Surface, Specific

an averaged characteristic of the dimensions of the internal cavities, canals, or pores of a porous body or of particles of a finely divided phase of a disperse system. The specific surface expresses the ratio of the total surface of a porous or dispersed substance in a given medium to its volume or mass. The specific surface is proportional to the dispersity; that is, it is inversely proportional to the particle size of the dispersed phase. The adsorptive capacity of adsorbents, the effectiveness of solid catalysts, and the properties of filter materials all depend on the magnitude of the specific surface.

Values of the specific surface are 500–1,500 m²/g for activated carbons, up to 800 m²/g for silica gels, not more than 70 m²/g macroporous ion-exchange resins, and less than 10 m²/g for diatomaceous carriers for gas-liquid chromatography. The specific surface characterizes the dispersity of such powdery materials as fillers, pigments, powdered fuels, and mineral binders. The values of the specific surface of these materials usually range from several tenths of 1 m²/g to several dozens m²/g.

The specific surface is usually determined by the amount of inert gas absorbed by the material or according to the air permeability of a layer of powder or porous material. The adsorption methods provide the most reliable values.