A STUDY OF MULTILAYER ADSORPTION AND PORE CONDENSATION OF PURE FLUIDS IN GRAPHITE SUBSTRATES ON APPROACHING THE BULK CRITICAL-POINT

The multilayer adsorption of sulfur hexafluoride (SF6) on three nonporous graphite materials (Grafoam and graphitized thermal carbon blacks) and in a mesoporous graphitic carbon (PGC) has been studied in a range of reduced temperatures T/T(c) from 0.85 to 0.985 (T(c) = 318.7 K) and pressures up to the critical pressure (p(c) = 37.6 bar). The thickness of the multilayer adsorbed film is calculated from the experimental surface excess amount on the basis of model density profiles p(z) which allow for the compressibility of the film. The resulting film thickness isotherms can be represented by the Frenkel-Halsey-Hill equation up to nearly the critical temperature of the fluid. The isotherms for the mesoporous substrate PGC coincide with those for the planar graphite substrates up to a point at which pore condensation commences. On approaching T(c), this point is shifted to higher relative fugacities f/f(o) and greater film thickness, as predicted by modern theories of pore condensation. The pore condensation hysteresis loop decreases in width and height on approaching T(c). These findings are analyzed on the basis of critical point scaling laws of fluids, and the influence of the pore size distribution of PGC on the hysteresis results is discussed.