J. Phys. Chem. 1996, 100, 12477-12482
12477
C60 and C70 Fullerene Ions in Nonpolar Liquids: Mobility and Radiation-Chemical Changes G. Bakale* Case Western ReserVe UniVersity, DiVision of Radiation Biology, 10900 Euclid AVenue, CleVeland, Ohio 44106-4942
K. Lacmann and W. F. Schmidt Hahn-Meitner Institut, Abteilung Strahlenchemie, Glienicker Strasse 100, 14109 Berlin, Germany ReceiVed: December 1, 1995; In Final Form: May 1, 1996X
The drift mobilities of fullerene C60 and C70 ions were measured at several temperatures in n-pentane, neopentane, cyclohexane, tridecane, and carbon disulfide to determine the degree of solute-solvent interaction in these model systems of spherical C60 ions and ellipsoidal C70 ions in solvents of different shapes. Onecomponent ion current decays were generally observed in the fullerene solutions using a time-of-flight pulseconductivity technique, which indicates that ions of only one size were present in the solutions. These ions are assumed to be fullerene anions and cations formed by charge transfer from the solvents due to the higher electron affinity and lower ionization potential of the fullerenes relative to the solvents. Marked decreases of the ion mobilities were observed following γ-irradiation of the fullerene solutions, which indicate that ions larger than C60 and C70 are produced by radiolysis. These lower mobility ions may be products of radical addition to the fullerenes and/or dimerization products formed from radical or ionic precursors. Analysis of the data with the Stokes-Einstein equation leads to the conclusion that the degree of solvation of the ions is cyclohexane > neopentane = n-pentane > tridecane > carbon disulfide. Dion ) kBTµion/eo
Introduction The facile synthesis of C60 and C70 in macroscopic quantities1 has triggered an avalanche of studies2 in which these unique molecules have been characterized sufficiently well1-3 to make them attractive candidates to serve as tracer molecules in a variety of physicochemical studies. Since C60 is characterized by a relatively low ionization potential of ∼7.6 eV and a high electron affinity of ∼2.5 eV, fullerene anions and cations can be prepared in various matrices from primary charge-carriers by electron transfer. Albrecht’s group used a time-of-flight (TOF) method to measure the mobility of fullerene anions in n-hexane4 in studies analogous to our preliminary TOF studies of fullerene ion mobility in liquid hydrocarbons before and after γ-irradiation.5 In the latter radiolysis studies, product ions were observed having mobilities almost 2-fold less than those of C60 ions, which is a manifestation of the rich chemistry of highly electrophilic fullerenes6 that are known as “radical sponges”.7 This reactivity and pulse8 and steady-state radiolysis studies of fullerene solutions9 indicate an extremely complex radiation chemistry of fullerene. The well-defined geometries of C60 and C70 ions and their large masses relative to other solute ions make the fullerenes ideal probes for studying the relationship between ion mobility and solvent viscosity η. The Stokes-Einstein (SE) equation relates the diffusion coefficient, D, of a particle of radius R to η by
D ) kBT/(χπηR)
(1)
where T is the temperature, kB is the Boltzmann constant, and χ denotes a factor that depends on the degree of solute-solvent interaction. A value of χ ) 6 denotes the “sticking” condition of maximum interaction, whereas χ ) 4 applies to minimal interaction in the “slipping” condition. For ions the diffusion coefficient, Dion, is proportional to the mobility, µion, X
Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(95)03569-6 CCC: $12.00
(2)
where eo is unit charge. Combining eqs 1 and 2 yields
µion ) eo/(χπηRion)
(3)
From eq 3 it is evident that the product of mobility and viscosity is a constant, i.e.,
µionη ) constant
(4)
which is also known as Walden’s rule. Numerous papers have been published on the validity of the SE equation for diffusing neutral species10 and of eq 3 for the mobility of ions,11 but deviations of measured data from eq 1 or 3 are frequent and the applicability of the macroscopic SE model to the molecular level has been questioned.12 Despite such criticism, the SE equation was recently referred to as “...a pillar of insight into the molecular dynamics of liquids”13 and is used to interpret the mobilities measured in this work. The results on µion of fullerenes in liquid hydrocarbons and carbon disulfide reported herein extend our studies5,14 of the applicability of the SE equation to quasi-spherical C60 ions and ellipsoidal C70 ions in solvents having different shapes. Our finding that preirradiation of fullerene/hydrocarbon solutions by 60Co γ-radiation changes the radius of the tracer fullerene ions prompted a study to identify the radiolytic product by smallangle X-ray scattering (SAXS) and mass spectral analysis that was recently reported.15 Experimental Section Cyclohexane, neopentane, and n-pentane (Phillips Research Grade, 99+ mol %) were purified using the same protocol as described earlier16 except the low vapor pressure of tridecane (also Phillips, 99.8 mol %) precluded vacuum distillation of this solvent. Therefore, tridecane was degassed by bubbling with high-purity helium (99.99%) and stored under helium in a glass cylinder that contained silica gel and a molecular sieve. © 1996 American Chemical Society
12478 J. Phys. Chem., Vol. 100, No. 30, 1996
Bakale et al.
Figure 1. Block diagram of the TOF pulse-conductivity setup used to measure the drift time of ions. Electric field across electrodes is provided by voltage supply U. Ions drifting in the field induce current across a resistance R that is amplified and monitored by a digitizer/scope.
The tridecane was transferred from this cylinder to an evacuated glass conductivity cell in which the ion mobility was measured. Carbon disulfide (Merck pro analysi, 99.7%) was degassed by refluxing at -78 °C under vacuum and distilled directly into the conductivity cell. In the initial phases of study,5 fullerene solutions were prepared from Strem Chemicals buckminsterfullerene, a mixture of 10-12% C70 in C60. However, for the µion measurements reported herein, C60 and C70 each of Fluka purum grade (>97%) were used. In addition, Hoechst C60 having a stated purity of >98% based on HPLC analysis was also used in µion measurements and was found to yield the same results as those obtained using Fluka C60. Stock solutions of C60 and C70 were prepared by dissolving the fullerenes in toluene (Phillips Research Grade, 99+ mol%) to yield ∼1-3 mM solutions from which appropriate aliquots were used to prepare hydrocarbon or CS2 solutions ranging from 2.5 µM for n-pentane to 12.5 µM for tridecane with care taken not to exceed the solubility limits reported by others.17 The solubility of C60 in tridecane was not measured in these studies, but Ruoff et al. reported that the solubility of C60 in n-decane is ∼100 µM and stated that the solubility of C60 in the alkanes increases with increasing number of carbon atoms in the solvent.17b Also, Sivaraman et al. reported that the solubility of C60 in dodecane increases from 91 to 126 µg/ mL in tetradecane,17a which corresponds to 130-175 µM. Thus, C60 should be soluble in tridecane at the 12.5 µM concentration studied in this work. The maximum concentration of the fullerenes in carbon disulfide was
