VAPOR-PRESSURE OF C-60 BY A TRANSPIRATION METHOD USING A HORIZONTAL THERMOBALANCE

A horizontal thermal analysis system was adopted for the measurement o f vapour pressure of C-60 using the vapour transport technique. The ex perimental precautions taken in order to ensure measurement of equilib rium vapour pressure by the transpiration method are described. The eq uilibrium nature of these measurements was ensured by the existence of plateau regions in the isothermal plots of apparent vapour pressure a s a function of flow rate of the carrier gas. To verify the applicabil ity 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) o ver the range 737-874 K yielding a value of 195.6 kK mol(-1) for the t hird-law enthalpy of sublimation, Delta H-sub,298(0):298 of CsI, the v alue which compares well with the literature data. The vapour pressure measurements on C-60 over the range 789-907 K could be represented by log(p/Pa) = 9.018 +/- 0.0.061 - (7955 +/- 0.280)/T(K). Third-law trea tment of the data yielded a value of 183.5 +/- 1.0 kJ mol(-1) for Delt a H-sub,298(0),298 of C-60 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 reli ably reported in the literature. (C) 1998 Elsevier Science B.V.