The Binary Phase Diagram Naphthalene-C 6 O
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ABSTRACT Molten naphthalene has considerable solubility for C60 (-5 10.3 in mole fraction) making it a potential solvent for fullerenes; the phase diagram is therefore important. It was also of interest to study whether C60 displays the same anomalous solubility behavior in naphthalene as in hexane, CS 2 , and toluene, where the solubility increases with temperature up to a maximum near room temperature and decreases subsequently. The naphthalene-C6 o eutectic temperature was determined by DSC; the solubility of C60 in the liquid solution up to -180 °C was derived by UV spectrophotometry and found to decrease monotonically with increasing temperature from a mole fraction of 5 • 10"3 at the eutectic (-79.5 °C) to 1.5 • 10"3 at 165 TC; thus, it shows a higher-temperature C60 solubility decrease (with increasing temperature) analogous to the other solutions, while a potential lowertemperature solubility increase (with increasing temperature) is masked by the eutectic. A binary phase diagram based on these data is proposed.
INTRODUCTION Since the discovery of a method to produce fullerenes in larger amounts, [1] thei. solubilities in organic solvents have been a common research subject. Also, there are many studies aimed at better methods for the extraction of fullerenes from soot and their improved separation from each other.
In these solvents, C60 presents an anomalous solubility behavior [2] that has not been fully explained, although recent efforts have been made to resolve this issue. [3, 4] Several solvent parameters have been examined and correlated to the solubility of C60 in many solvents [5] to determine which of these factors are essential for high solubility, but these correlations often show exceptions. [6] In general, solubilities (expressed as mole fractions) are low. Due to these low solubilities, the details of the deviation from ideality are not known; only general stability trends have been established. [5] In particular, it is known that the better solvents are aromatic compounds, with methyl-naphthalene and chloro-naphtalene having the highest solubilities. It appears that the field is still open to seeking new solvent alternatives that may be useful or at least can give more information about the behavior of the fullerenes in solution. One alternative that has not yet been explored is the use of aromatic fused-ring compounds in the series naphthalene, anthracene, phenantrene, tetracene, etc. These compounds are solids at room temperature and have to be melted to be able to dissolve fullerenes, but they allow solubility measurements to be made at higher temperatures than solvents liquid at room temperature. We present here the first of these studies, in which naphthalene (NP) was used as a solvent for C60 and in which a T-x phase diagram at P = 1 atm. is proposed for this binary mixture.
561 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
EXPERIMENTAL PROCEDURES AND DATA EVALUATION Solubility Study: The solubility of C60 in the liquid NP-C 60 solution over the temperature
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