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Characteristic Features of Amphiphilic P-Functionalized Calixarene Monolayers at the Air/Water Interface D. Vollhardt,*,† J. Gloede,‡ G. Weidemann,§ and R. Rudert§ Max-Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany, WITEGA, Applied Material Research, Kekule´ strasse 4, D-12489 Berlin, Germany, and Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12200 Berlin, Germany Received November 15, 2002. In Final Form: March 6, 2003 The combined application of surface pressure-area (π-A) isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD) offers a powerful possibility to characterize the amphiphilic P-functionalized calix[4]arene monolayers at the air/water interface. The studies are performed with two calix[4]arene bisphosphonates 1 and 2, which are phosphorylated in the same way at the upper rim and differ only in the number of alkyl chains at the lower rim (compound 1 with four n-dodecyl chains; compound 2 with two n-dodecyl chains in opposite positions). Significant differences exist in the π-A isotherms of the two calix[4]arene bisphosphonates 1 and 2 monolayers. In the case of derivative 1, they resemble those of usual amphiphiles with the characteristic temperature dependence and a coexistence region between a fluid and condensed phase. The π-A isotherms of derivative 2 deviate considerably from those of typical amphiphilic monolayers having no temperature dependence and no “plateau” region. The characteristics of the π-A isotherms of both derivatives 1 and 2 are not affected by interaction with counterions of different valency (Na+, Cd2+, Eu3+, Th4+) present in the subphase. Both derivatives 1 and 2 form two-dimensional condensed phase domains with a specific shape which is more or less modified by different-valent counterions in the aqueous subphase. The analysis of the BAM results corroborates the general differences between the monolayer properties of derivative 1 and 2. The GIXD results show that only the monolayers of derivative 1 form a 2D lattice structure. The data indicate a rectangular unit cell, which is not changed by the complexation of thorium ions by the phosphonate groups.
Introduction Since the 1970s calixarenes have been established as readily available platforms on which to build a wide variety of interesting cavity-containing and multifunctional molecules.1 Because of this fact, the increasing importance of the calixarenes is closely related with topics of current interest in host-guest structures and supramolecular chemistry such as design and synthesis of enzyme mimics, selective molecular recognition of biorelevant species, but also metal cation complexation.2 Functionalized amphiphilic calixarenes offer the possibility to prepare monolayers and LB films. Correspondingly, potential applications have been discussed in the fields of medical diagnostic and miniaturized sensor techniques.3,4 Phosphorus-functionalized calixarenes with P-containing moieties at the lower rim of the macrocyclic system have been prepared.5-7 Calixarenes containing phosphoryl groups at the upper rim are less studied.8-12 In a recent paper two novel amphiphilic calix[4]arenes phosphorylated at the upper rim (5,11,17,23-tetrakis(dihydroxyphosphonyl)-25,26,27,28-tetradecyloxycalix[4]arene and * Corresponding author. † Max-Planck Institute of Colloids and Interfaces. ‡ WITEGA. § Federal Institute for Materials Research and Testing. (1) Gutsche, C. D.; Dhawan, B.; No, K. H.; Muthukrishnan, R. J. Am. Chem. Soc. 1981, 103, 3782. (2) Asfari, Z., Bo¨hmer, V., Harrowfield, J., Vicens, J., Eds. Calixarenes 2001; Kluwer Academic Publishers: Dordrecht, Boston, London, 2001. (3) Hamuro, Y.; Calama, M. C.; Park, H. S.; Hamilton, A. D. Angew. Chem., Int. Ed. Engl. 1997, 36, 2680. (4) Spichiger-Keller, U. E. Chemical Sensors for Medical and Biological Application; Wiley-VCH: New York, 1998. (5) Gloede, J. Phosphorus, Sulfur, Silicon 1997, 127, 97. (6) Neda, I.; Kaukorat, T.; Schmutzler, R. Main Groups Chem. News 1998, 2-3, 4. (7) Wieser-Jeunesse, C.; Matt, D.; Yaftian, R. M.; Harrowfield, J. M. C. R. Acad. Sci. Paris, Se´ r. II 1998, 479.
5,11,17,23-tetrakis(dihydroxyphosphonyl)-25,26,27,28tetradodecyloxycalix[4]arene) were prepared and their interaction with some mono- and divalent cations was studied both at the air/water interface and in aqueous suspensions.13 The functionalized groups can generally increase the cavity size and provide additional complexation centers for guest molecules. It is of general interest that P-functionalized calixarenes allow the complexation of lanthanides and actinides.14-16 Consequently, their application for removing actinides from wastewater of nuclear power stations has been discussed.17 Organized molecular monolayers provide unique model systems for molecular interactions and consequently for molecular recognition. New molecular recognition systems can be developed by using monolayer systems. (8) Ami, M.; Arduini, A.; Casnati, A.; Pochini, A.; Ungano, R. Tetrahedron 1989, 45, 2176. (9) Kalchenko, V. I.; Atamas, L. I.; Pirozhenko, V. V.; Markovsky, L. N. J. Gen. Chem. USSR 1992, 62, 2623. (10) Hamada, F.; Fukugaki, T.; Murai, K.; Orr, G. W.; Atwood, J. L. J. Inclusion Phenom. 1991, 10, 57. (11) Ozegowski, S.; Costisella, B.; Gloede, J. Phosphorus, Sulfur, Silicon 1997, 119, 209. (12) Arnaund-Neu, F.; Boehmer, V.; Dozol, J. F.; Gruttner, C.; Jacobi, R. A.; Kraft, D.; Mauprives, O.; Rouguette, H.; Schwing-Weill, M. J.; Simon, N.; Vogt, W. J. Chem Soc., Perkin Trans. 2 1996, 1175. (13) Houel, E.; Lazar, A.; Da Silva, E.; Coleman, A. W.; Solovyov, A.; Cherenok, S.; Kalchenko, V. I. Langmuir 2002, 18, 1374. (14) Costisella, B.; Gloede, J. Phosphorus, Sulfur, Silicon 1994, 89, 39. (15) Arnaud- Neu, F.; Schwing-Weill, M. J.; Dozol, J. F. Calixarene 2001. In Calixarene 2001; Asfari, Z., Bo¨hmer, V., Harrowfield, Vicens, J., Eds.; Kluwer: 2001; p 642. (16) Dozol, J. F.; Schwing-Weill, M. J.; Arnaud-Neu, F.; Boehmer, V.; Ungaro, R.; van Veggel, F. C. J. M.; Wipff, G.; Costero, A.; Desreux, J. F.; de Mendoza, J. Extraction and selective separation of long-lived nucleides by funcionalized macrocycles; Contract F14W-CT-960022, EUR 19605 EN; 2000. (17) Reck, G.; Schneider, M.; Gloede, J.; Vollhardt, D. Z. Kristallogr. 1999, 214, 501.
10.1021/la026856u CCC: $25.00 © 2003 American Chemical Society Published on Web 04/04/2003
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The primary objective of the present work is to provide first information on the textural and structural features of the monolayers of amphiphilic P-functionalized calix[4]arenes which are bearing both hydrophobic and hydrophilic substituents. We focus on the two calix[4]arene bisphosphonates 1 und 2, which are phosphorylated at the upper rim and differ only in the amphiphilic character.
Compound 1 contains four n-alkyl chains of medium length (C12) whereas compound 2 has only two n-dodecyl chains in opposite positions. This suggests differences in the amphiphilic nature and the packing properties of the monolayers. Information on ordering and structure features of Langmuir monolayers has been effectively progressed by coupling the results of Brewster angle microscopy (BAM) and X-ray diffraction at grazing incidence (GIXD) with thermodynamic data. Consequently, the phase properties of monolayers of the two calixarenes are characterized thermodynamically by the surface pressure-area (π-A) isotherms at different temperatures. These results will show that both P-functionalized calix[4]arenes form condensed monolayer phases. So far, there is information on neither the microscropic textural features of any P-functionalized calixarene monolayers nor whether amphiphilic calixarene monolayers can form two-dimensional lattice structures. Therefore, the combined application of BAM and GIXD studies should have pioneering character in the understanding of calixarene monolayers. The comparison of the results obtained for the monolayers of both compounds 1 and 2 provides first information on the specific effect of the hydrophobic part of the calix[4]arene molecules on the ordering of the condensed monolayer phases. Finally, it
can be demonstrated in which way the presence of counterions of different valency affects the textural and structural features of the monolayer phases. Experimental Section The synthesis of the amphiphilic O-tetraalkylcalix[4]arene bisphosphonates 1 and 2 (see above) was performed according to Scheme 1.11 In the first step, calix[4]arene (3) was partially alkylated, and then the aromatic rings with free hydroxy groups were brominated in the para position. In the next step, the alkylation of the hydroxy groups was completed, and finally the bromine atoms were exchanged by P-containing groups to give the calix[4]arene bisphosphonates 1 and 2. Compound 1 [mp 87-88 °C. 31P NMR (CHCl3): 18.0 ppm] was purified by recrystallization from methanol, and compound 2 [mp 59-61 °C. 31P NMR (CHCl3): 18.8, 18.3 ppm. Addition of Eu(FOD)3: -80 to -96 (broad), -144.8 (broad) ppm], by column chromatography (acetone/hexene 3:1) followed by crystallization from water. The purity of the amphiphilic calix[4]arene derivatives 1 and 2 was g99%. Dissolved in CHCl3, calix[4]arene derivative 1 exists in the cone conformation and calix[4]arene derivative 2 exists in a cone conformation (2a) (Eu complex: δ ) -144.8 ppm) and in a partial cone conformation (2b) (.Eu complex: δ ) -80 to -96 ppm).11 This fact demonstrates that the amphiphilic calix[4]arene bisphosphonates are good compounds for complexation.
In the solid state, such calix(4)arene derivatives prefer the cone conformation.11,18 Ultrapure water having a specific resistance of 18.2 MΩ and used for the monolayer experiments was obtained from a Millipore desktop system. The spreading solvent was chloroform (p.a. grade, Baker, Holland). We selected Na+, Cd2+, Eu3+, and Th4+ as counterions of different valency and used 5 × 10-4 M aqueous solutions of NaCl, CdCl2, Eu(NO3)3, and Th(NO3)4 (Merck, Darmstadt) as subphase.
Scheme 1
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The experimental setup for the measurements of the surface pressure (π-A) isotherms and the BAM studies consisted of a self-made computer-interfaced film balance coupled with a Brewster angle microscope (BAM 1+, NFT, Go¨ttingen). The surface pressure was measured with the Wilhelmy method. Using a roughened glass plate, the accuracy of the surface pressure was reproducible to ( 0.1 mN m-1, and the area per molecule, to (5 × 10-3 nm2. The lateral resolution of the BAM1+ is approximately 4 µm. Simple imaging processing software was used to optimize the contrast. More detailed information on the BAM method is given elsewhere; see, for example, refs 19 and 20. The GIXD experiments were performed using the liquidsurface diffractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, Germany. For the experiments the trough is located in a sealed and He-filled container, as the helium reduces the background in the X-ray scattering experiments. A monochromatic synchrotron beam strikes the helium/ water interface at grazing incidence angle Ri ) 0.85Rc, where Rc is the critical angle for total reflection. The diffracted intensity is detected by a linear position-sensitive detector (PSD) (OED100-M, Braun, Garching, Germany) as a function of the vertical scattering angle Rf. A Soller collimator located in front of the PSD provides the resolution for the horizontal scattering angle 2θxy. The horizontal (in-plane) component of the scattering vector Q ) kf - ki is given by Qxy ≈ (4π/λ) sin(θxy), and the vertical (out-of-plane) component is given by Qz ≈ (2π/λ) sin(Rf), where λ is the X-ray wavelength.21,22 The diffracted intensities were corrected for polarization, effective area, and Lorentz factor. Model peaks were fitted to the corrected intensities. The lattice parameters can be obtained from the peak positions. The lattice spacing is given by d(hk) ) 2π/Qhk xy , where (h,k) denotes the order of the reflection.
Results and Discussion At first, we consider the π-A isotherms of the monolayers of compound 1 spread on pure water (Figure 1a) and on 1 mM aqueous Th(NO3)4 solution (Figure 1b). At first glance, it is seen that in both cases the shape of the isotherms depends strongly on the temperature in a similar way as known from typical amphiphilic monolayers. Above T > 26 °C, the π-A isotherms of compound 1 show a well developed “plateau” region indicating a main phase transition of first order from a fluidlike phase to a condensed phase. Even at the highest measured temperature of 39 °C the plateau exists over a broad area region but now with a more pronounced inclination. For the isotherms recorded at T e 26 °C, the two-phase transition region is already at π ≈ 0, so that formation of a condensed phase can be expected after the spreading of the monolayer. Despite the general agreement of the π-A isotherms of the two subsolutions, there exist minor differences in detail. The two-phase coexistence region is less extended on 1 mM aqueous Th(NO3)4 solution, and the plateau pressure of a respective temperature is decreased. Correspondingly, also the main phase transition point Ac is shifted to higher area values: For example, for compound 1, Ac ) 152 Å2/molecule spread on pure water at 35 °C and also on 1 mM aqueous Th(NO3)4 solution at 39 °C. The π-A isotherms of the monolayers of compound 2 are completely different (Figure 2). Again the differences between the isotherms recorded on pure water and those on 1 mM aqueous Th(NO3)4 solution are only small. However, their features deviate considerably from those of typical amphiphilic monolayers. It is surprising that (18) Meunier, J. Colloids Surf., A 2000, 171, 33. (19) Vollhardt, D. Adv. Colloid Interface Sci. 1996, 64, 143. (20) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246. (21) Kjaer, K. Physica B 1994, 198, 100. (22) Siegel, S.; Kindermann, M.; Regenbrecht, M.; Vollhardt, D.; von Kiedrowski, G. Prog. Colloid Polym. Sci. 2000, 115, 233.
Figure 1. Temperature dependence of the π-A isotherms of calix(4)arene 1 monolayers at different subphases: (a) on pure water; (b) on aqueous 1 mM thorium nitrate solutions.
the characteristics of the isotherms are nearly independent of temperature over the whole accessible region. The plateau region characteristic for the two-phase coexistence region in usual amphiphilic monolayers is missing. The surface pressure increase is somewhat more steep on 1 mM aqueous Th(NO3)4 solution and the cross-sectional area is slightly smaller in the tightest compressed state. The BAM studies show that two-dimensional condensed phase domains are formed in the monolayers of compounds 1 and 2 despite the fundamental differences in the π-A isotherms in both cases. Compound 1 behaves as a usual amphiphile. At compression, irregularly shaped domains are only formed in the two-phase coexistence region after the main phase transition point above T ) 26 °C. Figure 3 shows typical domain shapes of compound 1 spread on water at 30 °C. Depending on the size, the domains are more or less thickened in the center and can be lancetlike (Figure 3a) or sickle-shaped, either rather thin (Figure 3b) or thicker (Figure 3c), but always with a nonuniform periphery. All domains are homogeneously reflecting, which indicates perpendicular orientation of the alkyl chains at the lower rim. The presence of metal ions of different valency in the aqueous subsolution affects the domain morphology in a different way. We compared the effect of Th4+, Eu3+, Cd2+, and Na+ ions on the domain shape of compound 1. In all cases, the domains reflect homogeneously as those formed on water. Th4+ ions affect mostly the domain morphology in comparison to that on pure water. Figure 4 shows a sequence of condensed phase domain growth of compound
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Figure 3. BAM images of calix(4)arene 1 monolayers on pure water at 30 °C: (a) lancet-like condensed phase domains; (b) thin sickle-shaped condensed phase domains; (c) thick sickleshaped condensed phase domains. Image size: 500 µm × 500 µm.
Figure 2. Temperature dependence of the π-A isotherms of calix(4)arene 2 monolayers at different subphases: (a) on pure water; (b) on 1 mM aqueous thorium nitrate solution.
1 on 1 mM aqueous Th(NO3)4 solution during compression of the monolayer within the plateau region of the π-A isotherm at 30 °C. At the beginning after the phase transition point, the sickle-shaped domains are similar to those formed on water, but soon their size increases considerably (Figure 4a). Then, the domains extend at the periphery and the shape becomes more irregular (Figure 4b). Finally, they start to coalesce when touching each other at the end of the plateau (Figure 4c). Also, the mono- (Na+), di- (Cd2+), and trivalent (Eu3+) metal ions investigated affect the domain shape of compound 1 in comparison to the pure water subphase. Minor differences were found for Na+ (Figure 5a) and Eu3+ (Figure 5b). Although the domain shape in the selected images looks different for the two ions Na+ and Eu3+, it resembles that on pure water (see Figure 3). It is interesting to note that the divalent Cd2+ ions cause domain shapes more similar in size and shape to those obtained on Th4+ containing subsolutions (Figure 5c and d). However, besides large domains, also extended at the periphery, small domains exist, which are rather similar to those formed on pure water. The effect of the different-valent metal ions on the domain shape indicates specific interaction with the P-functionalized calixarene 1, which is determined not only by the increase of valency (Th4+) but also by the specifity of the metal ion (Cd2+, Th4+). Condensed phase domains are also formed by compound 2, although the π-A isotherms do not show the characteristic plateau region indicating the two-phase coexistence. The domains, already formed at the beginning of
Figure 4. BAM images of the domain growth at compression of a calix(4)arene 1 monolayer on a 1 mM aqueous thorium nitrate solution within the two-phase coexistence region of the π-A isotherm at T ) 30 °C: (a) sickle-shaped domains formed after the phase transition point; (b) domain growth to a more irregular shape at the periphery; (c) beginning coalescence of the domains at the end of the two-phase coexistence region. Image size: 500 µm × 500 µm.
the pressure increase, are not essentially affected by the temperature in the measured interval 25 °C e T e 35 °C. On pure water the predominant domain shape is rectangular platelike (Figure 6). The sides are slightly curved, the longer one inward and the shorter one outward. Only
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Figure 5. BAM images of calix(4)arene 1 monolayers on aqueous subphases containing 1 mM one-, two-, or three-valent cations: (a) on NaCl; (b) on Eu(NO3)3; (c and d) on CdCl2. T ) 30 °C. Image size: 500 µm × 500 µm.
rarely, segments of round domains can be observed (Figure 6b). Marked differences in the reflectivity indicate drastic specificity in the azimuthal orientation of the tilted molecules. The presence of the metal ions Th4+, Eu3+, Cd2+, and Na+ enhances considerably the tendency to form segments of round domains. This is demonstrated by a selected BAM image for each metal ion taken at 30 °C (Figure 7). It is seen that, in the case of the higher valent ions Th4+ and Eu3+, nearly all domains look like segments of more or less round disks frayed at the periphery. The domains formed on subphases containing the lower valent ions Cd2+ and Na+ consist often of one platelike part and a round segment. Consequently, their morphology can be seen as an intermediate form between those observed on pure water and on an aqueous subphase containing higher valent metal ions. It is noteworthy that especially the segment-shaped domains or domain parts have a filigree substructure. The high intensity of the reflected light,
Vollhardt et al.
particularly of the bright domains, suggests the presence of more than one molecule layer. The white lines in the gray domains formed on Th4+ and Eu3+ containing subsolution support this assumption. Well developed domains with filigree substructure were observed at heptadecylbenzamidinium chloride monolayers on aqueous sodium phenylacetate solution.23 In that case, AFM studies provided evidence for overgrowing the filigree strings of the first layer by a second layer. In the present study the BAM results point to a similar phenomenon. The objective of the GIXD experiments in the present work is to clarify whether, on one hand, the P-functionalized calix[4]arenes can form two-dimensional (2D) lattice structures and, on the other hand, the long-range orientational order of the condensed phase domains is related to a lattice structure. The GIXD results from the monolayers of the two calix[4]arenes 1 and 2 are completely different. Whereas diffraction peaks are obtained for monolayers of compound 1 (Figure 8), no diffraction is found for compound 2. Consequently, the monolayers of compound 2 do not form any 2D lattice structure. Compound 1 contains four n-alkyl chains of medium length (C12) whereas compound 2 has only two n-dodecyl chains in opposite positions (see the molecular structure in the Introduction). The hydrophilic part at the upper rim is the same in both compounds. According to the 31P NMR results, the calix[4]arene derivative 1 has the stable cone form and all four dodecyl groups are directed to the same side. On the other hand, calix[4]arene derivative 2 dissolved in chloroform occurs in the cone and partial cone conformations. In Langmuir monolayers the alkyl chains at the lower rim are directed to the air, corresponding to the amphiphilic nature of both calix[4]arenes. However, fundamental differences in the packing density of the alkyl chains of the two derivatives 1 and 2 should exist. The possibility of the existence of the partial cone conformation for derivative 2 should have an additional influence on packing in the headgroup region of the amphiphilic calix(4)arene monolayer. The GIXD results are in general agreement with these molecular differences of the two calix(4)arene derivatives. In the case of derivative 1, the stable cone conformation and the presence of four long alkyl chains allow the realization of a 2D lattice structure in the region of both the hydrophobic alkyl chains and the hydrophilic headgroups. However, it is obvious from the GIXD experiments that in the case of derivative 2 neither the small cross-sectional area of the two alkyl chains nor the mobility in the calyx region (manifested by the coexistence of the cone and partial
Figure 6. BAM images of calix(4)arene 2 monolayers on pure water at 30 °C: (a) usually rectangular platelike domains; (b) sometimes segments of round domains can be observed. Image size: 500 µm × 500 µm.
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Langmuir, Vol. 19, No. 10, 2003 4233 Table 1 reflex no. 1 2 3 4 5 6 7
Qxy(exp) (Å-1)
hk
Qxy(calc) (Å-1)
Calixarene Derivative 1 on Water 0.616 02 10 0.690 1 1/1 1 h 0.871 1 2/1 2 h 1.110 1 3/1 3 h 1.231 04 20 1.272 2 1/2 1 h 1.379 2 2/2 2 h 1 4/1 4 h
0.615 0.617 0.689 0.872 1.110 1.231 1.234 1.272 1.379 1.377
Calixarene Derivative 1 on 10-3 M Thorium Nitrate Solution 1 0.617 02 0.617 10 0.618 2 0.692 1 1/1 1 h 0.691 3 0.874 1 2/1 2 h 0.874 4 1.114 1 3/1 3 h 1.114 5 1.234 04 1.234 20 1.237 6 1.275 2 1/2 1 h 1.275 7 1.382 2 2/2 2 h 1.382 1 4/1 4 h 1.380
Figure 7. BAM images of calix(4)arene 2 monolayers on aqueous subphases containing 1 mM solutions of cations of different valency at T ) 30 °C: (a) on thorium nitrate; (b) on europium nitrate; (c) on cadmium chloride; (d) on sodium chloride. Image size: 500 µm × 500 µm.
Figure 8. GIXD results of the calix[4]arene derivative 1 monolayer on water, taken at π ) 8 mN/m and T ) 25 °C. Diffracted intensity as a function of the in-plane scattering vector component Qxy.
cone conformation) makes possible the formation of a 2D lattice structure. The GIXD experiments of calix[4]arene derivative 1 monolayers, spread on both pure water and 1 mM aqueous Th(NO3)4 solution, were performed at 8 mN/m and 25 °C. Although in both cases condensed phase domains are formed already at zero pressure (see Figure 1), the diffracted intensity was rather low under these conditions. Figure 8 shows the diffracted intensity as a function of (23) Clark, M.; Cramer, R. D., III; Van Opdenbosch, N. J. Comput.Chem. 1989, 10, 982.
the in-plane scattering vector component Qxy for the derivative 1 monolayer spread on water. It is seen that seven diffraction peaks can be identified. In the case that the derivative 1 monolayer is spread on a 1 mM aqueous Th(NO3)4 solution, also seven diffraction peaks are observed, the Qxy positions of which are similar. In both cases, all peaks can be explained on the basis of a rectangular unit cell with a ) 10.18 Å, b ) 20.42 Å, and an in-plane area Axy ) 208 Å2. The indices of the peaks of the derivative 1 monolayer on water and 1 mM Th(NO3)4 solution are listed in Table 1. For the determination of the lattice parameters, pairs of the three reflexes with the highest intensity are used, that means, the (0 4) reflex and either the degenerate (2 1), (2 1h ) or the degenerate (2 2), (2 2 h ) reflex. The choice of the two degenerate reflexes provides exactly the same lattice parameters. The (0 4) reflex was used instead of the (0 2) reflex because of the lower relative error of the reflex position. Some calculated reflexes coincide in the limit of resolution with other reflexes. The (0 4) reflex can be superimposed with the (2 0) reflex, and the degenerate (2 2), (2 2h ) reflex, with the degenerate (1 4), (1 4 h ) reflex, respectively. The indices and positions of these reflexes are given in Table 1 in italic letters and are in reasonable agreement with the chosen reflexes. It is worth mentioning that this superimposition can reduce the precision of the calculation of the lattice constants, but its effect is smaller than that using the less precise positions of the peaks with lower intensity. The unit cell parameter a deviates only slightly from b/2. This slight deviation of the a spacing from that of b/2 raises the question whether the reflexes could be explained also by a tetragonal lattice. The choice of the rectangular unit cell is supported by the symmetry of the molecule, as the calix[4]arene derivative 1 molecule has no 4-fold symmetry axis. Moreover, the reflex positions calculated for a tetragonal lattice lead to slightly higher deviations from the experimental positions. Consequently, the unit cell area is consistent with two molecules in the unit cell. Each calix[4]arene derivative 1 molecule contains four alkyl chains; that means, the area per alkyl chain with 26 Å2 is very high to realize a crystalline packing of the rather short alkyl chains. Thus, it seems to be reasonable to assume that the calix[4]arene bodies pack in a lattice. The in-plane peak profiles are Gaussians, indicating a
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Figure 9. Cone arrangement in a rectangular unit cell of the calix[4]arene derivative 1 monolayer viewed perpendicular to the plane, as predicted by the program HARDPACK. The rectangle in the middle represents the unit cell.
crystalline order. The width of the peaks is resolution limited (∼0.01 Å-1). Finally, it is interesting to note that according to the very similar GIXD reflexes obtained on aqueous 10-3 M Th(NO3)4 solution, the lattice structure of the calix[4]arene derivative 1 monolayers is not changed by the complexation of thorium ions with the phosphonate groups. Obviously, the phosphonate groups directed to the aqueous subphase are movable in the crystalline packed monolayers and do not affect the 2D lattice structure of calix[4]arene derivative 1. This can explain that also the complexation of counterions mainly by the phosphonate groups does not influence the 2D lattice structure of the calix[4]arene derivative 1 monolayers. These conclusions can be supported by model predictions of the area per molecule of a cone type calix[4]arene derivative 1 in a two-dimensional lattice. A model of the molecule was created and optimized with respect to their energy using the force field Tripos 5.2.24 According to the conclusions of the π-A isotherm and because the complete molecule is too large and too flexible for a reliable crystal structure prediction, the alkyl chains and the phosphonate groups were then replaced by hydrogen atoms without further optimization. In that way the cone conformation of the calyx was preserved. The four alkyl chains have together only a small cross section of
