Ellipsometry, Brewster Angle Microscopy, and Thermodynamic

Langmuir 1996, 12, 5407-5412

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Ellipsometry, Brewster Angle Microscopy, and Thermodynamic Studies of Monomolecular Films of Cryptophanes at the Air-Water Interface Lucile Gambut,† Jean-Paul Chauvet,*,† Chantal Garcia,† Bruno Berge,‡ Anne Renault,‡ Sophie Rivie`re,§ Jacques Meunier,§ and Andre´ Collet*,† Ste´ re´ ochimie et Interactions mole´ culaires,| E Ä cole normale supe´ rieure de Lyon, 46 alle´ e d’Italie, 69364 Lyon cedex 07, France, Laboratoire de Spectrome´ trie Physique,⊥ Universite´ Joseph Fourier, BP 87, 38402 Saint-Martin d’He` res, France, and Laboratoire de Physique Statistique, E Ä cole normale supe´ rieure,∇ 24 rue Lhomond, 75231 Paris cedex 05, France Received March 14, 1996. In Final Form: July 19, 1996X The cryptophanes are spherical or ovoid-shaped hollow molecules of nanometric scale capable of forming inclusion complexes with neutral or charged organic species. Certain cryptophanes such as 1-4 form thin films when they are compressed at the air-water interface. These films have been studied by surface pressure measurements, ellipsometry and Brewster angle microscopy. The medium-sized cryptophane 2 and the large cryptophanes 3 and 4 exhibit a reversible monomolecular layer domain limited to a surface pressure of 8-10 mN m-1 and generated after a plateau corresponding to a gas-liquid transition. In contrast, molecular films of the small cryptophane 1 are not stable and irreversibly collapse to 3Daggregates when they are compressed.

Introduction During the last decade, thin layers made of hollow molecules have received much attention, particularly in view of their potential applications as active materials for chemical detection.1 Works in this area have principally dealt with suitably modified cyclodextrins,2-4 calixarenes,5 or resorcin[4]arenes6 spread at the air-water interface or forming self-assembling monolayers on gold surfaces.7 These compounds are closely related to classical amphiphilic structures, as they generally present a hydrophobic side opposite to an hydrophilic planar face allowing them to stand flat8 on the water surface or parallel to a solid support. The cryptophanes9 (Figure 1) represent another family of spherical or ovoid hollow molecules of nanometric size, capable of lodging in their cavity neutral molecules including methane,10 halogenomethanes,11 isobutane,12 †

E Ä cole normale supe´rieure de Lyon. ‡ Universite ´ Joseph Fourier. § E Ä cole normale supe´rieure. | UMR CNRS 117; chaire de l’Institut Universitaire de France. ⊥ URA CNRS 08. ∇ URA CNRS 1306. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Go¨pel, W. Science 1994, 265, 1413. (2) Eddaoudi, M.; Baszkin, A.; Parrot-Lopez, H.; Boissonnade, M. M.; Coleman, A. W. Langmuir 1995, 11, 13. (3) Tchoreloff, P. C.; Boissonnade, M. M.; Coleman, A. W.; Baszkin, A. Langmuir 1995, 11,13. (4) For a review on cyclodextrins, see : Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803. (5) Conner, M. D.; Janout, V.; Kudelka, I.; Zhu, J.; Regen, S. L. Langmuir 1993, 9, 2389. (6) Aoyama, Y.; Tanaka, Y.; Sugahara, S. J. Am. Chem. Soc. 1989, 111, 5397. (7) Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1994, 116, 3597. Huisman, B.-H.; Thoden van Velzen, E. U.; van Vegel, F. C. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. Tetrahedron Lett. 1995, 36, 3273. (8) The different positions adopted by these molecules are defined by Laschewsky, A. Angew. Chem., Int. Ed. Engl., Adv. Mater. 1989, 28, 1574. (9) Collet, A. Tetrahedron 1987, 43, 5725. (10) Garel, L.; Dutasta, J.-P.; Collet, A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1169. (11) Canceill, J.; Cesario, M.; Collet, A.; Guilhem, J.; Lacombe, L.; Pascard, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 1249.

S0743-7463(96)00245-4 CCC: $12.00

Figure 1. Molecular structure of cryptophanes 1-4.

and piperidine nitroxides,13 as well as cationic species such as acetylcholine and related quaternary ammonium salts.14 Since the inclusion process is fast and reversible, they represent good candidates for the design of chemical sensors for organic species, and elaboration of thin layers of these molecules eventually transferable onto solid surfaces would represent a significant step in this direction. Although these compounds do not possess a conventional amphiphilic structure, their surface comprises well-defined hydrophobic and hydrophilic regions; the north and south poles are made of rigid, cone-shaped C3cyclotriverarylene benzylic units, the equator region is purely aliphatic in nature, and these hydrophobic areas are separated from one another by two hydrophilic tropical belts each containing six oxygen atoms. Preliminary experiments have shown that, despite this unusual combination of regions of different polarities, cryptophanes such as 1-4 give after spreading at the air-water interface surface pressure (π) vs molecular area (A) isotherms (12) Canceill, J.; Lacombe, L.; Collet, A. C. R. Acad. Sci. Se´ r. II 1987, 304, 115. (13) Garel, L.; Vezin, H.; Dutasta, J.-P.; Collet, A. J. Chem. Soc., Chem. Commun. 1996, 719. (14) Garel, L.; Lozach, B.; Dutasta, J.-P.; Collet, A. J. Am. Chem. Soc. 1993, 115, 11652. (15) Chauvet, J.-P.; Gambut, L.; Se´auve, A.; Garcia, C.; Collet, A. C. R. Acad. Sci., Se´ r. II 1994, 318, 771.

© 1996 American Chemical Society

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evidencing the formation of thin films.15 We now report further investigations of these films, aiming at a better knowledge of their structure and properties, and of the features determining the pseudoamphiphilic character of these molecules. We particularly focus here on the surface elasticity and state equation of the two-dimensional phases evidenced from π-A isotherms and characterized by ellipsometry and Brewster angle microscopy (BAM) at different steps of compression. These experiments have in fact substantiated our previous views that certain cryptophanes reversibly form monolayers on compression at the air-water interface and that their ability to spread increases with their size and flexibility. Experimental Section Cryptophanes. The small cryptophane-E 1 and mediumsized cryptophane-O 2 were prepared as described16 and are pure anti stereoisomers. The large cryptophanes 3 and 4 were synthesized by the template method17 as described separately18 and consist of equimolecular mixtures of anti and syn stereoisomers designated W/X and Y/Z, respectively.19 The structures and geometry of 1 and 2 have previously been established by NMR spectroscopy and single crystal X-ray diffraction studies. These cryptophanes are relatively rigid and almost spherical in shape with mean external diameter of the order of 1.1 to 1.2 nm for 1 and 1.3 to 1.4 nm for 2. In a 2D hexagonal close packed monolayer, these figures would translate into a minimum area per molecule of 1.05-1.25 nm2 and 1.46-1.70 nm2 for 1 and 2, respectively. The structures of 3 and 4 have been established by NMR and mass spectroscopy of their mixtures.18 These cryptophanes are certainly less rigid than 1 and 2, and examination of CPK models suggests that their most expanded conformation, with the aliphatic spacer bridges fully extended, is an ellipsoid with an equatorial diameter similar to that of 2 and a north-south axis around 2 to 2.1 nm in length. In large cryptophanes, there also exist in-out topoisomers,18,20 in slow equilibrium with the out-out ones depicted in Figure 1, and it is therefore somewhat illusory to define a minimum area per molecule otherwise than very roughly. A close packed monolayer of ellipsoids with their long axes perpendicular to the water surface would exhibit a minimum area per molecule around 1.6 nm2 and up to 2.3 nm2 with their long axes parallel to the surface. Langmuir Setup and Experimental Procedure. The films were prepared on a thermostated home-made Teflon Langmuir trough (51 × 13 cm) at 22.0 ( 0.2 °C. The cryptophanes were spread at the air-water interface from dichloromethane (distilled on CaCl2) solutions of concentration (0.2 to 0.7) × 10-3 mol dm-3. The subphase was pure water (resistivity greater than 1 MΩ), purified by a Milli-RQ system. The desired amount of dichloromethane solution was delivered onto the water subphase by means of a microsyringe (Unimetrix), and the spreading was followed by a standby period of 20-30 min, allowing the evaporation of the solvent and the establishment of the internal equilibrium of the expanded monolayer. Film compression and decompression were performed with a mobile barrier driven at a constant speed of 70 mm2 s-1; a slower motion, 14 mm2 s-1, was used in some experiments. The surface pressure π, measured by the Wilhelmy method,21 was continuously recorded during the barrier displacement, and the π-A isotherms discussed below were obtained immediately after the above mentioned standby period. Thermodynamic Analysis of π-A Isotherm Diagrams. We used the Gaines equation (1)22 which, among the state equations applied to 2D-phase systems,23 is generally appropriate (16) Canceill, J.; Collet, A. J. Chem. Soc., Chem. Commun. 1988, 43, 582. (17) Collet, A. Tetrahedron 1987, 43, 5725. (18) Garcia, C.; Aubry, A.; Collet, A. Bull. Soc. Chim. Fr. in press. (19) Collet, A. Cryptophanes. In Comprehensive Supramolecular Chemistry; Vo¨gtle, F., Ed.; Pergamon: Oxford, 1996; Vol 2, Chapter 11, pp 325-365. (20) Garel, L. The`se de doctorat, Universite´ Claude Bernard, Lyon, 1995. The barrier for the conversion of in-out into out-out topoisomers is of the order of 80-90 kJ mol-1. (21) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; John Wiley and Sons : New York, 1982; p 101. (22) Gaines, G. L., Jr. J. Chem. Phys. 1978, 63, 924.

Gambut et al. for modeling the liquid expanded state of neutral amphiphiles. In eq 1 kB is the Boltzmann constant, fe the activity coefficient of water in the two-dimensional phase, we and wc are the partial molecular area of water (we ) 0.097 nm2) and of the amphiphile molecules, respectively; values of fe and wc are derived from the best fit of experimental π-A isotherms using eq 1.

π)

([ (

kBT we ln 1 we A - wc

)]

)

- ln fe

(1)

Ellipsometry. The ellipsometric measurements were carried out with a conventional null ellipsometer,24,25 using a He-Ne laser operating at 632.8 nm. Since the cryptophane absorbance is negligible at this wavelength, the variation of the ellipsometric angle ∆ is a relevant probe for changes occurring at the interface. In practice, the difference δ∆ ) ∆f - ∆e (∆f being the angle measured for the film spread on water and ∆e the angle measured on pure water), which is related to the thickness and index of refraction of the interfacial film, was recorded simultaneously with π and A during compression and decompression cycles performed at slow speed of the mobile barrier (14 mm2 s-1). Some experiments were also performed on films relaxing at a fixed position of the barrier. The spatial resolution of this technique is defined by the cross section of the laser beam at the interface, ca. 1 mm2. Brewster Angle Microscopy (BAM). We used the microscope built up by one of us and mounted above a Langmuir trough of the same type as that described above.26 BAM images recorded at different stages of compression provide information on the nature and homogeneity of interfacial films on the 10-6 to 10-3 m scale. In practice, the BAM images shown below were taken after delay times of 10 min or more at a given position of the mobile barrier, in order to bring the interfacial film to a complete standstill.

Results and Discussion π-A Isotherms. Typical compression-decompression isotherms recorded for the medium-sized crytophane 2 are shown in Figure 2; isotherms a and b were obtained in the ranges 2.50-0.65 and 2.50-1.45 nm2 molecule-1, respectively. The long range isotherm (a) presents two ascents of low compressibility (I) and (II) separated by a plateau. We found similar isotherms for the large cryptophanes 3 and 4. Relevant numerical data for these long range isotherms of 2-4 are assembled in Table 1. These three compounds behave in the same way. The existence of a strong hysteresis occurring during the decompression of the film indicates that either irreversible 3D aggregates or condensed 2D phases with a very slow dispersion rate have formed during the plateau and ascent (II) of the compression. Only cycles limited to the first compression step for 2, 3, and 4 (short range isotherm (b)) proved to be reversible under these conditions. For 2, the termination of ascent (I) occurs at 1.45 nm2 molecule-1, a figure which reasonably matches the expected area for a close packed monolayer of this cryptophane (see above). For 3 and 4, the corresponding transitions take place at 1.55 and 1.50 nm2 molecule-1, respectively, which is still consistent with the formation of monolayers if one considers the lower limit of the molecular area evaluated for these cryptophanes. These monolayers only exist in a small range of surface pressure, 0-8 mN m-1. If this interpretation is correct, the end of ascent (I) is either the onset of an irreversible phase transition to multilayer arrangements of the cryptophanes or the beginning of the irreversible formation of 3D aggregates. Figure 3 shows a π-A isotherm obtained with the small cryptophane 1. Like 2-4, compound 1 exibits an initial compression pattern (I) with a relatively steep ascent (23) Smaby, J. M.; Bockman, H. L. Langmuir 1991, 7, 1031. (24) Theeten, J. B.; Aspnes, D. E. Annu. Rev. Mater. Sci. 1981, 11, 97. (25) Berge, B.; Renault, A. Europhys. Lett. 1993, 21, 773. (26) Henon, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936.

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Figure 4. Variations of surface pressure π (solid line) and ellipsometric parameter δ∆ (dotted line) vs time for a film of 2 submitted to two successive short (left) and long range (right) cycles.

Figure 2. (a) Short range and (b) long range π-A isotherm for a film of 2.

Figure 3. π-A isotherm for a film of 1. Table 1. Surface Pressure and Molecular Area Limits of the Compressibility Domains of π-A Isotherms of 1-4a 1 I

π 0-12 A ≈1.8-1.1 plateau π 12-20 A 1.10-0.50 II π A

2

3

0-8 0-8 1.75-1.45 1.85-1.55 7-9 8-12 1.45-1.00 1.55-1.00 9-48b 12-45b 1.00-0.60b 1.00-0.55b

4 0-7 1.80-1.50 7-9.5 1.50-0.90 9.5-40b 0.90-0.40b

a Units: π in mN m-1 and A in nm2 molecule-1. b These limits do not represent the collapse of the film, but the point where the direction of the barrier displacement was inverted.

observed between 1.4 and 1.1 nm2 molecule-1, which is not far from that expected for a close packed monolayer (see also Table 1). This is followed by a sort of slightly ascending plateau. This pattern is not reversible, and a series of compression and decompression cycles restricted to region (I) shows a shift of the isotherms toward smaller areas, indicating that there is a loss of material at the

interface at each compression. The lack of stability and of reversibility of the monolayer is presumably due to the tendency of this cryptophane to form stable, high-melting 3D crystals27 (mp > 300 °C). The elasticity coefficient E ) -A(δπ/δA)T (or the compressibility coefficient 1/E) derived from the π-A isotherms provide information about the cohesion of molecules in a monolayer. In region I, the elasticity of the monolayer increases with the molecular density up to a maximum Em ) 50 mN m-1 for 2 and 40 mN m-1 for 3 and 4, reached at a surface pressure slightly lower than 8-10 mN m-1, for π-A isotherms recorded at high speed of compression immediately after the standby period of 30 min. These values of Em correspond to the upper limit of the elasticity of liquid expanded phases in conventional amphiphiles (15-50 mN m-1).28 Moreover, region I of the π-A isotherms could be satisfactorily fitted with the Gaines equation (1), giving fe ) 1.13, a small deviation from ideality (fe ) 1), and wc ) 1.28, 1.20, and 1.15 nm2 for 2, 3, and 4, respectively. These values of wc and fe indicate that the monolayers of 2-4 are equivalent to a 2D quasi-ideal liquid expanded state of a classical amphiphile, with an incompressible molecular area in the range of 1.28-1.15 nm2. These values of wc are about 20% smaller than the expected molecular area at the end of region I, a difference which is reasonable in the context of this model. The decrease of wc from 1.28 to 1.15 nm2 when the length of the aliphatic bridges increases (n ) 5, 9, and 10) probably reflects the more flexible and more compressible structure of the larger cryptophanes and possibly the presence of significant populations of in-out topoisomers. These results also support the idea that the cryptophanes are freely spinning at the interface, with a weak anchoring to water, and without specific and strong interactions between them, even in the closest packed state that can be reached for the monolayer. Ellipsometry Data. Ellipsometric studies were performed in order to characterize the three main regions (I, plateau, and II) of the isotherms of 2 and 3 (Figures 4 and 5) and to get an estimate of the thickness of the interfacial layers. In regions of very low surface pressure, either before the first compression or at the end of a decompression, δ∆ was found equal to 6.5° and 8° for 2 and 3, respectively. At the upper limit of region I, where the existence of homogeneous monolayers is expected, δ∆ was found to increase with the compression to a maximum (27) Canceill, J.; Lacombe, L.; Collet, A. J. Am. Chem. Soc. 1986, 108, 4320. (28) Tomoaia-Cotisel, M.; Zsako, J.; Mocanu, A.; Lupea, M.; Chifu, E. J. Colloid Interface Sci. 1987, 117, 464.

5410 Langmuir, Vol. 12, No. 22, 1996

Figure 5. Variations of surface pressure π (solid line) and ellipsometric parameter δ∆ (dotted line) vs time for a film of 3 submitted to two successive short (left) and long range (right) cycles.

Figure 6. (a) Variation of the ellipsometric parameter δ∆ vs time for a film of 2 relaxing at a fixed molecular area (3 nm2 molecule-1) and zero surface pressure. (b) Record of ∆e vs time on pure water (same scale).

δ∆m ) 11° for 2 and 10° for 3; if our views are correct, then δ∆m should represent the value of δ∆ for monolayers of 2 and 3 in their closest packed state. Ellipsometric measurements performed on films of 2 relaxing over several hours at A ) 3 nm2 molecule-1, and surface pressure near zero, revealed the occurrence of erratic variations of δ∆ between 1° and 11° (Figure 6a). For comparison, a record of ∆e on pure water for several hours did not show any significant noise (Figure 6b). These sharp variations of δ∆ observed for the cryptophane film indicate the existence on the water surface of distinct interfacial domains presenting different molecular densities, namely, liquid domains (δ∆ ) 10-11°) surrounded by a gas phase or by almost pure water (δ∆ ca. 1°).29 These domains, having sizes of the order of the laser beam section, drift onto the surface and occasionally pass under the observation spot. This region of the phase diagram thus corresponds to a gas-liquid transition. In region II, δ∆ reaches values of 20° and 27° for 2 and 3, respectively, which is at least twice the δ∆m values. This increase of δ∆ is higher than that observed in region I, and suggests that a condensation of material more important than a simple increase of molecular density due to the compression takes place at the interface, eventually leading to the formation of bi- and multilayers. In contrast with the π-A isotherms, we did not observe any hysteresis of δ∆ during the decompression in long (29) Xue, J.; Jung, C. S.; Kim, M. W. Phys. Rev. Lett. 1992, 69, 474.

Gambut et al.

Figure 7. Theoretical variation of the ratio δ∆/ef vs film refractive index (ef is the film thickness).

range isotherms. This means that even at the onset of the decompression, a significant fraction of the condensed cryptophane molecules are released and spread again on the water surface. In fact, the sharp drop of surface pressure in this part of the π-A diagram corresponds to a loss of molecular cohesion in the remains of the interfacial film. In principle, the thickness ef of the interfacial film can be evaluated from δ∆ if the refractive index nf of the film is known (Figure 7).26 As there is no simple way to measure nf, we attempted to estimate this parameter from the refractive indexes of representative molecular subunits such as 2,3-dimethoxytoluene (n25D ) 1.514) and alkanes (n25D ) 1.386 for heptane and n20D ) 1.398 for octane). The contribution of each subunit was weighted as a function of its mass fraction in the molecular structure, leading to nf ≈ 1.50 and 1.49 for 2 and 3, respectively. This estimate is very crude as it does not take into account the possible presence of remaining dichloromethane (spreading solution) or of water molecules trapped in the interfacial film or in the cryptophane cavities, which could increase the density of the interfacial film (hence somewhat increase nf). Moreover, it is difficult if at all possible to estimate the optical anisotropy of the film perpendicularly to the interface. In fact, nf ) 1.50 is the value usually adopted as the refractive index of monolayers made of conventional organic amphiphiles.30 A higher value, nf ) 1.55, has been reported for a sample of 40 layers of calcium arachidate deposited on a silicon wafer.31 Using the above figures for nf and δ∆m, the thickness of the layer in region I was estimated at 1.95 and 1.85 nm for 2 and 3, respectively. Although these values are slightly too large with respect to the molecular dimensions, they remain consistent with the existence of a monolayer for these cryptophane films in region I. An increase of nf from 1.50 to 1.53 would indeed lead to a better agreement between the estimated film thickness and the size of these molecules (ca. 1.4 nm for 2) and, given the above discussion, a value of nf greater than 1.5 cannot be excluded. The reproducibility of δ∆m for a series of independent experiments is also an argument in favor of the existence of a monolayer in region I for 2, 3, and 4. In fact, this reproducibility indicates that the measured δ∆m are not averaged values corresponding to the coexistence, under the observation spot, of monolayer and multilayer domains associated to domains of pure water. Brewster Angle Microscopy Analysis. We sought to record BAM images for 1, 2, and 3 at different stages (30) Ulman, A. Ultrathin Organic Films; Academic Press: New York, 1991, pp 2-5. (31) Rabe, J. P.; Novotny, V.; Swallen, J. D.; Rabolt, J. F. Thin Solid Films 1988, 159, 359.

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Figure 8. BAM image of a liquid-gas phase transition (π ≈ 0 mN m-1) (a) for a film of 1 at A ) 1.89 nm2 molecule-1 and (b) for a film of 2 at A ) 2.24 nm2 molecule-1.

Figure 9. BAM image of a liquid-gas phase transition (π ≈ 0 mN m-1) (a) for a film of 3 at A ) 2.22 nm2 molecule-1 and (b) for the same film at A ) 2.10 nm2 molecule-1.

of compression, in order to complement the information provided at the macroscopic level by π-A isotherms and ellipsometry. Characteristic features of the most expanded phases of 1-3 at low surface pressure are shown in Figures 8 and 9. The film of 1 (Figure 8a) presents a heterogeneous

structure evidenced by gray domains of different intensities. This behavior illustrates the tendency of 1 to give condensed molecular arrangements even in the absence of applied pressure. Under the same conditions, 2 and 3 (Figures 8b and 9) show dark domains corresponding to pure water or to a gas phase, coexisting with homogeneous

5412 Langmuir, Vol. 12, No. 22, 1996

Gambut et al.

Figure 10. BAM image in the plateau and ascent (II) regions (a) for a film of 2 at 0.98 nm2 molecule-1 and 9.2 mN m-1 and (b) for a film of 3 at 0.48 nm2 molecule-1 and 31 mN m-1.

gray areas suggestive of a liquid phase. The brightness of these different areas is similar to that observed with layers of conventional amphiphilic compounds under the same states. For both compounds, we also observed the formation of 2D foams (Figure 9b). This behavior may explain the erratic variations of δ∆ observed during ellipsometric measurements and presented Figure 6. In the close packed monolayer region of 2 and 3, areas of low density no longer exist and both compounds show a homogeneous texture typical of a liquid phase (not shown). The same texture was observed for 1 at this step of compression (I), suggesting that this compound can form transient monolayers during the displacement of the barrier. At high compression, the BAM image of the mediumsized cryptophane 2 (Figure 10a) exhibits a grain structure of bright, gray, and dark microdomains of 1-2 µm size which seem to represent layers of different thickness or 3D-microcrystals. This collapse is quite different from that of compound 3 (Figure 10b), which displays bright domains of different sizes, suggesting a rigid film breaking up in directions parallel to the mobile barrier during the compression. These images were obtained with an incident laser beam of low intensity and are typical of films presenting regions of molecular density higher than a monolayer. Conclusion In conventional amphiphilic substrates, the interactions between the polar heads and the water surface are stronger than the interactions between the substrate molecules (32) Obeng, Y. S.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 6279. Milliken, J.; Dominguez, D. D.; Nelson, H. H.; Barger, W. R. Chem. Mater. 1992, 4, 252. Diederich, F.; Effing, J.; Jonas, U.; Jullien, L.; Plesnivy, T.; Ringsdorf, H.; Thilgen, C.; Weinstein, D. Angew. Chem., Int. Ed. Engl. 1992, 31, 1599. Malszewskyj, N. C.; Heiney, P. A.; Jones, D. R.; Strongin, R. M.; Cichy, M. A.; Smith, A. B., III. Langmuir 1993, 9, 1439.

themselves, preventing the formation of aggregated structures at low pressure and favoring the existence of monolayers anchored at the surface on compression. Cryptophanes are not conventional amphiphiles, yet they form interfacial films which in many respects behave like those of conventional amphiphiles. This is particularly true for the medium, 2, and large cryptophanes, 3 and 4, which reversibly form films of monomolecular thickness in the range of surface pressure 0-10 mN m-1. Even though their monolayer domain only exists at small surface pressure, the ability of these cryptophanes to spread at the air-water interface is noteworthy. This behavior is certainly related to the coexistence of well-delimited hydrophilic and hydrophobic regions on their surface. The anchoring of these cryptophanes to water is not very strong, however, (i) because ether functions are not very good polar heads, and only a fraction of the 12 ether oxygens are exposed to water at the same time, and (ii) probably because these spherical molecules remain free to spin onto the surface. For these reasons the monolayer is fragile. Another destabilizing factor, which mainly holds for the smallest members 1 and 2, is their tendency to crystallize (the same reason may explain the lack of stability of C60 monolayers32). At this stage, we focus on a modification of the peripheral groups aiming at increasing their polarity in order to achieve stronger interactions with water. The addition to the cryptophanes of classical amphiphiles, to form mixed layers, may represent another way to improve the stability of these films and to allow their transfer onto solid surfaces. Works in these directions are in progress and will be reported in due course. Acknowledgment. We are grateful to Dr. Laurent Garel and Dr. Jean-Pierre Dutasta for providing us with samples of cryptophane-E and cryptophane-O and to Agne`s Se´auve and Martine Simon for technical assistance. LA960245I