Impact of the Spacer on the Condensing Effect of Fluorinated Chains

Article pubs.acs.org/Langmuir

Impact of the Spacer on the Condensing Effect of Fluorinated Chains Investigated by Grazing Incidence X‑ray Diffraction on Ultrathin Langmuir Monolayers F. Giulieri,† F. Jeanneaux,‡ M. Goldmann,§ and M.P. Krafft*,∥ †

Laboratoire de Physique de la Matière Condensée (LPMC), UMR 7336, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 2, France. ‡ Concordia Station, Dome C, Antarctica. § Institut des Nano Sciences de Paris (INSP), UMR 7588 CNRS, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France ∥ Systèmes Organisés Fluorés à Finalités Téléonomiques (SOFFT), Institut Charles Sadron, UPR 22 CNRS, 23 rue du Loess, 67034 Strasbourg Cedex 2, France ABSTRACT: Langmuir monolayers of double perfluoroalkyl(alkyl) chain amphiphiles fitted with a monomorpholinophosphate polar head, [CnF2n+1(CH2)mO]2P(O)[N(CH2CH2)2O] (di(FnHm)MP with n = 6, 8, or 9; and m = 1 or 2), were investigated by surface pressure (π)/molecular area (A0) compression isotherms for temperatures ranging from 15 to 50 °C, and by grazing-incidence X-ray diffraction (GIXD) at 25 °C. Ultrathin monolayers were obtained for these short surfactants. Though the hydrocarbon spacer is short, it has a remarkable impact on the monolayer’s organization. At 25 °C, whereas di(F8H2)MP monolayer presents a liquid expanded (LE)/liquid condensed (LC) transition, simply replacing one CH2 by a CF2 in the latter compound’s structure at constant chain length, i.e. shortening the spacer from 2 to 1 CH2 (as in di(F9H1)MP), suppresses the LE phase. At 25°, GIXD established that for both di(F8H2)MP and di(F9H1)MP, the chains form an hexagonal lattice in the LC phase. The collective tilt of the two compounds is close to zero. The lattice of the dense phase can be compressed, as assessed by the continuous linear decrease of the d spacing with increasing pressure. This indicates that the azimuthal distribution of the molecular tilts is progressively reduced upon compression. The d value for di(F9H1)MP is significantly lower than that of di(F8H2)MP, providing evidence for strong condensing effect of the fluorinated chains. Molecular areas were determined directly from the compression curves and also from the X-ray data, the latter allowing reconstruction of the compression isotherms. The calculated lattice compressibilities are ∼30% and 50% of the macroscopic compressibilities for di(F9H1)MP and di(F8H2)MP, respectively. Comparison with the experimentally determined isotherms shows that the monolayer of di(F9H1)MP is more stable than that of di(F8H2)MP. The enthalpies and entropies determined for di(F9H1)MP and di(F8H2)MP, derived from the Clausius−Clapeyron equation, confirm that the observed transitions are both of the LE/LC type, although the triple point temperatures are strikingly different (27 °C vs −18 °C); this large difference further illustrates the stabilizing effect of the fluorinated chains. Disorder is hindered by the fluorinated chains and facilitated by a hydrocarbon spacer when larger than 1 CH2.

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INTRODUCTION

Introducing a perfluoroalkyl chain (F-chain) in the structure of an amphiphile strongly increases the stability of the monolayers5 and modifies their properties.6,7 Recent studies of the monolayer behavior of (perfluoroalkyl)alkylated amphiphiles include those of highly fluorinated functionalized phospholipids,8 glycolipids,9 and anchor-shaped bolaamphiphiles.10 The case of semifluorinated alkanes (CnF2n+1CmH2m+1, FnHm diblocks), which are amphiphiles devoid of polar head,

The structure and properties of Langmuir monolayers are known to depend on a delicate balance between the polar head(s) and the hydrophobic chain(s) of the amphiphiles that make them up. The main driving forces for monolayer formation are the repulsion between the heads, which allows the spreading of the amphiphiles at the air/water interface, and the hydrophobicity of the chains, which prevents the molecules from dissolving in the aqueous phase.1 Compression isotherms, combined with grazing incidence X-ray diffraction (GIXD), allow determination of the influence of the different parts of the amphiphiles on the organization of the thin films.2−4 © 2012 American Chemical Society

Received: May 18, 2012 Revised: July 5, 2012 Published: July 5, 2012 12022

dx.doi.org/10.1021/la302037n | Langmuir 2012, 28, 12022−12029

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equation from the molecular area values determined by GIXD at 25 °C. Grazing Incidence X-ray Diffraction. GIXD experiments were performed at line D41 of the LURE-DCI storage ring of Orsay, France. The λ = 1.488 Å radiation was selected using a Ge(111) monochromator. The incident beam hits the surface of water at an angle smaller than the critical angle of water (θ = 2.09 mrad). Under these conditions, the beam is totally reflected. There is formation of an evanescent wave that propagates in the plane of the water surface. If there is an organization of matter at the nanoscale at the surface of water, this evanescent wave is diffracted and Bragg peaks appear. The diffracted radiation with the in-plane wave vector transfer Qxy was selected using a Söller collimator (opening 1.43 mrad, i.e. 0.0056 Å−1 at 1.5 Å−1) and detected using a vertically mounted argon-filled position sensitive detector measuring the Qz intensity. Temperature was 25 ± 0.5 °C. The surface pressure π was measured using a Wilhemy balance and kept constant during a scan. In this study, only one phase has been observed: the untilted rotator hexagonal phase (LS). We will use the equivalent rectangular description of the chains’ lattice. For the LS phase, the diffraction pattern exhibits a single degenerate peak corresponding to the [1̅1], [11], and [02] reflections, the intensity of which, along the Bragg rod, is located in plane (Qz ∼0 Å−1). In this case, the lattice parameters a and b, were determined using a two atom lattice model according to ref,20 a = Qxy/2 and b = a√3. The positional correlation length ξ is related to the full width at half height (fwhh) of the in-plane peak when Qxy is fitted by a Gaussian, according to ξ = 0,9 × 2π/fwhh. The surface area per chain is calculated from At = ab/2, At being the surface area per molecule during compression. A0 is the cross-section normal to the molecular axis of an all trans configurated fluorinated chain. A0 is related to the molecular tilt by cos θ = A0/At. Relaxation of the Monolayers. GIXD data were acquired after a period of 3 to 4 h after monolayer compression to anneal defects.

has also been investigated and their monolayers were shown to consist of arrays of large surface micelles.11,12 Little is known on the role of the hydrocarbon junction or spacer (H-spacer) usually present between the polar head(s) and the F-chain(s). To assess the effect of this H-spacer on monolayer organization, it is necessary to investigate fluorinated surfactants fitted with small polar heads, that is, with a surface area comparable to the cross-section of an Fchain’s (∼30 Å2). The only available reports on such a situation concern (perfluoroalkyl)alkylated carboxylic acids CnF2n+1(CH2)mCOOH (FnHmCOOH). The monolayer behavior of FnHmCOOH was found to strongly depend on the length of the H-spacer.13,14 F-chains are stiffer than hydrocarbon chains (H-chains). The stiffness of the F-chain originates in the large energy difference between trans and cis conformations11,12 and favors stable crystalline order even for short chain amphiphiles.15 This stiffness was deemed to explain why the liquid expanded (LE) phase had not been observed for acids having no, or only one CH2 group (e.g., C11F23COOH (F11COOH) and C10F21CH2COOH (F10H1COOH)).16 When the H-spacer became longer, such as in F8H4COOH, the LE/liquid condensed (LC) phase transition was seen to occur.14,17 Molecular simulations of GIXD spectra of monolayers of FnHmCOOH indicated that the H-spacer brought disorder.18 Broader GIXD peaks were predicted, for example, for C7H15C7F14COOH and C7F15C7H14COOH than for C7H15COOH.13,14 We show here that the H-spacer, or H-linker, in partially fluorinated surfactants can have a considerable impact on monolayer organization, structure and stability. We report the monolayer behavior of three double-chain fluorinated amphiphiles with a small monomorpholinophosphate polar head: [CnF2n+1(CH2)2O]2P(O)[N(CH2CH2)2O] with n = 6 (di(F6H2)MP) or 8 (di(F8H2)MP), and [C9F21CH2O]2P(O)[N(CH2CH2)2O] (di(F9H1)MP). Although they have similar hydrophobic tails, these compounds were found to display substantially different monolayer behavior demonstrating the impact that a short alkyl spacer can have on organization of the monolayer. For this purpose, we have investigated the structure of the Langmuir monolayers of di(F8H2)MP and di(F9H1)MP (the two compounds that form LC phases) using GIXD.

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RESULTS Compression Isotherms. At 25 °C, di(F8H2)MP and di(F9H1)MP form Langmuir monolayers with high collapse pressures, πc, of 45 and 55 mN m−1, respectively (Figure 1,

EXPERIMENTAL SECTION

Synthesis. di(F6H2)MP, di(F8H2)MP and di(F9H1)MP were synthesized as described previously,19 and purified by repeated column chromatography and crystallization. Their purity (>99%) was controlled by 1H, 13C, 19F and 31P NMR, TLC and elemental analysis. Langmuir Monolayers. Monolayer studies were performed in a Teflon trough (60 × 250 mm) in a thermoregulated box at temperatures ranging from 5 to 50 °C (±0.2 °C) with a RK2 manual electrobalance (Riegler & Kirstein GMBH, Wiesbaden, Germany). Surface tension (±0.05 mN m−1) was measured according to the Wilhemy method using a paper filter plate. Monolayers were prepared by spreading 25 μL of a chloroform solution of the amphiphiles (5 × 10−4 M and 2.5 × 10−4 mol L−1 for di(F8H2)MP and di(F9H1)MP, respectively) on water. After evaporation of the spreading solvent (15 20 min), the π/A curves were recorded at a compression rate of 5.8 A2 molec.−1 min−1. All curves were run in triplicate with a reproducibility of 0.5 mN m−1. Macroscopic and Microscopic 2D Compressibilities. The , was calculated from the slope of macroscopic compressibility, Cmacro s the π/A compression isotherms for temperatures ranging from 5 to 50 = −1/A(dA/dπ), A being the molecular area °C according to: Cmacro s and π the surface pressure. The microscopic compressibility, that is the , was calculated using the latter compressibility of the lattice, Cmicro s

Figure 1. Compression isotherms of (a) di(F6H2)MP (at 5 °C), and (b) di(F8H2)MP, and (c) di(F9H1)MP at 25 °C.

Table 1). Di(F6H2)MP only forms a stable monolayer at low temperature (5 °C) and exhibits some dissolution at 25 °C. This is remarkable for amphiphiles with small polar heads and such short 8−10 carbon hydrophobic tails. As a comparison, the minimal chain length required for carboxylic fatty acids to form stable monolayers is 14 carbon atoms.21 Short chains result in ultrathin films. The calculated maximal thicknesses of the monolayers are ∼12.6 Å for di(F6H2)MP and ∼15.3 Å for di(F8H2)MP and di(F9H1)MP, assuming that the chains are fully extended, that is in an all-trans configuration and untilted.22,23 These values are confirmed by the GIXD data. 12023


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Table 1. Characteristics of the Compression Isotherms of di(FnHm)MPs; T Is the Temperature, πt Is the Surface Pressure on LE the LE/LC Coexistence Plateau, πc Is the Collapse Surface Pressure, ALC ∞ Is the Area Extrapolated in the LC Phase, A∞ Is the LC LE Area Extrapolated in the Linear Portion of the LE Phase, A0 Is the Limiting Area in the LC Phase, A0 Is the Limiting Area in the LE Phase, ΔH Is the Entropy and ΔS the Entropy Associated to the LE/LC Transition Di(FnHm) MP di(F6H2) MP di(F8H2) MP

di(F9H1) MP

a

T °C ± 0.5 °C

πt mN m−1 ± 1 mN m−1

5

a

5

10

15 20 25 30 35 45 15

15 16 18 21 23 28

20 30 35 40 50

a

a

2 3 6 10

πc mN m−1 ± 1 mN m−1 42

2 ALC ∞ Å ± 2 Å2 a

2 ALE ∞ Å ± 2 Å2

87

2 ALC 0 Å ± 2 Å2

2 ALE 0 Å ± 2 Å2

ΔH kJ mol−1

ΔS kJ mol−1

a

a

57

76

2311

8.3

1635 1480 1372 1368 1308 1166

5.7 5.0 4.6 4.5 4.2 3.7

3411 3326 2545 2370

11.2 10.8 8.1 7.3

53 53 54 54 54 51 45

60 61 62 62 62 64 59

86 88 88 88 88 88

57 58 59 58 58 59

70 70 69 68 68 67

a

a

a

45 48 49 50 50

59 59 59 60 60

a

a

a

92 92 92 92

59 59 58 59

86 84 78 77

No LE/LC transition.

Figure 2. Compression isotherms of (a) di(F8H2)MP and (b) di(F9H1)MP at various temperatures.

for di(F9H1)MP between 25 and 30 °C. For all temperatures, the pressure at which the LE/LC transition occurs (πt) is substantially higher for di(F8H2)MP than for di(F9H1)MP, which indicates that the energy required to obtain the LC phase is lower for the latter reflecting stronger chain interactions. The isotherms show that for di(F9H1)MP, in which the fluorinated character is the most pronounced and the spacer is reduced to one CH2, the domain of occurrence of the LC phase is much larger than for di(F8H2)MP. The 2D macroscopic compressibilities, Csmacro, of the monolayers were evaluated from the slope of the π/A isotherms for both phases at various T values. For both compounds, Cmacro s of the LC and LE phases did not change significantly with T. The monolayer behavior is characterized by the variation of πt as a function of temperature. Figure 3 shows that πt varies linearly with temperature in agreement.24 The slope of the π/T curve gives the degree of thermal expansion of the monolayer, that is 0.447 mN m−1 K−1 and 0.417 mN m−1 K−1 for di(F8H2)MP and di(F9H1)MP, respectively. These values are similar, indicating a similar behavior of the monolayers with T.

The compression isotherms (shown in Figure 1) are, however, strikingly different depending on the di(FnHm)MPs investigated. At 25 °C, the monolayer of di(F6H2)MP presents only a liquid expanded (LE) state as the condensed phase, that of di(F9H1)MP presents only a liquid condensed (LC) state, whereas that of di(F8H2)MP displays a LE/LC transition. The characteristics of the isotherms are displayed in Table 1. Di(F9H1)MP and di(F8H2)MP have the same chain length and differ only by the replacement of one CF2 by one CH2. This shows the strong effect of fluorinated chains on the monolayer organization. The compressibilities of the LC phase ∼2.5 m of di(F9H1)MP and of di(F8H2)MP are similar (Cmacro s N−1, compressibility modulus κ∼0.4 N m−1). It is a priori surprising that such a small change in spacer length has such a strong impact on the LE/LC coexistence plateau, while there would be no apparent effect on the compressibility of the LC phases. This situation has been addressed by GIXD (below). Influence of Temperature. The effect of temperature, T, on the compression isotherms of di(FnHm)MPs is summarized in Figure 2 and Table 1. The LE/LC phase transition appears 12024


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thickness). Two types of information were extracted from the X-ray diffraction data, the in-plane scattering peak position (Qxy), which is related to the Bragg peak and gives access to the d spacing of the diffraction planes, and the out-of-plane peak (Qz), which is related to the tilt angle and tilt direction. The results obtained at 25 °C are collected on Figures 4−7.

Figure 3. Linear variation of the LE/LC transition pressure πt as a function of temperature T for (a) di(F8H2)MP and (b) di(F9H1)MP.

The critical temperature of the LE/LC transition, (i.e., the temperature at which ΔH = 0) is calculated to be 115 °C for di(F8H2)MP and 95 °C for di(F9H1)MP. The extrapolated temperature, T0, corresponding to πt = 0, is the temperature at which the phase transition disappears. T0 is 27 °C for di(F9H1)MP and −18 °C for di(F8H2)MP. A large difference of 45 °C is thus observed for T0 between the two compounds. Knowing that the LE/LC transition reflects chain/ chain interactions, this means that interactions are stronger for di(F9H1)MP. Such analysis overlooks the triple point temperature that occurs at too low temperature to be detected. Having in mind that the typical increase in T0 per CH2 in a hydrogenated chain is typically of 11 °C,24 the value of 45 °C is exceptionally large especially for two molecules of same length. It is likely that the mobility induced by the presence of two CH2 (that can make a gauche defect) rather than only one (that is immobilized between the phosphate group and the Falkyl chain) brings disorder in the hydrophobic chains, thus favoring the LE phase. The effect of increased mobility of the spacer can be compared to an increase in polar head size. When lowering T, the defects tend to be reduced and a LC phase is eventually obtained at πt. The LE/LC transition is mainly driven by the competition between the chain entropy, which is proportional to the number of carbons N, and the cohesion energy. The latter is proportional to N when restricted to the interaction with the first neighbors, but varies as Nα (α > 1) when the interactions with the other neighbors are considered. Adding a CF2 group does not change the possible chain conformation number, contrary to the addition of a CH2 group in the spacer. Therefore, such a high variation in T0 demonstrates that suppressing a CH2 group has a strong influence on the chain entropy. The energy associated to the LE/LC transition in the monolayers was calculated using the Clausius−Clapeyron LC equation: dπt/dT = ΔH/T(ALE 0 − A0 ), ΔH being the enthalpy associated to the LE/LC transition, A0LE and A0LC the extrapolated molecular areas in the LE and LC phases, respectively. The entropy associated to the transition was calculated by ΔS = ΔH/T. The transition enthalpy and entropy are similar for both di(F9H1)MP and di(F8H2)MP (ΔH = 14 and 12 kJ mol−1 and ΔS = 44 and 43 kJ mol−1, respectively) meaning that the transitions are of same (LE/LC) nature. X-ray Diffraction. Compression isotherms give macroscopic information on monolayer’s behavior. GIXD allows characterization of the monolayer’s phase structure (unit cell, tilt angle, lattice compressibility, molecular area, monolayer

Figure 4. Grazing incidence X-ray diffraction (GIXD) data for di(F8H2)MP at different surface pressures. Temperature was 25 °C.

For di(F8H2)MP, Qxy diffraction peaks were measured with π ranging from 20 to 45 mN m−1 (Figure 4), no diffraction peak being detected at π < 20 mN m−1, that is in the LE phase. The first-order diffraction peak consists of a single Qxy peak, which varies from 1.22 Å−1 to 1.24 Å−1 depending on π together with a second-order peak detected at √3Qxy (2.13 Å−1) (Figure 5).

Figure 5. Grazing incidence X-ray diffraction (GIXD) data for di(F8H2)MP at 45 mN m−1. A second-order peak is detected at Qxy = 2.14 Å−1. Temperature was 25 °C.

A single peak (without splitting) is observed for both compounds, whatever the thermodynamical conditions. Therefore, it is logical to interpret these spectra as the signature of a hexagonal packing of the chains. Moreover, the corresponding hexagonal unit cell is in perfect agreement with the chain’s cross section. It is obvious, however, that the unit cell must include two chains. Therefore, we must consider a rectangular cell which two parameters a and b with b = a√3. This relation induced a degenerescence of the diffraction peaks of the rectangular lattice, which are located at the same Q value than the ones from the hexagonal lattice. At 45 mN m−1, the maximal π investigated, a = 5.85 Å, b = 10.13 Å (±0.01 Å) and A0 = 59.2 Å2 (±0.1 Å2). The characteristic a and b parameters are given in Table 2 for the various π. From 20 to 45 mN m−1, 12025


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the d spacing decreases significantly from 5.13 Å to 5.07 Å (±0.01 Å). Table 2. X-ray Diffraction Data for di(F9H1)MP and di(F8H2)MP; π Is the Surface Pressure, Qxy the in-Plane Wave Vector, d the Inter Reticular Distance, a and b the Parameters of the Hexagonal Lattice and A the Molecular Area di(F9H1)MP

di(F8H2)MP

π mN m−1

Qxy Å−1

dÅ

bÅ

aÅ

A0 Å 2

0.3 2.0 4.0 7.6 11.5 15.0 23.0 28.0 33.0 20.5 22.0 24.0 26.0 30.0 35.0 40.0 45.0

1.239 1.240 1.241 1.243 1.245 1.247 1.247 1.246 1.250 1.252 1.254 1.224 1.225 1.227 1.229 1.231 1.235 1.239 1.241

5.069 5.067 5.063 5.056 5.047 5.039 5.040 5.042 5.027 5.018 5.010 5.129 5.127 5.117 5.110 5.100 5.084 5.069 5.061

10.13 10.13 10.12 10.11 10.09 10.07 10.07 10.08 10.05 10.03 10.01 10.26 10.26 10.24 10.22 10.20 10.17 10.14 10.13

5.85 5.85 5.84 5.84 5.83 5.82 5.82 5.82 5.80 5.79 5.78 5.93 5.92 5.91 5.90 5.89 5.87 5.86 5.85

59.3 59.2 59.1 59.0 58.8 58.6 58.6 58.6 58.3 58.1 57.9 60.8 60.8 60.5 60.4 60.1 59.8 59.4 59.2

Figure 7. Grazing incidence X-ray diffraction (GIXD) data for di(F8H2)MP at 45 mN m−1. A second-order peak is detected at Qxy = 2.17 Å−1. Temperature was 25 °C.

between a LC phase and a LE or a gaseous phase, the number of the LC phase domains increasing upon compression. The positional correlation length ξ was calculated from the full width of the Bragg peak at its half height (fwhh). For di(F9H1)MP, ξ is resolution limited (ξ ≤ 0.007 Å−1), which corresponds to organized domains that can reach ∼800 Å in size. For di(F8H2)MP, ξ is ∼0.007 Å−1, which means domains of ∼700 Å, that is on the same order than for di(F9H1)MP. The determination of the out-of-plane wave vector Qz is key to precise description of the hexagonal lattices. Figure 8 shows the variation of the diffracted intensity as a function of Qz. In a perfect 2D crystal, the Bragg rods should not present any intensity variation along Qz. However, Langmuir monolayers do present a nonvanishing thickness related to the molecule length. This induces a modulation of the intensity along the rod following a cardinal sinus function behavior. If the diffracted plane formed by the molecules is oriented perpendicular to the surface, a simple calculation demonstrates that one should have a maximum of the cardinal sinus function located at Qz = 0. If the molecules are tilted, and consequently also the diffraction plane, the calculation indicates that the cardinal sinus function is dephased along Qz inducing a shift of the maxima along Qz. The maximum of intensity is almost at Qz = 0 Å−1 (∼0.05 −1 Å ), which means that the molecules are oriented normal to the water surface. The small shift does not result from the X-ray scattering by the monolayer but from an optical phenomena. Indeed, applying the principle of identical trajectory of light when the propagation is reversed, one understands that, if photons need to reach the interface at the critical angle of incidence to propagate parallel to the interface, they will leave this interface with the same angle if they are diffacted in the direction of the interface plane (that is Qz = 0). The slight maximum at Qz = 0.7 Å−1 corresponds to the second maximum of the sinus function, which is attenuated by geometrical effects. Also important is the observation that the position of the peak does not vary with π meaning that the molecules remain vertical throughout the pressure range investigated. The thickness of the monolayers can be calculated from these data to be ∼14.0 Å for both di(F8H2)MP and di(F9H1)MP). This value is in agreement with that estimated considering fully extended, nontilted chains (∼15.3 Å). The variation of the inter-reticular distance d as a function of π at 25 °C is given in Figure 9, which shows an essentially linear decrease for both compounds. The slopes of the two lines are different indicating that the lattice compressibilities are different. The microscopic 2D

For di(F9H1)MP, the first-order Bragg peak Qxy increases from 1.23 to 1.25 Å−1 when π increases from 2 to 33 mN m−1, which corresponds to a decrease of the lattice spacing from 5.06 Å to 5.02 Å (Figure 6, Table 2). As for di(F8H2)MP, an

Figure 6. Grazing incidence X-ray diffraction (GIXD) data for di(F9H1)MP at different surface pressures. Temperature was 25 °C.

hexagonal array is confirmed by a second-order peak at √3Qxy, that is at 2.17 Å−1 (Figure 7). At 45 mN m−1, the maximal π, a = 5.78 Å, b = 10.01 Å (±0.01 Å) and A0 = 57.9 Å2 (±0.1 Å2). It is noteworthy that the A0 value of di(F9H1)MP is significantly lower than that of di(F8H2)MP, which means that the hexagonal phase is more compact, likely due to reduced disorder transmitted to the hydrophobic chains by a spacer consisting of only one, nonmobile CH2. Also, contrary to di(F8H2)MP, scattering peaks were detected at very low surface density and at essentially zero π. The observation of diffraction peaks at zero π confirms a first-order transition 12026


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Figure 8. Left: Variation of the integrated intensity, Iint, as a function of the out-of-plane vector, Qz (rod scans); red: di(F8H2)MP, black: di(F9H1)MP. Right: Contours of equal intensity versus the in-plane and out-of-plane scattering vectors components Qxy and Qz. π = 30 mN m−1. Temperature was 25 °C.

di(F8H2)MP, respectively. This means that other phenomena such as domain coalescence or precollapse may occur and that their impact on the macroscopic compressibility differs between the two compounds. The fact that the molecules remain untilted (Qz ∼0) throughout the compression experiments originate from a decrease in lattice size, establishes that Cmicro s and not from a change in tilt angle, the latter situation being generally observed in typical nonfluorinated surfactant monolayers.20 Altogether, all our X-ray diffraction data establish that di(F8H2)MP and di(F9H1)MP are both organized in an essentially untilted hexagonal array. The d spacing was found to decrease continuously as a function of π. This decrease in d spacing is not related to a decrease in tilt angle but to a decrease in the surface area A of untilted chains. Fluorinated chains are more rigid than hydrocarbon chains and are characterized by low intermolecular interaction forces. They are thus prone to display an azimuthal distribution of individual molecular tilt angles rather than a collective tilt. An azimuthal tilt was introduced in molecular dynamics simulations of FnHmCOOH Langmuir monolayers (together with the helical conformation of the F-chain) and allowed to better match with the experimental result of the zero collective tilt.26 The azimuthal distribution of molecular tilts averages the collective tilt angle to near zero. However, this phenomenon has never been evidenced experimentally. In the case of the FnHmCOOH, the d spacing was not observed to decrease with π. We suggest that for di(FnHm)MP the molecular azimuthal tilts decrease when pressure increases leading to a decrease in the average d spacing. A calculation of the minimal surface area A0 at high π (Table 2) shows a significant difference in packing − the more fluorinated surfactant with the shortest spacer being the more compact. The compression isotherms were reconstructed by plotting the variations of π as a function of A0 determined by GIXD. Figure 11 shows that, for di(F9H1)MP, the reconstructed isotherm is close to the macroscopic isotherm experimentally obtained from film compression. The discrepancy seen at low π likely originates from compression of the LC phase domains present in the coexistence region. The discrepancy is more

Figure 9. Variation of the d spacing as a function of surface pressure, π, for (a) di(F8H2)MP and (b) di(F9H1)MP. Temperature was 25 °C.

compressibility Cmicro was calculated using the GIXD data. To s obtain further insight, the variation of A0 was plotted as a function of π (Figure 10). It shows that the slope of the straight line is higher for di(F8H2)MP than for di(F9H1)MP. For example, the microscopic compressibility values are 0.725 × 10−3 m mN−1 for di(F9H1)MP and 1.150 × 10−3 m mN−1 for di(F8H2)MP. These values correspond to LC phases although they are low for such phases.25 They correspond to 35% and 57% of the macroscopic compressibilities of di(F9H1)MP and

Figure 10. Variation of molecular area, A, as a function of surface pressure, π, as determined by GIXD for (a) di(F8H2)MP and (b) di(F9H1)MP. Temperature was 25 °C. 12027


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observed condensing effect of the hexagonal lattice. When the number of CH2 increases, such as in F8H4COOH, the flexibility of the spacer increases resulting in poor organization14 or a loss of orientational order as observed in Langmuir monolayers of semifluorinated alkanes.28,29 The diffraction data of both di(FnHm)MP and FnHmCOOH, showed no evidence of the existence of a collective tilt, as was observed in monolayers of arachidic acid.30,31 A model of close packed monolayer with an orientational ordering of the long axes of the molecules was found to be consistent with the diffraction data for F11COOH, while for F10H1COOH either of two models, that is free rotating molecules or azimuthal ordering, appear to be equally consistent with the data.16 By contrast with the FnHmCOOH case, the lattices of di(F8H2)MP and di(F9H1)MP are compressible in the LC phase indicating a reduction of the azimuthal tilt with increasing pressure. At the highest surface pressure, the molecular area of di(F9H1)MP becomes similar to the cross-section of a F-chain, which means that no molecular tilt remains.

Figure 11. Plots of the variation of surface pressure, π, as a function of molecular area A0 determined by GIXD (blue dots) as compared to experimental compression isotherms (black) at 25 °C for a) di(F9H1)MP and b) di(F8H2)MP.

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CONCLUSIONS We have shown that two surfactants with a monomorpholinophosphate headgroup and two short highly fluorinated hydrophobic chains (8 and 10 carbon atoms), di(F9H1)MP and di(F8H2)MP, form stable ultrathin monolayers. Changing the length of the H-spacer from 2 CH2 to 1 CH2 has an unexpectedly remarkable impact on chain/chain interactions, hence on monolayer organization, decreasing by 45 °C the LC/ LE temperature (for the same surface pressure). GIXD data establish that the hydrophobic chains of di(F9H1)MP and di(F8H2)MP are essentially untilted and form hexagonal lattices at the surface of water with a coherence length of ∼700 Å. The lattice of di(F9H1)MP is more compact than that of di(F8H2)MP. This is explained by the fact that the hydrocarbon spacer, one or two CH2, plays a key role in the packing of the molecules by providing, in the second case only, conformational mobility to the surfactant structure. In both cases, the dense phase is compressible indicating that, contrary to what was seen for hydrocarbon carboxylic acids, there likely exists an azimuthal distribution of the molecular tilt that is progressively reduced upon compression.

pronounced for di(F8H2)MP; there is in this case a significant loss of molecules at high π. It means that the monolayer of di(F8H2)MP is less stable than that of di(F9H1)MP in the LC phase likely due to the higher hydrophobicity of the latter. Comparing di(F9H1)MP and di(F8H2)MP leads to the conclusion that shortening the spacer from two CH2 to one CH2, hence reducing its mobility, contribute more significantly to the stability of the monolayer than the fact of having lengthened the chain by one CF2. Little is known concerning the compression and structure of monolayers of amphiphiles with partially fluorinated chains. It is therefore interesting to compare the X-ray diffraction data collected here with those reported for two partially fluorinated carboxylic acids (CnF 2n+1(CH2 )m COOH, FnHmCOOH), C 10 F 21 CH 2 COOH (F10H1COOH), and C 11 F 23 COOH (F11COOH).14,16,27 Though di(FnHm)MPs and FnHmCOOH present the common feature of both having a small polar head with respect to their cross section, they also present major structural differences. Whereas di(FnHm)MPs are neutral and double-chained, the carboxylic acids are charged at high pH and single-chained. The two FnHmCOOH were shown to present a first-order transition between an ordered condensed phase and a disordered dilute phase. The minimal A0 was smaller for F10H1COOH (29.0 Å2) than for F11COOH (29.7 Å2). The increase in A0 when 1 CH2 is substituted by 1 CF2 was explained by a change in the potential energy due to the presence of one CH2 near the headgroup. In the two FnHmCOOH investigated, the spacer is reduced to only one (F10H1COOH), or zero (F11COOH), CH2 groups, which prevents conformational mobility. In this study, it is the compound with the shortest spacer (di(F9H1)MP) that has the smaller area (29.0 Å2, as compared to 29.6 Å2 for di(F8H2)MP)). When the spacer consists of two CH2, formation of gauche defects becomes possible, thus allowing conformational mobility. This flexible character is prone to communicate disorder to the hydrophobic chains. However, in the case of di(F9H1)MP, the unique CH2 is immobilized between the phosphate group and F-alkyl chain. Interactions between F-alkyl chains can occur resulting in the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: kraff[email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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