When a gas comes into contact with a solid, some of the gas molecules may be absorbed into the solid, depending on the material’s porosity. Some gas molecules however do not fully permeate into the solid but do stay attached to its surface. In a process called physical adsorption, or physisorption, gas molecules form weak Van der Waals bonds with the surface (the adsorbent). Although this process has been known for centuries, its applications are quite modern and often used in nanoscience applications. Where the attraction is due to covalent bonding this is known as chemisorption.
Gas adsorption can be used to determine the surface area of a given solid sample. For the majority of materials, nitrogen is used as the adsorbate gas. The solid sample is first cleaned from contaminants, cooled under vacuum to 77 K and then exposed to several doses of nitrogen. As pressure increases with every new nitrogen dose, nitrogen molecules occupy the surface and fill the pores. According to gas adsorption theory, the specific surface area of a solid can be calculated from the amount of gas needed to form a molecular monolayer on the surface of the solid.
Dependence of the adsorbed gas quantity on relative pressure at constant temperature produces an isotherm curve. There are five types of isotherms used to calculate surface area. They have been originally described by Brunauer, Emmet and Teller (BET theory). Furthermore, differences between adsorption and desorption curves of an isotherm also provide information about pore size and volume.
Unlike other methods (e.g. electron microscopy) gas adsorption overcomes problems that arise from the presence and complexity of pores and channels in the material. It can be used to easily determine surface size of powders, porosity of materials, nanopore size and distribution
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