The surface properties of carbon-II the effect of capillary condensation at low relative pressures upon the determination of surface area

Abstract

With carbons which have a large internal volume, the problem of determining the surface area is extremely difficult because the range of usefulness of the BET and other adsorption equations which predict a monolayer capacity, extends into the pressure range where capillary condensation occurs. Although the adsorption isotherm may be represented closely by the BET equation, the impossibly high values for surface area which are predicted by the equation show that the assumptions of the equation are not being met. Considerations are outlined which led to the development of the use of the Polanyi Potential Energy equation as modified by Dubinin. This equation is thought to permit the determination, under certain conditions, of the monolayer capacity of a carbon as distinct from the volume capacity. The comparative use of the BET equation and the Potential Energy equation is reported for materials of known surface properties, and for activated carbons of unknown surface properties.