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Langmuir 1996, 12, 1402-1403
Buckminsterfullerene-Containing Microemulsions Wei Zhang, Robert V. Honeychuck, and Abul Hussam* Department of Chemistry, George Mason University, Fairfax, Virginia 22030 Received July 10, 1995. In Final Form: November 7, 1995
Introduction Microemulsions are thermodynamically stable dispersions of two immiscible fluids such as oil and water which are interfacially stabilized by surfactant molecules.1-3 Microemulsions made from charged surfactants may also contain electrolytes to further stabilize their microenvironment. Since microemulsions provide a unique environment capable of partitioning both hydrophobic and hydrophilic molecules, it would be most interesting to prepare microemulsions containing C60 in the hydrophobic oil core and an aqueous bulk phase. As a van der Waals solid, similar to the solid form of inert gas, C60 used as a probe could provide valuable information on the nature of the oil core via an examination of its ultraviolet and visible absorption spectrum. UV/visible absorption spectroscopy was an important tool in the early identification of buckminsterfullerene because of theoretical band predictions and the ease of collection of spectra. Spectra were taken in the solid state,4,5 aliphatic solvents,6-8 and benzene.7,8 Later work included an extensive examination in hexane and 3-methylpentane,9 a Beer’s Law study in chlorobenzene,10 transmittance studies at 532 nm in toluene and methylene chloride,11 a solid state visible/near IR spectrum taken on a BaF2 substrate,12 and a UV/visible spectrum on a Langmuir-Blodgett film containing C60.13 Only the Langmuir-Blodgett work13 represents an attempt to examine the behavior of this remarkable molecule in the presence of a surface active agent. We now present evidence that C60-containing microemulsions made with sodium dodecyl sulfate (SDS) are easily prepared and stable, and that they exhibit the UV/visible absorption characteristics of homogeneous solutions in aromatic solvents. Experimental Section Materials. Chromatographically pure buckminsterfullerene was obtained from SES Research, Houston, TX, and from MER Corp., Tucson, AZ. Sodium dodecyl sulfate (SDS, Aldrich, >98% (1) Dayalan, E.; Qutubuddin, S.; Texter, J. In Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH Publishers, Inc.: New York, 1992; p 119. (2) Hoar, T. P.; Schulman, J. H. Nature 1943, 152, 102. (3) Miller, C. A.; Qutubuddin, S. In Interfacial Phenomena in Apolar Media; Eicke, H. F., Parfitt, G. D., Eds.; Marcell Dekker: New York, 1987; p 117. (4) Kra¨tschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167. (5) Kra¨tschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (6) Ajie, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kra¨tschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L. J. Phys. Chem. 1990, 94, 8630. (7) Hare, J. P.; Kroto, H. W.; Taylor, R. Chem. Phys. Lett. 1991, 177, 394. (8) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J. Chem. Soc., Chem. Commun. 1990, 1423. (9) Leach, S.; Vervloet, M.; Despres, A.; Breheret, E.; Hare, J. P.; Dennis, T. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Chem. Phys. 1992, 160, 451. (10) Honeychuck, R. V.; Cruger, T. W. Anal. Lett. 1992, 25, 1755. (11) Tutt, L. W.; Kost, A. Nature 1992, 356, 225. (12) Kafafi, Z. H.; Lindle, J. R.; Pong, R. G. S.; Bartoli, F. J.; Lingg, L. J.; Milliken, J. Chem. Phys. Lett. 1992, 188, 492. (13) Milliken, J.; Dominguez, D. D.; Nelson, H. H.; Barger, W. R. Chem. Mater. 1992, 4, 252.
purity), 1-pentanol (Fisher, reagent grade), toluene (Aldrich, Gold label), and sodium bromide (Baker, analyzed reagent) were used as received. Methods. A stock solution of C60 (2.6 mmol L-1) in toluene was prepared by mixing 9.6 mg of C60 in 4.5019 g of toluene. The micellar solution was prepared by dissolving 5.5 g of SDS and 1.0 g of NaBr in 30-40 mL of water, and then 10.3 g of 1-pentanol was slowly added to the solution with constant stirring. Finally, the solution was diluted to 100.0 mL in a volumetric flask. At this point the solution appeared clear but viscous. C60-containing microemulsions were prepared by slowly dissolving the C60toluene stock solution into the SDS-pentanol micellar solution. The solution remained turbid until about 16% w/w toluene was achieved and then cleared with the characteristic deep magenta color of C60 up to 20% w/w toluene. At this point the solution became much less viscous. The fullerene microemulsions thus prepared remained stable for at least 3 weeks unless toluene evaporated. To obtain different concentrations of C60 microemulsions, solutions were serially diluted with microemulsions containing SDS, NaBr, H2O, 1-pentanol, and toluene in the above (identical) amounts. Similarly, oil-in-water ferrocene-containing microemulsions were also prepared. It should be noted that attempts to prepare oil free C60-containing microemulsions failed: clearly visible solid C60 persisted even after ultrasonication for 0.5 h. This was also true for oil and cosurfactant free micelles of SDS. UV/visible spectra were collected on a Hewlett-Packard 8452A diode array UV/visible spectrophotometer, using a collection time of 20 s. A solid quartz block with a width of slightly less than 1 cm was inserted into a 1 cm quartz cuvette, and the microemulsions were placed between the block and cuvette. The previously determined path length in this system is 0.0237 cm.14 Chronocoulometry and cyclic voltammetry were performed with either a custom-made electrochemical system or a BAS-100B Electrochemical analyzer. General methodology of diffusion coefficient measurement by electrochemical techniques is described elsewhere.15,16
Results and Discussion The C60-containing microemulsions were clear, magenta solutions which remained stable for weeks and gave reproducible UV/vis spectra. The fact that C60 has no detectable solubility in the water continuous bulk phase is indicative of the presence of C60 in a hydrophobic pseudophase containing toluene. The considerable stability of these microemulsions also signifies an exclusively hydrophobic core (or pool) with no exit or reentry of C60. The spectrum of the most concentrated sample is shown in Figure 1a. It exhibits peaks at 228, 274, and 334 nm in the UV region. It also shows a very small peak at 406 nm due to transitions of low probability in the visible region. Although we have obtained excellent background subtraction as observed by the residual absorbance ((0.05 au) values below 300 nm in a spectrum of toluene vs toluene, peaks below 300 nm in Figure 1a may still have some contribution due to π f π* transitions in toluene. However, λmax below 300 nm is in the range of λmax obtained in other solvents and surfaces.17 A 334 nm peak is typical of buckminsterfullerene in aromatic solvents. This peak is at 336 nm14 in toluene and 334 nm10 in chlorobenzene. In hexane the same peak appears at 328 nm,9 probably because of a reduction in transition energy due to dipoledipole interactions between the polar excited state of C60 and the aromatic solvents.18 The solvent cage around each fullerene molecule, from these considerations, appears to (14) Cruger, T. W. M.S. Thesis, George Mason University, 1993. (15) Choksi, K.; Qutubuddin, S.; Hussam, A. J. Colloid Interface Sci. 1989, 129, 315. (16) Davies, K. M.; Hussam, A. Langmuir 1993, 9, 3270. (17) Tomioka, Y. T.; Ishibashi, M.; Kajiyama, H.; Taniguchi, Y. Langmuir 1993, 9, 32. (18) Olsen, E. D. Modern Optical Methods of Analysis; McGraw-Hill: New York, 1975.
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Figure 1. UV/vis absorption spectra of C60 microemulsions: (a) spectrum over extended wavelength range with [C60] ) 0.43 mmol L-1, (b) spectra show concentration dependence of C60 microemulsion on the absorbance of peaks at 334 and 406 nm C60 concentration (top to bottom): 0.43, 0.21, 0.11, and 0.05 mmol L-1).
be largely aromatic in nature. The 406 nm peak in the microemulsion is at 408 nm in toluene14 and 406 nm in chlorobenzene.14 Because of the higher resolution available in less polar solvents, there are two peaks in this region, at 404 and 408 nm, in hexane.9 The concentration dependent spectra are presented in Figure 1b. The Beer’s Law plot at 334 nm shows excellent linearity (r2 ) 0.9997, n ) 7, intercept ) 0.003 ( 0.004) and yields a molar absorptivity of 6.0 × 104 L mol-1 cm-1.19 This value compares with 6.2 × 104 in toluene (336 nm)14 and 6.0 × 104 in chlorobenzene.14 Since C60 is not dissolved in micellar solution (toluene free) to any measurable extent, one can conclude that the micellar hydrophobic core environment formed by the dodecyl hydrocarbon chain of SDS is not like liquid dodecane or similar hydrocarbons. In dodecane (and in hexane) C60 solubility is appreciable (0.13 mmol L-1 in dodecane20 ); this imparts a characteristic light magenta color, which is absent in toluene (19) Similar Beer’s Law analysis gave 228 ) 9.0 × 104, 274 ) 7.0 × 104, and 406 ) 4.1 × 103 L mol-1 cm-1, all with r2 > 0.993.
Langmuir, Vol. 12, No. 5, 1996 1403
free micellar solution. It appears from these observations that C60 molecules are confined to a core environment composed primarily of toluene. The λmax and values indicate the microenvironment of C60 has a polarity similar to that of chlorobenzene. This weakly polar nature of the microenvironment can be caused by the presence of some 1-pentanol inside the core as cosurfactant. The microemulsions prepared in our studies contained a narrow range of toluene concentration (16-20% w/w). To understand the nature of microemulsions we measured the diffusion coefficient of microemulsion droplets by using ferrocene-containing microemulsions of exactly the same composition (without C60). The droplet diffusion coefficient decreased almost linearly with increased toluene concentration. The droplet diffusion coefficients at 17, 18, 19, and 20% w/w toluene were 15.2 × 10-7 cm2/s, 11.0 × 10-7 cm2/s, 6.1 × 10-7 cm2/s, and 3.43 × 10-7 cm2/s, respectively. Very similar diffusion coefficients were also reported for a lower phase microemulsion containing SDS, toluene, butanol, and NaCl.1 By using this data and assuming a water continuous bulk phase, the droplet diameter calculated from the Stokes-Einstein equation was 14.0 nm for the 20% w/w toluene.21 The average diameter of micelles of similar [SDS] is about 5.0 nm.22 The diffusion coefficient data shows about 20-fold expansion of volume of microemulsions by the incorporation of C60-toluene solution. Assuming that negligible free SDS exists in the bulk, and that all C60 is dissolved in the hydrophobic core containing toluene, we calculate that no more than one C60 per droplet is present.23 This potentially offers the opportunity for study of single buckminsterfullerene molecules in solution free of interaction with other C60 molecules, and in addition separated from other C60 molecules by a continuous aqueous bulk. Conclusions We have shown that C60-containing microemulsions can be readily prepared. This opens up the possibility of studying C60 molecules in a medium which has significant practical applications. One of the most intriguing aspects of this work is the revelation of a local C60 environment, inside the micelles, which is largely aromatic and weakly polar. This shows that van der Waals interactions between a fullerene molecule and the aromatic π cloud of the solvent molecules are important in bringing the solute into solution in a largely aliphatic micellar interior. LA950569R (20) Sivaraman, N.; Dhamodaran, R.; Kaliappan, I.; Srinivasan, T. G.; Vasudeva Rao, P. R.; Mathews, C. K. J. Org. Chem. 1992, 57, 6077. (21) Stokes-Einstein equation: D ) kT/(6πηr), where k is the Boltzmann constant, T is absolute temperature, η is the viscosity of water (0.8937 cP at 25 °C), and r is the radius of the droplet. (22) Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385. (23) The estimated value is the upper limit at the highest concentration of C60 used. Also, assuming an SDS aggregation number of 100 and [SDS] . cmc, an estimate of two C60 molecules per 10 droplets is obtained as the lower limit.
