Adsorption of the first layer of argon on graphite

2014 I have determined a set of adsorption isotherms for argon on graphite from 55 to 62 K. An interpretation of the results is proposed. LE JOURNAL DE PHYSIQUE LETTRES TOME 40, 1,,r JANVIER 1979, Classification Physics Abstracts 82.65 Argon on graphite isotherms have been determined by volumetry with a modified MacLeod gauge previously described by Larher [1]. The cooling bath is solid nitrogen obtained by pumping. The adsorption cell, enclosed in an aluminium block, is regulated to within 0.01 K. Using a nitrogen vapour pressure bulb and an oil manometer the temperature is known with a precision of 0.01 K. Pressures are corrected for thermal transpiration [2] and mercury drag effect [3]. The graphite sample is an uncompressed grafoil from Union Carbide which was previously heated in vacuum at 400 ~C. Its specific surface area is accurately determined from a nitrogen isotherm at 62 K (see Fig. 1. Adsorption isotherms for argon : 1) 54.61 K ; 2) 55.75 K; 3) 59.53 K; 4) 62.41 K, nitrogen; 5) 62.11 K (uncorrected pressures). Fig. 1) as 26.5 ml/g (the end of the rise of isotherm 5 corresponds to the density of a registered ~/3 x ~/3 solid [4, 5]). From the isotherms of figure 1 the critical temperature of the gas-dense phase transition was found to be 7~ = 58 ± 1 K. An accompanying isostere was made at 1.6 ~mole (see Fig. 2). This data follows the linear fit Fig. 2. Isosteres at 1.6 ~mole and 4.3 ~mole. The highest pressure point is from Larher’s work [4]. with standard deviations (607.5) = 2.4 and (7.14) = 0.04 . The relations between the vapour pressure of the transition p(l) and that of solid argon p ~ °°~ at the same temperature are [6] : Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:019790040010900 L-10 JOURNAL DE PHYSIQUE LETTRES