Langmuir 1996,11, 1435-1438
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C6o-propylamine Adduct Monolayers at the Air-Water Interface D. Vaknin,**tJJ. Y. Wang,#and R. A. Uphaussl" Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, and Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011 Received February 3, 1995. I n Final Form: March 20, 1995@ X-ray reflectivity measurements and surface-pressure isotherms of spread films of the fullerenepropylamine adduct (C~~-[NHZ(CHZ)ZCH~]~Z) indicate that despite the lack of definite amphiphilic character, this material forms a homogeneous monomolecular layer on water surfaces. X-ray reflectivity data, taken at various lateral pressures, were fitted to a model structure defined in terms of the dimensions of an average cell and its chemical compositions. At T = 18 "C, the lateral pressure versus area per molecule is indicative of a structural phase transition presumably from ope closely-packed phase to another, as the total thickness of the film is consistent with a single layer of 15 A thickness. At pressureqabove 30 mN/m the film consists of an inhomogeneous bilayer, consistent with a layer thickness of 26 A. 1. Introduction
Until recently, it was a common conception that in order to form a monomolecular layer at the air-water interface, the so-called Langmuir monolayer, the constituent molecules must be amphiphilic, i.e., possess a hydrophilic head moiety and a hydrophobic tail. However, a recent study showed that the nonpolar molecule perfluoro-neicosane (F(CF2)zoF) formed a stable ordered Langmuir monolayer on water, despite the lack of amphiphilic character.l Independently, we reported our results on the amine adduct of the c60 fullerene (C~O-[NHZ(CHZ)IICH&) that by symmetry lacks polarity, but may form a monolayer at the air-water interface.2 Extensive studies on spread films of pure c60 and C70 fullerenes on aqueous surfaces were performed as a step toward their exploitation in films on solid supports by the Langmuir-Blodgett deposition t e c h n i q ~ e . ~These - ~ studies indicated that the films are m ~ l t i l a y e r e d ~and . ~ therefore, for practical purposes, useless. In our study we derivatized fullerenes by reaction with amines2inan attempt to reduce the strong cohesive forces operative between individual fullerene molecules, thus reducing aggregation. At the same time, it was anticipated that the presence of amine groups would introduce regions with amphiphilic character to the adduct. In a similar approach, the fullerene has been placed in the nitrogen-rich, hydrophilic cavities of azacrown compounds that when spread on water resulted in the spontaneous formation of a monomolecular layer of the ~ o m p l e x . ~ Amines with varying chain lengths readily react with fullerenes in a two-step nucleophilic addition6z7that in Ames Laboratory and Department of Physics and Astronomy.
* e-mail,
[email protected]; Tel, (515) 294-6023; Fax, (515)
294-0689. 5 Ames Laboratory and Department of Chemistry. 'I Deceased.
(1)Li, M.;Acero, A. A.; Huang, Z.; Rice, S. A.Nature 1994,367,151. (2)Wang, J . Y.; Vaknin, D.; Losche, M.; Kjaer, K.; Uphaus, R. A. Thin Solid Films 1994,242,40. (3)Obeng, Y. S.; Bard, A. J . J . Am. CFem. SOC.1991,113,6279. (4)Back, R.; Lennox, R. B. J.Phys. Chem. 1992,96,8149. (5) Diederich, F.; Effing, J.; Jonas, E.; Jullien, L.; Plesnivy, T.; Ringsdorf, H.; Thilgen, C.; Weinstein, D. Angew. Chem., Int. Ed. Engl. 1992,31, 1599. (6)Suzuki,T.; Li, Q.; Khemani, K.; Wudl, F.; Almarsson, 0.Science 1991,254,1186.
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Figure 1. Surface pressure versus area per molecule of CSOpropylamine spread at the air-water interface, with three distinct regions labeled A, B, and C. Region A and B correspond to a single layer and C corresponds to a bilayer structure. Surface-pressure versus area-per-moleculemeasurements at room temperature and T = 18 "C yielded indistinguishable curves (within measurements errors).
general yields a mixture of adducts due to the multiple reaction sites on the fullerene and the inability to control the extent of addition. It was reported7that the average number of amine molecules that react with each fullerene varies with the hydrocarbon-chain length, the shorter the chain the more amine molecules react with the fullerene. This may be due in part to the attraction of the long hydrocarbon chains to the graphite-like surface of the fullerene and their flexibility to wrap around the same or adjacent fullerenes and block reaction sites by steric hindrance. The attraction of hydrocarbon chains to graphite is well-known from STM studies of monolayers of long alkanes and alkyl derivatives on the surface of graphite, that revealed that the molecules order with the carbon skeleton parallel to the graphitic ~ u r f a c e . ~ ~ ~ (7)Wudl, F.; Hirsch, A,;Khemani, K. C.; Suzuki, T.; Allemand, P.M.; Koch, A,; Eckert, H.; Srdanov, G.; Webb, H. M. In Fullerenes, Synthesis, Properties, and Chemistry ofLarge Carbon Clusters; Hammond, G. s.,Kuck,V.J.,Eds.; American ChemicalSociety: Washington, DC, 1992; p 161. (8) Hentschke, R.; Schurmann, B. L.; Rabe, J. P. J . Chem. Phys. 1992,96 (8), 6213.
0743-7463/95/2411-1435$09.00/00 1995 American Chemical Society
1436 Langmuir, Vol. 11, No. 5, 1995
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Figure 2. (a)X-ray reflectivity of (360-propylamine at the air-water interface. In part b, the reflectivity (a) normalized to the Fresnel reflectivity, RF,of water subphase, is presented. This normalized reflectivity is practically the genuine signal due t o the monolayer. Parts c and d show the normalized reflectivities as in (b) at higher surface pressure values. The solid lines are calculated from the electron-density profiles ( g ( z ) )shown as solid lines in the insets. The dashed lines in the insets illustrate the ideally sharp interfaces that are Gaussian-smeared due to surface roughness to yield the solid-line electron-density profiles. Wilhelmy pressure sensor. Monolayers were spread from a Here, we report for the first time on the lateral-pressure chloroform solution on Millipme Milli-Q plus HzO, and comversus area per molecule, n-A isotherms, and X-ray reflectivity studies of the material C ~ ~ - [ N H Z ( C H Z ) ~ C H ~pressed ~ ~ Z , a t a rate of about 1 (A2/molecule)/min. Reflectivity measurements of spread films were carried out that forms a monomolecular layer at the air-water on a novel liquid-surfaceX-ray reflectometerrecently constructed interface with an intriguing phase diagram. at Ames Laboratory. The apparatus is similar to the one
11. Experimental Section The fullerenes were purchased from the Texas Fullerene Corp., Houston, TX (purity 99.9%), and were used as received, and n-propylamine was purchased from the Aldrich Chemical Co. The amine adduct was prepared as described by Wudl et aL.,l and the method is summarized as follows: 15 mg of c 6 0 was mixed with 20 mL of freshly opened n-propylamine, forming a dark green solution that upon stirring for 24 h at room temperature gradually turned brown. The remaining unreacted n-propylamine was evaporated and the product was dried under vacuum. Further purificationby dissolving the product in toluene and passing the solution through a silica column eliminated the unreacted CSO. The product was then washed from the column using toluenehsopropylamine alcohol (9:l). Subsequently, the solvent was evaporated and the sample was dried under vacuum for 24 h. Elemental analysis was consistent with the addition 12 f 2 propylamine molecules per fullerene to yield c60[NHz(CHz)zCH3112. Note, that C ~contains O 12 pentagon-likerings that are likely to react with amines. Surface-pressure versus area-per-molecule isotherms were measured with a conventional Teflon Langmuir trough with a (9) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. Lett. 1993, 281, L297.
described by Als-Nielsen and Pershan,lo with the additional ability to rotate the 0 axis and 20 arm of the monochromator to obtain the additional degrees of freedomthat allow second-order correctionsofthe two axes.ll These corTectionsensure operation with a constant wavelength (A = 1.5404A,CuKa) at all scattering angles. In reflectivity experiments a monochromatic X-ray beam of intensitylo and at an angle ai is incident upon an aqueous surface, and the reflected intensity, I,, a t an angle a, (a,= ai = a, specular reflection conditions) is detected. The reflectivity as a function of the momentum transfer, Qz= (4zk) sin a, is defined as I?(&,) = l J l 0 . From the reflectivity, the electron density across the interface, e(z), can be determined and related to the molecular structure at the interface. The electron density of a homogeneous medium modifies the refractive index with respect to X-rays as12
where ro is the classical radius of the electron (TO = 2.82 x (10)As-Nielsen, J.; Pershan, P. S. Nucl. Instrum. Methods 1983, 208, 545. (11)Vaknin, D., unpublished angle calculations for liquid-surface X-ray reflectometer operating at a constant wavelength mode.
Langmuir, Vol. 11, No. 5, 1995
Letters cm). The reflectivity from a finite number and ideally sharp interfaces, Ro(Q,), can be exactly calculated using standard recursion methods.13J4 To accountfor capillarywaves and surface imperfections, and working under the assumption of Gaussiansmeared interfaces of the form exp(-0.5(z - zi)2/a2)for each interface, the reflectivity is modified by a Debye-Waller-like factorl5
111. Results and Discussion Figure 1 shows the surface-pressure versus area-permolecule isotherm for C6o-propylamine on the surface of H20. The film exhibits a well-reproduciblen-A isotherm which does not depend on the size of the sample or the concentration of the solution, and the limiting area per molecule is larger than that of the pure fullerene, c60.2 Detectable increases in the surfac9 pressure start a t an area-per-moleculelarger than 200 A2. From the isotherm we infer that the molecqles are already densely packed at area values of -150 A2 per molecule. Above n 15 mN/m the n-A slope changes, indicating a phase transition, possibly from one closely-packed structure to another. Above 30 mN/m the film consists of a bilayer of the amine adduct, as inferred from the total thickness of the film extracted from reflectivity measurements. Repeated compression-expansion of the film below about 15 mN/m results in essentially identical n-A curves, whereas above that pressure the curve is hysteretic suggestive of some irreversible phenomena. Figure 2a shows the X-ray reflectivity of c6O-propy1amine on water at n = 6.7 mN/m. Parts b-d of Figure 2 show the reflectivity normalized to the Fresnel reflectivity expected for an interface where the electron density, g(z), changes oabruptlyfrom zero to the density of water, g, = 0.334 e/A3, a t different lateral pressures. The solid lines are calculated from the scattering length densities shown in the insets. In calculating the reflectivities, it was assumed that the interface consists of a single slab15 of homogeneous electron density. The scattering length density is calculated assuming a box-model structure at the interface. The box contains one adduct and a variable number of water molecules, N,. The thickness of the box d ,N,, and the surface roughness, 0,are the free variables in this compositional space refinement technique.16 In the refinement, the area of the box A(n)was determined from the n-A isotherm. The nonlinear least-squares best fitted parameters of the model structure are listed and defined in Table 1for several values of lateral pressure. The average thickness of the aodduct monolayer a t pressures below 30 mN/m is 12- 15A which is larger than the value expected from a smoothly distributed fullerene layer, 3110 A. It is however, smaller than the expected end-to-end length of the adduct. We estimatp a linearly stretched amine-C6o-amine to be about 20 A in length. It is therefore possible that at the transition to a closelypacked monolayer, at an area per molecule of 150 A2, nearest-neighbor molecules interpenetrate the chains of
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(12) James, R. W. Optical Principles ofX-Rays;Ox Bow: Woodbridge,
CT,1982.
Table 1. Structural Parameters of CSO-[NH2(CH2)2CHsh Monolayer at the Air-Water Interface at Various Lateral Pressure Values, As Obtained from Fitting a Model to the Reflectivity Curvesa lateral pressure m (mN/m) 0.7 6.7 9.3 11.8 20 30
where o is defined as the surface roughness.
(13)Parratt, L. G.Phys. Rev. 1954,95,359. (14)Born, M.; Wolf, E. Principles of Optics; MacMillan: New York, 1959. (15)Als-Nielsen,J.;Kjaer,K. In Phase Transitions in Soft Condensed Matter; Riste, T., Sherrington, D., Eds.; Proc. NATO AS1 ser. B 211; Plenum Press: New York, 1989; p 113. (16)Vaknin, D.;Kjaer, K.; Als-Nielsen, J.; Losche, M. Makromol. Chem. Macromol. Symp. 1991,46, 383. Vaknin, D.;Kjaer, K.; AlsNielsen, J.; Losche, M. Biophys. J . 1991,59,1325.
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area per molecule (A2) f 10 157 148 145 139 118 66.5
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a The area per molecule was determinedfrom the m-A isotherms and the errors where estimated including the uncertainty in the area per molecule. Global estimated errors are given next to the dimensions of the parameters.
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adjacent adducts. Although smaller than expected for an adduct with its amine stretched out, the area per molecule is nonetheless larger $han that expected for a closelypacked fullerene (-87 A). This is consistent with a picture in which the adducts might be partially entangled with alkyl chains. This entanglement and interdigitation of hydrocarbon chains of neighboring adducts may be due to the van der Waals attraction of hydrocarbon chains to the fullerene and to neighboring chain^.^^^ At intermediate lateral pressures, 15-28 mN/m, the n-A curve changes slope, suggestive of a phase transition to a newly closelypacked phase. it should be emphasized that the film thickness of 15 A is just slightly larger than in phase A (cf. Figure 1);however it is still within the range of a single monolayer. Interdigitation rearrangement of hydrocarbon chains of neighboring adducts is one possible explanation to the change in slope assuming that at surface pressures lower than 15 mN/m the chains are not entangled. Another possible explanation is that this is a subtle structural phase transition (e.g., from square to triangular symmetry). Diffraction at grazing angles of incidence with the intense X-ray beam of a synchrotron may reveal the in-plane arrangement of the adducts and resolve the problem. At pressyres larger than 28 mN/m the film thickness is about 26 A which is consistent with the formation of an inhomogeneous bilayer of the adduct (cf. Figure 2d). The inhomogeneity of the bilayer gives rise to the relative increase in surface roughness (see Table 1). Figure 3 depicts schematically our interpretation of the conformation of the interdigtating adducts at the airwater interface below 30 mN/m. Despite the simple and the straightforward result presented here, it is remarkable that the isotropic fullerene
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1438 Langmuir, Vol. 11, No. 5, 1995 adduct, which lacks a clear amphiphilic character, forms a homogeneous monolayer at aqueous interfaces. It is widely appreciated that fullerenes possess unique combinations of chemical and physical properties. The present study confirms this view in the specific case of spread monolayers of derivatized fullerene adducts. Adducts with short hydrocarbon chains are more likely to form a homogeneous monolayer in contrast to the long-chain amine adduct which consist of a distribution of amine products and form inhomogenous monolayers.
Acknowledgment. We thank B. W. Gregory for bringing to our attention refs 8 and 9 and M. Losche for helpful discussions. Ames Laboratory is operated by Iowa State University for the U.S. Department of Energy under Contract No. W-7405-Eng-82 (1993). The work at Ames was supported by the Director for Energy Research, Office of Basic Energy Sciences. LA950081Q
