Adsorptive energies of calorigenic plesimorphs of graphite

Abstract

The point-discontinuity in the krypton isotherm on highly-graphitized carbon black is treated as a transition from: a mobile gas film to a lattice of localized adsorbate molecules. Mobile adsorption is described by the virial adsorption isotherm equation; the Fowler-Guggenheim refined isotherm equation is used to analyze localized adsorption. The criteria for success of the model includeequations of state which exhibit intersections in the same regions of coverage as do the adsorption isotherm equations.

The nitrogen adsorption isotherm at 77.0°K on Sterling MT FF, an energetically uniform carbon-black, can be analyzed as a mobile gas film at fractions of coverage up to 0 = 0.42, or as localized lattice up to full monolayer coverage. The mobile film is described by a 2-D virial isotherm adsorption equation which uses Lennard-Janes parameters to determine the adsorbate-adsorbate interaction and the effective molecular area. The Fowler-Guggenheim refined adsorption isotherm equation describes the localized lattice in which the effective area of each adsorbed molecule is determined by the geometry of the hexagonal graphite network.

Analyses of nitrogen adsorption isotherms at 77.0°K.are employed to monitor the changes induced by thermal treatment in the specific surface and in the development of the graphite basal plane of Spheron 6, an energetically heterogeneous carbon-black. The surface area of Spheron 6 is shown to increase after heat treatment at 2700°C by more than 20%. This result contradicts the conclusions of previous investigators, whose "point B" analysis of multilayer adsorption on a similar graphitized series appeared to show a decline in specific surface with thermal treatment.

Exfoliated graphites bearing varying amounts of substrate contamination by metallic iron display a line-discontinuity in the krypton-adsorption isotherm at a point where iron-free graphitized carbon-black displays a point discontinuity. Even an exfoliated graphite low enough in iron content that other features of the adsorption isotherm are identical with those observed for gr-aphitized carbon-black, still retains the form of the discontinuity found with the more grossly contaminated specimens of exfoliated. The highly -graphitized carbon-blacks, therefore, retain their pre-eminence as the purest and most uniform of available substrates.