compression. This points to different volumes and/or conformations of the fluorinated and nonsubstituted hydrophilic heads. Maps of molecular lipophilicity and molecular electrostatic potential, based on semiempirical quantum mechanical models of the two molecules in vacuo, relate the observed differences in monolayer properties to decreased hydrophilicity of the trifluoroethyl group and a stronger electrostatic repulsion between the hydrocarbon chains of TFEB. Such a repulsion results from polarization of the CH2 groups adjacent to the heads that is more significant for the trifluoroethyl behenate molecule.

Introduction Fluorinated surfactants possess unusual properties that determine their wide application in aqueous and nonpolar media.1 They serve as oil and fat repellents, solubilizing additives, and fire extinguishing agents. Adsorbed or deposited on solid surfaces they render them both hydrophobic and oleophobic and strongly decrease surface energy.2-5 Such modified surfaces exhibit much lower frictional coefficients.6 The relation between interfacial behavior and molecular structure of fluorinated surfactants has been a matter of continuous interest during the second half of this century. Various molecular aggregates such as micelles,7-10 biomembranes,11,12 and Langmuir monolayers at the air* To whom correspondence should be addressed. Permanent address: Institute of Biophysics, Bulgarian Academy of Sciences, 1 Acad. G. Bonchev Str., Block 21, 1113 Sofia, Bulgaria. † Max-Planck Institute of Colloids and Interfaces. ‡ ASTA Medica AG. (1) Fluorinated Surfactants: Synthesis, Properties, Applications; Kiss, E., Ed.; Marcel Decker: New York, 1994. (2) Hare, E. F.; Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1954, 58, 236. (3) Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1957, 61, 1046. (4) Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1962, 62, 740. (5) Kobayashi, H.; Owen, M. J. TRIP 1995, 3, 330. (6) 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. (7) Arrington, C. H., Jr.; Patterson, G. D. J. Phys. Chem. 1953, 57, 247. (8) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (9) Funasaki, N.; Hada, S. J. Phys. Chem. 1983, 87, 342. (10) Burkitt, S. J.; Ingram, B. T.; Ottewill, R. H. Prog. Colloid Polym. Sci. 1988, 76, 247. (11) Kumano, A.; Kajiyama, T.; Takayanagi, M.; Okahata, Y.; Kunitake, T. Bull. Chem. Soc. Jpn. 1985, 58, 1205. (12) Flewelling, R. F.; Hubbell, W. L. Biophys. J. 1986, 49, 541.

water interface13-17 have been considered. Fox13 has found that monolayers of ω-trifluorooctadecanoic acid and ω-trifluorooctadecylamine are significantly less stable than those of stearic acid and stearylamine. They exhibit negative surface potentials while the nonfluorinated compounds show positive ∆V values. Barnet and Zisman14 observed a similar effect for Langmuir films of progressively fluorinated fatty acids and determined the specific influence of different terminal X-CH2 groups (X ) F, Cl, Br, I) on surface pressure-area and surface potentialarea isotherms of ω-halogenated acids and alcohols.15 The comparison of surface potentials of monolayers of ω-trifluorostearic and stearic acids and amines was used in the formulation of the two-capacitor14-16 and threecapacitor17,18 electrostatic models of Langmuir films. It is interesting to know how the incorporation of a CF3 group in the hydrophilic head affects the mechanical and electrostatic properties, morphology, and structure of a Langmuir monolayer. Does it bring about a decrease in monolayer stability, and if so, why? Is this decrease due to a stronger lateral repulsion, as assumed in ref 15, or is it due to weaker adhesion between headgroups and water?3,4 Would the surface potential of such monolayers change significantly or even reverse its sign? Is the hydration of the fluorinated and nonfluorinated heads different, and would such a difference influence the surface (13) Fox, H. W. J. Phys. Chem. 1957, 61, 1058. (14) Barnett, M. K.; Zisman, W. A. J. Phys. Chem. 1963, 67, 1534. (15) Barnett, M. K.; Jarvis, N. L.; Zisman, W. A. J. Phys. Chem. 1964, 68, 3520. (16) Vogel, V.; Mo¨bius, D. J. Colloid Interface Sci. 1988, 126, 408. (17) Demchak, R. J.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191. (18) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. J. Colloid Interface Sci. 1990, 139, 508.

10.1021/la991557z CCC: $19.00 © 2000 American Chemical Society Published on Web 08/16/2000

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potential significantly?16-19 Does the fluorination of the headgroup affect the arrangement of the hydrocarbon chains? Does the presence of a CF3 group change the conformation of the hydrophilic head, and is its effect similar to the one that trifluoroethanol has on the structure of peptides and proteins?20-22 In a first attempt to answer some of these questions, we have presented in a previous paper23 surface pressurearea and surface potential-area isotherms as well as area-time relationships of trifluoroethyl behenate (TFEB) monolayers. For this system we found a low collapse pressure and significant monolayer instability. Spontaneous formation of a “heads-to-tails” bilayer during monolayer collapse was observed, as had been previously registered for long chain ethyl esters and acetates by Lundquist24,25 and for triglycerides by Larsson.26,27 However, the most interesting result of this study was the reversal of the sign and a 200% change in the magnitude of the surface potential caused by the substitution of the ester CH3 group by a CF3 group. ∆V of the TFEB monolayer was found to be negative in contrast to the positive ∆V values observed for long chain amphiphils with nonfluorinated uncharged heads. In ref 23 we compared our results with literature data for Langmuir monolayers of nonfluorinated fatty alcohols and methyl and ethyl esters, ignoring the difference in experimental conditions. Analyzing the structure of the TFEB monolayer on the basis of π/A isotherms, we assumed a priori that fluorination of the ester group does not change the packing and orientation of the hydrocarbon chains. Thus, postulating a structural analogy with ethyl ester monolayers and utilizing the phase diagrams of Lundquist,25 we concluded that the TFEB monolayer undergoes a phase transition from tilted to vertical chain orientation under compression. This assumption should be checked by direct comparison of the properties of trifluoroethyl and ethyl behenate monolayers and by investigation of their structure under the same experimental conditions. Our results gave no clear answer as far as the affinity of the ester CF3 group toward water and its effect on monolayer stability is concerned. As Shafrin and Zisman4 showed, solid surfaces covered with closely packed ω-fluorinated hydrocarbon chains are more hydrophilic than those covered with nonfluorinated chains. This observation implies that the TFEB monolayer should be more stable than the methyl or ethyl ester monolayer, which is not in agreement with the experimental results in ref 23. Furthermore, an explanation for the main outcome of our work, namely the reversal of the sign and the significant change in the magnitude of surface potential caused by the CF3 versus CH3 substitution in the hydrophilic head, has not been delivered as yet. If the condensed TFEB film has the structure attributed to the monolayers of long chain ethyl esters by Alexander and Schulman28 and that is widely accepted to the present,29-32

Optimization of the molecular geometries in vacuo was performed by means of the MNDO method.33 Four conformations of the hydrophilic heads discussed in the literature17 have been considered, namely the cis-cis, cistrans, trans-cis, and trans-trans conformations shown in Figure 1. Maps of the molecular lipophilicity (MLP) and molecular electrostatic potentials (MEP) were obtained for EB and TFEB molecules. The MLP displays the lipophilicity pseudopotential on the solvent accessible surface of the molecule34 utilizing the concept of Ghose and Crippen35,36 that overall molecular lipophilicity can be presented as the sum of single atom contributions. The method is described in ref 37 and is incorporated in the graphics program MOLCAD.38 Although the MLP has no rigorous physical meaning, it can be successfully applied for a quantitative comparison of the lipophilicity of different molecules or molecular conformations. The map of the MEP displays the electrostatic potential on the molecular surface. It is determined from the molecular structure and the partial atomic charges calculated by the MNDO method. Display of the MEP maps is also incorporated in the MOLCAD graphics program. All calculations were performed on a Silicon Graphics INDY R4400 workstation.

(19) Petrov, J. G.; Polymeropoulos, E. E.; Mo¨hwald, H. J. Phys. Chem. 1996, 100, 9860. (20) Kemmink, J.; Greighton, T. E. Biochemistry 1995, 34, 12630. (21) Narhi, L. O.; Philo, J. S.; Li, T.; Zhang, M.; Samal, B.; Arakawa, T. Biochemistry 1996, 35, 11447. (22) Rajan, R.; Balarm, P. Int. J. Pept. Protein Res. 1996, 48, 328. (23) Petrov, J. G.; Mo¨hwald, H. J. Phys. Chem. 1996, 100, 18458. (24) Lundquist, M. Chem. Scripta 1971, 1, 5. (25) Lundquist, M. Chem. Scripta 1971, 1, 197. (26) Larsson, K. Chem. Phys. Lipids 1973, 10, 177. (27) Larsson, K. In Surface and Colloid Science; Mattijevich, E., Ed.; Wiley: New York, 1973; Vol. 16, p 261. (28) Alexander, A. E.; Schulman, J. H. Proc. R. Soc. London A, 1937, 161, 115. (29) Stenhagen, E. In Determination of Organic Structures by Physical Methods; Braude, Nachod, Eds.; New York, 1955; Chapter 8, p 356.

(30) Fort, T., Jr.; Alexander, A. E. J. Colloid Interface Sci. 1959, 14, 190. (31) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1961; p 72. (32) Gericke, A.; Huenerfuss, H. Ber. Bunseges. Phys. Chem. 1995, 99, 641. (33) MNDO.; MOPAC (version 5.0) QCPE, No. 455, 1989. (34) Connolly, M. L. Science 1983, 221, 709. (35) Ghose, A.; Crippen, G. J. Comput. Chem. 1986, 7, 565. (36) Viswanadhan, V. N.; Ghose, A. K.; Revankar, G. R.; Robins, R. K. J. Chem. Inf. Comput. Sci. 1989, 29, 163. (37) Heiden, W.; Moeckel, G.; Brickmann, J. J. Comput.-Aided Mol. Des. 1993, 7, 503. (38) Waldherr-Teschner, M.; Goetze, T.; Heiden, W.; Knoblauch, M.; Volhardt, H.; Brickmann, J. In Advances in Scientific Visualisation; Post, F. H., Hin, A. J. S., Eds.; Springer: Heidelberg, 1992; pp 58-67.

then the CH2CF3 group should be located beneath the chains with the F-atoms pointing toward water. Such an orientation, however, gives a positive contribution to the surface potential and disagrees with the negative ∆V potential found experimentally. In the present series of investigations, consisting of three closely related papers, we directly compare mechanical properties and stability, morphology and structure, and electrostatic potential and ellipsometric thickness of trifluoroethyl behenate (TFEB) and ethyl behenate (EB) monolayers under the same experimental conditions. Maps of molecular lipophilicity potential (MLP) and molecular electrostatic potential (MEP) based on semiempirical quantum mechanical calculations of TFEB and EB molecules in vacuo are presented in Part 1. With these models we attempt to establish a relationship between the properties of the monolayer and the isolated molecules. Differences in surface pressure-area isotherms, compression-expansion hysteresis, compressional and dilational moduli, and monolayer stability are delineated in the first part. In Part 2 we compare the morphology of TFEB and EB monolayers registered via Brewster angle microscopy, and the arrangement of their hydrocarbon chains at the air-water interface studied in situ via X-ray diffraction. Surface potential and ellipsometric isotherms of TFEB and EB monolayers are presented in Part 3. With the help of a simple molecular model for the trifluoroethyl behenate monolayer, we propose a plausible explanation for the observed negative ∆V potential. Molecular Models

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Figure 1. Comparison of the calculated in vacuo conformations of the headgroups of ethyl behenate and trifluoroethyl behenate. Ethyl behenate is on the left side of each couple with the same headgroup conformation.

Comparison of MLP and MEP of TFEB and EB Molecules in Vacuo. The variation of the energy (heat of formation as defined in MNDO33) of single TFEB and EB molecules with changing conformation of the polar heads is rather complicated, because free rotations are possible around the H2C-CO2, OC-O, and CO-CH2 bonds. For this reason we compared only the conformations illustrated in Figure 1. The results of the calculations showed that the cis-trans and trans-trans conformations have very close energy values. They have the most negative energies, which indicates greater stability of these conformations in vacuo. The MLP maps of EB and TFEB molecules are shown in Figure 2. Their comparison leads to the following conclusions: (1) In all conformations considered, the CF3 group of TFEB is significantly more hydrophobic than the alkyl CH3 group of EB and, therefore, would cause weaker affinity of the fluorinated head toward water. (2) For both substances, and for all considered conformations, the first two or three methylene groups adjacent to the carbonyl C-atom are more hydrophilic than the more distant ones. (3) The lipophilicity distribution in the chains differs for EB and TFEB molecules but seems to be independent of the headgroup conformation of the respective molecule. From the maps of MEP of EB and TFEB presented in Figure 3 we can draw the following conclusions: (1) The atomic charge distribution and the local electrostatic potential in the hydrocarbon chains depend on the hydrophilic head, or in other words, the heads polarize the chains. For the same conformation of the EB and TFEB heads, the polarization of the chain is larger for the TFEB molecule. (2) Both EB and TFEB chains have more positive CH2 groups closer to the heads and less positive distant ones. Such a charge distribution is equivalent to a negative dipole

moment of the behenyl chain. Comparison of the charge distribution along the chains of EB and TFEB suggests that this dipole moment is greater for TFEB. Experimental Section Materials and Methods. Ethyl behenate with 99% purity was purchased from Sigma and used as received. The synthesis of trifluoroethyl behenate was described in our previous publication.23 The final 5-fold recrystallized product had a melting point of 55-56 °C. Gas chromatography gave a purity of 98% and the elemental analysis confirmed the theoretical amount of C and H within 0.2%. The 13C NMR spectrum reinforced the above data. Both substances were dissolved in chloroform to a concentration of 1 × 10-3 M. Several drops of ethanol were added to a 10 mL flask to facilitate dissolution of the fluoroester. A 200 µL aliquot of these solutions was spread on Milli-Q Millipore water in a Langmuir Teflon trough and left 5 min before compression for evaporation of the solvent. The Lauda film balance FW-2 with a floating barrier was used to register the surface pressure within 0.1 mN/m. Its large analytical area of 927 cm2 enabled determination of the area per molecule with an error of (0.1 Å2. A thermostating plate underneath the trough maintained a constant subsolution temperature of 20.0 ( 0.1 °C and a serpentine with circulating water kept the air temperature in the closed cabinet also at this value. Surface Pressure-Molecular Area Isotherms of TFEB and EB Monolayers. In Figure 4 the surface pressure-molecular area isotherms of TFEB and EB monolayers are compared. To approach equilibrium conditions, compression was performed at a low velocity (0.8 Å2/molecule.min), and the isotherms were recorded continuously to better visualize structural changes. The most interesting result in Figure 4 is the significant difference between the two π/A dependencies caused by

7414 Langmuir, Vol. 16, No. 19, 2000 Figure 2. Maps of the local molecular lipophilicity potential (MLP) shown on the water accessible surface of ethyl behenate and trifluoroethyl behenate molecules. The conformations of the molecules were calculated in vacuo. The ethyl behenate models are on the left side of each couple with the same headgroup conformation.

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Figure 3. Maps of the local molecular electrostatic potential (MEP) shown on the water accessible surface of ethyl behenate and trifluoroethyl behenate molecules. The conformations of the molecules were calculated in vacuo. Ethyl behenate is on the left side of each couple.

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Figure 4. Surface pressure-molecular area isotherms of the monolayers of ethyl behenate (full circles) and trifluoroethyl behenate (open circles) on water. Table 1. Comparison of the Characteristic Surface Pressures and Molecular Areas of the Compression Isotherms of TFEB and EB Monolayers Presented in Figure 4 monolayer TFEB

EB

characteristic pointsa InI TS IP CP InI TS K IP CP

π, b mN/m

A,b A2

0.5 10.5-11.6 (11.0) 20.0 23.7 0.5 11.5-16.5 (13.5) 28.0 38.5 44.3

25.5 20.6-20.3 (20.4) 19.4 19.0 23.5 19.9-19.6 (19.7) 19.2 19.0 18.8

a InI, initial increase in surface pressure; TS, transition section; K, kink; IP, inflection point; CP, collapse point. b The transition surface pressure and molecular area are given in parentheses.

the substitution of the alkyl CH3 group by a CF3 group. The isotherm of TFEB is much more shallow; surface pressure starts increasing at a larger molecular area, and monolayer collapse occurs at much lower πcol and almost the same Acol as for EB (Table 1). The surface pressure at the inflection point, where d2π/dA2 changes its sign and the monolayer loses stability, is almost twice as low for TFEB. The isotherms intersect at 19.5 Å2, and this molecular area practically coincides with the abscissa of the inflection point in the isotherm of TFEB (19.4 Å2). Thus, in comparing stable TFEB and EB monolayers we should refer to molecular areas above this value. The isotherm of TFEB does not show horizontal parts, indicating first-order phase transitions. However, by enlarging the regions where the slope changes and inspecting the dependence dπ/dA vs A obtained via differentiation of the isotherm, we can distinguish several sections. In the first one the slope increases monotonically with decreasing molecular area to 20.6 Å2. Some sort of structural change occurs between 20.6 and 20.3 Å2 which could be characterized by the coordinates of the intersection of the extrapolated adjacent linear parts, 20.4 Å2 and 11.0 mN/m (Table 1). Further compression first brings about a decrease followed by a subsequent increase in the slope to finally reach a maximum at the inflection point. The isotherm of TFEB does not have a steep linear section characteristic to a solid condensed monolayer with vertical closely packed hydrocarbon chains. The same analysis applied to the EB monolayer shows that surface pressure starts to increase at a smaller molecular area and with a slower rate than the TFEB

Figure 5. Compression-expansion hysteresis of the surface pressure-area dependence. Compression was followed to 75% of the corresponding collapse pressures in Figure 4. The full circles indicate ethyl behenate and the open circles represent trifluoroethyl behenate.

monolayer, but below 20.6 Å2 this isotherm becomes steeper. A linear section extends to 19.9 Å2, followed by a gradual transition (19.9-19.6 Å2) to another linear part with larger slope. The extrapolations of these linear parts intersect at 19.7 Å2 and 13.4 mN/m. A third even steeper linear region begins with a kink at 19.2 Å2. Extrapolation of the linear sections to π ) 0 yields the following limiting areas, A0 ) 21.2 Å2, A0 ) 20.1 Å2, and A0 ) 19.8 Å2. Table 1 compares the characteristic molecular areas and surface pressures of TFEB and EB monolayers. The transition surface pressure (given in parentheses), surface pressure at the inflection point, and collapse surface pressure of TFEB are lower than the corresponding values for EB. All characteristic molecular areas of TFEB exceed those of EB. This difference decreases with increasing surface pressure; it is 2.0 Å2 at 0.5 mN/m, 0.4 Å2 at the inflection points, and 0.2 Å2 at the collapse points. Compression-Expansion Hysteresis of the π/A Isotherms. Compression-expansion cycles of the π/A dependence yield additional information about the rearrangement and molecular interactions in Langmuir monolayers. Moreover, the expansion isotherms are often preferably employed to construct monolayer phase diagrams because of the smeared character of the transitions at compression.24,25,39 For this reason we have recorded the hysteresis loops of TFEB and EB monolayers at the same low velocity. The compression was followed up to 18 mN/m for TFEB and 33 mN/m for EB. These values are below the inflection points in Figure 4 and correspond to 75% of the collapse surface pressures (Table 1). On this basis we assume that the expansion branches of the loops are not affected by collapse of the monolayer during compression. Figure 5 shows that both π/A dependencies exhibit hysteresis, which implies irreversible or slowly reversible molecular rearrangements of both TFEB and EB monolayers occurring during compression. These rearrangements, characterized by the difference between Acmp and Aexp at a given surface pressure, are more significant for the TFEB monolayer. The hysteresis loop of TFEB is located at larger molecular areas and is more shallow than the EB loop. Both expansion branches of the isotherms have linear low-pressure sections, but these sections have different (39) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591.

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Table 2. Comparison of the Characteristic Surface Pressures and Molecular Areas of the Expansion Parts of the Hysteresis Loops of TFEB and EB Monolayers Shown in Figure 5 monolayer TFEB expansion EB expansion

characteristic pointa

π, b mN/m

A,b A2

K TS ENC K TS ENC

11.8 11.8-5.5 (8.6) 0 19.5 16.5-13.0 (14.0) 0

20.1 20.1-20.6 (20.2) 21.7 19.3 19.3-19.5 (19.4) 20.9

a K, kink; TS, transition section; ENC, extrapolation to no compression. b The transition surface pressure is given in parentheses.

slopes for TFEB and EB. Their extrapolation to π ) 0 gives a larger limiting area for TFEB, A0 ) 21.7 Å2, versus A0 ) 20.9 Å2 for EB. Between 20.6 and 20.1 Å2 for TFEB, and from 19.5 to 19.3 Å2 for EB, one finds transition sections of varying slope. The transition surface pressure is lower and the transition molecular area is larger for the TFEB (see the values in brackets in Table 2). Both expansion isotherms have kinks, indicating structural changes in the monolayer, but the TFEB kink is located at a lower surface pressure and larger molecular area. Integration of the π/A curves from Akink to A0 gives the work of expansion, Wexp, against the cohesion forces in TFEB and EB monolayers. Assuming the same initial and final structure of these films, we can compare the values of Wexp ) 45 J/mol for TFEB and Wexp ) 74 J/mol for EB. They show that the cohesion forces in the TFEB monolayer are weaker (or that repulsion is stronger). Unfortunately, similar comparison of the work of compression is impossible, because of the smeared phase transitions in this case. Analyzing the expansion isotherms of several long chain ethyl esters, Lundquist25 found three monolayer phases with vertical closely packed molecules, namely LS, S, and CS. In the LS phase the molecules are free to rotate around their axes and the positional order is small. In the S phase this rotation is impeded and positional order is higher, and the CS phase is a crystal with a long-range positional order. Two “tilted” phases, L2′ and L2′′, were also observed. They have closely packed but inclined molecules with maximum tilt angle from the normal to the water surface of 25°-27° for the L2′ and 23°-24° for the L2′′ phase. The transition between tilted and vertical orientation occurs in the range of 18.4-18.6 Å2 when the vertical phase is CS, 18.8-19.1 Å2 for the S phase, and 19.1-19.7 Å2 for the LS phase. The kink in the EB expansion isotherm in Figure 5 is located at 19.3 Å2, which implies that below this molecular area the EB monolayer exists in the LS state. The limiting molecular area of the low-pressure part, A0 ) 20.9 Å2, falls in the range of 20.4-21.5 Å2, which was found by Lundquist to be characteristic for the L2′ phase.25 On this basis one could conclude that the kink in the expansion isotherm of the EB monolayer corresponds to an LS-L2′ phase transition. The ratio of the molecular area at the kink and the limiting area at π ) 0 yields the maximum tilt angle:25

The expansion isotherm of TFEB is qualitatively similar to the one of the EB monolayer. This similarity leads us to the assumption that the TFEB kink also indicates transition between a vertical and a tilted molecular orientation. Application of eq 1 to the corresponding molecular areas, Akink ) 20.1 Å2 and A0 ) 21.7 Å2, gives practically the same maximum tilt angle, Θmax ) 22°. If the above assumption is correct, then the obtained result would mean that the low-pressure parts of the expansion isotherms of TFEB and EB correspond to liquid-condensed states with the same tilt angle but different molecular areas. Applied to the limiting areas, this interpretation generalizes Adam’s40 conclusion (considering only vertical chains) that larger values of A0 imply larger hydrophilic heads. Compressional and Dilational Moduli of TFEB and EB Monolayers. Thermodynamic analysis shows41,25 that a first-order phase transition is accompanied by a discontinuity in molecular area, that is, the isobaricisothermal derivative of monolayer chemical potential, µ, with respect to surface pressure:

(∂π∂µ)

P,T

)A

(2)

During a second-order phase transition the thermodynamic states of monolayer and molecular area change gradually, but the second derivative of the chemical potential

( ) ( ) ∂2µ ∂π2

)

P,T

∂A ∂π

(3)

P,T

exhibits a discontinuity. This means that the monolayer compressibility

(A1 ∂A ∂π )

CS ) -

P,T

(4)

(1)

should abruptly change at the kinks of the isotherms and that such discontinuities should indicate second-order phase transitions. Instead of CS, one often uses the reciprocal quantity, CS-1, called compressional modulus.42 The studies of longitudinal capillary waves43 utilize the notation “dilational modulus”, ES-1, that has the same definition but corresponds to an increase in the monolayer area. For reversible π vs A dependence, CS-1 and ES-1 coincide, representing the equilibrium compressional modulus of the Langmuir film. The above thermodynamic criteria for phase transitions could be rigorously applied only for such data while for nonequilibrium isotherms or those exhibiting hysteresis they might give ambiguous results. On the other hand, CS-1 and ES-1 are real physical quantities that could be used for comparison of nonequilibrium Langmuir monolayers and monolayers states. Their abrupt changes indicate structural transformations at given monolayer density, although these changes cannot be clearly identified as phase transitions. Comparison of CS-1 and ES-1 of TFEB and EB films would demonstrate the effect of fluorination of the hydrophilic head on mechanical properties of Langmuir monolayers, but the

Substitution of our data for EB Akink ) 19.3 Å2 and A0 ) 20.9 Å2 in eq 1 gives Θmax ) 23°. This value coincides with the maximum tilt angle of the L2′′ phase but is also very close to the values for the L2′ phase reported by Lundquist.25

(40) Adam, N. K. Proc. R. Soc. (London) 1922, A101, 452. (41) Harkins, W. D. The Physical Chemistry of Surface Films; New York, 1954; pp 106-109. (42) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1961; p 265. (43) Lucassen, J.; van den Tempel, M. J. Colloid Interface Sci. 1972, 41, 491.

Akink ) cos Θmax A0

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Figure 6. Comparison of the compressional moduli of TFEB and EB monolayers obtained via differentiation of the isotherms in Figure 4 plotted versus molecular area. Full circles correspond to EB and open circles represent TFEB monolayer.

conclusion of phase transitions made on this basis should be considered with precaution. From a systematic analysis of surface pressure-area isotherms, Davis and Rideal42 found that CS-1 of a liquidexpanded film varies between 12.5 and 50 mN/m, a liquid condensed monolayer shows CS-1 between 100 and 250 mN/m, and a solid condensed state is characterized by CS-1 from 1000 to 2000 mN/m. On the same basis Harkins41 has specified the limits for the LS phase to be 600-2000 mN/m and for the intermediate liquid phase to be 5-300 mN/m. The latter “is believed to consist of small islands of the condensed liquid phase in a sea of the expanded phase”.41 The dependencies of the compressional modulus on molecular area calculated from the isotherms of TFEB and EB in Figure 4 are shown in Figure 6. One can see that the values of CS-1 for TFEB and EB and the two CS-1 vs A curves differ considerably below 19.7 Å2. All CS-1 values for TFEB are below the limits characteristic for solid condensed films. They are typical for a liquid condensed or intermediate liquid phase, but the corresponding molecular areas are considerably smaller. There is no discontinuity indicating a phase transition in the TFEB monolayer before the inflection point. In contrast to TFEB, two jumps at 19.7 and 19.2 Å2 can be clearly seen in the CS-1 vs A dependence of EB. Comparison of the CS-1 values at these points (200 and 650 mN/m) with literature data implies that at 19.7 Å2 the EB monolayer undergoes a transition between a liquid-condensed and a solid-condensed (probably LS) state. Another transition between two solid phases (a LS-S transition?) seems to occur at 19.2 Å2. Figure 7 presents the dilational modulus calculated from the expansion isotherms in Figure 5. For TFEB one finds alternative changes of ES-1 between 770 and 310 mN/m in the very narrow range (20.0-20.2 Å2) around the abscissa of the kink (see Table 2). In the transition section 20.2-20.6 Å2 the dilational modulus decreases from 210 to 105 mN/m and remains constant at this value up to 21.6 Å2. Another discontinuity occurs here (cf. Figure 5) with alternative variations of ES-1 between 100 and zero. A similar trend is observed for the ES-1 vs A dependence of EB that, however, is shifted to lower molecular areas. Between 19.1 and 19.3 Å2 the dilational modulus fluctuates from a maximum value of 1650 mN/m to a minimum of 630 mN/m. In the transition region of the expansion isotherm, 19.3-19.5 A2, ES-1 sharply decreases from 1270 to 170 mN/m and drops down to zero between 20.9 and

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Figure 7. Dependence of the dilational modulus on molecular area evaluated from the expansion branches of the isotherms in Figure 5. Full circles correspond to EB and open circles represent TFEB monolayer.

Figure 8. Dependence of the molecular area at a constant surface pressure of 9 mN/m with time. Ethyl behenate is presented via full circles and the open circles correspond to trifluoroethyl behenate.

21.1 Å2. Although no literature data for dilational modulus is available, comparison of ES-1 for EB with the values of CS-1 collected by Harkins41 and Davis and Rideal42 supports the conclusion expressed in the former section that a solid-to-liquid condensed state transition occurs in both monolayers under expansion. Area-Time Dependence of TFEB and EB Monolayers. In Figures 8 and 9 we compare the molecular area-time dependence for TFEB and EB monolayers at constant surface pressures of 9 and 18 mN/m. These surface pressures were chosen as characteristic for different states of these films (cf. Figure 4). Compression at 0.8 Å2/molecule.min was performed up to the desired value of π that was then automatically maintained while the variation of A with time was recorded. For EB both A vs t curves do not show accelerating loss of monolayer area with time that would indicate collapse of the film.44 Instead, the rate of area loss decreases at both surface pressures, thus inferring some relaxation of the EB monolayer toward more stable structures. The decrease of molecular area of TFEB at 18 mN/m shown in Figure 9 is much more significant. The initial part of this dependence is typical for nucleation and growth of a bulk (44) Brooks J. H.; Alexander, A. E. In Retardation of Evaporation by Monolayers; La Mer, V. K., Ed.; Academic Press: New York, 1962; p 252.

Langmuir Monolayers with Fluorinated Groups

Figure 9. Variation with time of the molecular area of TFEB and EB monolayers at 18 mN/m. The S-shape of TFEB dependence (open circles) implies transition to a more stable formation.

phase in Langmuir films,45 but the following reversal of curvature indicates transition to a more stable formation. After long enough time the ratio between the initial and the final molecular areas asymptotically approaches 2.0, implying formation of a bilayer during monolayer collapse. Similar monolayer-to-bilayer transition has been observed in monolayers of long chain acetates,24 ethyl esters,25 and triglycerides,26,27 but not in alcohols and fatty acids, probably due to greater hydrophilicity of the polar heads. Detailed inspection of the very weak A vs t dependence of TFEB at 9 mN/m (Figure 8) also reveals an inflection point. The rate of area loss first decreases but at 20.6 Å2 it starts increasing, indicating that below 20.6 Å2/molecule the TFEB monolayer undergoes collapse. It is surprising that this molecular area coincides exactly with the beginning of the transition section of the TFEB compression isotherm (cf. Table 1), and with the value of A at which the compressional modulus of the EB monolayer becomes greater than CS-1 for the TFEB monolayer (cf. Figure 6). This coincidence implies that 20.6 Å2 might be the molecular area at which the equilibrium collapse of the TFEB monolayer takes place. Discussion The molecular models in Figures 2 and 3 hold for isolated TFEB and EB molecules in vacuo. In the real monolayer at the air-water interface the headgroups are hydrated and the water dipoles affect their polarization. A mutual polarization of the adjacent closely packed chains in the condensed films would also modify the maps of MLP and MEP. The three-capacitor electrostatic model of uncharged condensed Langmuir monolayers17-19 neglects the mutual polarization of the hydrocarbon chains, headgroups, and hydration water. Within the framework of this simplification the contribution of the hydration water is very small.17-19 Accounting for the polarization effects in a Langmuir film considered as an ensemble of interacting hydrated molecules is a rather complicated task that is out of the scope of the present study. A simplified model interpreting the opposite ∆V values of TFEB and EB films will be presented in Part 3. Our maps of MLP and MEP of isolated TFEB and EB molecules in vacuo make the first step in the attempt to understand the relationship between monolayer and (45) Smith, R. D.; Berg, J. C. J. Colloid Interface Sci. 1980, 74, 273.

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molecular properties. Despite the simplicity of this approach, it leads to some important conclusions. The maps of molecular lipophilicity of TFEB and EB in Figure 2 show that the commonly accepted segregation of an amphiphilic molecule into a “hydrophilic head” and a “hydrophobic tail” is a rather rough approximation. The polar group is not the only hydrophilic part of the molecule, and the CH2 groups of the hydrocarbon chain do not all have the same hydrophobicity. A similar conclusion follows from the maps of molecular lipophilicity of C17H35OH and ω-BrC16H32OH molecules reported in ref 19 and from other our (unpublished) models of long chain fatty acids and amides. Figure 2 also shows that the lipophilicity of the CF3 group in the TFEB head is higher so that its affinity toward water must be lower than the corresponding affinity of the CH3 group in the EB head. This important conclusion is opposite to the one of Shafrin and Zisman4 concerning the ω-trifluoromethyl and methyl groups of hydrocarbon chains. However, it enables a plausible explanation of our experimental results showing a decrease in the collapse and transition surface pressures of TFEB monolayer. Substitution of the CH3 group in the hydrophilic head by CF3 group makes TFEB similar to the fatty acid esters with longer alkyl chains whose monolayers collapse at lower surface pressure and larger molecular area.46,47 According to the theory of the equivalent states in amphiphilic lamellae by Peterson et al.,48 the decreased hydrophobicity of the TFEB head could be responsible for the observed decrease in the transition surface pressures of the TFEB monolayer as compared to those of the EB film. Similar decrease of the transition surface pressures of ethyl ester monolayers with respect to the fatty acid monolayers was found in ref 48, and this effect was related to the more hydrophobic character of the ethyl ester head. Adam46 discovered that monolayers of esters of long chain fatty acids rearrange under compression in different ways, depending on the length of the alkyl chain. He concluded that if the alkyl chain contains less than four carbon atoms and the temperature is low, then a condensed film is formed at high surface pressure in which the alkyl chains attain a vertical position opposing the long acidic chains. For longer alkyl chains or at higher temperature both chains are directed toward air, and the esters form liquid expanded films. Alexander and Schulman28 confirmed this conclusion for ethyl ester Langmuir films by comparing the measured dipole moment with the one calculated from atomic group contributions for different conformations of the hydrophilic head. Since the higher lipophilicity of the CF3 group of TFEB is energetically equivalent to a longer alkyl chain, one could expect that the CF3 group will tend to orient itself toward the air. It is difficult to say to which extent this tendency will affect the conformation of the hydrophilic heads of the TFEB monolayer, but it will make the cis-trans conformation energetically more preferable at the air-water interface than the trans-trans conformation having the same energy in vacuo. Thus, the different affinity of the CF3 and CH3 ester terminals toward water might lead to a different conformation of the headgroups of TFEB and EB monolayers. The shift of both the compression and expansion branches of the π/A hysteresis loop of TFEB to larger molecular areas might be caused by such a difference but may also result (at least partially) from (46) Adam, N. K. Proc. R. Soc. (London) 1929, A126, 366. (47) Stenhagen, E. Trans. Faraday Soc. 1938, 34, 1328. (48) Peterson, I. R.; Brzezinski, V.; Kenn, R. M.; Steitz, R. Langmuir 1992, 8, 2995.

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different volumes of the hydrated TFEB and EB heads having the same conformation. The ratio of the molecular areas at the kinks of the expansion isotherms of TFEB and EB is 1.04 (see Table 2). The Connolly volumes49 of the cis-trans headgroups in vacuo including the two adjacent CH2 groups, 147.4 Å3 for TFEB and 128.9 Å3 for EB, give a ratio of 1.14. Almost the same values, namely 144.7 Å3 and 127.6 Å3, and a ratio of 1.13 are obtained for the trans-trans conformations of the TFEB and EB heads. This ratio becomes 1.15 for a cis-trans TFEB head and a trans-trans EB head. All these values considerably exceed the ratio of the kink molecular areas 1.04, but for the monolayer at the airwater interface this difference should become smaller. The volumes of the hydrated TFEB and EB polar groups should be closer to each other, because the more hydrophobic TFEB head would build up a smaller hydration shell. The close values of the Connoly volumes of the same substance for different headgroup conformations on first sight suggests that the conformation of the head does not affect the molecular area. However, the same volumes of the cis-trans and trans-trans conformations correspond to a different orientation of the heads with respect to the hydrocarbon chain (Figure 2) that will specifically influence the packing, orientation, and mean area per molecule. The maps of molecular electrostatic potential in Figure 3 show that the headgroups tend to polarize the adjacent parts of the chains. The MEPs of C17H35OH and ωBrC16H32OH in ref 19 reinforce this conclusion. Such an outcome is in contrast with the accepted opinion that the CH2 groups of a normal hydrocarbon chain do not contribute to the dipole moment of a Langmuir monolayer.14-19 However, it is supported by recent theoretical models50 which take into account intermolecular and intramolecular polarization in such films and predict a contribution of the methylene groups to the dipole moment of the hydrocarbon chains. The degree of polarization of the chains depends on the chemical structure of the heads while the conformation of the headgroup seems to play a secondary role. Figure 3 shows that the distribution of the molecular electrostatic potential is different for the chains of trifluoroethyl and ethyl behenate and that the chains of TFEB are more polarized than the EB chains. Therefore, the resulting electrostatic repulsion between the parallel chains in the condensed TFEB monolayer should be stronger and its cohesion energy should be smaller than in the EB film. The increased repulsive component of the interaction between the chains could be the reason for some of the differences delineated in the Experimental Section. These are the earlier rise of the surface pressure in the isotherm of TFEB, the higher surface pressure of the stable TFEB monolayer as compared with EB monolayer with the same density (Figure 4), and the smaller work for expansion of TFEB monolayer. The stronger electrostatic repulsion between the TFEB chains may also lower the stability and collapse pressure of TFEB compared to those of the EB monolayer. A similar explanation was put forward by Barnet, Jarvis, and Zisman15 regarding the terminal CF3CH2 dipoles of monolayers of ω-halogenated hexadecanoic acid. Specific dipole-dipole repulsion between the hydrophilic heads of TFEB and EB may also cause the above differences, but such an interaction should be strongly (49) The volume of an isolated molecule or atomic group that is accessible for water molecules. (50) Taylor, D. M. Thin Solid Films 1998, 331, 1. See also ref 30 herein.

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reduced or even eliminated by their hydration at the airwater interface. Decrease of the rate of area loss as observed in the initial sections of TFEB and EB area-time dependence in Figure 8 has been also registered for other Langmuir films.45 Such a trend in the area-time dependence was interpreted as “a time-dependent structural rearrangement within the monolayer to relieve surface inhomogeneities resulting from the pushing together of islands of condensed film to form a coherent close-packed monolayer”.45 Such an interpretation finds support in other literature sources. Formation of 2D crystals at large molecular areas was registered51 in the ellipsometric isotherms of undissociated arachidic, behenic, and tetracosanoic acid. Schlossmnan et al.52 and Bommarito et al.53 found the same for tetracosanoic and behenic acid on 0.01 M HCl applying gracing incidence X-ray diffraction. As will be shown in Part 2 of this investigation, Brewster angle microscopy of TFEB and EB monolayers also supports this interpretation. Compression of the “island phase” seems to form a polycrystalline film containing voids that are expelled (perhaps partially) at high surface pressure. Such a mosaic structure may exhibit much lower compressional modulus (higher compressibility) than the compact solid condensed phase. On the other hand, disintegration of the compact monolayer during expansion would obey a different forcedistance law and kinetics than the mosaic structure existing during compression. This difference offers another interpretation of the hysteresis of the surface pressurearea isotherms. The experimental data in this paper do not unambiguously show phase transitions in the TFEB monolayer. The shallow compression π/A isotherm exhibits one (subtle) change in the slope but no steep linear part characteristic of a solid-condensed film. On the other hand, the expansion isotherm and the kink in this isotherm imply a secondorder “tilting” transition. Another indication of the existence of a solid-condensed phase at high surface pressure comes from the A/t dependence of TFEB in Figure 9, which implies a monolayer-to-bilayer transition. According to the results of Lundquist24,25 and Larssen,26,27 such a transition takes place in solid-condensed but not in liquidcondensed monolayers. The compression isotherm of EB contains three sections of different slopes, confirmed by two jumps in the compressional modulus in Figure 6. Analysis of our expansion π/A curve for EB on the base of Lundquist’s25 data for other long chain ethyl esters implies that this monolayer undergoes L2′-LS and LS-S phase transitions. However, this conclusion disagrees with the phase diagram of Stenhagen54 for ethyl behenate predicting only an L2-CS transition at 2.5 mN/m. This contradiction and the ambiguous identification of the monolayer phases on the basis of surface pressure-area isotherms and values of compressional modulus clearly point out to the necessity for independent investigations of monolayer structure, which will be reported in part 2 of this study. LA991557Z (51) Petrov, J. G.; Pfohl, Th.; Mo¨hwald, H. J. Phys. Chem. 1999, 103, 3417. (52) Schlossman, M. L.; Schwartz, D. K.; Pershan, P. S.; Kawamoto, E. H.; Kellog, G. J.; Lee, S. Phys. Rev. Lett. 1991, 66, 1599. (53) Bommarito, G. M.; Foster, W. J.; Pershan, P. S.; Schlossman, M. L. J. Chem. Phys. 1996, 105, 5265. (54) Stenhagen, E. In Determination of Organic Structures by Physical Methods; Braude, Nachod, Eds.; New York, 1955; Chapter 8, p 335.