Article pubs.acs.org/Langmuir
Evidence for Interaction with the Water Subphase As the Origin and Stabilization of Nano-Domain in Semi-Fluorinated Alkanes Monolayer at the Air/Water Interface Philippe Fontaine,*,† Marie-Claude Fauré,‡,§ Lisa Bardin,†,‡ Eduardo J. M. Filipe,∥ and Michel Goldmann†,‡,§ †
Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette, France Institut des NanoSciences de Paris (INSP), UMR 7588 CNRS, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France § Faculté des Sciences Fondamentales et Biomédicales, Université Paris Descartes, 45 rue des Saints Pères, 75006 Paris, France ∥ Centro de Química Estrutural, Instituto Superior Técnico, 1049-001 Lisboa, Portugal ‡
ABSTRACT: Multilayer films of semifluorinated alkanes (SFAs) at the air/water interface were studied in situ by grazing incidence small-angle X-ray scattering (GISAXS). The results provide evidence that the first layer in contact with the water subphase, buried below the overlayers, exhibits the same supramolecular hexagonal structure that is observed in the monolayer before the collapse, at non-zero surface pressure. We believe this result clearly demonstrates the major role of the interactions between the first layer of SFAs and the water subphase to the formation of the structure.
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INTRODUCTION Formation of Langmuir monolayers of completely hydrophobic molecules is a surprising property exhibited by semifluorinated alkanes (SFAs, CnF2n+1CmH2m+1, FnHm for short). Such molecules associate the hydrophobic character of hydrocarbon chains to the lipophobic and hydrophobic characters of fluorinated chains in diblocks that self-assemble in various structures either in bulk,1−4 in solution,5−8,2 or at the air/water interface.9−16 Following the pioneering work of Gaines,9 many studies have been carried out with monolayers of pure SFAs on the free surface of water (Langmuir films).11,13,17−21 Performing grazing incidence small-angle X-ray scattering (GISAXS) experiments on FnHm films at the water surface, we observed a hexagonal lattice of large parameter evidencing the presence of an array of perfectly organized and monodispersed domains, having a few tens of nanometers wide.13,17 The driving force of this self-assembling process seems specific to the FnHm molecules, because it is not observed for other similar short-chain organic amphiphiles. Indeed, n-alkanes (hydrogenated) do not spread as a monolayer on the surface of water but form droplets. Perfluoroalkanes,22 perfluorinated fatty acids,23 and hydrogenated fatty acid24 form homogeneous monolayers, without any evidence of large monodispersive domain formation. SFAs, therefore, seem to be an exception. Semenov et al.25 proposed the interaction between the strong dipoles appearing at the junction between the fluorinated and hydrogenated part as the driving force of such self-assembly. In © 2014 American Chemical Society
this description, the interaction is only in plane and should not depend upon the nature of the upper phase or subphase. In the literature, FnHm layers have been studied at the surface of substrates other than the aqueous substrate and various domain shapes have been observed. However, using the spincoating technique, we were only able to recover this crystalline organization at the surface of a water prewetted silicon substrate.26 Also, FnHm Gibbs films on 4-n-octyl-4-cyanobiphenyl (8CB) and 4-dodecyl-4′-cyanobiphenyl (12CB) layers exhibit similar behavior.27 Conversely, no evidence of nanostructuration was found at the surface of bulk FnHm28,29 or on a dry silicon surface.26 These experimental observations raise the question of the influence of the substrate nature on the equilibrium structure of the FnHm monolayers. To discriminate between molecule−molecule interactions and molecule−substrate (either liquid or solid) interactions, one may look at a multilayer situation obtained when the collapse of the monolayer is completed at the air/water interface. Indeed, in the case of multilayer formation, the upper layers are not interacting directly with the substrate anymore. Then, if interactions with the substrate are relevant, this should induce a variation of the in-plane structure of the layers. Received: September 30, 2014 Revised: November 24, 2014 Published: November 25, 2014 15193
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was slightly deflected downward by a double mirror setup to an incidence angle with the water surface of 2 mrad, below the critical angle of total reflection on the air/water interface (2.8 mrad at 8 keV). The incident beam sizes, fixed by optical slits, were 100 μm vertical and 300 μm horizontal. This glancing incidence method is a standard technique to reduce the bulk scattering and to reveal the surface signal.38,39 The scattered signal was recorded using a one-dimensional (1D), gas-filled (Ar−CO2) position sensitive detector (PSD), the counting wires of which are oriented vertically. In this configuration, the spectra are obtained by scanning the in-plane 2θ angle. At each point, the vertical scattered intensity is recorded. To obtain the integrated spectra, the vertical distribution of the scattered intensity is integrated along Qz between 0 and 8 nm−1. For the GISAXS signal (below 1° in 2θ), a high-resolution setup is mandatory. This is achieved using a small size and low divergence incident beam (provided by the undulator source and optics of the ID10B beamline) and a collimator with two vertical slits (300 and 500 μm horizontal gaps separated by 650 mm) for the scattered intensity. Such a setup leads to an in-plane angular resolution of about 1 mrad.
Studies of FnHm in multilayers mainly focused on mixtures of FnHm with other amphiphilic molecules, such as phospholipids,30 polymers,31 peptides,32 fatty alcohols, and fatty acids.33 They mainly show that, upon compression, FnHm molecules are vertically segregated on the top of the other compounds, i.e., the upper layer in contact with air being a pure FnHm layer but with no information about the structure at the nanometer length scale. Although some papers show surface pressure versus molecular area (π−A) isotherms beyond the collapse of pure FnHm monolayers,34−36 only two papers focused on this phenomenon. Broniatovsky et al. studied the relaxation of collapsed FnHm Langmuir monolayers by surface pressure time evolution measurement and Brewster angle microscopy (BAM). They were interested in the two-dimensional (2D)− three-dimensional (3D) transition itself, but again, no information is deduced concerning the sub-micrometric structure of the layer after collapse.36 de Gracia Lux et al. studied the collapse of FnHm layers at the air/water interface by π−A isotherms, BAM, and atomic force microscopy (AFM) after transfer on a silicon substrate. π−A isotherms and BAM experiments reveal a reversible first-order transition between the monolayer and 3D states. AFM images of the collapsed film show thick domains surrounded by regions of monomolecular thickness.15 These regions of remaining monolayers still exhibit domains, similar to those observed on the monolayer transferred at non-zero surface pressure before collapse, although they appear dispersed and non-organized. The bulk phase resulting from the collapse is proposed to be a continuous bilayer without in-plane structuration. However, the authors assumed that this bulk phase stands over a monolayer composed of domains described as hemimicelles, but no observation justifies such assumption, because the buried monolayer cannot be observed by AFM. In this work, we studied the microscopic structure of the F8H18 multilayers at the air−water interface, during and especially after completion of the collapse transition. We used in situ GISAXS to probe the possible existence of the nanostructures within the buried monolayer in contact with water.
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RESULTS π−A Isotherms. Figure 1 presents the π−A isotherm of a F8H18 monolayer at 20 °C. At a large area per molecule, above
MATERIALS AND METHODS
Figure 1. Surface pressure versus area per molecule (π−A) isotherm of a F8H18 monolayer at the air/water interface at T = 20 °C. Arrows indicate the state where the GISAXS spectra of Figure 2 were measured.
Molecules and Sample Preparation. F8H18 was synthesized according to ref 37 and thoroughly purified by column chromatography and recrystallization. The final purity was assessed above 99% by thin-layer chromatography, nuclear magnetic resonance (NMR), and elemental analysis. The semifluorinated molecules were dissolved in chloroform to form spreading solutions of a typical concentration of 1 mmol L−1. F8H18 monolayers were spread on a Langmuir trough equipped with a movable barrier for film compression. The accuracy of the area measurement is estimated to 0.1 cm2. The compression speed is 20 cm2 min−1. The surface pressure was measured according to the Wilhelmy plate method with an accuracy of 0.1 mN m−1. The plate was made of filter paper, and the measurement device was a microbalance from Riegler & Kirstein GmbH (Germany). Milli-Q Millipore ultrapure water (resistivity of 18.2 MΩ cm) was used for the subphase. The temperature was regulated at 20 ± 0.5 °C using a circulating water bath. Surface X-ray Scattering (GISAXS). The GISAXS measurements were performed on the ID10B beamline of the ESRF synchrotron source (Grenoble, France) using a dedicated Langmuir trough. It was enclosed within a gastight box with Kapton windows and flushed with water-saturated helium gas. Although the film remains stable, a feedback maintains the surface pressure constant during the scans. The energy of the incoming X-ray beam, 8 keV (0.154 nm), was selected using a diamond double-crystal monochromator. The incident beam
0.28 nm2, the compression is characterized first by a near zero surface pressure plateau, followed by a sharp increase of the surface pressure at about 0.33 nm2/molecule and finishing by the collapse of the monolayer at the area per molecule of 0.29 nm2 and surface pressure of 11 mN m−1. This is the usual behavior for this class of compounds.10−12,14−17 After a small decrease (probably corresponding to an activation energy), the surface pressure remains almost constant over a wide range of area per molecule (0.27−0.11 nm2). After this second plateau, the surface pressure rises to 16.5 mN m−1 at 0.09 nm2/ molecule, followed by a slope increase leading to a value of 31 mN m−1 at 0.05 nm2/molecule. This second plateau can be interpreted as corresponding to a first-order transition from the monolayer state to a 3D phase. Indeed, the collapse area at 0.29 nm2/molecule is very similar to the limit area observed on a monolayer of vertical fluorinated chains, 0.27 nm2/molecule.40 The end of the plateau is at about 0.10 nm2/molecule, roughly one-third of the initial area, suggesting that the film is mainly 15194
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19.8 mN m−1 (0.15 and 0.1 nm2/molecule, respectively) exhibit the same seven diffraction peaks, leading to parameters of 38.5 and 37.2 nm, respectively, for the hexagonal network. Again, such a structure is attributed to the presence of monodispersed nanodomains in the layer, which remains organized on a hexagonal lattice. At these surface areas, thermodynamic measurements (π−A isotherms) indicate that the transition is completed and that no monolayer regions are likely to remain at the water surface. It can be noted that the peak intensities are reduced by a factor of about 2 with respect to the single monolayer phase at 3 mN m−1 (Table 1 and bottom panel of Figure 2). Because the surface is considered as homogeneous, such an intensity decrease indicates that the hexagonal structure is less accessible to X-rays after the collapse. In a first approximation, the intensity at depth z starting from the upper FnHm−air interface varies as I(z) = I0e−z/ξ, where ξ is the penetration depth of the X-ray. At the working energy and incidence, the penetration depth is about 4.5 nm. Thus, on can estimate the intensity ratio between a free monolayer and one buried below two similar monolayers by the relation
formed by a trilayer of upright F8H18 molecules, in agreement with ref 15. The constant surface pressure and the reversibility of the isotherms mainly validate the first-order nature of this transition. The same conclusion was given in ref 15 supported by AFM images. Surface X-ray Scattering. The top panel of Figure 2 shows GISAXS spectra measured on a F8H18 monolayer at the air/
3e
R=
∫2e e−z / ξ dz e −z / ξ
∫0 e
dz
=
e − 2e / ξ − e − 3e / ξ 1 − e −e / ξ
(1)
where e is the monolayer thickness (which is assumed identical for the three monolayers). One obtains R ≈ 0.33 for e = 2.5 nm and R ≈ 0.415 for e = 2 nm. This rough calculation (absorption has been neglected) is in good agreement with the values from Table 1, with the ratio between 3 mN/m (single monolayer) and 19.8 mN/m (full collapse) ranging from 0.46 to 0.15, depending upon the peak [intensities of (21), (31), and (33) are too weak to be considered]. From these experimental results, one concludes that the hexagonal nanostructure observed for the monolayer not only remains along the transition but also still persists after the transition is completed. The lower intensities of the peaks after the plateau indicate that, after the 2D/3D transition, the organized structure of domains is covered by a material that does not contribute to the diffraction. The hexagonal lattice is then buried below this material. Figure 3 shows the out of the plane intensity distribution before (Figure 3a) and after (Figure 3b) completion of the monolayer collapse. For comparison purposes between the two situations, the rod scan measured at the in-plane peak position (hk) were fitted to extract parameters using a first rough approach by the scattering form factor of a disk of thickness t.
Figure 2. (Top) GISAXS spectra measured on a F8H18 monolayer at different area per molecule along the compression isotherm of Figure 1. The curves were vertically shifted for clarity. (Bottom) Peak intensity versus peak positions along the π−A isotherm.
water interface at different surface pressures. At 3 mN m−1, before the collapse, the spectrum exhibits seven diffraction peaks. Each peak has been fitted using a Gaussian function according to an analysis procedure given in ref 17. The results are gathered in Table 1. The peaks can be indexed using a hexagonal network, in which the lattice parameter was adjusted using the ERACEL software41 at 37.5 nm. Such a structure is in agreement with the formation of monodisperse nanodomains at the surface of water, which organize on this crystalline hexagonal network. After the collapse, the GISAXS spectra do not change much. The seven diffraction peaks are still observed at 11.8 mN m−1, which corresponds on the isotherm to 0.23 nm2/molecule. The deduced lattice parameter at this surface pressure is 38.5 nm2. This result demonstrates the presence of nanodomains that remain organized and monodispersed on the liquid surface during the 2D/3D transition, although they appear disorganized on the AFM images of ref 15. However, because the film is in a coexistence region, no information about the first monolayer of the bulk phase and interaction with the water subphase can be deduced, but this result also demonstrates the first-order character of the transition. After the plateau, the spectra measured at 13.8 and
⎛ (qz − qzmax )t ⎜ sin 2 I(qz) = I0⎜ (q − q max ⎜ z z )t ⎝ 2
⎞2 ⎟ ⎟⎟ ⎠
(2)
Results before and after collapse are gathered in Table 2. As seen, regardless of the state of the film, the parameters are comparable and no evolution is noticeable. In particular, the thickness of the organized structure, i.e., the mean values from the thicknesses deduced from the width of the out-of-plane scattered intensity, ranges between 2.33 nm before collapse to 2.47 nm after collapse. This indicates that the thickness of the organized structure contributing to the scattered signal is the same before and after collapse. Especially, the order of magnitude of these two values is not compatible with a multilayer diffracting structure but in agreement with a 15195
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Table 1. Parameters [Position, Indexation, Full Width at Half Maximum (fwhm), and Integrated Intensity] of the Diffraction Peaks of the GISAXS Spectra of Figure 2 π (mN m−1)
a (nm)
hk
max Q
(nm−1)
fwhm (nm−1)
3
37.53
0.183 0.321 0.369 0.499 0.659
± ± ± ± ±
0.0001 0.001 0.001 0.001 0.0003
0.0269 0.0386 0.0357 0.0367 0.659
± ± ± ± ±
0.00025 0.002 0.002 0.0025 0.0004
11.8
38.53
13.8
38.53
19.8
37.16
10 11 20 21 22 31 33 10 11 20 21 22 31 33 10 11 20 21 22 31 33 10 11 20 21 22 31 33
0.981 0.186 0.328 0.373 0.487 0.663 0.734 0.996 0.193 0.339 0.381 0.491 0.673 0.72 0.944 0.182 0.324 0.371 0.496 0.671 0.750 0.98
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.002 0.0002 0.0003 0.0004 0.0005 0.002 0.007 0.001 0.0001 0.001 0.002 0.001 0.003 0.06 0.002 0.0001 0.0008 0.001 0.001 0.003 0.015 0.01
0.074 0.0231 0.0301 0.0306 0.03544 0.061 0.064 0.086 0.028 0.033 0.037 0.024 0.064 0.09 0.101 0.0291 0.046 0.041 0.042 0.071 0.054 0.04
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.005 0.0004 0.0005 0.0007 0.0013 0.001 0.007 0.003 0.0002 0.001 0.004 0.002 0.010 0.05 0.005 0.0004 0.001 0.001 0.003 0.006 0.02 0.02
peak intensity (×10−7) 5.79 4.17 1.95 0.075 0.337
± ± ± ± ±
0.06 0.06 0.09 0.007 0.006
0.022 ± 0.002 5.13 ± 0.09 2.29 ± 0.034 1.14 ± 0.03 0.146 ± 0.006 0.181 ± 0.009 0.044 ± 9.2 × 10−3 0.026 ± 1.1 × 10−3 2.68 ± 0.025 1.82 ± 0.13 0.94 ± 0.13 0.037 ± 8.85 × 10−3 0.096 ± 0.001 0.080 ± 0.001 0.012 ± 9.5 × 10−4 3.72 ± 0.03 1.45 ± 0.05 0.67 ± 0.03 0.065 ± 0.007 0.051 ± 0.005 6.76 × 10−3 ± 4.3 × 10−3 1.14 × 10−3 ± 6.5 × 10−4
interacting with the water surface, although the second monolayer interacts with the structured first monolayer. A first simple explanation to justify the presence of domains in FnHm monolayers could be considering these molecules as intermediates between perfluoroalkanes (that form homogeneous monolayers)22 and alkanes, which are known to form drops coexisting with a monolayer on the water surface. Using a different type of meaning, Semenov et al. proposed a detailed theoretical model, which justifies the domain formation by considering a repulsive interaction between dipoles.25 In this model, under compression, molecules self-assemble vertically with the same orientation and the formed domains, which present a strong dipole, resulting from the sum of all molecular dipoles at the fluorinated and hydrogenated junctions (CH2− CF2). The number of molecules per domain and, consequently, their size are then limited to minimize the repulsive interactions between the domains. The formation of the domains and their monodispersity appear then as independent from the interaction of the molecule with the substrate/subphase, although this model considers a transition from a parallel to a vertical orientation of the molecules (when the monolayer is compressed at a large area). However, we have previously observed that the dimension of the domains changes with the molecular structure and mainly with the length of the hydrogenated block.17 Moreover, we observed that the hexagonal structure remains stable under compression, with a compressibility coefficient of about 30 m/N. This indicates that the stability in size of the domains prevents their possible coalescence, even when the monolayer is compressed up to a
monolayer. Thus, if before collapse the diffracting structure is obviously a monolayer, it remains a similar monolayer after collapse. The GISAXS results show that it is reasonable to assume that during and even after completion of collapse, there is only one organized monolayer, buried below and in contact with water. On top of it, F8H18 multilayers grow as the 2D/3D transition progresses, scattering and absorbing the X-ray signal coming from the lower organized layer. We underline that the upper layers do not present the large hexagonal structure.
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DISCUSSION SFAs have already revealed surprising properties when spread as the monolayer on the water surface with the crystalline structuration in nanodomains. The collapse, i.e., the transition of the monolayer to the third dimension upon 2D compression, also reveals interesting features. The collapse proceeds via what could be considered a first-order transition between the 2D state and the 3D state. The change of area per molecule during the transition is compatible with the formation of a multilayer, probably a trilayer of upright molecules.15 However, our experimental results demonstrate that the structure of these upper layers strongly differs from the one of the first monolayer, which always remains crystalline without coalescence of the nanodomains. We note that this is the case even when it is buried below continuous domains of F nH m molecules. The structure of this monolayer remains unchanged (nanostructured) when it is in contact with air or covered with other F8H18 molecules. On the contrary, this organization is not maintained in the upper layers, which are not directly 15196
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Figure 3. (a) Out-of-plane scattered intensity distribution (Qxy−Qz map, top) and rod scan [Ihk(Qz), bottom] at the peak positions of a F8H18 monolayer before collapse at π = 3 mN m−1. (b) Out-of-plane scattered intensity distribution (Qxy−Qz map, top) and rod scan [Ihk(Qz), bottom] at the peak positions of a F8H18 monolayer after collapse at π = 19.8 mN m−1.
Table 2. Out-of-Plane Peak Position (qmax z ) and Layer Thickness (t) Obtained by Fitting the Rod Scans of Figure 3 by the Form Factor of a Disk before and after Collapse hk 10 11 20 21 22 31 10 11 20 21 22 31
qmax (nm−1) z before Collapse (π = 3 mN m−1) 0.71 ± 0.04 0.87 ± 0.06 1.19 ± 0.04 1.49 ± 0.04 1.89 ± 0.04 2.23 ± 0.03 after Collapse (π = 19.8 mN m−1) 0.64 ± 0.05 0.79 ± 0.06 0.91 ± 0.05 1.61 ± 0.03 1.89 ± 0.04 2.08 ± 0.04
interactions, as proposed by Semenov et al., and/or from the interaction with the underlying substrate. To discriminate between these possibilities, we should analyze the conditions known to be necessary for obtaining the FnHm monolayer nanostructuration. Organized nanodomains are obtained at the surface of water,17 fully hydrated silicon wafers,26 and thermotropic liquid crystals (8CB and 12CB).27 Conversely, no evidence of nanostructuration was found at the surface of bulk FnHm28,29 or on a dry silicon surface.26 Moreover, our results as well as results from other groups15 indicate that the upper layers of FnHm molecules are not structured after the collapse. This indicates that single inplane interactions (as interactions between dipoles) are not sufficient to induce self-assembly and nanostructuration. The presence of a substrate surface bearing strong dipoles seems mandatory for the onset of the phenomena. We consider that, regardless of the 2D pressure, only a part of the FnHm molecules in the monolayer interacting with the polar substrate can switch to the vertical configuration. Indeed, we suggest that this limitation results from the high energetic cost of preventing the dipoles carried by FnHm molecules (located at about the center of the linear molecule) from interacting with the polar substrate when these molecules go from a horizontal configuration to a vertical configuration. This leads to a minimum surface concentration of lying molecules that, under compression, will rearrange with the vertical molecules to form the observed nanostructure. These lying molecules (we suppose surrounding the domains formed by
t (nm) 3.04 2.60 2.44 2.09 2.00 1.80
± ± ± ± ± ±
0.16 0.15 0.09 0.08 0.06 0.04
3.08 2.71 2.48 2.46 2.13 1.98
± ± ± ± ± ±
0.20 0.17 0.13 0.06 0.08 0.05
high 2D pressure. FnHm monolayers collapse at a rather low surface pressure, typically 10−20 mN/m.15 The fact that the bottom layer remains nanostructured independent of its upper surface being in contact with air or with other SFA molecules indicates that the interaction of this layer with the upper medium is not the driving force for the self-organization of the layer. It could thus result from in-plane 15197
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(8) Marzuk, P.; Lang, P.; Möller, M. Bulk structure and surface activity of semifluorinated alkanes. Colloids Surf., A 2000, 163, 103− 113. (9) Gaines, G. L. Surface activity of semifluorinated alkanes: F(CF2)m(CH2)nH. Langmuir 1991, 7, 3054. (10) Krafft, M. P.; Giulieri, F. Stabilizing colloids with fluorocarbon− hydrocarbon diblocks: Examples of vesicles made from single-chain fluorinated surfactants. In Fluorinated Surfaces, Coatings, and Films; Castner, D. G., Grainger, D. W., Eds.; American Chemical Society (ACS): Washington, D.C., 2001; ACS Symposium Series, Vol. 787, Chapter 4, pp 48−56. (11) Krafft, M. P.; Goldmann, M. Monolayers made from fluorinated amphiphiles. Curr. Opin. Colloid Interface Sci. 2003, 8, 243−250. (12) Zhang, G.; Marie, P.; Maaloum, M.; Muller, P.; Benoit, N.; Krafft, M. P. Occurrence shape, and dimensions of large surface hemimicelles made of semifluorinated alkanes. Elongated versus circular hemimicelles. Pit- and tip-centered hemimicelles. J. Am. Chem. Soc. 2005, 127, 10412−10419. (13) Fontaine, P.; Goldmann, M.; Muller, P.; Fauré, M.-C.; Konovalov, O.; Krafft, M.-P. Direct evidence for highly organized networks of circular surface micelles of surfactant at the air−water interface. J. Am. Chem. Soc. 2005, 127, 512−513. (14) Broniatowski, M.; Dynarowicz-Latka, P. Direct evidence for highly organized networks of circular surface micelles of surfactant at the air−water interface. Adv. Colloid Interface Sci. 2008, 138, 63−83. (15) de Gracia Lux, C.; Gallani, J. L.; Waton, G.; Krafft, M. P. Compression of self-assembled nano-objects: 2D/3D transitions in films of (perfluoroalkyl)alkanesPersistence of an organized array of surface micelles. Chem.Eur. J. 2010, 16, 7186−7198. (16) Nakahara, H.; Krafft, M. P.; Shibata, A.; Shibata, O. Interaction of a partially fluorinated alcohol (F8H11OH) with biomembrane constituents in two-component monolayers. Soft Matter 2011, 7, 7325−7333. (17) Bardin, L.; Fauré, M.-C.; Limagne, D.; Chevallard, C.; Konovalov, O.; Filipe, E. J. M.; Waton, G.; Krafft, M.-P.; Goldmann, M.; Fontaine, P. Long-range nanometer-scale organization of semifluorinated alkane monolayers at the air/water interface. Langmuir 2011, 27, 13497−13505. (18) Kato, T.; Kameyama, M.; Ehara, M.; Iimura, K. I. Monodisperse two-dimensional nanometer size clusters of partially fluorinated longchain acids. Langmuir 1998, 14, 1786−1798. (19) Kmetko, J.; Datta, A.; Evmenenko, G.; Dutta, P. The effects of divalent ions on langmuir monolayer and subphase structure: A grazing-incidence diffraction and Bragg rod study. J. Phys. Chem. B 2001, 105, 10818−10825. (20) Maaloum, M.; Muller, P.; Krafft, M. P. Monodisperse surface micelles of nonpolar amphiphiles in langmuir monolayers. Angew. Chem., Int. Ed. 2002, 41, 4331−4334. (21) Mourran, A.; Tartsch, B.; Gallyamov, M.; Magonox, S.; Lambreva, D.; Ostrovskii, B. I.; Dolbnya, I. P.; De Jeu, W. H.; Moeller, M. Self-assembly of the perfluoroalkyl-alkane F14H20 in ultrathin films. Langmuir 2005, 21, 2308−2316. (22) Li, M.; Acero, A. A.; Huang, Z.; Rice, S. A. Formation of an ordered Langmuir monolayer by a non-polar chain molecule. Nature 1994, 367, 151−153. (23) Goldmann, M.; Nassoy, P.; Rondelez, F.; Renault, A.; Shin, S.; Rice, A. J. In-plane X-ray diffraction from monolayers of perfluorinated fatty acids: Evidence for azimuthal ordering in the condensed phase. Phys. II 1994, 4, 773−785. (24) Kaganer, V. M.; Möhwald, H.; Dutta, P. Structure and phase transitions in Langmuir monolayers. Rev. Mod. Phys. 1999, 71, 779− 819. (25) Semenov, A. N.; Gonzalez-Perez, A.; Krafft, M. P.; Legrand, J. F. Theory of surface micelles of semifluorinated alkanes. Langmuir 2006, 22, 8703−8717. (26) Bardin, L.; Fauré, M.-C.; Filipe, E. J. M.; Fontaine, P.; Goldmann, M. Highly organized crystalline monolayer of a semifluorinated alkane on a solid substrate obtained by spin-coating. Thin Solid Films 2010, 519, 414−416.
self-assembly of the vertical molecules) prevent the coalescence between domains, leading to this surprising effect that the monolayer collapses before forming a homogeneous layer at the molecular scale. In a way, such cooperative association between the lying molecules (interacting strongly with water) and the vertical molecules replaces, on the average of the monolayer, the hydrophilic moieties of “classical” amphiphiles that interact strongly with water and contribute to stabilize Langmuir films.
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CONCLUSION Fully collapsed Langmuir films of SFAs at the air/water interface were studied in situ by GISAXS. The collapse occurs through a first-order phase transition between a pure monolayer state and a 3D state, most likely a trilayer. The results show that the layer interacting with the water subphase remains organized on a hexagonal lattice even after the complete collapse of the film. These domains never coalesce, contrary to the upper layers (bilayer), which do not exhibit similar organization and form a homogeneous film. These results combined with others from the literature clearly demonstrate the importance of the interactions between SFA molecules and the water subphase to the formation of such a robust structure. The final structure of the film results from a subtle balance between horizontally oriented SFA molecules, interacting with water through their dipoles, and a homogeneous film configuration, in which SFA molecules, all in a vertical orientation, would be prevented from interacting with water, thus being in a less favorable energetic state.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Oleg Konovalov from the ID10B beamline (European Synchrotron Radiation Facilities) for help and assistance during the experiment.
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REFERENCES
(1) Krafft, M. P.; Giulieri, F.; Riess, J. G. Can single-chain perfluoroalkylated amphiphiles alone form vesicles and other organized supramolecular systems? Angew. Chem., Int. Ed. Engl. 1993, 32, 741−743. (2) Krafft, M. P.; Riess, J. G. Chemistry, physical chemistry, and uses of molecular fluorocarbon−hydrocarbon diblocks, triblocks, and related compoundsUnique “apolar” components for self-assembled colloid and interface engineering. Chem. Rev. 2009, 109, 1714−1792. (3) Lo Nostro, P. Aggregates from semifluorinated n-alkanes: How incompatibility determines self-assembly. Curr. Opin. Colloid Interface Sci. 2003, 8, 223−226. (4) Riess, J. G. Aggregates from semifluorinated n-alkanes: How incompatibility determines self-assembly. Tetrahedron 2002, 58, 4113− 4131. (5) Turberg, M. P.; Brady, J. E. Semifluorinated hydrocarbons: Primitive surfactant molecules. J. Am. Chem. Soc. 1988, 110, 7797− 7801. (6) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. Semifluorinated alkanes as primitive surfactants in apolar hydrocarbon and fluorocarbon solvents. J. Mol. Liq. 1997, 72, 177−190. (7) Marzuk, P.; Lang, P. A structural X-ray study on semifluorinated alkanes (SFA): SFA revisited. Macromolecules 1998, 31, 9013−9018. 15198
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(27) Feng, X.; Mourran, A.; Möller, M.; Bahr, C. AFM study of gibbs films of semifluorinated alkanes at liquid crystal/air interfaces. ChemPhysChem 2013, 14, 1801. (28) Gang, O.; Ellmann, J.; Möller, M.; Kraack, H.; Sirota, E. B.; Ocko, B. M.; Deutsch, M. Surface phases of semi-fluorinated alkane melts. Europhys. Lett. 2000, 49, 761−767. (29) Ocko, B. M. Private communication. (30) Krafft, M.-P.; Giulieri, F.; Fontaine, P.; Goldmann, M. Reversible stepwise formation of mono- and bilayers of a fluorocarbon/hydrocarbon diblock on top of a phospholipid langmuir monolayer. A case of vertical phase separation. Langmuir 2001, 17, 6577−6584. (31) Simoes Gamboa, A.-L.; Filipe, E. J. M.; Brogueira, P. Nanoscale pattern formation in Langmuir−Blodgett films of a semifluorinated alkane and a polystyrene−poly(ethylene oxide) diblock copolymer. Nano Lett. 2002, 2, 1083−1086. (32) El Abed, A.; Ionov, R.; Goldmann, M. Structural and electric properties of two semifluorinated alkane monolayers compressed on top of a controlled hydrophobic monolayer substrate. Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys. 2007, 76, 041606. (33) Simões Gamboa, A. L. Ordering in Langmuir and Langmuir− Blodgett films. Ph.D. Thesis, Instituto Superior Técnico, Lisbon, Portugal, 2006. (34) El Abed, A.; Pouzet, E.; Fauré, M.-C.; Sanière, M. Air−water interface-induced smectic bilayer. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 62, R5895−R5898. (35) De Viguerie, L.; Keller, R.; Jonas, U.; Berger, R.; Clark, C.G.; Klein, C.O.; Geue, T.; Müllen, K.; Butt, H.-J.; Vlassopoulos, D. Effect of the molecular structure on the hierarchical self-assembly of semifluorinated alkanes at the air/water interface. Langmuir 2011, 27, 8776−8786. (36) Broniatowski, M.; Vila-Romeu, N.; Nieto-Suarez, M.; Dynarowicz-Latka, P. Nucleation and growth in the collapsed Langmuir monolayers from semifluorinated alkanes. J. Phys. Chem. B 2007, 111, 12787−12794. (37) Brace, N. O. Radical addition of iodoperfluoroalkanes to vinyl and allyl monomers. J. Org. Chem. 1962, 27, 3033−3038. (38) Fontaine, P.; Goldmann, M.; Bordessoule, M.; Jucha, A. Fast and adjustable-resolution grazing-incidence X-ray liquid surface diffraction. Rev. Sci. Instrum. 2004, 75, 3097−3106. (39) Schlossmann, M. L.; Pershan, P. S. Liquid Surfaces and Interfaces, Synchrotron X-ray Methods; Cambridge University Press: Cambridge, U.K., 2012. (40) Schwickert, H.; Strobl, G.; Kimmig, M. Molecular dynamics in perfluoro-n-eicosane. I. Solid phase behavior and crystal structures. J. Chem. Phys. 1991, 95, 2800−2805. (41) Laugier, J.; Filhol, A. ERACEL, Program for the Refinement of Cell Parameters; Institut Laue−Langevin (ILL): Grenoble, France, 1978.
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Evidence for Interaction with the Water Subphase As the Origin and Stabilization of Nano-Domain in Semi-Fluorinated Alkanes Monolayer at the Air/Water Interface Philippe Fontaine,*,† Marie-Claude Fauré,‡,§ Lisa Bardin,†,‡ Eduardo J. M. Filipe,∥ and Michel Goldmann†,‡,§ †
Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette, France Institut des NanoSciences de Paris (INSP), UMR 7588 CNRS, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France § Faculté des Sciences Fondamentales et Biomédicales, Université Paris Descartes, 45 rue des Saints Pères, 75006 Paris, France ∥ Centro de Química Estrutural, Instituto Superior Técnico, 1049-001 Lisboa, Portugal ‡
ABSTRACT: Multilayer films of semifluorinated alkanes (SFAs) at the air/water interface were studied in situ by grazing incidence small-angle X-ray scattering (GISAXS). The results provide evidence that the first layer in contact with the water subphase, buried below the overlayers, exhibits the same supramolecular hexagonal structure that is observed in the monolayer before the collapse, at non-zero surface pressure. We believe this result clearly demonstrates the major role of the interactions between the first layer of SFAs and the water subphase to the formation of the structure.
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INTRODUCTION Formation of Langmuir monolayers of completely hydrophobic molecules is a surprising property exhibited by semifluorinated alkanes (SFAs, CnF2n+1CmH2m+1, FnHm for short). Such molecules associate the hydrophobic character of hydrocarbon chains to the lipophobic and hydrophobic characters of fluorinated chains in diblocks that self-assemble in various structures either in bulk,1−4 in solution,5−8,2 or at the air/water interface.9−16 Following the pioneering work of Gaines,9 many studies have been carried out with monolayers of pure SFAs on the free surface of water (Langmuir films).11,13,17−21 Performing grazing incidence small-angle X-ray scattering (GISAXS) experiments on FnHm films at the water surface, we observed a hexagonal lattice of large parameter evidencing the presence of an array of perfectly organized and monodispersed domains, having a few tens of nanometers wide.13,17 The driving force of this self-assembling process seems specific to the FnHm molecules, because it is not observed for other similar short-chain organic amphiphiles. Indeed, n-alkanes (hydrogenated) do not spread as a monolayer on the surface of water but form droplets. Perfluoroalkanes,22 perfluorinated fatty acids,23 and hydrogenated fatty acid24 form homogeneous monolayers, without any evidence of large monodispersive domain formation. SFAs, therefore, seem to be an exception. Semenov et al.25 proposed the interaction between the strong dipoles appearing at the junction between the fluorinated and hydrogenated part as the driving force of such self-assembly. In © 2014 American Chemical Society
this description, the interaction is only in plane and should not depend upon the nature of the upper phase or subphase. In the literature, FnHm layers have been studied at the surface of substrates other than the aqueous substrate and various domain shapes have been observed. However, using the spincoating technique, we were only able to recover this crystalline organization at the surface of a water prewetted silicon substrate.26 Also, FnHm Gibbs films on 4-n-octyl-4-cyanobiphenyl (8CB) and 4-dodecyl-4′-cyanobiphenyl (12CB) layers exhibit similar behavior.27 Conversely, no evidence of nanostructuration was found at the surface of bulk FnHm28,29 or on a dry silicon surface.26 These experimental observations raise the question of the influence of the substrate nature on the equilibrium structure of the FnHm monolayers. To discriminate between molecule−molecule interactions and molecule−substrate (either liquid or solid) interactions, one may look at a multilayer situation obtained when the collapse of the monolayer is completed at the air/water interface. Indeed, in the case of multilayer formation, the upper layers are not interacting directly with the substrate anymore. Then, if interactions with the substrate are relevant, this should induce a variation of the in-plane structure of the layers. Received: September 30, 2014 Revised: November 24, 2014 Published: November 25, 2014 15193
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was slightly deflected downward by a double mirror setup to an incidence angle with the water surface of 2 mrad, below the critical angle of total reflection on the air/water interface (2.8 mrad at 8 keV). The incident beam sizes, fixed by optical slits, were 100 μm vertical and 300 μm horizontal. This glancing incidence method is a standard technique to reduce the bulk scattering and to reveal the surface signal.38,39 The scattered signal was recorded using a one-dimensional (1D), gas-filled (Ar−CO2) position sensitive detector (PSD), the counting wires of which are oriented vertically. In this configuration, the spectra are obtained by scanning the in-plane 2θ angle. At each point, the vertical scattered intensity is recorded. To obtain the integrated spectra, the vertical distribution of the scattered intensity is integrated along Qz between 0 and 8 nm−1. For the GISAXS signal (below 1° in 2θ), a high-resolution setup is mandatory. This is achieved using a small size and low divergence incident beam (provided by the undulator source and optics of the ID10B beamline) and a collimator with two vertical slits (300 and 500 μm horizontal gaps separated by 650 mm) for the scattered intensity. Such a setup leads to an in-plane angular resolution of about 1 mrad.
Studies of FnHm in multilayers mainly focused on mixtures of FnHm with other amphiphilic molecules, such as phospholipids,30 polymers,31 peptides,32 fatty alcohols, and fatty acids.33 They mainly show that, upon compression, FnHm molecules are vertically segregated on the top of the other compounds, i.e., the upper layer in contact with air being a pure FnHm layer but with no information about the structure at the nanometer length scale. Although some papers show surface pressure versus molecular area (π−A) isotherms beyond the collapse of pure FnHm monolayers,34−36 only two papers focused on this phenomenon. Broniatovsky et al. studied the relaxation of collapsed FnHm Langmuir monolayers by surface pressure time evolution measurement and Brewster angle microscopy (BAM). They were interested in the two-dimensional (2D)− three-dimensional (3D) transition itself, but again, no information is deduced concerning the sub-micrometric structure of the layer after collapse.36 de Gracia Lux et al. studied the collapse of FnHm layers at the air/water interface by π−A isotherms, BAM, and atomic force microscopy (AFM) after transfer on a silicon substrate. π−A isotherms and BAM experiments reveal a reversible first-order transition between the monolayer and 3D states. AFM images of the collapsed film show thick domains surrounded by regions of monomolecular thickness.15 These regions of remaining monolayers still exhibit domains, similar to those observed on the monolayer transferred at non-zero surface pressure before collapse, although they appear dispersed and non-organized. The bulk phase resulting from the collapse is proposed to be a continuous bilayer without in-plane structuration. However, the authors assumed that this bulk phase stands over a monolayer composed of domains described as hemimicelles, but no observation justifies such assumption, because the buried monolayer cannot be observed by AFM. In this work, we studied the microscopic structure of the F8H18 multilayers at the air−water interface, during and especially after completion of the collapse transition. We used in situ GISAXS to probe the possible existence of the nanostructures within the buried monolayer in contact with water.
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■
RESULTS π−A Isotherms. Figure 1 presents the π−A isotherm of a F8H18 monolayer at 20 °C. At a large area per molecule, above
MATERIALS AND METHODS
Figure 1. Surface pressure versus area per molecule (π−A) isotherm of a F8H18 monolayer at the air/water interface at T = 20 °C. Arrows indicate the state where the GISAXS spectra of Figure 2 were measured.
Molecules and Sample Preparation. F8H18 was synthesized according to ref 37 and thoroughly purified by column chromatography and recrystallization. The final purity was assessed above 99% by thin-layer chromatography, nuclear magnetic resonance (NMR), and elemental analysis. The semifluorinated molecules were dissolved in chloroform to form spreading solutions of a typical concentration of 1 mmol L−1. F8H18 monolayers were spread on a Langmuir trough equipped with a movable barrier for film compression. The accuracy of the area measurement is estimated to 0.1 cm2. The compression speed is 20 cm2 min−1. The surface pressure was measured according to the Wilhelmy plate method with an accuracy of 0.1 mN m−1. The plate was made of filter paper, and the measurement device was a microbalance from Riegler & Kirstein GmbH (Germany). Milli-Q Millipore ultrapure water (resistivity of 18.2 MΩ cm) was used for the subphase. The temperature was regulated at 20 ± 0.5 °C using a circulating water bath. Surface X-ray Scattering (GISAXS). The GISAXS measurements were performed on the ID10B beamline of the ESRF synchrotron source (Grenoble, France) using a dedicated Langmuir trough. It was enclosed within a gastight box with Kapton windows and flushed with water-saturated helium gas. Although the film remains stable, a feedback maintains the surface pressure constant during the scans. The energy of the incoming X-ray beam, 8 keV (0.154 nm), was selected using a diamond double-crystal monochromator. The incident beam
0.28 nm2, the compression is characterized first by a near zero surface pressure plateau, followed by a sharp increase of the surface pressure at about 0.33 nm2/molecule and finishing by the collapse of the monolayer at the area per molecule of 0.29 nm2 and surface pressure of 11 mN m−1. This is the usual behavior for this class of compounds.10−12,14−17 After a small decrease (probably corresponding to an activation energy), the surface pressure remains almost constant over a wide range of area per molecule (0.27−0.11 nm2). After this second plateau, the surface pressure rises to 16.5 mN m−1 at 0.09 nm2/ molecule, followed by a slope increase leading to a value of 31 mN m−1 at 0.05 nm2/molecule. This second plateau can be interpreted as corresponding to a first-order transition from the monolayer state to a 3D phase. Indeed, the collapse area at 0.29 nm2/molecule is very similar to the limit area observed on a monolayer of vertical fluorinated chains, 0.27 nm2/molecule.40 The end of the plateau is at about 0.10 nm2/molecule, roughly one-third of the initial area, suggesting that the film is mainly 15194
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19.8 mN m−1 (0.15 and 0.1 nm2/molecule, respectively) exhibit the same seven diffraction peaks, leading to parameters of 38.5 and 37.2 nm, respectively, for the hexagonal network. Again, such a structure is attributed to the presence of monodispersed nanodomains in the layer, which remains organized on a hexagonal lattice. At these surface areas, thermodynamic measurements (π−A isotherms) indicate that the transition is completed and that no monolayer regions are likely to remain at the water surface. It can be noted that the peak intensities are reduced by a factor of about 2 with respect to the single monolayer phase at 3 mN m−1 (Table 1 and bottom panel of Figure 2). Because the surface is considered as homogeneous, such an intensity decrease indicates that the hexagonal structure is less accessible to X-rays after the collapse. In a first approximation, the intensity at depth z starting from the upper FnHm−air interface varies as I(z) = I0e−z/ξ, where ξ is the penetration depth of the X-ray. At the working energy and incidence, the penetration depth is about 4.5 nm. Thus, on can estimate the intensity ratio between a free monolayer and one buried below two similar monolayers by the relation
formed by a trilayer of upright F8H18 molecules, in agreement with ref 15. The constant surface pressure and the reversibility of the isotherms mainly validate the first-order nature of this transition. The same conclusion was given in ref 15 supported by AFM images. Surface X-ray Scattering. The top panel of Figure 2 shows GISAXS spectra measured on a F8H18 monolayer at the air/
3e
R=
∫2e e−z / ξ dz e −z / ξ
∫0 e
dz
=
e − 2e / ξ − e − 3e / ξ 1 − e −e / ξ
(1)
where e is the monolayer thickness (which is assumed identical for the three monolayers). One obtains R ≈ 0.33 for e = 2.5 nm and R ≈ 0.415 for e = 2 nm. This rough calculation (absorption has been neglected) is in good agreement with the values from Table 1, with the ratio between 3 mN/m (single monolayer) and 19.8 mN/m (full collapse) ranging from 0.46 to 0.15, depending upon the peak [intensities of (21), (31), and (33) are too weak to be considered]. From these experimental results, one concludes that the hexagonal nanostructure observed for the monolayer not only remains along the transition but also still persists after the transition is completed. The lower intensities of the peaks after the plateau indicate that, after the 2D/3D transition, the organized structure of domains is covered by a material that does not contribute to the diffraction. The hexagonal lattice is then buried below this material. Figure 3 shows the out of the plane intensity distribution before (Figure 3a) and after (Figure 3b) completion of the monolayer collapse. For comparison purposes between the two situations, the rod scan measured at the in-plane peak position (hk) were fitted to extract parameters using a first rough approach by the scattering form factor of a disk of thickness t.
Figure 2. (Top) GISAXS spectra measured on a F8H18 monolayer at different area per molecule along the compression isotherm of Figure 1. The curves were vertically shifted for clarity. (Bottom) Peak intensity versus peak positions along the π−A isotherm.
water interface at different surface pressures. At 3 mN m−1, before the collapse, the spectrum exhibits seven diffraction peaks. Each peak has been fitted using a Gaussian function according to an analysis procedure given in ref 17. The results are gathered in Table 1. The peaks can be indexed using a hexagonal network, in which the lattice parameter was adjusted using the ERACEL software41 at 37.5 nm. Such a structure is in agreement with the formation of monodisperse nanodomains at the surface of water, which organize on this crystalline hexagonal network. After the collapse, the GISAXS spectra do not change much. The seven diffraction peaks are still observed at 11.8 mN m−1, which corresponds on the isotherm to 0.23 nm2/molecule. The deduced lattice parameter at this surface pressure is 38.5 nm2. This result demonstrates the presence of nanodomains that remain organized and monodispersed on the liquid surface during the 2D/3D transition, although they appear disorganized on the AFM images of ref 15. However, because the film is in a coexistence region, no information about the first monolayer of the bulk phase and interaction with the water subphase can be deduced, but this result also demonstrates the first-order character of the transition. After the plateau, the spectra measured at 13.8 and
⎛ (qz − qzmax )t ⎜ sin 2 I(qz) = I0⎜ (q − q max ⎜ z z )t ⎝ 2
⎞2 ⎟ ⎟⎟ ⎠
(2)
Results before and after collapse are gathered in Table 2. As seen, regardless of the state of the film, the parameters are comparable and no evolution is noticeable. In particular, the thickness of the organized structure, i.e., the mean values from the thicknesses deduced from the width of the out-of-plane scattered intensity, ranges between 2.33 nm before collapse to 2.47 nm after collapse. This indicates that the thickness of the organized structure contributing to the scattered signal is the same before and after collapse. Especially, the order of magnitude of these two values is not compatible with a multilayer diffracting structure but in agreement with a 15195
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Table 1. Parameters [Position, Indexation, Full Width at Half Maximum (fwhm), and Integrated Intensity] of the Diffraction Peaks of the GISAXS Spectra of Figure 2 π (mN m−1)
a (nm)
hk
max Q
(nm−1)
fwhm (nm−1)
3
37.53
0.183 0.321 0.369 0.499 0.659
± ± ± ± ±
0.0001 0.001 0.001 0.001 0.0003
0.0269 0.0386 0.0357 0.0367 0.659
± ± ± ± ±
0.00025 0.002 0.002 0.0025 0.0004
11.8
38.53
13.8
38.53
19.8
37.16
10 11 20 21 22 31 33 10 11 20 21 22 31 33 10 11 20 21 22 31 33 10 11 20 21 22 31 33
0.981 0.186 0.328 0.373 0.487 0.663 0.734 0.996 0.193 0.339 0.381 0.491 0.673 0.72 0.944 0.182 0.324 0.371 0.496 0.671 0.750 0.98
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.002 0.0002 0.0003 0.0004 0.0005 0.002 0.007 0.001 0.0001 0.001 0.002 0.001 0.003 0.06 0.002 0.0001 0.0008 0.001 0.001 0.003 0.015 0.01
0.074 0.0231 0.0301 0.0306 0.03544 0.061 0.064 0.086 0.028 0.033 0.037 0.024 0.064 0.09 0.101 0.0291 0.046 0.041 0.042 0.071 0.054 0.04
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.005 0.0004 0.0005 0.0007 0.0013 0.001 0.007 0.003 0.0002 0.001 0.004 0.002 0.010 0.05 0.005 0.0004 0.001 0.001 0.003 0.006 0.02 0.02
peak intensity (×10−7) 5.79 4.17 1.95 0.075 0.337
± ± ± ± ±
0.06 0.06 0.09 0.007 0.006
0.022 ± 0.002 5.13 ± 0.09 2.29 ± 0.034 1.14 ± 0.03 0.146 ± 0.006 0.181 ± 0.009 0.044 ± 9.2 × 10−3 0.026 ± 1.1 × 10−3 2.68 ± 0.025 1.82 ± 0.13 0.94 ± 0.13 0.037 ± 8.85 × 10−3 0.096 ± 0.001 0.080 ± 0.001 0.012 ± 9.5 × 10−4 3.72 ± 0.03 1.45 ± 0.05 0.67 ± 0.03 0.065 ± 0.007 0.051 ± 0.005 6.76 × 10−3 ± 4.3 × 10−3 1.14 × 10−3 ± 6.5 × 10−4
interacting with the water surface, although the second monolayer interacts with the structured first monolayer. A first simple explanation to justify the presence of domains in FnHm monolayers could be considering these molecules as intermediates between perfluoroalkanes (that form homogeneous monolayers)22 and alkanes, which are known to form drops coexisting with a monolayer on the water surface. Using a different type of meaning, Semenov et al. proposed a detailed theoretical model, which justifies the domain formation by considering a repulsive interaction between dipoles.25 In this model, under compression, molecules self-assemble vertically with the same orientation and the formed domains, which present a strong dipole, resulting from the sum of all molecular dipoles at the fluorinated and hydrogenated junctions (CH2− CF2). The number of molecules per domain and, consequently, their size are then limited to minimize the repulsive interactions between the domains. The formation of the domains and their monodispersity appear then as independent from the interaction of the molecule with the substrate/subphase, although this model considers a transition from a parallel to a vertical orientation of the molecules (when the monolayer is compressed at a large area). However, we have previously observed that the dimension of the domains changes with the molecular structure and mainly with the length of the hydrogenated block.17 Moreover, we observed that the hexagonal structure remains stable under compression, with a compressibility coefficient of about 30 m/N. This indicates that the stability in size of the domains prevents their possible coalescence, even when the monolayer is compressed up to a
monolayer. Thus, if before collapse the diffracting structure is obviously a monolayer, it remains a similar monolayer after collapse. The GISAXS results show that it is reasonable to assume that during and even after completion of collapse, there is only one organized monolayer, buried below and in contact with water. On top of it, F8H18 multilayers grow as the 2D/3D transition progresses, scattering and absorbing the X-ray signal coming from the lower organized layer. We underline that the upper layers do not present the large hexagonal structure.
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DISCUSSION SFAs have already revealed surprising properties when spread as the monolayer on the water surface with the crystalline structuration in nanodomains. The collapse, i.e., the transition of the monolayer to the third dimension upon 2D compression, also reveals interesting features. The collapse proceeds via what could be considered a first-order transition between the 2D state and the 3D state. The change of area per molecule during the transition is compatible with the formation of a multilayer, probably a trilayer of upright molecules.15 However, our experimental results demonstrate that the structure of these upper layers strongly differs from the one of the first monolayer, which always remains crystalline without coalescence of the nanodomains. We note that this is the case even when it is buried below continuous domains of F nH m molecules. The structure of this monolayer remains unchanged (nanostructured) when it is in contact with air or covered with other F8H18 molecules. On the contrary, this organization is not maintained in the upper layers, which are not directly 15196
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Figure 3. (a) Out-of-plane scattered intensity distribution (Qxy−Qz map, top) and rod scan [Ihk(Qz), bottom] at the peak positions of a F8H18 monolayer before collapse at π = 3 mN m−1. (b) Out-of-plane scattered intensity distribution (Qxy−Qz map, top) and rod scan [Ihk(Qz), bottom] at the peak positions of a F8H18 monolayer after collapse at π = 19.8 mN m−1.
Table 2. Out-of-Plane Peak Position (qmax z ) and Layer Thickness (t) Obtained by Fitting the Rod Scans of Figure 3 by the Form Factor of a Disk before and after Collapse hk 10 11 20 21 22 31 10 11 20 21 22 31
qmax (nm−1) z before Collapse (π = 3 mN m−1) 0.71 ± 0.04 0.87 ± 0.06 1.19 ± 0.04 1.49 ± 0.04 1.89 ± 0.04 2.23 ± 0.03 after Collapse (π = 19.8 mN m−1) 0.64 ± 0.05 0.79 ± 0.06 0.91 ± 0.05 1.61 ± 0.03 1.89 ± 0.04 2.08 ± 0.04
interactions, as proposed by Semenov et al., and/or from the interaction with the underlying substrate. To discriminate between these possibilities, we should analyze the conditions known to be necessary for obtaining the FnHm monolayer nanostructuration. Organized nanodomains are obtained at the surface of water,17 fully hydrated silicon wafers,26 and thermotropic liquid crystals (8CB and 12CB).27 Conversely, no evidence of nanostructuration was found at the surface of bulk FnHm28,29 or on a dry silicon surface.26 Moreover, our results as well as results from other groups15 indicate that the upper layers of FnHm molecules are not structured after the collapse. This indicates that single inplane interactions (as interactions between dipoles) are not sufficient to induce self-assembly and nanostructuration. The presence of a substrate surface bearing strong dipoles seems mandatory for the onset of the phenomena. We consider that, regardless of the 2D pressure, only a part of the FnHm molecules in the monolayer interacting with the polar substrate can switch to the vertical configuration. Indeed, we suggest that this limitation results from the high energetic cost of preventing the dipoles carried by FnHm molecules (located at about the center of the linear molecule) from interacting with the polar substrate when these molecules go from a horizontal configuration to a vertical configuration. This leads to a minimum surface concentration of lying molecules that, under compression, will rearrange with the vertical molecules to form the observed nanostructure. These lying molecules (we suppose surrounding the domains formed by
t (nm) 3.04 2.60 2.44 2.09 2.00 1.80
± ± ± ± ± ±
0.16 0.15 0.09 0.08 0.06 0.04
3.08 2.71 2.48 2.46 2.13 1.98
± ± ± ± ± ±
0.20 0.17 0.13 0.06 0.08 0.05
high 2D pressure. FnHm monolayers collapse at a rather low surface pressure, typically 10−20 mN/m.15 The fact that the bottom layer remains nanostructured independent of its upper surface being in contact with air or with other SFA molecules indicates that the interaction of this layer with the upper medium is not the driving force for the self-organization of the layer. It could thus result from in-plane 15197
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self-assembly of the vertical molecules) prevent the coalescence between domains, leading to this surprising effect that the monolayer collapses before forming a homogeneous layer at the molecular scale. In a way, such cooperative association between the lying molecules (interacting strongly with water) and the vertical molecules replaces, on the average of the monolayer, the hydrophilic moieties of “classical” amphiphiles that interact strongly with water and contribute to stabilize Langmuir films.
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CONCLUSION Fully collapsed Langmuir films of SFAs at the air/water interface were studied in situ by GISAXS. The collapse occurs through a first-order phase transition between a pure monolayer state and a 3D state, most likely a trilayer. The results show that the layer interacting with the water subphase remains organized on a hexagonal lattice even after the complete collapse of the film. These domains never coalesce, contrary to the upper layers (bilayer), which do not exhibit similar organization and form a homogeneous film. These results combined with others from the literature clearly demonstrate the importance of the interactions between SFA molecules and the water subphase to the formation of such a robust structure. The final structure of the film results from a subtle balance between horizontally oriented SFA molecules, interacting with water through their dipoles, and a homogeneous film configuration, in which SFA molecules, all in a vertical orientation, would be prevented from interacting with water, thus being in a less favorable energetic state.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Oleg Konovalov from the ID10B beamline (European Synchrotron Radiation Facilities) for help and assistance during the experiment.
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REFERENCES
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