J. Phys. Chem. 1992,96,8149-8152 (8) Aparicio, L. M.;Hall, W. K.; Fang, S.-M.;Ulla, M.A.; Millman, W. S.;Dumesic, J. A. J . Cord. 1987, 108, 233. (9) Aparicio, L. M.;Dumesic, J. A,; Fang, S.-M.; Long, M. A,; Ulla, M. A.; Millman, W.S.;Hall, W. K. J. Carol. 1987, 104, 381. (10) Aparicio, L. M.;Ulla, M.A,; Millman, W. S.; Dumesic, J. A. J.
Carol. 1988, 110. 330. (ll)Amiridis,M.D.;Puglisi,F.;Millman,W.S.;To~,N.-Y.;Dumesic, J. A. J. Coral., to be submitted. (12) Reston, R.S.; Hanna, S.S.;Herberle, J. Phys. Rev. 1%2,128,2207. (13) Hobson. M.C., Jr.; Gager, H. M.J . Colloid Interface Sci. 1970, 34, 357. (14) Gager, H. M.;Lefelhocz, J. F.;Hobson, M. C., Jr. Chem. Phys. Leu. 1973, 23, 386. (15) Clausen, B. S.;Msrup, S.;Top=, H. Surf. Sci. 1981, 106, 438.
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(16) Yuen, S.; Chen, Y.;Kubsh, J. E.; Dumesic, J. A.; Top% N.; T o ~ ~ c , H.J . Phys. Chem. 1982,86, 3022. (17) Aparicio, L. M.;Dumesic, J. A. Lungmuir 1988, I , 1044. (18) Delpss,W. N.; Garten, R.L.; Boudart, M. J. Phys. Chem. 1969,73, 2970. (19) Garten, R. L.; Delpss, W. N.; Boudart, M. J . Carol. 1970,18,90. (20) Greenwood, N. N.; Gibb, T. C. MBssbauer Spectroscopy; Chapman and Hall: London, 1971. (21) Bauerle, G. L.; Wu,S. C.; Nobe, K. I d . Eng. Chem. Prod. Res. Dcv. 1975, 14, 268. (22) Odenbrand, C.U.I.; Lundin, S. T.;Andersson, L.A.H. Appl. Carol. 1982, 18, 335. (23) Inomata, M.; Miyamoto, A.; Ui, T.;Kobayashi, K.; Murakami, Y. Ind. Eng. Chem. Prod. Res. Deu. 1982, 21, 424.
C60and Cl0 at the Air-Water Interface Roberta Back and R. Bruce L~MOX* Department of Chemistry, McGill University, 801 Sherbrooke St. West, Montreal, Quebec, Canada H3A 2K6 (Received: February IO, 1992;In Final Form: April 29, 1992)
CsoIC,& and a Ca/G0 (8515)mixture have been investigated at the ah-water interface using the Langmuir trougb experiment. For each sample, isotherms obtained under a variety of experimental conditions are consistent with the formation of multilayered in the initial dilute state. Calculation of the work done on each multilayer film as a fimction of the extent of compreasiOn do" reveals differing properties for the Cso and C70 films. These results are discussed in the context of the balance between fullerenelfullerene and fullerenelwater interactions.
Introductiw
Monolayer films formed at the air-water (a/w) interface are of great interest because of the insights they provide about m o l e c u l m l d e interactions and their applications to important technologies related to coatings and surface modifications.'p2 The vast majority of compounds studied to date at the a/w interface have been amphiphilic molecules, whose structures have both hydrophobic and hydrophilic domains. This property of amphiphilicity often leads to aggregation (self-assembly) into reasonably well-defined structures and is the basis for diverse phenomena such as cell membrane organization, soap micelle formation, and the spontaneous monolayer coating of metal surface~.'-~ Much less studied have been the film-forming materials which do not contain the structural elements of amphiphilicity (i.e., rigid polycyclics,6 hydrophobic polymers such as polystyrene,' or large macrocycles such as phthalocyanines8). Often these 'nontraditionaP types of materials prove to be difficult or impossible to study at the a/w interface because their propensity for self-association is much greater than their ability to spread on the water surface. In many cases, this competition leads to the formation of macroscopic lenses of the material on the water surface. We have recently undertaken a number of studies related to the question of unusual Langmuir film-forming materials, including diblock copolymers,e12thiophene/ferrocene conjugates," and bipolar and tripolar pht~pholipids.'~ In this report we describe a test of the concept of film formation from 'nontraditional" materials, where we have studied the properties of c 6 0 and C70 spread at the a/w interface. The rcccnt a~ailability'~ of Cso/C70mixtures in reasonable quantities and detailed documentation of methods to purify each species from this mixture have led to a number of studies dealing with both characterization and the chemical manipulation of these molecules. In some of these studies thin films of Cso, prepared have been used. by vapor-phase deposition1618of solvent ~asting,'~ In cases of experiments where molecular rcpolution is possible (i.e., STM and AFM), the solid substrate surface has been shown to strongly influence the ordering phenomena and properties of the
adsorbed Cso. This, combined with the fact that bulk Cbomolecules have been shown by NMR spectroscopy to be only very weakly self-interacting?O leads us to believe that the interactions of Cso and related compounds with metal and polymer surfaces is potentially of great importance. Given this, we have studied the properties of Cso and C70 on water to qualitatively determine the balance between the molecule/molecule and molecule/substrate interactions. During the course of this work, Obeng and Bard presented a study of Csoat the a/w interfad' and Nakamura et al." described the formation of Langmuir-Blodgett films of Cso in monolayers of two matrix molecules. Our study expands upon the former report in that, in addition to Car both C70 and a CM/C70 mixture are also studied and compared under a variety of experimental conditions. Experimental Section A c6O/c70 mixture was obtained from Research Materials (Boulder, CO). A fast atom bombardment mass spectrometry (FAB/MS) characterization (Biomedical Mass Spectrometry Unit, McGill University) indicated that the mixture contained approximately 85% Cm and 15% C70 and no higher fullerenes in >I% quantities. This material is referred to in the text as Ca/C,,. Another sample was obtained from Aldrich (nominally 9010 Cm/C70)and was purified by column chromatography (alumina, Aldrich, hexanes as the eluting solvent) as per a literature proc e d ~ r e .Purity ~ ~ was checked by FAB/MS and UV-vis spect r o ~ c o p y .The ~ ~ latter technique is particularly useful because of the very different absorbances of Csoand C70in the 4O(MSO-nm range. The FABIMS was consistent with a purity of >99% for both the c 6 0 and C70 obtained by chromatography. Two types of Langmuir troughs were used in this study as a check of methodologies and as a probe of the origin of the surface pressure (r)measured. The previous reports of airlwater meawere conducted solely with the torsion balance surements of Cso2122 type of measuring device. A Lauda Model D film balance, o p erated under thermostated control (Haake), u8e8 a L.angmuir-style floating barrierltorsion balance measuring device and has a total
0022-365419212096-8 149$03.00/0 0 1992 American Chemical Society
Back and Lennox
8150 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
spreading surface of a975 cm2. The other film balance used, a KSV 3000, employs a Wilhelmy plate (WP) measuring device and two moving barriers. The WP was positioned perpendicularly to the direction of compression and remained so throughout the experiment. The WP is a sandblasted Pt plate which was flamed to redness and quenched in methanol between runs. The KSV device has a total spreading surface of -740 cm2. We note that C, and C70 tenaciously stick to most surfaces (even Teflon), and rigorous cleaning of the Langmuir troughs was found to be necessary. This affinity is readily noticed because these materials are highly colored and are strongly absorbing" in the visible range. CM/C70, C,, and C70 spreading solutions were prepared by dissolving weighed quantities in an appropriate spreading solvent. Toluene, benzene, and CH2Clzwere all used, either pure or as mixtum, with similar results in all cases. Samples of these solvents were spread on the Langmuir troughs as a check for their purity in terms of interfacially-active contaminants. The fullerene to 6.7 X M. In concentrations used ranged from 2.7 X each experiment, a known volume (25-500 pL) of solution was carefully added to the surface in 5-10-pL increments using a Hamilton syringe. Typically, 15 min elapsed so as to allow for evaporation of the spreading solvent before compression proceeded. Evaporation times of up to 900 min had no discernible effect on the results. Compression rates ranged from 4 to 62 cm2/min (Lauda) and 15 to 60 cm2/min (KSV). Again, no significant differences in the resulting isotherm were noted, and intermediate compression rates were used throughout. In order to clearly distinguish the origin of the measurements reported here, data obtained from the Langmuir type of film balance (Lauda) is designated as rL and that obtained from the WP experiment (KSV) is designated as rw. Because both the concentration of the spreading solution and the total quantity of material (per cm2 of surface area) are important factors in these particular experiments?l a ratio C,/C, is quoted throughout. Ci is the total area expected to be covered by the sample if the area per molecule is a100 A2. C, is the total area available to this quantity of material for spreading (i.e., 740-975 cm2). A Ci/Ct ratio of 0.10 is a statement that the initially spread sample occupies a theoretical maximum of 10% of the surface. This ratio provides a simple indicator as to how dilute the initial surface concentration actually is. Although numerous studies have shown that the two types of measuring devices used here yield equivalent isotherms, it must be emphasized for the following discussion that they are each measuring different features of the a/w interface. The Langmuir balance technique detects displacement of a floating barrier which separates a pure H 2 0 surface and the filmaated surface, so that rL = 6, - 6 = bclepn- 6film-coated.For nonamphiphilic molecules such as polystyrene, the rLmeasured is due solely to the mechanical force exerted on the float by the film,' as the surface tension of water is unaffected. rLis therefore more properly considered to be an apparent rLin this case. The WP technique, on the other hand, directly measures the surface tension of the film-coated surface, rw. Since a nonpolar material such as polystyrene (or C,) should not, in principle, alter the surface tension of the water subphase, mass changes at a WP m a y arise from modification of the plate by the film material7 Figure la shows the ~ L - cAu m for a C,/C,, sample deposited on a pure water surface using a Langmuir balance system. Several features of this isotherm are notable. Firstly, the surface film is clearly quite compressible, with a very gradual increase in rL beginning at A >lo0 A2/molecule and, eventually, steeper increases at A < 40 A2/molecule. Higher surface pressures are observable with greater initial loadings; this is not a feature of the film but is rather an inherent part of both trough configurations. Because the compression process often ends before substantial r values or allapse of the film can occur,the fact that values are reported does not necessarily reflect only modest rmax the properties of the surface f h . If enough molecules are applied to the water surface at the beginning of the experiment, the film
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80
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Molecular Area (.&'/molecule)
Figwe 1. (a) Surface preasure (q,)as a function of mean molacular area for Cso/C70.A weighted average for the Cso/C70mixture is used. Temperature = 25 O C , CJC, = 0.108. Measurements were obtained using a Langmuir balance type of measuring system. (b) Surface pressure (ww) as a function of area per molecule for C60/C70.Conditionsas per (a) except that CJC, = 0.529. Measurements were obtained using a Wilhelmy plate type of measuring system.
can be observed to collapse at -45 mN/m. Figure 1b shows an isotherm of the Cm/C70 mixture, but this time measured with the Wilhelmy plate (WP) technique. Compared with the isotherm in Figure la, the magnitude of the entire rwresponse is reduced considerably. As a result, any changes in r above baseline values are difficult to discem at A 1 30 A2/molecule. Ali, values obtained with the WP technique were consistently smaller than those obtained with the Langmuir balance technique. This was observed under many experimental conditions, particularly over a range of Ci/Ctvalues. In addition, if C,/Ct is large (e.g., 2.0) on the WP trough, a collapse pressure of -65 mN/m is obserwd, again indicating that the two meesUring devices arc detecting somewhat different features of the Lmgmuir film. Nonetheless, despite the different measurement principles of the two techniqucs,2s the features of the r~and rw w A curves appear to parallel one another well, so that WP-derived measurements are a useful indicator of packing changes of these hydrophobic molecules on the water surface. The pure C, film behaves similarly to the 8515 Cm/Cm film (Figure 2a). The pure C70 film (Figurc2b) is similar to the C, film in that a rwvalue of substantially greater than 0 mN/m is only expressed at greater than the theoretical monolayer merage (Le., -25-33 Az/molecule instead of 1W110 A2/molecule). Notably, the Cm film also undergoes an apparent phase tramition, with a pronounced inflection occurring at rw = 3 mN/m. Diacuasion The data in Figures 1 and 2 clearly are consistent with a multilayering of the C,, C70, and Cso/C70samples at the a/w interface, and the most prominent isotherm features are associated with multilayers composed of 3-10 monolayer equivalents. Considerable effort was directed to the study of these films at
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8151
Cmand C70 at the Air-Water Interface
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I'
16 15 14 13
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/
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Figure 3. Data from Figures lb, Za, and 2b replotted as TW vs I', the mean molecular surface densities of C60/C7o9Cm, and C70 on the water surface. 17 16
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Figure 2. (a) xw as a function of mean molecular area for pure CW Temperature= 25 "C,Ci/C, = 0.903. (b) As per (a) except that sample is C70and CJC, 1.016.
coverages expected to be consistent with a gas-analogous phase. Conditions such as highly dilute initial surface concentrations (where Ci/Ct = 0.10), long evaporation times (Le., 15 h) of the spreading solvent, large solvent quantities, and elevated temperatures during spreading (i.e., 35 "C) were all imposed. None of these protocols, however, led to isotherms which differed in substance from those in Figures 1 and 2; i.e., only isotherms which exhibited signifcant features at multilayer rather than monolayer coverage2' were found. Much can be learned from this multilayering behavior regarding the nature of Ca/C70, C70, and Ca interactions. In all the isotherms generated, only modest ?r values ( Am, the C m / C 7Car be (i) organized in a single layer of noninteracting, isolated molecules, (ii) self-assembled in defined, ordered domains or
1 2
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Figure 4. Work required to compress fih from N - 1 to N monolayer equivalents, plotted as work vs N. Thae values were determined from the area under the x vs molecular area isotherms. One monolayer equivalent is assumed to be 100 A* per fuuerene molecule. Cso/C70(-), Cso
and
c70
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islands one layer thick, or (iii) organized into islands of multiple layer thickness. Situation (i) probably does not describe the system because the surface pressures expected at close-to-monolayer coverage (Le., A = 125 A2/molecuie) from simple ideal gas law considerations are not experimentally observed. The experimental T-A curves indicate that for situation (ii) to be operative the collapse of the close-packed monola er film and formation of a bilayer film, for example, at A < 100 i 2 / m o l d e would OCCUT at surface prarsures below the measuring capabilities of the Langmuir trough experiment (Le., h0.1 mN/m). Although this cannot be discounted, the magnitude of the energies involved with compression of multilayers suggests that r values of -0 mN/m at A = 100 A2/molecule arise because the surface is only partially covered at this area by multilayered domains and much of the surface is uncoated water (i.e., condition iii). Of particular note is the fact that the isotherms are reproducible over the range of CJC, ratioa investigated (0.1-2.1) on each particular trough. This suggests that the postulated islands, when formed by evaporation of the spreading solvent, adopt an average height (and therefore probably also an average width) which is reproducible from experiment to experiment. The isotherm of pure C70 is unique in our measurements in that it exhibits a distinct kink consistent with there being a phase transition. This kink varies with temperature, becoming more pronounced and arising at higher r values at lower temperatures. Unlike the Cmsample, which features a steadily increasing value of work per layer, N (Figure 4), the work per layer for the C70 sample g m through a maximum at the N = 6 N = 7 transition. It is not known at the present time whether the larger work values
-.
8152 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
in the C70 compared to the Cm are due to a pressure-induced reorganization from a side-on to end-on orientation (given that C70 is an oblate ellipsoid) of C70 in the multilayer film. Interestingly, the C,j,3/C70 mixture combines features of both materials, with substantial r beginning only at a trilayer coverage but with substantially more work required to effect an N = 4 N = 5 transition than in the pure Cm case. Figure 4 is therefore useful in commrintz the energetics associated with multilayer evolution. Mtiough-cm is oniy weakly self-interacting, there is clearly a strong driving force for it to self-assemble at the a/w interface. Expansion of compressed films does not occur, showing that the condensed multilayer films are very cohesive and that c m / c 6 0 interactions are strong relative to Cso/water interactions. A recent STM study of Cm (submonolayer coverage) adsorbed to a GaAs (1 10) surface reveals parallel behavior, where the Cm spontaneously forms islands and, in places, forms a duplex film.16 In the GaAs case,as in the H20case discussed here, the balance between the C@-substrate and & C O interactions favors the latter to some extent. In retrospect, it is perhaps remarkable that the Cm and C70 species form compressible films of any form at the a/w interface. The finite affinity of Cm and C70for the water surface probably originates from their large polarizabilitie~.~~ The role of this polarizability in selective adsorption is also evident from differences in thin films of Cm deposited on Au( 1lo)'* and GaAs.16 Some aspects of the form of the multilayer compression curves are also of interest. In general, compression of an amphiphile film past a close-packed monolayer state results in film buckling and collapse.' This process is generally ill-defined and uncharacterizable; multilayering may occur, and fragments of the film may be forced into the subphase. The *-A signature of this process is a sudden drastic decrease in x at the film collapse point. Multilayering, on the other hand, can be manifested as a plateau in the is0therms,l4J7consistent with the film partitioning between two states. Neither the Cm nor C70 sample exhibits collapse or plateau behavior at N = 1 N = 10, however. Evidently, the (N - 1) N reorganizations are energetically costly, but accessible, and reflect the energies associated with multilayer evolution.28 The film balance studies thus provide access to details about the structure and energies of different multilayer states on a liquid surface, all in terms of the lateral pressure exerted by the films. In the context of other a/w studies of surfactants, these measurements are also of interest because island/cluster formation has been observed in x-ray and neutron reflectivity experiments.29~30 We note that multilaym at the a/w interfaceare much less studied than at the &/solid or vacuum/solid interfaces, primarily because most molecules studied at the a/w interface are strongly adsorbed by polar or ionic groups. This strong adsorption leads to film collapse and undefinable morphologies at surface densities of greater than one monolayer coverage. It is precisely because Cso and C70have such weak affinity for the water surface that the compression-induced multilayering phenomenon appears to be so accessible. Recognizing this, it is apparent that the Langmuir
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Back and Lennox trough experiment offers further opportunities in the study and manipulation of surface-modifying materials. Acknowledgment. Support for this work from NSERC (Canada) and the McGill Graduate Faculty is gratefully a p preciated. R@try
NO. Ca,99685-96-8; C70, 115383-22-7.
References and Notes (1) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Lungmuit-Blodgett to Self-Assembly; Academic Press: New York, 1991. (3) Tanford, C. The Hydrophobic Effect; 2nd ed., Wiley: New York, 1980. (4) Fendler, J. Membrane Mimetic Chemistry; Wiley: New York, 1984. (5) Bain, C. D.; Whitesides, G. M. Angew. Chem., In?. Ed. Engl. 1989, 28, 506. (6) Kenny, P. W.; Miller, L. L.; Rak, S. F.; Jozefiak, T. H.; Christopfel. W. C. J. Am. Chem. Soc. 1988,110, 4445-4446. (7) Kumaki, J. Macromolecules 1988, 21, 749. (8) (a) A r m , R.; Jennings, C.; Kovacs, G. J.; Loutfy, R. 0.; Vincett, P. S. J . Phys. Chem.1985,89,4051-4054. (b) Baker, S.; Petty, M. C.; Roberts, G. G.; T w i g , M. V. Thin Solid Films 1983, 99, 53. (9) Zhu, J.; Eisenberg, A.; Lennox, R. B. J . Am. Chem. Soc. 1991, 113, 5583.
(IO) Zhu, J.; Eisenberg, A.; Lennox, R. B. Makromol. Chem., Mucromol. Symp. 1992, 53, 211. (1 1) Zhu, J.; Lennox, R. B.; Emnberg, A. J. Phys. Chem. 19!32,96,4727. (12) Zhu. J.: Lennox. R. B.: Eisenbern. A. Lunmuir 1991.. 7.. 1579. (1 3) Back, R.;Lennox, R. B. Submit& to Luigmuir. (14) Beck, A.; Hebert, N.; Just, G.; Wei, L.; Lennox, R. B. Unpublished results. (15) Kritschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (16) Li, Y. Z.; Patrin, J. C.; Chander, M.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Science 1991, 252, 547-548. (17) Snyder, E. J.; Anderson, M. S.; Tong, W. M.; Williams, R. S.;Anz, S.J.; Alvarez, M. M.; Rubin, Y.; Diederich, F. N.; Whetten, R. L. Science 1991, 253, 171. (18) W h n , R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; deVries, M. S.;Hunziker, H. E.; Wendt, H.R. Nature 1990,348,621. (19) Jehoulet, C.; Bard, A. J. J . Am. Chem. Soc. 1991, 113, 5456. (20) Yannoni, C. S.; Johnson, R. D.; Meijer, G.; Bethune, D. S.; Salem, J. R. J. Phys. Chem. 1991,95, 9. (21) Obeng, Y.S.; Bard, A. J. J . Am. Chem. Soc. 1991, 113, 6279. (22) Nakamura, T.; Tachibana, H.; Yumura, M.; Matsumoto, M.; Azumi, R.; Tanaka, M.; Kawabata, Y. Lungmuir 1992,8,4. (23) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J . Chem. Soc., Chem. Commun. 1990, 1423. (24) Allemand, P.-M.; Koch, A.; Wudl, F.; Rubin, Y.; Dicderich, F.; Alvarez, M. M.; Anz, S. T.; Whetten, R. J. J. Am. Chem. Soc. 1991, 113, 1050. (25) Adamson, A. W. Physical Chemistry of Surfaces, 5th cd.; Wiley: New York, 1990. (26) This value has been estimated from models, assuming that the van der Waals radii are as per observed in CW (27) Yamauchi, K.; Moriya, A,; Kinoshita, M. Biochim. Biophys. Acta 1989, 1003, 151. (28) Dash, J. G. Proc. Natl. Acad. Sei. U.S.A. 1987, 84, 4690. (29) Jacuuemein. D.: Leveiller. F.: Weinbach. S.P.: Lahav. M.: Leiserow&,'L.; a a e r , K.'; Als-Nielsen, J. J. Am. Chem. Soc. 1991, 113, 7684. (30) Landau, E. M.; Grayer Wolf, S.;Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. J . Am. Chem. Soc. 1989, 1 1 1 , 1436.
