Specific surface area


Specific surface area is a property of solids defined as the total surface area of a material per unit of mass, or solid or bulk volume.
It is a physical value that can be used to determine the type and properties of a material. It has a particular importance for adsorption, heterogeneous catalysis, and reactions on surfaces.

Measurement

Values obtained for specific surface area depend on the method of measurement. In adsorption based methods, the size of the adsorbate molecule, the exposed crystallographic planes at the surface and measurement temperature all affect the obtained specific surface area. For this reason, in addition to the most commonly used Brunauer-Emmett-Teller adsorption method, several techniques have been developed to measure the specific surface area of particulate materials at ambient temperatures and at controllable scales, including methylene blue staining, ethylene glycol monoethyl ether adsorption, electrokinetic analysis of complex-ion adsorption and a Protein Retention method. A number of international standards exist for the measurement of specific surface area, including ISO standard 9277.

Calculation

The SSA can be simply calculated from a particle size distribution, making some assumption about the particle shape. This method, however, fails to account for surface associated with the surface texture of the particles.

Adsorption

The SSA can be measured by adsorption using the BET isotherm. This has the advantage of measuring the surface of fine structures and deep texture on the particles. However, the results can differ markedly depending on the substance adsorbed. The BET has been proven incorrect and yields incorrect answers, However it may yield relative answers if the solid are very similar chemically. It is better to use the Quantum Mechanical model and the Thermodynamic Model which yield consistent results.

Gas permeability

This depends upon a relationship between the specific surface area and the resistance to gas-flow of a porous bed of powder. The method is simple and quick, and yields a result that often correlates well with the chemical reactivity of a powder. However, it fails to measure much of the deep surface texture.