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Compression driven 2D nematic phase in a columnar Langmuir monolayer
Optical Materials 34 (2012) 1692–1696
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Compression driven 2D nematic phase in a columnar Langmuir monolayer A. El Abed a,⇑, M. Goldmann b a b
UMR S775, Translational Research and Microfluidics Group, Paris Descartes University, 45 rue des Saints-Pres, 75006 Paris, France INSP, Pierre & Marie Curie University, 4 Place Jussieu, 75005 Paris, France
a r t i c l e
i n f o
Article history: Received 26 January 2012 Received in revised form 21 February 2012 Accepted 22 February 2012 Available online 31 March 2012 Keywords: Langmuir films Molecular organization Grazing incidence X-ray diffraction Liquid–crystals Langmuir monolayers Nematic ordering
a b s t r a c t Langmuir films of pyramidic liquid crystals were studied using surface pressure versus molecular area isotherms and synchrotron grazing incidence X-ray diffraction. The used molecule, named 3BCN/14, consists of a pyramidal central core to which are bound symmetrically six lateral C14 alkyl chains. These molecules spread spontaneously at the air–water interface in a metastable side-on phase which relax rapidly upon compression towards a stable edge-on phase. Our results suggest that the new edge-on phase consists of an in-plane organization of columns which are made of about 11 stacked edge-on molecules. This structure remains stable after several expansion–compression cycles. Comparing these results with those obtained previously on two other pyramidic liquid crystals with shorter and longer lateral alkyl chains, C9 and C15 respectively, we attribute the formation of the obtained 2D nematic phase to a suitable lateral chains length which allow for the establishing of strong short smectic order within of the 3BCN/14 columns. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The development of very sensitive techniques implemented at the air–water interface, such as grazing X-ray diffraction [1], allowed for the understanding of the phase diagram of Langmuir films [2] in terms of orientation of hydrocarbon chains of amphiphiles by regards to the normal to the air–water interface: nowadays, several condensed phases of classical Langmuir monolayers can be viewed as 2D mesophases [3]. Nevertheless, since the top and the bottom of classical amphiphilic molecules are distinct, the observed 2D mesophases can be mainly considered as 2D polar nematics and not as genuine 2D nematic phases as originally proposed by de Gennes [4]. Langmuir monolayers of discotic molecules [5–9] or those formed by pyramidic molecules [10,11], which differ from discotics only by the pyramidal geometry of the central core (see Fig. 1e), have the potential to exhibit genuine 2D nematic and 2D smectic phases at the air–water interface. These molecules consist of a central core to which several lateral hydrocarbon chains may be attached. They generally exhibit columnar mesophases in the bulk state [12–16]. They present also an amphiphilic feature which allows for the formation of original side-on and edge-on monolayers at the air–water interface. Such nomenclature refers to the orientation of the central core of the molecules by regards to the air–water interface. It corresponds respectively to cores lying with their base ⇑ Corresponding author. E-mail addresses: [email protected] (A. El Abed), M.Goldmann@ insp.jussieu.fr (M. Goldmann). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.02.034
in a plane parallel and normal to the air–water interface as sketched in Fig. 1. An interesting feature of edge-on monolayers is that the columns where molecules stack may be ordered in one direction and not in the other. In such a manner, the resulting columnar organization may be regarded as a 2D smectic [8]. However, it has been shown that thermal fluctuations destroy quasi-longrange smectic order in 2D systems [17]: only nematic order is possible on sufficiently long scales whereas a smectic order could still persist on a shorter scale. The present study concerns the original behavior exhibited by the Langmuir monolayer of pyramidic liquid crystal, namely 3BCN/14, where the six alkyl chains consist of myristoyl chains C13H25C(O)OA. We show, using p A isotherms and grazing incidence X-ray diffraction (GIXD), that the length of C14 lateral alkyl chains is suitable to allow for strong short smectic order within each column, up to about 11 molecules, which persists after several cycles of compression-expansion. Such smectic order impedes the retrieval of the original side-on phase and leads to the formation of a genuine nematic Langmuir monolayer made of nanorods-like edge-on columns. 2. Experimental The 3BCN/14 compound was synthesized and purified by Zimmermann [13]. It exhibits in the bulk state columnar mesophases, between 73 °C and 135 °C approximately, as observed by polarizing optical microscopy [13]. Its exact structure was not determined yet by X-ray diffraction as it was done for many other pyramidic compounds [14–16].
A. El Abed, M. Goldmann / Optical Materials 34 (2012) 1692–1696
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Fig. 2. Surface pressure p A isotherms of a 3BCN/14 Langmuir monolayer obtained upon three successive compressions. Molecules may adopt a side-on or edge-on orientations upon compression as sketched in Fig. 3.
Fig. 1. Schematic representations of the edge-on (a, c) and side-on (b, d) organizations of discotic and pyramidic molecules; (e) molecular structure of pyramidic compounds: R = Cn1H2n1C(O)OA, n = 14 in the present study.
The grazing incidence X-ray diffraction (GIXD) were performed on the D41B beam line at the LURE synchrotron source (Orsay, France). The experimental set-up and procedure are described in more detail in references [18,19]: a bent Ge (1 1 1) crystal was used to select the X-ray wavelength, k = 0.1646 nm and a silica glass mirror was used to deflect the X-ray beam onto the monolayer spread at the air–water interface at an angle equal to 2.0 mrad. The beam is collimated between the mirror and the Langmuir trough through Huber slits with a vertical height of 100 lm and a width of 5 mm. Scattered X-rays are collected by a vertical circular Position Sensitive Detector (PSD) of a radius equal to 0.550 m which allows for rod scan measurements from 0° to 13°. The Qxypattern, corresponding to the diffracted intensity integrated over the vertical wave-vector transfer Qz, allows for the determination of the cell parameters of the two-dimensional lattice. The tilt angle is deduced from the intensity distribution along the surface normal z (Bragg rod). Soller slits are used for horizontal collimation, their full range angle of acceptance being 2.6 mrad. The resolution in the wave vector transfer, Q, is about 0.01 Å1 (FWHM). The integration range along Qz is from 0 to 0.8 Å1 owing to the vertical size of Soller slits. GIXD measurements were realized at the air–water interface under a helium saturated atmosphere which allow for the avoidance of damaging the probed monolayer by the X-ray beam. This was checked by measuring twice the diffracted intensity at each single point. The 3BCN/14 molecules were spread from chloroform solution (Normapur, Prolabo). Subphase water was purified using a UHQ II Elga system. Surface film pressure p versus molecular area A isotherm diagrams were obtained using a R & K Langmuir balance at 20 °C and pH = 5.7. Air humidity was about 50%. The trough temperature was controlled by using RM6 Lauda thermostat. The surface pressure was measured with an accuracy of about 0.1 mN/m.
area of about 3.0 nm2, until the occurrence of a hump at p ’ 9 mN/m and A ’ 2.3 nm2; the appearance of such humps is generally interpreted as a nucleation barrier for the formation of a new stable phase. Taking into account of the molecular area values, one may suggest that 3BCN/14 molecules may adopt a side-on arrangement, as sketched in Fig. 3a, where each lateral hydrocarbon chain would occupy an area Ach of about 0.5 nm2. On further compression, the p A isotherm diagram exhibits a quasi-plateau at p ’ 8 mN/m. If one stops compressing the film in this region of the p A isotherm diagram, as shown in curve (a) of Fig. 2, then one observes a rapid relaxation of the film surface pressure until p 0, depending on the mean molecular area value at which the compression stop is realized. The observed p relaxation process indicates, as we show in this paper, the occurring of phase transition from a metastable side-on phase to a new edge-on stable phase, as sketched in Fig. 3c. If one expands and re-compresses the film, one obtains new reproducible p A isotherm diagrams which differ significantly from the one obtained upon the first compression, as shown from curves (b) and (c) of Fig. 2. The new phase corresponds to a monolayer where each 3BCN/14 molecule
3. Results and discussion 3.1. p A isotherm diagrams Fig. 2a shows the p A isotherm diagram recorded upon a first continuous compression of a freshly spread 3BCN/14 monolayer. The surface pressure p starts to increase regularly at a molecular
Fig. 3. Suggested molecular organization models as adopted upon compression (a) side-on, (b) edge-on and (c) nematic.
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occupies an area of about 1.30 nm2 at p = 0 mN/m. Its measured compressibility coefficient 21 m N1, defined as A1 ddAp , indicates more likely that this phase is a liquid-like [2]. Taking into account of the molecular area values, 3BCN/14 molecules may organize at the air–water interface either side-on or edge-on. In the first model, the area Ach occupied by each hydrocarbon chain would be about 0.21 nm2 and the corresponding phase would be a solidelike. In the second model, each alkyl chain would occupy an area of about 0.42 nm2 or 0.32 nm2, in agreement with the liquid-like feature of the observed phase, depending on whether three or four alkyl chains are assumed to extend upwards, respectively. Moreover, the nonreversible feature of the observed side-on/edge-on phase transition indicates more likely that edge-on 3BCN/14 molecules aggregate into nanoscopic edge-on coulmns, i.e., nanorods. Such nanorods start to interact upon compression at a molecular area of about 1.30 nm2, as sketched in Fig. 3c. It is interesting to compare p A isotherm diagrams obtained on successive compression and expansion of the 3BCN/14 compound with those obtained on the 3BCN/9 compound (which are reported for the first time in the present study). Indeed, if one stops compressing (or expanding) the 3BCN/9 film, one observes, as shown on Fig. 4, a small decrease (or increase) of the surface pressure to an equilibrium value which is located between the compression value and the expansion value. One should note also that the p A isotherm diagram obtained on further successive compressions of the 3BCN/9 are similar to the one obtained upon the first compression. By comparing the behavior of the 3BCN/14 and the 3BCN/9 films upon compression and expansion, one may conclude that the side-on ? edge-on phase transition is indeed reversible in the case of the 3BCN/9 film and it is not in the case the 3BCN/14 film. From a fundamental point of view, the formation of a side-on or an edge-on Langmuir monolayer from a ‘‘columnar’’ liquid crystal (LC) should be governed by the hydrophilic/hydrophobic balance, i.e., the length of the alkyl side chains: on one hand, intermolecular LC/LC interactions within the film promote the formation of columns, and on the other hand, interactions between LC and the subphase molecules orient individual LC molecules at the air–water interface and prevent the formation of columns. For instance, El Abed et al. have shown, in two previous studies [10,11], that 3BCN/9 pyramidic molecules with shorter C9 lateral alkyl chains undergo a reversible side-on ? edge-on phase transition upon compression, whereas 3BCN/15 pyramidic molecules with longer lateral C15 alkyl chains exhibit a transition from a liquid side-on
monolayer to a condensed side-on monolayer [11]. To underline the effect of alkyl chains length on the molecular organization of Langmuir monolayers, one may cite the original study reported by Weinbach et al. [20] on the spreading of alkanes H(CH2)nHat the air–water interface. These authors showed that for n > 30 alkanes spread spontaneously as stable monolayers, whereas for 20 < n < 30, alkanes spread as multilayers at the air–water interface. One may cite also another interesting study reported more recently by Vaknin et al. [21] where it is shown that arachidic acid molecules may organize, during the collapse process in presence of Ca2+ cations, as trilayers where hydrophobic alkyl chains are in contact with the water subphase. Also, one may estimate roughly the cost in free energy per molecule from a side-on to an edge-on orientation, by assuming that interactions in unionized monolayers should be dominated by hydrogen bonding and dipolar interactions [21]. (i) The cost in free energy per alkyl chain needed to bring an alkyl chain in contact with the water subphase is about 6kBT [21], i.e., 12kBT if two chains are assumed to be brought in contact with the water subphase. Indeed, one may remark that, in the edge-on phase, alkyl side chains should be asymmetrically distributed around the central core as shown for example by El Abed et al. in another study on Langmuir monolayers of bowlic compounds [22]. A simple molecular organization model indicates that in order to allow for an edge-on arrangement, pyramidic molecules may get only two alkyl chains in contact with the water. (ii) The cost in free energy needed to expel a hydrophilic group from the water subphase is about 6kBT, originating mainly from the loss of hydrogen bonding [21], i.e., 36kBT for the six hydrophilic groups. (iii) The gain in free energy per alkyl chain when hydrocarbon chains are brought close to each other in the edge-on columnar organization (VdW interactions), is about 3kBT per added ACH2A [25]. This corresponds to a difference in free energy between the 3BCN/9 and the 3BCN/14 by about 15kBT per chain, i.e., 60kBT for the four upper alkyl chains. We assumed that the lower alkyl chains are in a disordered state, due to the presence of the water subphase, in a such manner we may neglect their VdW interactions. As one can notice, the net gain in free energy balance when increasing the length of the alkyl side chains from C9 to C14 would be (12kBT). Such an estimation indicates that the edge-on orientation may be indeed more stable than the side-on orientation for the 3BCN/14 monolayer. Also, comparing p A isotherm diagrams obtained on first and second compressions may allow for a calculation of the energy barrier needed to transit from the metastable side-on phase to the stable edge-on phase of the 3BCN/14 monolayer. Such energy should be equal to the film compression work pDA 3.5kBT, where p ’ 8mN/m and DA ’ (3.0 1.3)nm2. We may suggest that the energy barrier for the side-on/edge phase transition in the case of the 3BCN/15 film is probably too high which cannot not be overcome upon compression before the collapse of the film occurs.
π
3.2. Grazing incidence X-ray diffraction
2
Fig. 4. Surface pressure p A isotherm of 3BCN/9 Langmuir monolayer obtained upon successive compression and decompression. Dashed curve represents the equilibrium isotherm diagram. 3BCN/9 molecules undergo a reversible side-on/ edge-on phase transition upon compression as sketched in Fig. 3.
In order to characterize the structure of the new edge-on phase, we performed grazing incidence X-ray diffraction (GIXD) experiments on a single monolayer of 3BCN/14 molecules re-compressed to a surface pressure of about p = 6 mN/m. At such pressure, the molecular area is about 116 Å2. One observes a single X-ray diffraction peak at an in-plane Qxy scattering wavevector of about 1.43 Å1 as shown in Fig. 5. The observed peak is rather large, with a FWHM of about 0.127 Å1 which indicates that the correlation length is about 50 Å. A rodscan analysis shows (Fig. 6) that this peak has its maximum intensity in the diffraction plane. Consequently, the diffracting planes should be oriented normal to the
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Fig. 5. X-ray diffraction pattern of the 3BCN/14 monolayer recorded at p = 6 mN/m and a molecular area of about 116 Å2: a single large diffraction peak is observed at Qxy = 1.43 Å1. It corresponds to an ordering of 3BCN molecules in one direction along the edge-on column axis.
monolayer interface. On the basis of these data, two models may be considered for the in-plane molecular organization. In the first model, one may assume that the observed diffraction peak at Qxy = 1.43 Å1 is due to the ordering of alkyl side chains. The resulting 2D hexagonal chain would possess a lattice parameter equal to a = 5.08 Å and an area per alkyl chain of about 2 AC ¼ 22:3 Å . This model should be rejected as argued hereafter. Indeed, if one assumes that the pyramidic molecules adopt a side-on organization, in order to allow for the formation of a dense packed alkyl chains, then the resulting 3BCN/14 molecular area 2 2 would be equal to 134 Å ð¼ 22:3 Å 6Þ which is significantly 2 greater than the measured 116 Å value deduced from the p A isotherm at p = 6 mN/m. Though we are aware that the behavior of tethered chains may be very different by regards to that of free chains, as observed for examples in studies on Langmuir monolayers of hairy-rod polypeptides [23] or on bowlics [22], the observed GIXD peak in the present study could not originate from an hexagonal packing of the lateral chains. Moreover, since alkyl chains 2 should possess in this model an area of about AC ¼ 22:3 Å , then one should admit that they should be tilted by an Consequently, one should observe two GIXD peaks: a first peak located at low Qxy values which should correspond to the two degenerate [11] ~ out-of-plane diffraction peaks and a second peak located and ½11 at higher Qxy which should correspond to the [02] in-plane diffraction peak [1,24]. Since only one peak is detected at Qxy = 1.43 Å1 1 and Q z ¼ 0 Å , one should reject this model. Also, rodscan analysis of the observed peak gives a value of about DQz = 0.57 Å1 which allows for the determination of the thickness of the diffracting slabs of about h ¼ D2Qpz ’ 11 Å. This value is much smaller than the calculated 19 Å for the length of a C14 alkyl chain. In the second model, we assume that the observed diffraction peak at Qxy = 1.43 Å1 originates from an in-plane organization of the pyramidic cores thanks to their columnar edge-on organization. Hence, the observed diffraction peak should correspond to an intracolumnar core-core spacing distance of about d = 4.4 Å. This value is in a very good agreement with the value reported by Poupko et al. [16] from X-ray diffraction data of the columnar mesophase exhibited by the 3BCN/13 compound [16]. Nevertheless, one may notice that our measured core-core distance value is smaller than the 4.8 Å value measured by Levelut et al. [15] for the 3BCN/10 and 3BCN/12. Such noticeable difference between these compounds which differ only by a small variation in the length of their alkyl side chains may appear surprising. Nevertheless, one may remark that the phase diagram of these compounds
Fig. 6. Contour plot of the X-ray diffraction scan presented in Fig. 5. Rod scan analysis shows that the peak observed at Qxy = 1.43 Å1 has its maximum intensity approximately in the diffraction plane, at Qz = 0.1 Å1.
depends strongly on the length of the alkyl side chains as reported by Zimmermann et al. [13]. For example, the 3BCN/10 compound exhibits a single biaxial mesophase in the 30 °C–150 °C tempearture range (labeled PD), the 3BCN/15 compound exhibits a single uniaxial mesophase in the 80 °C–150 °C temperature range (labeled PC), whereas the 3BCN/14 compound exhibits both PD and PC mesophases versus temperature, observed between 73 °C– 82 °C and 82 °C–135 °C, respectively [13]. As indicated above, the observed peak is rather large. This leads to a small correlation length of about 50 Å which indicates that the intracolumnar core-core positional ordering extents over about 11 molecules. Taking account of these data and of the p A isotherm diagram of the new edge-on monolayer, we suggest that 3BCN/14 molecules form a columnar phase where molecules adopt an edgeon configuration for all molecular area values. Such behavior could be explained by the formation, during the first compression, of nanoscopic columns (nanorods) which do not disaggregate upon the film decompression. The mean length of such nanorods should be of about 5 nm, i.e., correlation length as determined from GIXD data. In order to show clearly the formation of the columnar structure of the film, one may search for the presence of any GIXD peak which may originate from intercolumnar spacing. If such GIXD peak exists, it should be large, of weak intensity and should be detected at small
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wavevectors, corresponding to an intercolumnar spacing distance of about Unfortunately, in our experiments, the very low signal/ noise ratio, due to a large diffusion at such small wavevectors, does not allow for the detection of any GIXD peaks even when they exist. However, though we did not obtain clear evidence from X-ray data alone supporting the formation of nanorods, the obtaining of a reproducible p A isotherm diagrams (after a preliminary compression) of the 3BCN/14 film supports clearly the formation of a new ‘‘species’’ which start to interact at a ’’molecular’’ area of a about 1.30 nm2, i.e., at a chain area ’ 0.43 nm2. Such value indicates that, after the first compression, the 3BCN/14 molecules should remain oriented edge-on even at zero surface pressure. The building of such nanorods and their compression at the air– water interface should allow for the formation of a genuine nematic Langmuir monolayer as sketched in Fig. 3c. 4. Conclusion We have shown in this study that the pyramidic 3BCN/14 molecules spread spontaneously at the air–water interface to form a side-on phase which transits irreversibly to an edge-on phase upon compression. In this new phase, grazing incidence X-ray diffraction shows that the molecules pack to form stable columns of about 10 molecules length with strong short smectic order. The built-up nanorod-like structure remains stable upon several expansioncompression cycles. This stability impedes the retrieval of the side-on phase upon expansion and allows for the formation of a genuine nematic monolayer.
[2] [3]
[4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
References [1] K. Kjaer, J. Als-Nielsen, C.A. Helm, L.A. Laxbauer, H. Möhwald, Phys. Rev. Lett. 58 (1987) 2224;
[24] [25]
P. Dutta, J.B. Peng, B. Lin, J.B. Ketterson, M. Prakash, P. Georgopoulos, S. Ehrlich, Phys. Rev. Lett. 58 (1987) 2228. G. Gaines Jr., Insolubles Monolayers at Liquid-Gas Interfaces, Interscience, New York, 1966. A.M. Bibo, C.M. Knobler, I.R. Peterson, J. Phys. Chem. 95 (1991) 5591; G.A. Overbeck, D. Möbius, J. Phys. Chem. 97 (1993) 7999; V.M. Kaganer, H. Möhwald, P. Dutta, Mod. Phys. 71 (1999) 779. P.G. de Gennes, Faraday Symp. Chem. Soc. 5 (1971) 16. F. Rondelez, D. Koppel, J. Physique 43 (1982) 1371. O. Albrecht, W. Cumming, W. Kreude, A. Laschewsky, H. Ringsdorf, Colloid Polym. Sci. 264 (1986) 659. J.Y. Josefowicz, N.C. Maliszewskyj, S.H.J. Idziak, P.A. Heiney, J.P. McCauley Jr., A.B. Smith III, Science 260 (1993) 323. D. Gidalevitz, O.Y. Mindyuk, P.A. Heiney, B.M. Ocko, M.L. Kurnaz, D.K. Schwartz, Langmuir 14 (1998) 2910. O.Y. Mindyuk, Y. Oksana, P.A. Heiney, Adv. Mater. 11 (1999) 341. A. El Abed, P. Muller, P. Peretti, F. Gallet, J. Billard, J. Phys. II Fr. 3 (1993) 851. A. El Abed, J. Daillant, P. Peretti, Langmuir 9 (1993) 3111. S. Shandrasekhar, G.S. Raganath, Rep. Prog. Phys. 53 (1990) 57; S. Chandrasekhar, in: D. Demus, J. Goodby, G.W. Gray, H.-W. Spiess, V. Vill (Eds.), Handbook of Liquid Crystals, vol. 2B, Wiley-VCH, Weinheim, 1998, pp. 749–780 (chapter. 8). H. Zimmermann, R. Poupko, Z. Luz, J. Billard, Z. Naturforsch. 40a (1985) 149. J. Malthête, A. Collet, Nouv. J. Chimie 9 (1985) 151. A.M. Levelut, J. Malthête, A. Collet, J. Physique 47 (1986) 351. R. Poupko, Z. Luz, N. Spielberg, H. Zimmermann, J. Am. Chem. Soc. 111 (1989) 6094. J. Toner, D.R. Nelson, Phys. Rev. B 23 (1981) 316. P. Fontaine, M. Goldmann, M. Bordessoule, A. Jucha, Rev. Sci. Instrum. 75 (2004) 3097. A. Renault, C. Alonso, F. Artzner, B. Berge, M. Goldmann, C. Zakri, Eur. Phys. Journal. B 1 (1998) 189. S.P. Weinbach, I. Weissbuchi, K. Kjaer, W.G. Bouwman, J.A. Nielsen, M. Lahav, L. Leiserowitz, Adv. Mater. 7 (1995) 857. D. Vaknin, W. Bu, A. Travesset, Langmuir 23 (2007) 1888. A. El Abed, R. Ionov, M. Goldamann, P. Fontaine, J. Billard, P. Peretti, Europhys. Lett. 56 (2001) 234. M. Fukuto et al., Phys. Rev. E 66 (2002) 010601; M. Fukuto et al., J. Chem. Phys. 119 (2003) 6254. V.M. Kaganer, H. Mohwald, P. Dutta, Rev. Mod. Phys. 71 (1999) 779. J.M. Israelachvili, Intermolecular and Surface Forces, 2nd ed., Academic Press Inc., San Diego, 1987. pp. 71.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Compression driven 2D nematic phase in a columnar Langmuir monolayer A. El Abed a,⇑, M. Goldmann b a b
UMR S775, Translational Research and Microfluidics Group, Paris Descartes University, 45 rue des Saints-Pres, 75006 Paris, France INSP, Pierre & Marie Curie University, 4 Place Jussieu, 75005 Paris, France
a r t i c l e
i n f o
Article history: Received 26 January 2012 Received in revised form 21 February 2012 Accepted 22 February 2012 Available online 31 March 2012 Keywords: Langmuir films Molecular organization Grazing incidence X-ray diffraction Liquid–crystals Langmuir monolayers Nematic ordering
a b s t r a c t Langmuir films of pyramidic liquid crystals were studied using surface pressure versus molecular area isotherms and synchrotron grazing incidence X-ray diffraction. The used molecule, named 3BCN/14, consists of a pyramidal central core to which are bound symmetrically six lateral C14 alkyl chains. These molecules spread spontaneously at the air–water interface in a metastable side-on phase which relax rapidly upon compression towards a stable edge-on phase. Our results suggest that the new edge-on phase consists of an in-plane organization of columns which are made of about 11 stacked edge-on molecules. This structure remains stable after several expansion–compression cycles. Comparing these results with those obtained previously on two other pyramidic liquid crystals with shorter and longer lateral alkyl chains, C9 and C15 respectively, we attribute the formation of the obtained 2D nematic phase to a suitable lateral chains length which allow for the establishing of strong short smectic order within of the 3BCN/14 columns. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The development of very sensitive techniques implemented at the air–water interface, such as grazing X-ray diffraction [1], allowed for the understanding of the phase diagram of Langmuir films [2] in terms of orientation of hydrocarbon chains of amphiphiles by regards to the normal to the air–water interface: nowadays, several condensed phases of classical Langmuir monolayers can be viewed as 2D mesophases [3]. Nevertheless, since the top and the bottom of classical amphiphilic molecules are distinct, the observed 2D mesophases can be mainly considered as 2D polar nematics and not as genuine 2D nematic phases as originally proposed by de Gennes [4]. Langmuir monolayers of discotic molecules [5–9] or those formed by pyramidic molecules [10,11], which differ from discotics only by the pyramidal geometry of the central core (see Fig. 1e), have the potential to exhibit genuine 2D nematic and 2D smectic phases at the air–water interface. These molecules consist of a central core to which several lateral hydrocarbon chains may be attached. They generally exhibit columnar mesophases in the bulk state [12–16]. They present also an amphiphilic feature which allows for the formation of original side-on and edge-on monolayers at the air–water interface. Such nomenclature refers to the orientation of the central core of the molecules by regards to the air–water interface. It corresponds respectively to cores lying with their base ⇑ Corresponding author. E-mail addresses: [email protected] (A. El Abed), M.Goldmann@ insp.jussieu.fr (M. Goldmann). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.02.034
in a plane parallel and normal to the air–water interface as sketched in Fig. 1. An interesting feature of edge-on monolayers is that the columns where molecules stack may be ordered in one direction and not in the other. In such a manner, the resulting columnar organization may be regarded as a 2D smectic [8]. However, it has been shown that thermal fluctuations destroy quasi-longrange smectic order in 2D systems [17]: only nematic order is possible on sufficiently long scales whereas a smectic order could still persist on a shorter scale. The present study concerns the original behavior exhibited by the Langmuir monolayer of pyramidic liquid crystal, namely 3BCN/14, where the six alkyl chains consist of myristoyl chains C13H25C(O)OA. We show, using p A isotherms and grazing incidence X-ray diffraction (GIXD), that the length of C14 lateral alkyl chains is suitable to allow for strong short smectic order within each column, up to about 11 molecules, which persists after several cycles of compression-expansion. Such smectic order impedes the retrieval of the original side-on phase and leads to the formation of a genuine nematic Langmuir monolayer made of nanorods-like edge-on columns. 2. Experimental The 3BCN/14 compound was synthesized and purified by Zimmermann [13]. It exhibits in the bulk state columnar mesophases, between 73 °C and 135 °C approximately, as observed by polarizing optical microscopy [13]. Its exact structure was not determined yet by X-ray diffraction as it was done for many other pyramidic compounds [14–16].
A. El Abed, M. Goldmann / Optical Materials 34 (2012) 1692–1696
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Fig. 2. Surface pressure p A isotherms of a 3BCN/14 Langmuir monolayer obtained upon three successive compressions. Molecules may adopt a side-on or edge-on orientations upon compression as sketched in Fig. 3.
Fig. 1. Schematic representations of the edge-on (a, c) and side-on (b, d) organizations of discotic and pyramidic molecules; (e) molecular structure of pyramidic compounds: R = Cn1H2n1C(O)OA, n = 14 in the present study.
The grazing incidence X-ray diffraction (GIXD) were performed on the D41B beam line at the LURE synchrotron source (Orsay, France). The experimental set-up and procedure are described in more detail in references [18,19]: a bent Ge (1 1 1) crystal was used to select the X-ray wavelength, k = 0.1646 nm and a silica glass mirror was used to deflect the X-ray beam onto the monolayer spread at the air–water interface at an angle equal to 2.0 mrad. The beam is collimated between the mirror and the Langmuir trough through Huber slits with a vertical height of 100 lm and a width of 5 mm. Scattered X-rays are collected by a vertical circular Position Sensitive Detector (PSD) of a radius equal to 0.550 m which allows for rod scan measurements from 0° to 13°. The Qxypattern, corresponding to the diffracted intensity integrated over the vertical wave-vector transfer Qz, allows for the determination of the cell parameters of the two-dimensional lattice. The tilt angle is deduced from the intensity distribution along the surface normal z (Bragg rod). Soller slits are used for horizontal collimation, their full range angle of acceptance being 2.6 mrad. The resolution in the wave vector transfer, Q, is about 0.01 Å1 (FWHM). The integration range along Qz is from 0 to 0.8 Å1 owing to the vertical size of Soller slits. GIXD measurements were realized at the air–water interface under a helium saturated atmosphere which allow for the avoidance of damaging the probed monolayer by the X-ray beam. This was checked by measuring twice the diffracted intensity at each single point. The 3BCN/14 molecules were spread from chloroform solution (Normapur, Prolabo). Subphase water was purified using a UHQ II Elga system. Surface film pressure p versus molecular area A isotherm diagrams were obtained using a R & K Langmuir balance at 20 °C and pH = 5.7. Air humidity was about 50%. The trough temperature was controlled by using RM6 Lauda thermostat. The surface pressure was measured with an accuracy of about 0.1 mN/m.
area of about 3.0 nm2, until the occurrence of a hump at p ’ 9 mN/m and A ’ 2.3 nm2; the appearance of such humps is generally interpreted as a nucleation barrier for the formation of a new stable phase. Taking into account of the molecular area values, one may suggest that 3BCN/14 molecules may adopt a side-on arrangement, as sketched in Fig. 3a, where each lateral hydrocarbon chain would occupy an area Ach of about 0.5 nm2. On further compression, the p A isotherm diagram exhibits a quasi-plateau at p ’ 8 mN/m. If one stops compressing the film in this region of the p A isotherm diagram, as shown in curve (a) of Fig. 2, then one observes a rapid relaxation of the film surface pressure until p 0, depending on the mean molecular area value at which the compression stop is realized. The observed p relaxation process indicates, as we show in this paper, the occurring of phase transition from a metastable side-on phase to a new edge-on stable phase, as sketched in Fig. 3c. If one expands and re-compresses the film, one obtains new reproducible p A isotherm diagrams which differ significantly from the one obtained upon the first compression, as shown from curves (b) and (c) of Fig. 2. The new phase corresponds to a monolayer where each 3BCN/14 molecule
3. Results and discussion 3.1. p A isotherm diagrams Fig. 2a shows the p A isotherm diagram recorded upon a first continuous compression of a freshly spread 3BCN/14 monolayer. The surface pressure p starts to increase regularly at a molecular
Fig. 3. Suggested molecular organization models as adopted upon compression (a) side-on, (b) edge-on and (c) nematic.
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occupies an area of about 1.30 nm2 at p = 0 mN/m. Its measured compressibility coefficient 21 m N1, defined as A1 ddAp , indicates more likely that this phase is a liquid-like [2]. Taking into account of the molecular area values, 3BCN/14 molecules may organize at the air–water interface either side-on or edge-on. In the first model, the area Ach occupied by each hydrocarbon chain would be about 0.21 nm2 and the corresponding phase would be a solidelike. In the second model, each alkyl chain would occupy an area of about 0.42 nm2 or 0.32 nm2, in agreement with the liquid-like feature of the observed phase, depending on whether three or four alkyl chains are assumed to extend upwards, respectively. Moreover, the nonreversible feature of the observed side-on/edge-on phase transition indicates more likely that edge-on 3BCN/14 molecules aggregate into nanoscopic edge-on coulmns, i.e., nanorods. Such nanorods start to interact upon compression at a molecular area of about 1.30 nm2, as sketched in Fig. 3c. It is interesting to compare p A isotherm diagrams obtained on successive compression and expansion of the 3BCN/14 compound with those obtained on the 3BCN/9 compound (which are reported for the first time in the present study). Indeed, if one stops compressing (or expanding) the 3BCN/9 film, one observes, as shown on Fig. 4, a small decrease (or increase) of the surface pressure to an equilibrium value which is located between the compression value and the expansion value. One should note also that the p A isotherm diagram obtained on further successive compressions of the 3BCN/9 are similar to the one obtained upon the first compression. By comparing the behavior of the 3BCN/14 and the 3BCN/9 films upon compression and expansion, one may conclude that the side-on ? edge-on phase transition is indeed reversible in the case of the 3BCN/9 film and it is not in the case the 3BCN/14 film. From a fundamental point of view, the formation of a side-on or an edge-on Langmuir monolayer from a ‘‘columnar’’ liquid crystal (LC) should be governed by the hydrophilic/hydrophobic balance, i.e., the length of the alkyl side chains: on one hand, intermolecular LC/LC interactions within the film promote the formation of columns, and on the other hand, interactions between LC and the subphase molecules orient individual LC molecules at the air–water interface and prevent the formation of columns. For instance, El Abed et al. have shown, in two previous studies [10,11], that 3BCN/9 pyramidic molecules with shorter C9 lateral alkyl chains undergo a reversible side-on ? edge-on phase transition upon compression, whereas 3BCN/15 pyramidic molecules with longer lateral C15 alkyl chains exhibit a transition from a liquid side-on
monolayer to a condensed side-on monolayer [11]. To underline the effect of alkyl chains length on the molecular organization of Langmuir monolayers, one may cite the original study reported by Weinbach et al. [20] on the spreading of alkanes H(CH2)nHat the air–water interface. These authors showed that for n > 30 alkanes spread spontaneously as stable monolayers, whereas for 20 < n < 30, alkanes spread as multilayers at the air–water interface. One may cite also another interesting study reported more recently by Vaknin et al. [21] where it is shown that arachidic acid molecules may organize, during the collapse process in presence of Ca2+ cations, as trilayers where hydrophobic alkyl chains are in contact with the water subphase. Also, one may estimate roughly the cost in free energy per molecule from a side-on to an edge-on orientation, by assuming that interactions in unionized monolayers should be dominated by hydrogen bonding and dipolar interactions [21]. (i) The cost in free energy per alkyl chain needed to bring an alkyl chain in contact with the water subphase is about 6kBT [21], i.e., 12kBT if two chains are assumed to be brought in contact with the water subphase. Indeed, one may remark that, in the edge-on phase, alkyl side chains should be asymmetrically distributed around the central core as shown for example by El Abed et al. in another study on Langmuir monolayers of bowlic compounds [22]. A simple molecular organization model indicates that in order to allow for an edge-on arrangement, pyramidic molecules may get only two alkyl chains in contact with the water. (ii) The cost in free energy needed to expel a hydrophilic group from the water subphase is about 6kBT, originating mainly from the loss of hydrogen bonding [21], i.e., 36kBT for the six hydrophilic groups. (iii) The gain in free energy per alkyl chain when hydrocarbon chains are brought close to each other in the edge-on columnar organization (VdW interactions), is about 3kBT per added ACH2A [25]. This corresponds to a difference in free energy between the 3BCN/9 and the 3BCN/14 by about 15kBT per chain, i.e., 60kBT for the four upper alkyl chains. We assumed that the lower alkyl chains are in a disordered state, due to the presence of the water subphase, in a such manner we may neglect their VdW interactions. As one can notice, the net gain in free energy balance when increasing the length of the alkyl side chains from C9 to C14 would be (12kBT). Such an estimation indicates that the edge-on orientation may be indeed more stable than the side-on orientation for the 3BCN/14 monolayer. Also, comparing p A isotherm diagrams obtained on first and second compressions may allow for a calculation of the energy barrier needed to transit from the metastable side-on phase to the stable edge-on phase of the 3BCN/14 monolayer. Such energy should be equal to the film compression work pDA 3.5kBT, where p ’ 8mN/m and DA ’ (3.0 1.3)nm2. We may suggest that the energy barrier for the side-on/edge phase transition in the case of the 3BCN/15 film is probably too high which cannot not be overcome upon compression before the collapse of the film occurs.
π
3.2. Grazing incidence X-ray diffraction
2
Fig. 4. Surface pressure p A isotherm of 3BCN/9 Langmuir monolayer obtained upon successive compression and decompression. Dashed curve represents the equilibrium isotherm diagram. 3BCN/9 molecules undergo a reversible side-on/ edge-on phase transition upon compression as sketched in Fig. 3.
In order to characterize the structure of the new edge-on phase, we performed grazing incidence X-ray diffraction (GIXD) experiments on a single monolayer of 3BCN/14 molecules re-compressed to a surface pressure of about p = 6 mN/m. At such pressure, the molecular area is about 116 Å2. One observes a single X-ray diffraction peak at an in-plane Qxy scattering wavevector of about 1.43 Å1 as shown in Fig. 5. The observed peak is rather large, with a FWHM of about 0.127 Å1 which indicates that the correlation length is about 50 Å. A rodscan analysis shows (Fig. 6) that this peak has its maximum intensity in the diffraction plane. Consequently, the diffracting planes should be oriented normal to the
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Fig. 5. X-ray diffraction pattern of the 3BCN/14 monolayer recorded at p = 6 mN/m and a molecular area of about 116 Å2: a single large diffraction peak is observed at Qxy = 1.43 Å1. It corresponds to an ordering of 3BCN molecules in one direction along the edge-on column axis.
monolayer interface. On the basis of these data, two models may be considered for the in-plane molecular organization. In the first model, one may assume that the observed diffraction peak at Qxy = 1.43 Å1 is due to the ordering of alkyl side chains. The resulting 2D hexagonal chain would possess a lattice parameter equal to a = 5.08 Å and an area per alkyl chain of about 2 AC ¼ 22:3 Å . This model should be rejected as argued hereafter. Indeed, if one assumes that the pyramidic molecules adopt a side-on organization, in order to allow for the formation of a dense packed alkyl chains, then the resulting 3BCN/14 molecular area 2 2 would be equal to 134 Å ð¼ 22:3 Å 6Þ which is significantly 2 greater than the measured 116 Å value deduced from the p A isotherm at p = 6 mN/m. Though we are aware that the behavior of tethered chains may be very different by regards to that of free chains, as observed for examples in studies on Langmuir monolayers of hairy-rod polypeptides [23] or on bowlics [22], the observed GIXD peak in the present study could not originate from an hexagonal packing of the lateral chains. Moreover, since alkyl chains 2 should possess in this model an area of about AC ¼ 22:3 Å , then one should admit that they should be tilted by an Consequently, one should observe two GIXD peaks: a first peak located at low Qxy values which should correspond to the two degenerate [11] ~ out-of-plane diffraction peaks and a second peak located and ½11 at higher Qxy which should correspond to the [02] in-plane diffraction peak [1,24]. Since only one peak is detected at Qxy = 1.43 Å1 1 and Q z ¼ 0 Å , one should reject this model. Also, rodscan analysis of the observed peak gives a value of about DQz = 0.57 Å1 which allows for the determination of the thickness of the diffracting slabs of about h ¼ D2Qpz ’ 11 Å. This value is much smaller than the calculated 19 Å for the length of a C14 alkyl chain. In the second model, we assume that the observed diffraction peak at Qxy = 1.43 Å1 originates from an in-plane organization of the pyramidic cores thanks to their columnar edge-on organization. Hence, the observed diffraction peak should correspond to an intracolumnar core-core spacing distance of about d = 4.4 Å. This value is in a very good agreement with the value reported by Poupko et al. [16] from X-ray diffraction data of the columnar mesophase exhibited by the 3BCN/13 compound [16]. Nevertheless, one may notice that our measured core-core distance value is smaller than the 4.8 Å value measured by Levelut et al. [15] for the 3BCN/10 and 3BCN/12. Such noticeable difference between these compounds which differ only by a small variation in the length of their alkyl side chains may appear surprising. Nevertheless, one may remark that the phase diagram of these compounds
Fig. 6. Contour plot of the X-ray diffraction scan presented in Fig. 5. Rod scan analysis shows that the peak observed at Qxy = 1.43 Å1 has its maximum intensity approximately in the diffraction plane, at Qz = 0.1 Å1.
depends strongly on the length of the alkyl side chains as reported by Zimmermann et al. [13]. For example, the 3BCN/10 compound exhibits a single biaxial mesophase in the 30 °C–150 °C tempearture range (labeled PD), the 3BCN/15 compound exhibits a single uniaxial mesophase in the 80 °C–150 °C temperature range (labeled PC), whereas the 3BCN/14 compound exhibits both PD and PC mesophases versus temperature, observed between 73 °C– 82 °C and 82 °C–135 °C, respectively [13]. As indicated above, the observed peak is rather large. This leads to a small correlation length of about 50 Å which indicates that the intracolumnar core-core positional ordering extents over about 11 molecules. Taking account of these data and of the p A isotherm diagram of the new edge-on monolayer, we suggest that 3BCN/14 molecules form a columnar phase where molecules adopt an edgeon configuration for all molecular area values. Such behavior could be explained by the formation, during the first compression, of nanoscopic columns (nanorods) which do not disaggregate upon the film decompression. The mean length of such nanorods should be of about 5 nm, i.e., correlation length as determined from GIXD data. In order to show clearly the formation of the columnar structure of the film, one may search for the presence of any GIXD peak which may originate from intercolumnar spacing. If such GIXD peak exists, it should be large, of weak intensity and should be detected at small
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wavevectors, corresponding to an intercolumnar spacing distance of about Unfortunately, in our experiments, the very low signal/ noise ratio, due to a large diffusion at such small wavevectors, does not allow for the detection of any GIXD peaks even when they exist. However, though we did not obtain clear evidence from X-ray data alone supporting the formation of nanorods, the obtaining of a reproducible p A isotherm diagrams (after a preliminary compression) of the 3BCN/14 film supports clearly the formation of a new ‘‘species’’ which start to interact at a ’’molecular’’ area of a about 1.30 nm2, i.e., at a chain area ’ 0.43 nm2. Such value indicates that, after the first compression, the 3BCN/14 molecules should remain oriented edge-on even at zero surface pressure. The building of such nanorods and their compression at the air– water interface should allow for the formation of a genuine nematic Langmuir monolayer as sketched in Fig. 3c. 4. Conclusion We have shown in this study that the pyramidic 3BCN/14 molecules spread spontaneously at the air–water interface to form a side-on phase which transits irreversibly to an edge-on phase upon compression. In this new phase, grazing incidence X-ray diffraction shows that the molecules pack to form stable columns of about 10 molecules length with strong short smectic order. The built-up nanorod-like structure remains stable upon several expansioncompression cycles. This stability impedes the retrieval of the side-on phase upon expansion and allows for the formation of a genuine nematic monolayer.
[2] [3]
[4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
References [1] K. Kjaer, J. Als-Nielsen, C.A. Helm, L.A. Laxbauer, H. Möhwald, Phys. Rev. Lett. 58 (1987) 2224;
[24] [25]
P. Dutta, J.B. Peng, B. Lin, J.B. Ketterson, M. Prakash, P. Georgopoulos, S. Ehrlich, Phys. Rev. Lett. 58 (1987) 2228. G. Gaines Jr., Insolubles Monolayers at Liquid-Gas Interfaces, Interscience, New York, 1966. A.M. Bibo, C.M. Knobler, I.R. Peterson, J. Phys. Chem. 95 (1991) 5591; G.A. Overbeck, D. Möbius, J. Phys. Chem. 97 (1993) 7999; V.M. Kaganer, H. Möhwald, P. Dutta, Mod. Phys. 71 (1999) 779. P.G. de Gennes, Faraday Symp. Chem. Soc. 5 (1971) 16. F. Rondelez, D. Koppel, J. Physique 43 (1982) 1371. O. Albrecht, W. Cumming, W. Kreude, A. Laschewsky, H. Ringsdorf, Colloid Polym. Sci. 264 (1986) 659. J.Y. Josefowicz, N.C. Maliszewskyj, S.H.J. Idziak, P.A. Heiney, J.P. McCauley Jr., A.B. Smith III, Science 260 (1993) 323. D. Gidalevitz, O.Y. Mindyuk, P.A. Heiney, B.M. Ocko, M.L. Kurnaz, D.K. Schwartz, Langmuir 14 (1998) 2910. O.Y. Mindyuk, Y. Oksana, P.A. Heiney, Adv. Mater. 11 (1999) 341. A. El Abed, P. Muller, P. Peretti, F. Gallet, J. Billard, J. Phys. II Fr. 3 (1993) 851. A. El Abed, J. Daillant, P. Peretti, Langmuir 9 (1993) 3111. S. Shandrasekhar, G.S. Raganath, Rep. Prog. Phys. 53 (1990) 57; S. Chandrasekhar, in: D. Demus, J. Goodby, G.W. Gray, H.-W. Spiess, V. Vill (Eds.), Handbook of Liquid Crystals, vol. 2B, Wiley-VCH, Weinheim, 1998, pp. 749–780 (chapter. 8). H. Zimmermann, R. Poupko, Z. Luz, J. Billard, Z. Naturforsch. 40a (1985) 149. J. Malthête, A. Collet, Nouv. J. Chimie 9 (1985) 151. A.M. Levelut, J. Malthête, A. Collet, J. Physique 47 (1986) 351. R. Poupko, Z. Luz, N. Spielberg, H. Zimmermann, J. Am. Chem. Soc. 111 (1989) 6094. J. Toner, D.R. Nelson, Phys. Rev. B 23 (1981) 316. P. Fontaine, M. Goldmann, M. Bordessoule, A. Jucha, Rev. Sci. Instrum. 75 (2004) 3097. A. Renault, C. Alonso, F. Artzner, B. Berge, M. Goldmann, C. Zakri, Eur. Phys. Journal. B 1 (1998) 189. S.P. Weinbach, I. Weissbuchi, K. Kjaer, W.G. Bouwman, J.A. Nielsen, M. Lahav, L. Leiserowitz, Adv. Mater. 7 (1995) 857. D. Vaknin, W. Bu, A. Travesset, Langmuir 23 (2007) 1888. A. El Abed, R. Ionov, M. Goldamann, P. Fontaine, J. Billard, P. Peretti, Europhys. Lett. 56 (2001) 234. M. Fukuto et al., Phys. Rev. E 66 (2002) 010601; M. Fukuto et al., J. Chem. Phys. 119 (2003) 6254. V.M. Kaganer, H. Mohwald, P. Dutta, Rev. Mod. Phys. 71 (1999) 779. J.M. Israelachvili, Intermolecular and Surface Forces, 2nd ed., Academic Press Inc., San Diego, 1987. pp. 71.