Elsevier

Thermochimica Acta

Volume 316, Issue 1, 26 May 1998, Pages 101-108
Thermochimica Acta

Vapour pressure of C60 by a transpiration method using a horizontal thermobalance

https://doi.org/10.1016/S0040-6031(98)00304-9Get rights and content

Abstract

A horizontal thermal analysis system was adopted for the measurement of vapour pressure of C60 using the vapour transport technique. The experimental precautions taken in order to ensure measurement of equilibrium vapour pressure by the transpiration method are described. The equilibrium nature of these measurements was ensured by the existence of plateau regions in the isothermal plots of apparent vapour pressure as a function of flow rate of the carrier gas. To verify the applicability of this TG based transpiration method, vapour pressure of CsI was measured to be log(p/Pa)=11.667±0.013−(9390±0.078)/T (K) over the range 737–874 K yielding a value of 195.6 kJ mol−1 for the third-law enthalpy of sublimation, ΔH0sub,298 of CsI, the value which compares well with the literature data. The vapour pressure measurements on C60 over the range 789–907 K could be represented by log(p/Pa)=9.018±0.061−(7955±0.280)/T(K). Third-law treatment of the data yielded a value of 183.5±1.0 kJ mol−1 for ΔH0sub,298 of C60 which is in good agreement with some of the other vapour pressure measurements in the literature, if subjected to third-law processing using the same set of free energy functions reliably reported in the literature.

Introduction

Following the discovery of fullerenes in 1985 by Kroto et al. [1], considerable work on their synthesis and characterization were reported in the literature 2, 3, 4. Notable among the physico-chemical measurements made on the fullerenes were the vapour pressure measurements which clearly demonstrated the metastability of the fullerenes in comparison with other polymorphic forms of carbon. However, the number of reports in the literature 5, 6, 7, 8, 9, 10, 11, 12on the vapour pressure of C60, C70 and their solid solutions are quite a few which included techniques such as the conventional transpiration Quartz Crystal Microbalance (QCM), Knudsen Cell Mass Spectrometry (KCMS), Knudsen Effusion Weight Loss (KEWL) and Optical Absorption Spectra (OAS) measurements. Despite a reasonable number of such vapour pressure measurements employing diverse techniques, there is a considerable scatter in the vapour pressure data and in the values of the standard enthalpy of sublimation, ΔH0sub,298 for even the well-studied fullerene C60. Hence, the present investigation was undertaken in order to assess the reliability of the vapour pressure data by employing a modified transpiration method. Since this method facilitates adaptation of a commercial thermoanalyser functioning in the horizontal configuration as a transpiration apparatus, the reliability of such a technique needs to be established by measurements on a well characterised material whose vapour pressure is well assessed in the literature. This was accomplished by carrying out vapour pressure measurements on cesium iodide and comparing the results with the literature values prior to the initiation of vapourisation studies on the fullerene C60.

Section snippets

Materials

Reagent grade CsI of purity better than 99.9% (supplied by Aldrich Chemicals, USA) pulverised to a mesh size of between 200 and 350 was used for the transpiration runs.

The Kratschmer's carbon arc method 2, 3was used to produce the fullerenes. The soot so obtained by arcing the graphite electrodes was extracted by using Soxhlet extractor followed by chromatographic separation on an alumina column. Toluene/Hexane mixtures which was used as the eluent was removed by vacuum annealing of the product

Results

If W is the mass loss of the sample at the isothermal temperature T, which was caused by the flow of Vc (dm3) of the carrier gas, the apparent vapour pressure p could be calculated using the Dalton's law of partial pressure for ideal gas mixtures as given bypapp=WRT/MVcwhere M is the molecular weight of the sample.

Vapour pressure of CsI(s)

The results of the vapour pressure measurements on CsI using an inert gas (namely He) as the carrier, shown in Fig. 1, was represented by Eq. (2). These measurements were carried out essentially to verify whether the modified transpiration method (adopting a horizontal thermobalance) could yield reliable vapour pressure data for a well characterised material whose vapour pressure values were well established in the literature. Measurement of vapour pressure on standard metal powders might be

Acknowledgements

The authors are indebted to Dr. V.S. Raghunathan, Head, Metallurgy Division, and to Dr. T.S. Radhakrishnan, Head, Materials Science Division, for their useful discussions and keen interest throughout the course of this work. The authors express their deep gratitude to Dr. Baldev Raj, Director, Metallurgy and Materials Group of this Centre for his constant encouragement and support for carrying out these investigations.

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