physisorption

Physisorption, or physical adsorption, is the adhesion of atoms or molecules (adsorbates) to a solid surface (adsorbent) driven by weak van der Waals forces. It is noncovalent and generally reversible, with no formation of chemical bonds between the adsorbate and the surface. This distinguishes it from chemisorption, where chemical bonds are formed and the process often involves higher energies and specificity.

Characteristics of physisorption include relatively small heats of adsorption, typically in the range of a few to several tens of kilojoules per mole. The interaction is usually weak, non-specific, and can be easily reversed by increasing temperature or decreasing pressure. Physisorption can involve multilayer formation, particularly on porous or high-surface-area materials, and is commonly described using adsorption isotherms such as the Brunauer–Emmett–Teller (BET) model for multilayer adsorption or Freundlich-type models for heterogeneous surfaces.

Factors that influence physisorption include the nature of the adsorbate and adsorbent (nonpolar molecules on nonpolar surfaces tend to physisorb readily), surface area and porosity, temperature, and the partial pressure of the adsorbate. Higher surface area materials like activated carbon, silica, and porous metals often exhibit significant physisorption, especially for noble gases and small nonpolar molecules.

Common applications rely on physisorption phenomena, including gas storage and separation, purification, catalysis support, and sensing. Measurements of adsorption isotherms, calorimetry, and surface characterization techniques help quantify the extent and energetics of physisorption, contributing to the design of materials with tailored adsorption properties.