Physisorptioa

Physisorption, short for physical adsorption, is a type of adsorption where the adsorbate is held to the adsorbent surface by weak van der Waals forces. This is in contrast to chemisorption, which involves the formation of chemical bonds between the adsorbate and adsorbent. Physisorption is a reversible process, meaning that as the temperature increases or the pressure of the adsorbate decreases, the adsorbate molecules detach from the surface.

The strength of the van der Waals forces in physisorption is relatively low, typically in the range of 20-40 kJ/mol. This means that physisorption occurs readily at low temperatures and high pressures. The process does not involve the breaking or formation of chemical bonds, and the electronic structure of the adsorbate is generally not altered. Physisorption can occur on any solid surface, and it is not specific to particular chemical interactions.

Key characteristics of physisorption include its reversibility, low enthalpy of adsorption, and lack of specificity. The process is often described by adsorption isotherms, such as the Langmuir isotherm (though this is more accurately for chemisorption, it's often used as a starting point) or the BET (Brunauer, Emmet, and Teller) isotherm, which relates the amount of adsorbate adsorbed to the pressure of the adsorbate at a constant temperature. The BET isotherm is particularly useful for determining the surface area of porous materials.

Physisorption plays a role in various phenomena, including the adsorption of gases on solid surfaces at low temperatures, the separation of gases, and the functioning of activated carbon filters.