J. Phys. Chem. 1996, 100, 9815-9819
9815
Vapor Pressure of C70 Fullerene V. Piacente,* G. Gigli, P. Scardala, and A. Giustini Dipartimento di Chimica, UniVersita` di Roma “La Sapienza”, Piazzale A. Moro 5, 00185 Roma, Italy
G. Bardi CNR, Centro per la Termodinamica Chimica alle Alte Temperature, c/o Dipartimento di Chimica, UniVersita` di Roma “La Sapienza”, Piazzale A. Moro 5, 00185 Roma, Italy ReceiVed: October 27, 1995; In Final Form: February 26, 1996X
The equilibrium pressures over C70 were measured in the temperature range 783-904 K by the torsion effusion and Knudsen effusion methods. The obtained data are well represented by the following selected equation: log(p/Pa) ) (11.38 ( 0.15) - (9917 ( 160)/(T/K). Assuming the vapor phase constituted by only C70(g), the sublimation standard enthalpy of this compound, ∆subH°(298) ) 200 ( 6 kJ mol-1, as derived by second law elaboration of the experimental data, was derived. From the vapor pressure data, an evaluation of the free energy functions for solid C70 has been made. The heat of formation of gaseous C70, ∆forH°(298) ) 2755 ( 18 kJ mol-1, was also derived.
Introduction Apart from the value estimated by Guo1 from cohesive energy calculations [∆subH°(739) ) 194.1 kJ mol-1], apparently the sublimation enthalpies of C70 reported in the literature were evaluated from the temperature dependence of its vapor pressure all directly or indirectly measured by the Knudsen effusion method. Pan et al.,2 employing this method coupled to a mass spectrometer, determined the sublimation enthalpy of C70 from a polycrystalline mixture of C60 and C70 by plotting ln(I+70T) vs 1/T. The authors found in six vaporization runs sublimation enthalpy values ranging from 174 to 196 kJ mol-1 and proposed, at the middle temperature of 739 K, the average value ∆subH°(739) ) 180 ( 9 kJ mol-1. By the Knudsen effusion method Abrefah et al.3 employing a thermogravimetric assembly measured eight vapor pressure values from which they derived a ∆subH°(788) equal to 188 ( 4 kJ mol-1. Subsequently Sai Baba et al.4 measured mass spectrometrically the partial pressure of C70(g) in equilibrium over mixtures at several compositions along the binary C60-C70 system. In the same paper they also reported the temperature dependence of the vapor pressure of pure C70 and its sublimation enthalpy [∆subH°(750) ) 195.7 ( 1.5 kJ mol-1] both derived from data reported by the same authors in a previous work.5 More recently Popovic et al.,6 again using the Knudsen method coupled with a mass spectrometer, measured a new vapor pressures set in the temperature range 652-779 K from which they derived the sublimation enthalpy, ∆subH°(753) ) 174 ( 3 kJ mol-1. In their work the authors report that fullerenes containing small amounts of residual organic solvent partially decompose to amorphous carbon under thermal treatment. Considering that the sublimation enthalpy values for C70 reported in the literature are rather scattered and that most of the vapor pressures were determined by the mass-spectrometric technique so that their absolute values are subject to a number of uncertainties inherent to this technique (ionization cross section of C70, fragmentation phenomena, electron multiplier gain, etc.), after the study of the sublimation of C607 a new set of vapor pressure values of C70 were measured by the torsion X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03166-2 CCC: $12.00
and Knudsen effusion methods from which a new standard sublimation enthalpy value was derived. Experimental Details and Results In this work the absolute vapor pressures of C70 were measured by using the torsion apparatus described in detail elsewhere.8 Some pressure measurements were also carried out by the Knudsen effusion method determining the rate of mass loss of C70 samples by an Ugine-Eyraud B60 Setaram vacuum thermobalance.9 Two conventional graphite torsion cells (A and B) having effusion orifices different in the nominal diameter (A ) 0.8 mm and B ) 2 mm) and one graphite Knudsen cell (K) with a nominal diameter of 1 mm were used in the investigation. The instrument calibration constants were determined by vaporizing pure cadmium and lead having wellknown vapor pressures.10 The C70 used in this study was supplied by “Term USA”. Its purity, as checked by chromatographic analysis, was about 98.5%, the principal impurities being constituted by C60, by solvent residue, and probably, by undetermined minor impurities. When heating a C70 sample, at about 450 K a weight loss of about 1.0% of its original mass was observed (probably due to the vaporization of the impurities) while no other appreciable vaporization occurred up to about 800 K. At higher temperature, after a further small weight loss (about 0.3% by weight, probably due to the vaporization of the C60 impurity) the sample showed vapor pressures sufficiently reproducible in the first heating and cooling cycles, while going on the vaporization up to ∼1300 K their values, slowly but continuously, decreased to below the limit of detectability. The same behavior was also observed in the Knudsen effusion measurements. The residue, about 6-8% of the original weight of the sample, is constituted almost entirely by amorphous carbon as observed by SEM analysis. The formation of this form of carbon is probably due to a partial decomposition of C70 initiated by the presence of small amounts of solvent, as also observed by Popovic et al.6 and by us7 while studying C60. Using original C70 samples and taking into account only a limited number of vapor pressures measured in the very first step of four vaporization runs (three carried out by the torsion assembly and one by the thermobalance), the pressure-temperature equation reported in Table 1 was obtained © 1996 American Chemical Society
9816 J. Phys. Chem., Vol. 100, No. 23, 1996
Piacente et al.
TABLE 1: Vapor Pressures of Solid C70 log (p/Pa) ) A - B/(T/K) sample
method
cell
original
torsion (three runs) Knudsen (one run) torsion torsion torsion torsion torsion torsion Knudsen
A K A A A B differentialc differentiald K
distilled distilled distilled distilled distilled distilled distilled
Aa
∆T/K
no. of points
810-891
21
11.09b
813-898 805-895 810-885 783-861 840-904 831-896 802-893
10 12 9 11 6 10 7
11.51 ( 0.06 11.38 ( 0.12 11.39 ( 0.08 11.41 ( 0.23 11.34 ( 0.18 11.31 ( 0.12 11.25 ( 0.20e
Ba 9708b 10161 ( 38 10052 ( 112 9811 ( 77 9903 ( 201 9676 ( 106 9795 ( 87 9880 ( 200e
a The associated errors are the standard deviations. b Calculated taking into account all the torsion and Knudsen data (see text). c With C opposed 60 to C70. d With thallium opposed to C70. e Error estimated.
by a least-squares fitting of all data points. When the particular procedure adopted to obtain this equation is considered, the errors to associate to its constants were not evaluated. To rule out any solvent contamination, the original C70 was purified by distillation under high vacuum. The amount of amorphous carbon residue was found to be comparable to that observed in the previous vaporization runs. A further redistillation followed by suitable condensation of the fullerene yielded no appreciable residue. The vapor pressures of the redistilled C70, measured in various heating and cooling cycles, were found to be more reproducible than those measured above the original sample, even if at each thermal cycle their values showed again a very small decrease and small amounts of amorphous carbon were observed at the end of the vaporizations. The presence of this soot is surprising and can be explained by advancing the hypothesis that the decomposition of C70 is due not only to the presence of solvent, as supposed by Popovic et al.,6 but also occurs spontaneously when the fullerene, purified by distillation, is heated. Indeed in the experiments in which the time for the vaporization of the sample was relatively long, the soot residues were evident, while when utilizing a comparable amount of sample, the measurements were carried out at higher temperatures (so that the vaporizations duration were shorter) the residues were found to be negligible. In particular, when the distilled sample was quickly resublimated, as reported above, almost no residue was found. These observations lead one to conclude that the amount of the partial thermal decomposition of C70 into amorphous carbon might then be connected to the permanence of the sample at high temperature. When only the absolute vapor pressures measured above the redistilled C70 in the first step of each vaporization run are taken into account and the points not well reproducible are rejected, for each run a log p vs 1/T equation was evaluated by a least-squares fitting of the data. These equations are reported in Table 1. A set of vapor pressures of C70 was measured by using the torsion method in a different way. Considering reliable the vapor pressures of C60 that we measured in a previous work7, using a differential cell, i.e., a torsion cell having the effusion orifices of both housings on the same side, the difference between the vapor pressure of C60 and C70 was measured. By loading one housing of the cell with C60 and the other with C70, at a fixed temperature this difference, ∆p, was determined from the measurement of the torsion angle, ∆R, by the expression
∆p ) K∆R + p(C60)/K°
(1)
where p(C60) is the vapor pressure of C60 at this temperature, K is the instrument constant of the housing in which the C70 is loaded, and K° is the average of some p°/∆R° ratios between the vapor pressure, p°, of a pure reference substance10 (lead in
our case) and the corresponding torsion angle, ∆R°, measured in preliminary experiments in which both housings were filled with the same substance. This ratio actually represents a correction factor that takes into account possible geometrical differences in the two effusion orifices. As experimentally verified, the cell used here presented K° values decidedly negligible even at high pressures (∼10-1 kPa). The two instrument constants of the housings (K in eq 1 and the other constant necessary to measure p(C60)) were determined in separate runs by loading alternatively only one housing with a reference material.10 In a separate run the vapor pressure of C60 was again determined employing the same sample used in the previous investigation.7 The values obtained confirmed that the vaporization of this fullerene occurs in two steps, the first of which we deem to be representative of its real sublimation process. The vapor pressures measured in the first step fit well the equation
log(p/Pa) ) (11.26 ( 0.12) - (9.112 ( 98)/(T/K) (2) (the errors are standard deviations). This equation agrees fairly well with that selected in the previous work7 [log(p/Pa) ) (11.28 ( 0.20) - (9154 ( 150)/(T/K)]. Of course in the differential measurements only the points determined in the first step of the vaporization of C60 were taken into account. To obtain a further set of vapor pressure data for C70, the differential torsion cell was also used to measure differences between the vapor pressures of C70 and of a standard substance, pure thallium, which exhibits a comparable vapor pressure.10 Adding at each experimental temperature the ∆p values obtained by eq 1 and the vapor pressure of C607 or thallium,10 two vapor pressures sets for solid C70 were obtained. A least-squares fitting yielded the two pressure-temperature equations for C70 which are reported in Table 1 for comparison with the other experimental equations. A set of seven vapor pressure values was also obtained from thermogravimetric measurements. The corresponding log p vs 1/T equation is also given in Table 1. All of the equations in Table 1 are in mutual agreement so that by weighting their slopes and intercepts proportionally to the number of experimental points, the following equation was finally selected:
log(p/Pa) ) (11.38 ( 0.15) - (9917 ( 160)/(T/K) (3) where the associated uncertainties were estimated considering an overall error of about 2 K in the temperature values, of about 5% in the instrument constants, and of 10% in torsion angles
