Limited Propagation of Lattice Distortion in Trilayer LangmuirâBlodgett

Article pubs.acs.org/Langmuir

Limited Propagation of Lattice Distortion in Trilayer Langmuir− Blodgett Films: Correlation with Mesoscopic Structure Sophie Cantin,*,† Françoise Perrot,† Philippe Fontaine,‡ and Michel Goldmann‡,§,∥ †

Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI, EA 2528), Institut des Matériaux, Université de Cergy-Pontoise, 5 Mail Gay-Lussac Neuville/Oise, 95031 Cergy-Pontoise Cedex, France ‡ Synchrotron Soleil, L’Orme des Merisiers, Saint Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France § Institut des Nanosciences de Paris (INSP, UMR 7588), 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 ABSTRACT: The structure of trilayer Langmuir−Blodgett (LB) films on oxidized silicon wafers has been investigated using grazing incidence X-ray diffraction at various incidence angles and atomic force microscopy (AFM). These films are formed by two behenic acid (BA) layers and a third monolayer of amphiphilic molecules having different architectures. These molecules have the same polar head and differ from each other by the chain, either saturated or unsaturated hydrogenated or semi-fluorinated. The structure of the first BA monolayer appears as unchanged in all cases, whereas a condensation of the second BA monolayer is evidenced when the third layer is not formed with the saturated hydrogenated chain. We interpret this condensation as resulting from the mismatch between the lattices of the second BA layer and the external monolayer, possibly associated with the formation of a new monolayer−air interface creating line tension effects. Line tension estimation has also been made from the size of the holes observed in the different LB films.

1. INTRODUCTION The transfer of Langmuir monolayers onto solid substrates using the Langmuir−Blodgett (LB) technique may lead to various useful applications.1,2 However, the structure of the transferred layers may differ from the one observed on the monolayer at the air−water interface.3−8 Indeed, the monolayer over the water surface can be considered as unstrained. Characterization techniques, such as atomic force microscopy (AFM), allow for detecting defects at the mesoscopic scale resulting from possible molecular rearrangements. Indeed, holes may be observed in the layers, thus creating new device−air interfaces. The relationship between these defects and the microscopic structure of the transferred monolayers is not obvious, and connection between their observation by AFM (or similar surface-imaging techniques) and film structure experiment has so far received little attention.3,4 Usually, the structure at the molecular scale of LB films is determined by grazing incidence X-ray diffraction (GIXD) experiments. In the usual setup, X-ray beam impinges the surface with an incidence angle set below the critical angle for total internal reflection of the substrate. The scattered signal is generated by all of the transferred monolayers.9−11 However, when the grazing incidence angle is varied, the penetration depth of the X-rays can be controlled, and this allowed one to distinguish the structure of the top layer from the underlying layers.12 When this method is coupled with AFM measurements, it provides a good tool for analyzing a possible link between these structures and the presence of holes or aggregates in the films. © 2013 American Chemical Society

In this paper, we have investigated by AFM and GIXD the structure of several trilayer LB films deposited on air-oxidized silicon wafers, each one consisting of two behenic acid [CH3(CH2)20COOH, noted BA] layers covered by a third external monolayer. Three molecules have been used to form this external monolayer: a saturated hydrogenated fatty acid, stearic acid [CH3(CH2)16COOH, noted SA], an unsaturated hydrogenated fatty acid, ω-tricosenoic acid [CH2CH(CH2)20COOH], and a partially fluorinated fatty acid [CF3(CF2)5(CH2)10COOH], noted F6H10COOH. These molecules have the same polar head, but in comparison to BA, only the SA presents the same type of aliphatic chain and the fluorinated chain of F6H10COOH has a larger cross-section than the hydrocarbon chain. The lattice organizations in each monolayer of the LB films have been investigated and correlated to the mesoscopic structure. We have analyzed the results by also considering the previous results obtained with other trilayer LB films, where BA or a semi-fluorinated molecule called FEP has been used to form the external layer.12,13

2. EXPERIMENTAL SECTION BA, SA, and ω-tricosenoic acid were purchased from Aldrich, whereas F6H10COOH was a gift from Atochem. All chemicals were used without Received: May 18, 2013 Revised: July 16, 2013 Published: August 8, 2013 11046

dx.doi.org/10.1021/la401860j | Langmuir 2013, 29, 11046−11054

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Table 1. Rectangular Cell Parameters a and b and Molecular Area A Measured for the Different Monolayers in BA−BA−BA, BA− BA−SA, BA−BA−ω-Tricosenoic Acid, BA−BA−F6H10COOH, and BA−BA−FEP LB Films cell parameters

BA−BA−BA LB film

a (Å) b (Å) A (Å2)

BA−BA−SA LB film

a (Å) b (Å) A (Å2)

BA−BA−ω-tricosenoic acid LB film

a (Å) b (Å) A (Å2)

BA−BA−F6H10COOH LB film

a (Å) b (Å) A (Å2)

BA−BA−FEP LB film

a (Å) b (Å) A (Å2)

purification and dissolved in chloroform. Monolayers were spread over cadmium chloride (CdCl2, Sigma) solutions, 1.9 × 10−6 mol/L, adjusted to pH 7.5 by sodium bicarbonate addition (NaHCO3, Sigma). Such a low concentration of divalent ions in the subphase allows for a best anchorage of the layers on the substrate. Indeed, Cd2+ ions lead to a half neutralization of the fatty acid headgroups at a pH of 5.1.14 A strong condensation of the fatty acid monolayer is observed at pH 7.5, even at a very low Cd2+ concentration.15 Three-layer BA LB films without any holes or aggregates can be thus obtained as shown by means of AFM.13 The LB films were prepared by vertical deposition on polished oxidized silicon wafers (Si/SiO2 wafers) [Applications Couches Minces (ACM)]. The first BA bilayer was transferred at a surface pressure of 35 mN/m (S phase), while the external third monolayer was transferred at 28 mN/m. A constant dipping speed of 10 mm/min was used for all of the layers. AFM experiments were performed with a Nanoscope IIIA Dimension 3100 microscope from Veeco (Bruker). Measurements were carried out in air at room temperature. Standard silicon cantilevers (Digital Instruments) were used in “tapping” mode near the resonant frequency close to 300 kHz. The images were obtained with the height mode and a 256 × 256 dot resolution. The vertical heights were measured by crosssection analysis. GIXD experiments were performed on the D41B beamline at the LURE synchrotron source (Orsay, France). The experimental setup has been previously described.16 The wavelength λ = 1.646 Å was selected using a focusing Ge(111) crystal. The incidence angle of the X-ray beam was fixed to 2.1 mrad, slightly below the air−water interface critical angle for total reflection (2.5 mrad), for the experiment on the liquid surface. Two angles of incidence were used for measurement on the solid substrate. The first angle, 3.5 mrad, is below the critical angle for total internal reflection for Si/SiO2 wafers (3.9 mrad) and allows us to probe the three LB monolayers. The second angle, 1.5 mrad, is below the critical angle for total internal reflection for the hydrocarbon chains (2.4 mrad). The penetration depth of the evanescent wave is close to 45 Å; consequently, mainly the two external layers are probed, while about 15% of the beam probe the first deposited BA monolayer. The diffracted intensity was recorded by a vertical Ar/CO2-filled position sensitive detector (PSD), as a function of the in-plane component of the scattering vector, Qxy, selected using a Soller collimator. The Qxy resolution was 0.007 Å−1. The Qxy pattern, corresponding to the diffracted intensity integrated over the vertical wave-vector transfer Qz, allows for the determination of the cell parameters of the two-

first layer

second layer

third layer

BA 4.95 7.69 19.0 BA 4.98 7.71 19.2 BA 4.97 7.74 19.2 BA 4.98 7.74 19.3 BA 4.98 7.67 19.1

BA 4.95 7.69 19.0 BA 4.98 7.71 19.2 BA 4.92 7.50 18.5 BA 4.95 7.48 18.5 BA 4.91 7.51 18.4

BA 4.95 7.69 19.0 SA 4.80 8.31 19.9 ω-Tricosenoic Acid 4.92 7.50 18.5 F6H10COOH 5.73 9.92 28.4 FEP 5.92 10.26 30.4

dimensional lattice. The tilt angle is deduced from the intensity distribution along the surface normal z (Bragg rod).

3. RESULTS The structure of trilayer LB films consisting of two BA layers covered by SA, ω-tricosenoic acid, or F6H10COOH monolayer was investigated by coupling GIXD and AFM characterizations. Previously studied trilayer BA LB films (BA−BA−BA) were used as reference to analyze the possible changes in BA structure in the case of molecules with different chain architectures as the third monolayer. In BA−BA−BA LB films, it was shown, with the same experimental setup, that the diffraction spectra remain identical regardless of the incidence angle, meaning that the three monolayers have the same structure. The molecules are arranged on a rectangular cell of parameters a = 4.95 ± 0.02 Å and b = 7.69 ± 0.02 Å (Table 1).12 In addition, AFM characterizations of these films show the absence of any holes at the probed scale.13 Before LB transfer, the structure of the SA, ω-tricosenoic acid, and F6H10COOH monolayers was systematically analyzed by GIXD at the surface of dilute CdCl2 solutions. 3.1. BA−BA−SA LB Film. 3.1.1. SA Monolayer. On the liquid surface, the diffraction pattern of the SA monolayer presents a single in-plane peak at Qxy = 1.517 ± 0.007 Å−1, in agreement with the LS phase observed at this surface pressure over pure water (Figure 1).17−19 The untilted molecules are thus arranged on a hexagonal lattice of parameter a = 4.78 ± 0.02 Å or equivalently on a non-primitive centered rectangular cell of parameters a = 4.78 ± 0.02 Å and b = 8.28 ± 0.02 Å. 3.1.2. AFM and GIXD LB Film Characterization. Figure 2 shows an AFM image obtained for the BA−BA−SA LB film. It appears very smooth, with a low amount of small holes (coverage < 2%). The diffraction pattern of the trilayer is shown in Figure 3. It displays three in-plane peaks, independent of α, the incidence angle of the X-ray beam. Only the intensity ratio of the peaks varies with α. Two of the peaks (Qxy = 1.495 ± 0.007 and 1.630 ± 0.007 Å−1) have the same maximum as those observed in BA− BA−BA LB films (Qxy = 1.506 ± 0.007 and 1.638 ± 0.007 Å−1). They correspond to the degenerate (11)/(11̅) and non11047


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arrangement, an untilted hexagonal LS phase of parameter a = 4.80 ± 0.02 Å or, equivalently, a rectangular cell of parameters a = 4.80 ± 0.02 Å and b = 8.31 ± 0.02 Å (Table 1). Consequently, in BA−BA−SA LB films, the BA bilayer has the same arrangement as in three-layer BA films. 3.2. BA−BA−ω-Tricosenoic Acid LB Film. 3.2.1. ωTricosenoic Acid Monolayer. We investigated by GIXD the structure of ω-tricosenoic acid monolayers on the surface of dilute CdCl2 solutions at a surface pressure of π = 28 mN/m, corresponding to the transfer pressure onto the substrate (panels a and b of Figure 4). Two out-of-plane diffraction peaks are detected at Qxy = 1.496 ± 0.007 and 1.555 ± 0.007 Å−1. Because the Qz value of the second peak is twice the Qz value of the first peak, this pattern is the signature of a L′2 phase, with the first peak being the degenerate (11)/(11̅) and the second peak being the non-degenerate (02). The parameters of the associated rectangular cell are a = 4.91 ± 0.02 Å and b = 8.08 ± 0.02 Å, and the molecules tilted from 14.5° toward the next nearest neighbor. This is consistent with the isotherm showing a narrow first-order plateau at π = 24 mN/m, marking the L2−L′2 phase transition, and a kink at π = 42 mN/m, corresponding to the transition toward the S phase, as evidenced from the diffraction spectra measured at π = 45 mN/m (panels c and d of Figure 4). The ωtricosenoic acid molecule differs from BA by one more CH2 group but especially by the double bond at the end of the chain. This double bond leads to a significant increase in the surface pressure occurrence of the phase transitions (14 and 26 mN/m for BA over CdCl2),15 with the tilt angle value of the molecules remaining significant at high pressure. 3.2.2. AFM and GIXD LB Film Characterization. Figure 5 shows an AFM image of the surface of the BA−BA−ωtricosenoic trilayer. The film appears very smooth on a large scale, while on a 1 × 1 μm scale, one observes holes whose depth, close to 60 Å, corresponds to a thickness of the ω-tricosenoic acid monolayer and one BA monolayer. The coverage of these holes is about 15%. The diffraction pattern measured for the BA−BA−ωtricosenoic acid LB film at two angles of incidence α of the Xray beam (1.5 and 3.5 mrad) is presented in Figure 6. It allows us to distinguish the structure of each of the three monolayers. One first notices the absence of the out-of-plane peak, meaning that the ω-tricosenoic acid molecules, which were tilted over the liquid surface (L′2 phase), have stood up after transfer. Such differences in molecular tilt angle before and after monolayer deposition have already been reported for fatty acid molecules.20 Each of the scans obtained for the two incidence angles α of the X-ray beam shows four in-plane peaks located at the same scattering vectors Qxy = 1.499 ± 0.007, 1.522 ± 0.007, 1.623 ± 0.007, and 1.680 ± 0.007 Å−1. Only the intensity of the peaks varies with α. Two peaks (Qxy = 1.499 and 1.623 Å−1) are located at the same position as the BA−BA−BA trilayers or a ωtricosenoic acid monolayer compressed at a surface pressure π = 45 mN/m (panels c and d of Figure 4).15 This means that the four diffraction peaks correspond to two S-phase packing with different cell parameters. For α = 1.5 mrad, the beam mainly probes the ω-tricosenoic acid monolayer and the second BA monolayer, while the first deposited BA monolayer is probed by about 15% of the beam. From the variation of the peak intensities with α, one can thus conclude that both the ω-tricosenoic acid monolayer and the second deposited BA monolayer have the same molecular S-phase packing with rectangular cell parameters a = 4.92 ± 0.02 Å and b = 7.50 ± 0.02 Å. The first deposited BA monolayer remains in a

Figure 1. X-ray diffraction data in the horizontal plane, integrated over Qz for a SA Langmuir monolayer spread at room temperature over a CdCl2 solution of 1.9 × 10−6 mol/L adjusted to pH 7.5 and compressed at a surface pressure of 28 mN/m. The surface pressure versus mean molecular area isotherm is presented in the inset.

Figure 2. AFM image (1 × 1 μm2) of a BA−BA−SA LB film (a SA monolayer transferred onto a BA bilayer).

Figure 3. X-ray diffraction data in the horizontal plane, integrated over Qz for a BA−BA−SA LB film (a SA monolayer transferred onto a BA bilayer). The position of the three diffraction peaks is independent of the angle of incidence of the X-ray beam.

degenerate (02) peaks, respectively, associated to a S-phase packing with rectangular cell parameters a = 4.98 ± 0.02 Å and b = 7.71 ± 0.02 Å. The maximum of the third peak (Qxy = 1.510 ± 0.007 Å−1) is almost at the same Qxy position as the Qxy detected for the stearic film over the liquid surface at π = 28 mN/m. The transferred SA monolayer has thus kept the same molecular 11048


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Figure 4. X-ray diffraction data in the horizontal plane, integrated over Qz (left) and contours of equal intensity versus the in-plane and out-of-plane scattering vector components Qxy and Qz (right) for a ω-tricosenoic acid monolayer spread at room temperature over a CdCl2 solution of 1.9 × 10−6 mol/L adjusted to pH 7.5. The monolayer is compressed at a surface pressure of (a and b) 28 mN/m and (c and d) 45 mN/m. The surface pressure versus mean molecular area isotherm is presented in the inset of panel a.

Figure 5. (a) AFM image (1 × 1 μm2) of a BA−BA−ω-tricosenoic acid trilayer. (b) Height profile obtained from panel a.

5.77 ± 0.02 Å and b = 9.99 ± 0.02 Å. The deduced molecular area per molecule is 28.8 ± 0.1 Å2, in agreement with the cross-section of fluorinated segments.21 This molecular area measured by GIXD is significantly lower than the 30 Å2 value measured on the isotherm at 28 mN/m and, of course, also the 36 Å2 limiting molecular area corresponding to a monolayer determined from the slope of the isotherm in the condensed phase (Figure 7). A similar behavior was evidenced for Langmuir monolayers of semi-fluorinated alkanes, and the difference in molecular area was attributed to the formation of organized domains coexisting with non-organized molecules.22−24 About 20% of the surface is thus occupied by a disordered phase. 3.3.2. AFM and GIXD LB Film Characterization. Figure 8 shows AFM images of the BA−BA−F6H10COOH LB films. The samples appear very smooth on a large scale (10 × 10 μm), whereas on a 1 × 1 μm scale, a non-negligible proportion, i.e., 18% coverage, of large elongated holes is noticeable. Their

S-phase packing, in which cell parameters a = 4.97 ± 0.02 Å and b = 7.74 ± 0.02 Å (Table 1) are similar to those of the BA−BA−BA films. Because the transfer ratio is close to unity, one can consider that the appearance of holes results from the standing of the ωtricosenoic acid molecules after transferring of this monolayer. Indeed, the unit cell area decreases from 19.8 to 18.4 Å2, i.e., about 7%. 3.3. BA−BA−F6H10COOH LB Film. 3.3.1. F6H10COOH Monolayer. The F6H10COOH monolayer was first characterized over the dilute CdCl2 subphase. Figure 7 presents the diffraction profile obtained at a surface pressure of 28 mN/m. The measured diffraction scan displays one large in-plane (Qz = 0) degenerate peak located at Qxy = 1.257 ± 0.007 Å−1, corresponding to the (11), (11̅), and (02) peaks. This indicates untilted molecules arranged on a hexagonal lattice of parameter a = 5.77 ± 0.02 Å or, equivalently, on a centered rectangular lattice of parameters a = 11049


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Figure 6. X-ray diffraction data in the horizontal plane, integrated over Qz (left) and contours of equal intensity versus the in-plane and out-of-plane scattering vector components Qxy and Qz (right) for a BA−BA−ω-tricosenoic acid LB film. Two angles of incidence of the X-ray beam were used: (a) α = 1.5 mrad and (b) α = 3.5 mrad.

constraints, resulting from the bulky fluorinated helix and van der Waals interactions between the hydrogenated segments.26 The proportion of holes in the BA−BA−F6H10COOH LB film is close to the estimated surface covered by the disordered phase at the air−water interface. This means that the disordered F6H10COOH molecules probably do not transfer onto the BA bilayer, inducing instability in the second BA monolayer as the polar heads remain in contact with air. This leads to the generation of holes whose depth corresponds to a BA monolayer and the semi-fluorinated monolayer. The diffraction spectra obtained for the BA−BA− F6H10COOH LB film is presented in Figure 9. In the Qxy range corresponding to the fluorinated layer (1.15−1.40 Å−1), a weak intensity peak is detected at Qxy = 1.265 ± 0.007 Å−1, nearly the same position as over the liquid surface. The external F6H10COOH monolayer has thus kept its hexagonal arrangement (a = 5.73 ± 0.02 Å), with a limited positional correlation length close to 80 Å. It can be noticed that the scans measured in the Qxy range of hydrocarbon chains for the two angles α of the incident X-ray beam are exactly the same as those obtained for BA−BA−ω-tricosenoic acid films. Consequently, for the BA−BA−F6H10COOH LB film, considering the peak intensities obtained for the two incidence angle α, one can conclude that the two BA monolayers exhibit a S-phase packing with untilted molecules but with different cell parameters. The BA unit cell in the first deposited monolayer is the same as that obtained with the other trilayer films (a = 4.98 ± 0.02 Å and b = 7.74 ± 0.02 Å), while the BA unit cell of the second deposited monolayer is more condensed: a = 4.95 ± 0.02 Å and b = 7.48 ± 0.02 Å (Table 1).

Figure 7. X-ray diffraction data in the horizontal plane, integrated over Qz for a F6H10COOH Langmuir monolayer spread at room temperature over a CdCl2 solution of 1.9 × 10−6 mol/L adjusted to pH 7.5 and compressed at a surface pressure of 28 mN/m. The surface pressure versus mean molecular area isotherm is presented in the inset.

combined depth, 58 Å, corresponds to the semi-fluorinated and BA monolayer thickness. Moreover, at smaller scale (300 × 300 nm), the upper surface presents a periodic undulation of very low amplitude (about 6 Å), with a period close to 6 nm. Such ribbon texture is similar to the texture already observed in semifluorinated alkane LB films.25 These structures, also evidenced in partially fluorinated fatty acid LB films as well as at the air−water interface, have been explained in terms of balance between steric 11050


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Figure 8. AFM images of a BA−BA−F6H10COOH LB film (a F6H10COOH monolayer transferred onto a BA bilayer). The size of the images is (a) 1 × 1 μm2 and (b) 300 × 300 nm2. (c) Height profile obtained from panel b.

Figure 9. X-ray diffraction data in the horizontal plane, integrated over Qz (left) and contours of equal intensity versus the in-plane and out-of-plane scattering vector components Qxy and Qz (right) for a BA−BA−F6H10COOH film. Two angles of incidence of the X-ray beam were used: (a) α = 1.5 mrad and (b) α = 3.5 mrad.

4. DISCUSSION Two striking points can be noted from all of these measurements. First, the structure of the first transferred BA monolayer remains identical, regardless of the nature of the third monolayer. It presents the same area per molecule as that on the liquid

subphase before transfer. Second, the structure of the second BA monolayer can be either identical to the structure of the first BA monolayer or condensed. This seems related to the nature of the third transferred monolayer. Moreover, this contraction is associated with the formation of holes, as observed by AFM. 11051


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BA−BA−FEP LB films.13 All of these films display a large proportion of holes (15−50%). On the contrary, BA trilayer films and BA−BA−SA films present a homogeneous surface. In addition, in a previous work, samples consisting of a BA bilayer, without an external subsequent monolayer, have been studied.13 Because such a film is not stable in air, a reorganization of the second BA monolayer occurs, resulting in hole creation. Indeed, about 50% of the molecules in this second monolayer flip upside down to create an external third layer with hydrocarbon chains pointed out. This leads to a labyrinthine structure quite similar to the structure obtained for BA−BA−FEP films, although it is less regular (Figure 10).13 Such films are thus

Trilayers formed with saturated aliphatic chains BA−BA−BA or BA−BA−SA present identical two-dimensional (2D) structure of the BA monolayers and a homogeneous surface. This indicates also that the difference in surface tensions (CH3/ CH3 and CH3/air) is not sufficient to induce a distortion of the buried monolayers. Trilayers formed with saturated aliphatic chains, BA−BA−SA, present identical 2D structure for the two first BA monolayers and a very similar 2D structure for the last SA monolayer, which differs by less than 10% in each direction, less than 5% in area (Table 1). Moreover, the surface remains homogeneous without marked holes. On the contrary, when the third layer is formed by a fatty acid with different chain main architecture, such as ω-tricosenoic acid or fluorinated acid, its structure of course differs strongly from the BA structure, and we evidenced a condensation of the second BA monolayer with respect to the first BA monolayer deposited on the substrate. The same result was previously observed on trilayers terminated by a semi-fluorinated molecule [CF3− (CF 2 ) 5 −(CH 2 ) 2 −S−CH 2 −CHOH−CH 2 −O−CH 2 −CH− (C2H5)(C4H9), FEP], i.e., BA−BA−FEP LB films.12 The condensation of the second BA monolayer in these three LB films must be considered in the framework of the in-plane and interplane molecular interactions. Indeed, SA and BA molecules interact via very similar van der Waals aliphatic chain interactions (the chain lengths are very close) on one hand and acid headgroup interactions on the other hand. Then, the equilibrium distance between molecules is similar, and the structure of each monolayer is almost identical at the same surface density. The chain−chain interaction is of course different in the case of unsaturated aliphatic or fluorinated chain. It is noticeable that the difference between the first and second BA monolayer is increased when the chains differ from saturated aliphatic chains. The condensation could then be considered as resulting from the misfit between the second BA layer and the third monolayer structures before transfer. For example, in the case of the BA− BA−ω-tricosenoic acid LB film, there is a strong difference between the cell parameters of the ω-tricosenoic acid monolayer at the air−water interface (a = 4.91 ± 0.02 Å and b = 8.08 ± 0.02 Å) and those of the BA−BA−BA LB film (a = 4.95 ± 0.02 Å and b = 7.69 ± 0.02 Å). Upon transfer of the ω-tricosenoic acid monolayer onto the BA bilayer, this leads to a rearrangement of both the second BA monolayer and the ω-tricosenoic acid monolayer. It is known that incommensurate structures minimize the overall energy by avoiding a strong mismatch between the two neighboring lattices.27 This can be obtained by either modulating the periodicity of one of the networks28 or creating localized defects, so-called walls.29 A modulation of a network should lead to satellite diffraction peaks that were not observed on the spectra; however, they should be difficult to detect because their intensity is expected to be weak, while the main diffraction peaks are quite large. Another solution, a “light wall” defect in the third layer, would lead locally to an increase of the intermolecular distance of about one lattice parameter. Then, the corresponding molecules of the underneath BA layer would present their acid head uncovered, which appears as unstable. A solution for the system would be to flip these molecules and condense the layer in the way to avoid such a situation. Indeed, considering the AFM images of the different studied LB films, one can notice that the contraction of the BA layer occurs when the third monolayer is incomplete. It is evidenced for F6H10COOH, ω-tricosenoic acid, and previously studied

Figure 10. AFM image (1 × 1 μm2) of a BA bilayer LB film.

close to BA trilayer films but with holes in a large proportion in the second and third layers. By means of GIXD, one obtains exactly the same diffraction spectra as those associated with the two BA monolayers when they are covered by the ω-tricosenoic acid or the fluorinated molecules. Indeed, the diffraction patterns measured for the two angles of incidence of the X-ray beam indicate a condensation of the two incomplete external BA monolayers, whereas the first BA monolayer deposited on the substrate displays the same unit cell parameters as in three-layer BA LB films. In such a case, in the absence of any incommensuration, a condensation of the incomplete monolayers occurs. This could be attributed to either the formation of new monolayer−air linear interfaces inside the two external monolayers or different amounts of cadmium ions in each of the layers. Two origins of the holes could thus be identified, either a mismatch between the lattices of the two external monolayers or the instability of the external monolayer. The shape of condensed domains in a monolayer or, equivalently, holes in a condensed phase is related to the electrostatic interactions between molecules.30 From the size of the holes, one can estimate the line tension value of the two monolayer−air linear interfaces inside the two external monolayers. Indeed, the shape and size of domains (or holes) inside one monolayer result from a competition between dipole−dipole interactions and domain line tension.31,32 These competitive interactions were mainly evidenced in monolayers at the air−water interface, where the strength of the molecular dipole leads to a selection of domain size and shape.33,36 Quite different line tension effects were nevertheless observed for thin films of dipalmitoylphosphatidylcholine (DPPC) obtained by deposition of vesicles, i.e., disordered surfactant films.34 Indeed, a linear scratch inside the film was observed to evolve with time to a circular hole as a result of a competition between line tension and friction at the film− substrate interface.32 11052


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Considering that interactions between fluorinated chains change from less than a factor of 2 with respect to those between hydrocarbon chains,35 line tension estimation can be made from the size of the holes evidenced by AFM and considering holes in two external BA monolayers. The equilibrium radius (Req) of a domain resulting from line tension and dipolar interactions is given by the equation36

relaxation induced by surface reconstruction in three-dimensional (3D) solids.

■

Corresponding Author

*E-mail: [email protected]. Notes

⎛ 4πε λ ⎞ eδ = exp⎜ 20 ⎟ 4 ⎝ μ ⎠

The authors declare no competing financial interest.

3

R eq

AUTHOR INFORMATION

■

where λ is the line tension, μ is the difference in dipole density between the two phases, and δ is the nearest neighbor dipole− dipole distance, which can be estimated to be about 5 Å from GIXD measurements. In the case of the two monolayer−air interfaces, μ corresponds to the BA phase dipole density and can be expressed as a function of the surface potential ΔV measured for condensed phases of fatty acid monolayers: μ = ε0ΔV. ΔV values close to 390 mV have been reported for long-chain fatty acids.37,38 With hole diameters ranging between approximately 20 and 200 nm, the line tension of the BA−air interface can be estimated in the range of (2−5) × 10−13 N m−1. One can notice that the error bar is important because of the logarithmic variation with the radius. However, this estimation is of the same order of magnitude as the values reported for liquid condensed−gas, liquid condensed−liquid expanded, and solid−liquid interfaces.36,39−41 These observations provide a basis for further understanding the structure of LB films, the influence of a mismatch between two subsequent monolayers, and the role of two-dimensional domains of dipoles on the molecular arrangement. One also notes the remarkable stability of the first BA monolayer structure with respect to organization of the third external monolayer. An explanation based on a strong interaction of the molecule headgroups with the substrate seems unsatisfying because the solid surface does not present definite order. We suggest that the distortion resulting from the mismatch between the structure of the external surface and the underneath monolayer does not propagate over a single monolayer. Indeed, theoretical calculation of such distortion based on elastic relaxation shows that it does not penetrate over a unit cell depth in an isotropic solid crystal.42 Moreover, we note that this first BA layer interacts with the second BA layer by van der Waals interaction, while the two other layers interact through dipole interactions.

REFERENCES

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5. CONCLUSION AFM observations and GIXD measurements were associated to analyze the structure of trilayer films with respect to the nature of the external monolayer. A condensation of the intercalated monolayer was evidenced when the structure of the external monolayer differs from the first deposited monolayer. This is attributed to a mismatch between the lattices of the second BA monolayer and the external monolayer. In addition, line tension effects cannot be excluded in this condensation phenomenon. We evidenced the presence of holes in all of the corresponding samples leading to BA monolayer−air linear interfaces. The study of BA bilayer films without an external subsequent monolayer shows that the creation of holes also results in a condensation of the incomplete monolayers. The stability of the first monolayer structure can be justified by the models of elastic 11053


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Limited Propagation of Lattice Distortion in Trilayer LangmuirâBlodgett

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