Most atoms that make up a solid are bound on all sides by other atoms in the bulk of the solid. The atoms on the surface of the solid, however, are incompletely bound. Due to van der Waals forces of interaction, these surface atoms are more reactive and they attract gas, vapor and liquids to satisfy the imbalance of atomic forces.
Surface area helps determine such things as how solids burn, dissolve, and react with other materials. To determine the surface area, solid samples are pretreated by applying some combination of heat, vacuum and/or flowing gas to remove adsorbed contaminants acquired from atmospheric exposure. The solid is then cooled, under vacuum, usually to cryogenic temperature. An adsorptive (typically nitrogen) is admitted to the solid in controlled increments. After each dose of adsorptive, the pressure is allowed to equilibrate and the quantity of gas adsorbed is calculated. The gas volume adsorbed at each pressure (at one constant temperature) defines an adsorption isotherm, from which the quantity of gas required to form a monolayer over the external surface of the solid and its pores is determined. With the area covered by each adsorbed gas molecule known, the surface area can also be calculated.
Although nitrogen is the most commonly used adsorptive, many samples, especially those with low surface areas, may require the use of other gases such as krypton or argon. Micromeritics offers multigas options that include high vacuum pumps and 10-mmHg and 1-mmHg transducers on most of our surface area analyzers. These options make it possible to achieve and measure the low pressures required for low surface area determinations, micropore analysis, and chemisorption.