Surface area and pore size analyzers are used to measure the surface area and pore size of a sample. Surface area helps determine parameters such as how solids dissolve, burn, and react with other materials. Pore size is often a secondary determination. When selecting surface area and pore size analyzers, test media or material is an important parameter to consider. Choices include adhesives and coatings, asphalt and pavement, building and construction materials, ceramics and glass, chemicals, concrete and cement, food and drugs, fuels and petroleum, and geological specimens. Surface area and pore size analyzers are also used to test metals, oils and lubricants, packaging materials, polymers, powder and granular materials, pulp and paper, rock, soil, textiles, and wood.
Fundamentals of Adsorption: Free Space 101
Surface area and pore size analyzers may be used to analyze a number of factors, including pore distribution, pore volume, specific surface area, and total surface area. Pore distribution is the frequency of occurrence of the pores found within the given sample. Pore volume is the average volume (size) of the pores present in the sample. Specific surface area relates to powdered samples only. The specific surface area of a powder is determined by physical adsorption of a gas on the surface of the solid, and by measuring the amount of adsorbate gas corresponding to a monomolecular layer on the surface. Total surface area may be measured by determining the amount of material required to form a single layer (a monolayer) on the surface of the sample. If the area per molecule or ion adsorbed is known, then surface area and pore size analyzers can calculate the total surface area from such a measurement.
To determine the surface area, surface area and pore size analyzers measure solid samples that have been pretreated by some combination of heat, vacuum and/or flowing gas to remove adsorbed contaminants acquired from atmospheric exposure. The solid is then cooled, under vacuum conditions, usually to cryogenic temperature. An adsorptive (typically nitrogen) is admitted to the solid in controlled increments. After each dose of adsorptive, the pressure within the surface area analyzer or pore size analyzer 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, surface area and pore size analyzers can calculate the surface area.