On the microscopic scale, the surface area of particles and materials has an important effect on their behavior. Since the surface of a material is the primary point of contact between it and the outside environment, this is where many chemical and physical effects take place. Combustion, dissolution, filtration, catalysis and other processes are sensitive to the relative surface areas of the materials being used. Consequently, instruments have been developed to measure the surface area of materials.

The primary method for measuring surface area relies on the adsorption of gases. One specific method that is quite common is the BET method, developed by Drs. Brunauer, Emmett and Teller in 1938. BET theory provides a relationship between the pressure of a gas, and the volume of the adsorbed monolayer across the surface of the material. In practice, a sample is thoroughly cleaned and degassed by heating it under vacuum conditions to remove any oils or adsorbed species. While still under vacuum, the sample is cooled. Gas is then introduced in a regulated fashion into the sample chamber. Nitrogen is the most commonly used gas, although many others may be used, including reactive or corrosive gases. Another specialty gas is krypton, which is used with more demanding samples that have small surface areas. Some of the gas will adsorb onto the surface, while the remainder stays in gas phase. Once equilibrium is reached, the pressure in the chamber is measured and further introductions of gas allow a curve to be plotted. The slope and intercept of the BET plot can be mathematically manipulated to provide the BET surface area of the sample.

Further introduction of gas into the sample chamber, beyond that needed to produce a monolayer of adsorbed gas on the surface, results in the filling of the pores and cavities in the sample. Thus, plotting out the full sorption and desorption curve can provide information on the porosity and pore size distribution of the sample. Most instruments provide both surface area and porosity measurements.

Surface area and porosity measurements are used with many different sample types. Activated carbon is prized for its high porosity, which makes it valuable for purification and other purposes. Catalysts are active at their surfaces, so surface area provides information on quality. Surface area also has relevance to the dissolution of pharmaceutical products. Other applications exist for polymers, chemicals, cements and other materials.

The total market for surface area analyzers was about $38 million in 2007, and is growing about 5% annually. The major vendors are Micromeritics, Beckman Coulter and Quantachrome, which all participate more broadly in the particle characterization market, although Quantachrome is quite focused on this segment. Other participants include Thermo Fisher Scientific, Porous Materials and TSI. This year, Horiba acquired Beta Scientific (see IBO 8/31/08), including its surface area analyzer technology.

Surface Area at a Glance:

Leading Suppliers

• Micromeritics

• Beckman Coulter

• Quantachrome

Largest Markets

• Chemicals

• Pharmaceuticals

• Cement

Instrument Cost

• $18,000–$40,000