Langmuir Monolayers Characteristic of - American Chemical Society

J. Phys. Chem. B 2004, 108, 13403-13411

13403

Langmuir Monolayers Characteristic of (Perfluorodecyl)-Alkanes M. Broniatowski,† I. Sandez Macho,‡ J. Min˜ ones, Jr.,‡ and P. Dynarowicz-Ła¸ tka*,† Jagiellonian UniVersity, Faculty of Chemistry, Ingardena 3, 30-060 Krako´ w, Poland and UniVersity of Santiago de Compostela, Faculty of Pharmacy, Department of Physical Chemistry, 15-706 Santiago de Compostela, Spain ReceiVed: March 25, 2004

A series of semifluorinated n-alkanes (SFAs) of the general formula F(CF2)m(CH2)nH, (in short FmHn) where m ) 10 and n ) 6-20, have been synthesized and employed for Langmuir monolayer characterization. Surface pressure and electric surface potential measurements were obtained under a variety of experimental conditions such as spreading volume, subphase temperature, and compression speed. The Langmuir monolayer experiments have been complemented with Brewster angle microscopy results which enabled both direct visualization of the monolayers structure and estimation of the monolayer thickness at different stages of compression. Our results show that these “nonclassical” film-forming materials, which are completely hydrophobic in nature and do not possess any polar group in their structure, are capable of monolayer formation at the air/water interface. It has been observed that with the increase in the molecule’s length, its stability at the free water surface increases. The negative sign of the measured surface potential, ∆V, proves that SFA molecules are oriented at the air/water interface with their perfluorinated parts directed toward the air. The effective dipole moments reach the value of -0.65 ( 0.1 D at the minimum for all stable SFAs. The analysis of the direction of the molecular dipole moment in respect to the main axis enabled us to estimate that the minimum effective dipole moment is achieved for a molecule oriented at the angle of about 35° to the surface normal. The relative intensity measurements allow one to conclude that film molecules are tilted in respect to the surface normal at the vicinity of collapse.

Introduction A very different structure and physical properties of alkanes and their perfluorinated analogues1,2 have stimulated efforts to synthesize a very interesting class of materials, namely, semifluorinated alkanes (SFAs). These compounds were already known in the 1970s; however, the growing interest in such molecules has been observed since 1984 when Rabolt et al. published their paper entitled “Structural Studies of Semifluorinated n-Alkanes. 1. Synthesis and Characterization of F(CF2)m(CH2)nH in the Solid State”.3 The presence of two opposing segments within one molecule makes semifluorinated alkanes a very unusual group of compounds, which show a particular behavior both in solutions and at interfaces. Their highly asymmetric structure, arising from the incompatibility of both constituent parts, results in surface activity of these molecules (so-called primitiVe surfactants) when dissolved in organic solvents (both in liquid hydrocarbons and perfluorohydrocarbons)4-9 and allows for the Langmuir monolayer formation when spread at the air/water interface,10,11 despite the fact that these molecules are purely hydrophobic and lack any polar group in their structure. Although film-forming abilities of some SFA have already been reported by Gaines in 1991,12 only a few articles describing their monolayer properties have appeared so far,10-13 whereas the majority of papers deal with their structure and physicochemical properties in bulk phase.14 The aim of the present work is to provide a thorough characteristic of a series of SFA at the air/water interface. By changing systematically * Corresponding author. Tel.: +48-12-6336377 ext. 2236; fax: +4812-6340515; e-mail: [email protected]. † Jagiellonian University. ‡ University of Santiago de Compostela.

the length of the hydrogenated chain while keeping the same length of the perfluorinated part, we expect to gain information about how the increase in methylene units affects the stability and characteristic of their monolayers. For this purpose, we have employed surface pressure (π) and electric surface potential area (∆V) measurements, complemented with quantitative Brewster angle microscopy (BAM). Experimental Section The following compounds of the general formula F-(CF2)10(CH2)m-H, where m ) 6 and 8-20, were synthesized following the procedure described elsewhere.3 Perfluorodecyl iodide (97%) was purchased from Fluorochem while the respective n-alkenes (98-99%) were supplied by Aldrich. All the substrates were used for synthesis as received, without further purification. The spreading solutions for Langmuir experiments were prepared by dissolving each compound in chloroform (Aldrich, HPLC grade) with a typical concentration of ca. 0.5-1 mg/mL. To study the influence of the spreading solvent, other organic liquids such as hexane, isooctane, or toluene (Aldrich, HPLC grade) were used. In a typical experiment, 50-100 mL of chloroform solution was spread with a Microman Gilson microsyringe, precise to (0.2 µL. After spreading, the monolayers were left for 10 min to enable the solvent to evaporate, after which compression was initiated with a barrier speed of 25 cm2/min unless otherwise specified. Ultrapure water (produced by a Nanopure water purification system coupled to a Milli-Q water purification system, resistivity ) 18.2 MΩ cm) was used as a subphase. The subphase temperature was controlled to within 0.1 °C by a circulating water system from Haake. Experiments were carried out with a

10.1021/jp0402481 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/06/2004

13404 J. Phys. Chem. B, Vol. 108, No. 35, 2004

Figure 1. Surface pressure (π)-area (A) isotherm of F10H6, F10H8, F10H9, F10H10, and F10H11 spread at 20 °C on water. Inset: Compression modulus (Cs-1)-surface pressure (π) dependencies.

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Figure 3. Surface pressure (π)-area (A) isotherm of F10H17, F10H118, F10H19, and F10H20 spread at 20 °C on water. Inset: Compression modulus (Cs-1)-surface pressure (π) dependencies.

thickness and film optical properties. These parameters can be measured by determining the intensity of light detected by the camera and analyzing the polarization state of the reflected light employing the method based on the Fresnel reflection equation.15 At the Brewster angle

I ) |Rp|2 ) Cd2

(1)

where I is the relative intensity (defined as the ratio of the reflected intensity Ir and the incident intensity I0), C is a constant, d is the film thickness, and Rp is the p-component of the light. The lateral resolution of the microscope was 2 µm, and the micrographs were further digitized to obtain high quality BAM images. Results Figure 2. Surface pressure (π)-area (A) isotherm of F10H12, F10H13, F10H14, F10H15, and F10H16 spread at 20 °C on water. Inset: Compression modulus (Cs-1)-surface pressure (π) dependencies.

single barrier NIMA 601 trough (Coventry, United Kingdom) (total area ) 550 cm2) placed on an antivibration table. Surface pressure was measured with the accuracy of (0.1 mN/m using a Wilhelmy plate made of chromatography paper (Whatman Chr1) as the pressure sensor. The monolayer stability was verified by monitoring the change in surface pressure while holding the area constant. Surface potential measurements were performed with the Kelvin probe (model KP2, NFT, Germany) mounted on a NIMA trough. The vibrating plate was located ca. 2 mm above the water surface while the reference electrode, made from platinum foil, was placed in the water subphase. The surface potential measurements were reproducible to (10 mV. Both surface pressure-area and electric surface potentialarea isotherms reported here are the averages of at least three experiments. Brewster angle microscopy images and relative intensity measurements were performed with BAM2 plus (NFT, Germany), equipped with a 30 mW laser emitting p-polarized light at 690-nm wavelength. The incident light was reflected off the air/water interface at approximately 53.1° (Brewster angle). The intensity at each point of a BAM image depends on the local

The surface pressure-area (π-A) dependencies of the investigated SFAs have been separated into three graphs: Figure 1, 2, and 3. Each of these figures presents the isotherms of such compounds, which behave similarly at the air/water interface. The first group of semifluorinated alkanes, shown in Figure 1, includes those with hydrocarbon chain length smaller or approximately equal to the perfluorinated part (n ) 6 and 8-11). These compounds are rather poor materials as regards Langmuir monolayer formation, and their characteristics strongly depend on the experimental conditions. Especially, F10H6 and F10H8 form typically dissolving monolayers, which are very unstable at the interface and therefore will not be discussed further. The remaining molecules from this group of SFA, however, exhibit isotherms of a liquid-expanded character with a kink transition appearing at surface pressures of about 2-5 mN/m. This transition is clearly visible on the compression modulus, Cs-1 [reciprocal of compressibility, defined as Cs-1) -A (dπ/dA)] versus surface pressure dependencies (inset of Figure 1). The liquid-expanded state of the monolayers can be quantified with Cs-1 values, which upon full compression do not exceed a value of 50 mN/m. This meets the criterion of the LE character given by Davies and Rideal16 (12.5 mN/m < Cs-1 < 50 mN/m) as well as Harkins17 and Reis18 (14.3 mN/m < Cs-1 < 50 mN/m). The onset area for the surface pressure is 20-30 Å2, and the monolayer collapse appears at ca. 7 mN/m except for F10H9 which collapses at a 2 times lower pressure (3 mN/m).

Langmuir Monolayers

J. Phys. Chem. B, Vol. 108, No. 35, 2004 13405

Figure 5. Stability of F10H11 monolayer at different surface pressures (a) and compression-decompression cycles (b-c). Figure 4. The influence of spreading solvent (a), compression speed (b), number of spread molecules on the surface (c), and subphase temperature (d) on F10H11 monolayer.

The influence of various experimental parameters on the isotherm for this group of SFAs has been studied (Figure 4). Since the three compounds (F10H9, F10H10, and F10H11) show similar behavior, we have chosen F10H11 as a representative for this group of SFAs for a detailed monolayer characteristic. First, we have observed that the spreading solvent is not important for the shape of the π-A isotherm (Figure 4a). The liftoff area shifted toward higher values with increasing compression speed (Figure 4b) as well as with increasing number of deposited molecules at the surface (Figure 4c). Both collapse pressure and transition pressure were slightly displaced while changing the spreading volume and velocity of compression. The subphase temperature also affects the π-A isotherm as illustrated in Figure 4d. With lowering temperature, the kink transition shifts to a slightly higher surface pressure value and the monolayer compressibility significantly decreases. These experiments (4b-d) undoubtedly indicate low stability of monolayers from F10H11. This has been corroborated by the stability experiments where change in surface pressure was monitored with time (Figure 5a). For both studied π values (2.5 and 4 mN/m), a continuously falling surface pressure was observed while keeping the area constant. In the first 10 min, around 50% decrease in π occurred, and after 45 min it reached ca. 30% of its initial value. Further evidence for a low stability has been obtained with the subsequent compression-expansion cycles (Figure 5b, c). During experiments carried out at both investigated pressure regions, noticeable hysteresis associated

with the systematic shift toward smaller area in a subsequent cycle can be observed. Much better stability of monolayers can be observed for SFA containing longer polimethylene part, that is, 12 e m e16 (Figure 2). For these compounds, the surface pressure starts to rise at 35-40 Å2/molecule and film collapse is reached between 8 and 12 mN/m. The compression modulus-surface pressure dependencies (Figure 2, inset) clearly indicate that the characteristic minimum (at ca. 5 mN/m), corresponding to the kink transition, separates two regions of similar compressibility values. For F10H12 and F10H16, both regions are of a liquid character (50 mN/m < Cs-1 < 100 mN/m) while the remaining perfluorodecyl alkanes show a liquid-condensed character (Cs-1 for both regions falls within the range of 100-150 mN/m). The influence of various experimental parameters is discussed for F10H15 as a representative of this group of semifluorinated alkanes (Figure 6). Different spreading solvent does not modify in any significant way the character of the isotherm (Figure 6a). Neither a change of compression velocity (Figure 6b) nor the number of deposited molecules (Figure 6c) influences the course of the π-A isotherm. Subphase temperature, on the other hand, although not important for the liftoff area, has an impact on the collapse pressure value as well as the kink transition. The latter disappears at 10 °C as evidenced by the lack of a minimum on the Cs-1 versus π plot. The monolayer stability is much higher as compared to F10H11. Especially at surface pressure region at the transition or below, the surface pressure practically remains constant (Figure 7). Only at higher pressures, above the transition, the monolayer is slightly less stable, with a 20% decrease in π occurring during the first 10 min, and afterward the surface pressure stabilizes. No hysteresis is observed upon


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Figure 7. Stability of F10H15 monolayer at different surface pressures (a) and compression-decompression cycles (b-d).

Figure 6. The influence of spreading solvent (a), compression speed (b), number of spread molecules on the surface (c), and subphase temperature (d) on F10H15 monolayer.

subsequent compression-expansion cycles for the monolayer compressed both to pressures below and above transition. Quite similar behavior can be noticed for the third group of SFAs, containing long hydrogenated part (17e n e20) (Figure 3). The influence of a change of experimental parameters on the π/A isotherm is shown for F10H19, as an example of this group of compounds, in Figure 8. Similarly to F10H15, the variation in the compression velocity and spreading volume practically does not alter the isotherms. With temperature decrease, the collapse pressure rises and the kink transition disappears. The monolayer is very stable (Figure 9) with nearly constant surface pressure with time and lack of hysteresis, independent of the isotherm region. Table 1 compiles the characteristic parameters for the whole series of the investigated perfluorodecylalkanes, and Figure 10 collectively illustrates their stability at the air/water interface. Electric surface potential, ∆V, was measured simultaneously with π-A isotherms, and the results are shown in Figures 1113 for the three representatives. In general, surface potential starts to change at the so-called critical area.19 This occurs between 40 and 50 Å2/molecule, that is, before surface pressure starts to rise. Upon compression, ∆V drops, initially very steeply, and afterward stabilizes. The minimum value of ∆V is reached at the area corresponding to the initial phase of the surface pressure rise. A semiquantitative analysis of the surface potential isotherm was made. Using the Helmholtz equation16 in the form

∆V ) µ⊥/(A0)

(2)

Figure 8. The influence of spreading solvent (a), compression speed (b), number of spread molecules on the surface (c), and subphase temperature (d) on F10H19 monolayer.

Langmuir Monolayers

J. Phys. Chem. B, Vol. 108, No. 35, 2004 13407 monolayer, one may calculate the apparent dipole moment µΑ ) µ⊥/ at different stages of compression. The results are presented in Figures 11-13, which also contain π-A and ∆V-A isotherms for the purpose of comparison. Simultaneously with the electric surface potential, the apparent dipole moment decreases and reaches its minimum (of -0.65 ( 0.1 D, with the exception of the smallest investigated molecules which are less stable, see Table 1) at molecular areas close to the liftoff of surface pressure, and afterward it increases. We have also applied Brewster angle microscopy (BAM) for a visualization of the film structure and for a quantitative analysis of the film thickness upon compression. Since the observed structures corresponding to different regions of the isotherm are identical for all the investigated molecules, we show BAM photographs for F10H15 as an example. At areas between 50 and 40 Å2, foamlike structures appear (Figure 14a), which condense upon compression forming structures typical for a gas-liquid coexistence (Figure 14b). With the rise in surface pressure, the monolayer becomes homogeneous (Figure 14c), indicating its liquid character. No change in the monolayer’s structure can be noticed on reaching the kink transition and upon further compression until ca. 11 mN/m, where 3D domains start to appear (Figure 14d). They become thicker as the collapse proceeds. For a more quantitative analysis, the relative reflectivity (I/ Io) was measured along with the π/A isotherms, after the camera calibration. As seen (Figures 11-13), the relative intensity starts to increase at slightly larger areas as compared to the surface pressure liftoff and rises steeply, reaching its maximum at areas close to film collapse.

Figure 9. Stability of F10H19 monolayer at different surface pressures (a) and compression-decompression cycles (b-d).

Discussion

where µ⊥ is the vertical component of the dipole moment (socalled effectiVe dipole moment), A is the area per molecule, 0 is the vacuum permittivity, and is permittivity of the

The analysis of the π-A isotherms formed by the investigated perfluorodecylalkanes indicate that although their structure is not typical for film-forming materials, they are capable of

Figure 10. The decrease in surface pressure with time for all the investigated SFA spread on water at 20 °C.


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Figure 11. Surface pressure (π), electric surface potential (∆V), apparent dipole moment (µA), and relative intensity (I/Io)-area (A) isotherms for F10H11 monolayer spread on water at 20 °C. Compression speed: 25 cm2/min.


Langmuir monolayers formation when spread at the air/water interface. The addition of polymethylene units to F10 segment increases their monolayers stability as evidenced by higher collapse pressure values (Table 1), observed no shift in area/ molecule for monolayers compressed in subsequent compression-expansion cycles, and reduced drop in surface pressure value while holding the area constant for longer analogues. Characteristic kink transitions, rather poorly marked in the course of their π-A isotherms, are very clearly displayed on the compression modulus-surface pressure dependencies. BAM images recorded below and above the transition are identical, that is, both are homogeneous, which indicate that the phases of these two regions are of the same nature. The comparison of the maximum Cs-1 values before and after the minimum proves that the transition separates two states (either liquid L-L′ or liquid-condensed LC-LC′) in which the ordering of molecules is different. The existence of two LC surface states in monolayers is known and has been observed for typical film-forming molecules (fatty acids).20 Although in many cases they are not visible on the π-A isotherm, however, their presence has been confirmed by other methods. According to recent interpretation,21-25 the traditional generalized monolayer phases (e.g., liquid, liquid-condensed, or solid) include a host of phases differing in molecular ordering, which can be closely related to the liquid-crystalline phases.21 Indeed, smectic ordering has been confirmed for some SFAs in bulk.26 Such “ordered-less ordered” transitions as well as rotational transitions in nalkanes27 are triggered by the increase in temperature, as it is clearly visible for the SFAs investigated here, where the kink transition already appears at 20 °C but is still not visible at 10 °C.


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TABLE 1: Characteristic Parameters for Langmuir Monolayers of the Investigated SFA compound

A0a (Å2/ molec.)

Acollb (Å2/ molec.)

πcollc (mN/m)

CS-1d (mN/m) 1st maximum

CS-1 (mN/m) 2nd maximum

Acrit.e (Å2/ molec.)

µA minf (D)

Aming (Å2/ molec.)

(I/I0)collh

F10H9 F10H10 F10H11 F10H12 F10H13 F10H14 F10H15 F10H16 F10H17 F10H18 F10H19 F10H20

22.7 29 35 34.8 36 36 39 35 35 37 37.6 36

19.3 18 23.7 25.8 31.9 28 32.5 24.5 30.7 25.4 27.5 23.3

2.93 7.05 6.95 8.58 8.5 11.1 12.5 12.4 14.0 15.0 16.8 16.7

39.6 23.7 40.6 54.4 130.9 115.9 122.3 76.7 129.3 93.1 73.9 71.9

34.2 36.9 62.2 94.8 156.9 151.1 90.3 200.1 138.3 145.6 106.1

32.8 32.4 52.3 36 44 49 47 49 50 40 40 46.5

-0.122 -0.295 -0.712 -0.57 -0.67 -0.63 -0.74 -0.68 -0.69 -0.630 -0.625 -0.653

23.5 25.4 37.3 29.7 32.8 32.3 39.6 37.5 33.3 33.5 30.4 36.5

4.4 4.6 4.1 4.6 7.2 6.7 19.4 19.1 21.0 20.9

a Liftoff area of surface pressure. b Area/molecule corresponding to the film collapse. c Collapse pressure. d Compression modulus. e Critical area. f Μinimum value of the apparent dipole moment. g Area/molecule corresponding to µA min. h Value of relative intensity at the collapse.

Figure 14. BAM images taken at different stages of compression for F10F15 monolayer spread on water at 20 °C. Each image corresponds to 228 × 170 µm2 segments of the monolayer, the larger dimension being from side to side of the page.

Electric surface potential measurements allowed us to calculate the apparent dipole moment for the investigated molecules at different stages of compression. The negative value of the measured surface potentials leads to a most probable molecular orientation of SFA molecules at the air/water interface, in which the perfluorinated part, introducing large negative contribution, is directed toward the air side. The minimum value of µA for all the investigated SFA, which form stable monolayers, is nearly the same. A very interesting feature is that contrary to model film-forming molecules, like unsubstituted, neutral long-chain amphiphiles (in which the maximum value of µA is reached in the condensed region where molecules are believed to orient vertically in respect to the surface28) for SFA, the maximum value of |µA| appears in the initial region of the surface pressure rise. Of course, the maximum value of µA vector can be directly related to the vertical orientation of film molecules only when the total molecular dipole moment lies along the main axis of the molecule. To get insight into the location of the dipole moment of the free molecule, µ, in respect to its main axis, we

Figure 15. Schematic representation of the direction of the dipole moment in respect to the main axis for an SFA molecule.

have performed semiempirical dipole moment computations in a vacuum using PM3 method29 (a semiempirical method of the


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Figure 16. Two different orientations of an SFA molecule at the interface together with the respective dipole moments.

ZDO type) and eigenvector following algorithm in iteration cycles with the quantum chemistry computer program (HyperChem30) for the wide spectrum of free molecules of SFA, differing both in the hydrogenated and perfluorinated part. The value of the dipole moment was found to be almost identical for the whole group of SFA (m ) 4-12, n) 1-20), that is, 2.8 ( 0.1 D, independent of the length of both segments in the molecule. Interestingly, the resultant dipole moment vector lies away from the molecule’s main axis (defined as a line intersecting all the C-C bonds (all-trans configuration) in the middle of their length, Figure 15a). In the hydrocarbon chain, the resultant dipole moments of the methylene groups cancel out in pairs.31 The same applies to CF2 group moments in perfluorinated chain. In the perfluorodecylalkanes investigated here, the dipole moments of eight CF2 groups cancel out and, as regards the perfluorinated segment, the only contribution to the total dipole moment comes from the terminal -CF2-CF3 group. The second contribution results from the terminal -CH3 group, however, because of a small value of the group moment of -CH3 (ca. 0.3 D31-34) as compared to -CF3 (-2.32 D),34 the resultant dipole moment of SFA molecule is mainly governed by the negative contribution from the perfluorinated part. Taking into consideration tetrahedral configuration of all carbon atoms in the chain, we have calculated the angle between the total dipole moment of an SFA and the main axis of the molecule (35.25°, Figure 15). Our coordinate system is defined as follows: its origin is located in the center of mass of a molecule and z-axis lies along the main axis. The resultant dipole moment, µ, is situated in the x-z plane. In our analysis, we are interested only in µz, which is the projection of the dipole moment on z-axis. From our considerations, it comes out that the maximum value of |µA| does not correspond to the vertical orientation of the molecule. Figure 16 shows two different orientations together with the respective µz components. Figures 11-13 in addition to the models presented in Figure 16 clearly show that the maximum |µA| is achieved for a tilted molecule (∼35° in respect to the surface normal) while perpendicular orientation of a molecule corresponds to a smaller value of |µA|. These results are corroborated by the relative intensity measurements, which prove that after the maximum |µA| is reached, the monolayer thickness still increases upon compression although |µA| decreases.

Upon analyzing the results of electric surface potentials and relative intensity measurements, complemented with BAM images, we may suggest the following molecular arrangements of SFA molecules spread at the air/water interface. At large areas, film molecules are well separated in a gas-liquid monolayer. In this region, both surface pressure and electric surface potential depict the value for pure water. Adapting the interpretation suggested by Leite et al.19 to SFAs, a network of weak hydrogen bonds can be formed between F atoms of horizontally oriented SFAs and water molecules. At so-called critical area,19 the hydrogen bonds break and electric surface potential becomes nonzero. The negative value of ∆V indicates that the perfluorinated part of the molecules is exposed to the air. Upon further compression, surface pressure starts to increase as the molecules approach each other and rise up, forming a liquid-type monolayer, seen as a homogeneous image in Brewster angle microscope. When molecules are still tilted in respect to the water surface, the absolute value of effective dipole moment attains its maximum, and with further compression a slight decrease can be observed while molecules continue to lift upright. At the area of about 25-30 Å2, SFA molecules are closely packed and further compression causes the molecules to be pushed out from the monomolecular film and form multilayer domains, which are clearly visible in BAM images. For the majority of the investigated perfluorodecyl-n-alkanes, quantitative BAM measurements were performed, and the dependencies of relative intensity of the reflected light (I/Io) versus A were obtained (see Table 1 and Figures 11-13). As it was already mentioned, I/Io signal is triggered off at larger areas as compared to surface pressure liftoff, similarly to ∆V. However, critical area for ∆V-A does not coincide with that for (I/Io)-A. This is simply because other phenomena are responsible for surface pressure rise as compared to other parameters such as surface potential or film thickness.35 While surface pressure is sensitive to the density of film molecules, I/I0 depends on the film molecules tilt angle, whereas ∆V starts to change when the hydrogen bonds between film molecules and water start to break. According to Rodrı´guez Patino et al.,36 eq 1 enables one to estimate the changes of the monolayer’s thickness upon compression. From eq 1, it is evident that I/I0 ) d2/d02, where d0 is the thickness of the monolayer at the beginning of compression, where one can suppose that the film-

Langmuir Monolayers forming molecules lie horizontally on the water surface. The cross section of an SFA is 28.6 Å2,37 and thus the thickness of the horizontally oriented SFA molecule is ca. 6 Å (twice the radius of the rigid rod, which is often used as a model of an SFA molecule). The experimental thicknesses of SFA monolayers at any point of the compression can thus be obtained by multiplication of the square root of I/I0 by 6. According to Viney et al.,26 the theoretical length of an SFA molecule in all-trans configuration can be calculated from the following equation:

dtheor ) (m - 1)*1.3 + (n - 1)*1.25 + 1.28 + 1.1 + 0.9 (3) where m is the number of carbon atoms in the fluorinated moiety, n is the number of carbon atoms in the hydrogenated moiety, 1.28 is the CH2-CF2 bond contribution, 1.1 is the terminal C-F bond contribution, and 0.9 is the terminal C-H bond contribution. As an example, for F10H19 dtheor) 37.5 Å, and the experimentally estimated film thickness (in the vicinity of monolayer collapse) d ) 27.5 Å. It is thus evident that film molecules are tilted in respect to the surface normal, for instance, in F10H19 the tilt angle is ca. 40°. Acknowledgment. The State Committee for Scientific Research (KBN, Poland) is gratefully acknowledged for financial support (Grant No. 3 TO9A 105 27). References and Notes (1) Jarvis, N. L.; Zisman, W. A. In Encyclopedia of Chemical technology; Wiley: New York, 1966; Vol. 7. (2) Hoffmann, H.; Wu¨rz, J. J. Mol. Liq. 1997, 72, 191. (3) Rabolt, J. F.; Russell, T. P.; Twieg, R. J. Macromolecules 1984, 17, 2786. (4) Turberg, M. P.; Brady, J. E. J. Am. Chem. Soc. 1988, 110, 7797. (5) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. J. Mol. Liq. 1997, 72, 177. (6) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. Langmuir 1995, 11, 977. (7) Hayami, Y.; Findenegg, G. H. Langmuir 1997, 13, 4865.

J. Phys. Chem. B, Vol. 108, No. 35, 2004 13411 (8) Binks, B. P.; Fletcher, P. D. I.; Kotsev, S. N.; Thompson, R. L. Langmuir 1997, 13, 6669. (9) Marczuk, P.; Lang, P.; Mo¨ller, M. Colloids Surf., A 2000, 163, 103. (10) Huang, Z.; Acero, A. A.; Lei, N.; Rice, S. A.; Zhang, Z.; Schlossman, M. L. J. Chem. Soc., Faraday Trans. 1996, 92, 545. (11) El Abed, A.; Faure´, M. C.; Pouzet, E.; Abillon, O. Phys. ReV. E 2002, 65, art. no. 051603. (12) Gaines, G. L., Jr. Langmuir 1991, 7, 3054. (13) Kim, N.; Shin, S. J. Chem. Phys. 1999, 110, 10239. (14) Lo Nostro, P. Curr. Opin. Colloid Interface Sci. 2003, 8, 223. (15) Azzam, R. M. A.; Bashara, N. M. In Ellipsometry and Polarized Light, 1st ed.; North-Holland: Amsterdam, 1992. (16) Davies, J. T.; Rideal, E. K. In Interfacial Phenomena, 2nd ed.; Academic Press: New York, 1963; pp 265 and 65. (17) Harkins, W. D. In The Physical Chemistry of Surface Films; Reinhold: New York, 1952; p 107. (18) Reis, T. In Introduction a´ la Chimie-Physique des Surfaces; Dunod: Paris, 1952; p 111. (19) Leite, V. B. P.; Cavalli, A.; Oliveira, O. N., Jr. Phys. ReV. E 1998, 57, 6835. (20) Schwartz, D. K.; Knobler, C. M. J. Phys. Chem. 1993, 97, 8849. (21) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591. (22) Kaganer, V. M.; Loginov, E. B. Phys. ReV. E 1995, 51, 2237. (23) Zangi, R.; Rice, S. A. Langmuir 2003, 19, 5949. (24) Lawrie, G. A.; Barnes, G. T. J. Colloid Interface Sci. 1994, 162, 36. (25) Lo¨sche, M.; Kru¨ger, P. In Morphology of Condensed Matter; Mecke, K. R., Stoyan, D., Eds.; Springer-Verlag: Berlin, 2002. (26) Viney, C.; Russell, T. P.; Depero, L. E.; Twieg, R. J. Mol. Cryst. Liq. Cryst. 1989, 168, 63. (27) Marconcelli, M.; Qi, S. P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6237. (28) Davies, J. T.; Rideal, E. Can. J. Chem. 1955, 33, 947. (29) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 221. (30) HyperChem Professional Release 5.1, A Molecular Visualization and Simulation Software Package; Hypercube Inc.: Gainesville, FL, 1998. (31) Taylor, D. M.; Bayes, G. F. Phys. ReV. E 1994, 49, 1439. (32) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. J. Colloid Interface Sci. 1990, 139, 508. (33) Vogel, V.; Mo¨bius, D. J. Colloid Interface Sci. 1988, 126, 408. (34) Minkin, V. I.; Osipov, O. A.; Zhdanov, Y. A. In Dipole moments in organic chemistry; Chimia: Leningrad, 1968; p 91. (35) Oliveira, O. N., Jr.; Riul, A., Jr.; Leite, V. B.; Braz, P. J. Phys. 2004, 34, 73. (36) Rodrı´guez Patino, J. M.; Carrera Sanchez, C.; Rodrı´guez Nin˜o, M. R. Langmuir 1999, 15, 2484. (37) Bunn, C. W.; Howells, E. R. Nature 1954, 174, 549.

Langmuir Monolayers Characteristic of - American Chemical Society

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