Physisorbed

Physisorbed refers to a state where a substance, typically a gas or liquid, is held onto the surface of a solid or liquid by weak intermolecular forces, primarily van der Waals forces. This type of adsorption is called physisorption, and the substance in this state is physisorbed. The forces involved are similar to those that cause condensation of gases into liquids.

Unlike chemisorption, where chemical bonds are formed between the adsorbate and the adsorbent, physisorption does not involve the formation of new chemical bonds. The interactions are non-specific and are primarily attractive forces. This means that almost any gas can physisorb onto almost any solid surface, although the extent of physisorption will depend on factors like temperature, pressure, and the specific nature of the adsorbate and adsorbent.

Physisorption is typically reversible. When the pressure of the adsorbate is reduced or the temperature is increased, the physisorbed molecules can easily detach from the surface. This reversibility is a key characteristic that distinguishes it from chemisorption. Physisorption often occurs at low temperatures and high pressures, as these conditions favor the condensation of gases onto surfaces.

The process of physisorption can be described by adsorption isotherms, such as the Langmuir or BET (Brunauer-Emmett-Teller) models, which relate the amount of gas adsorbed to the pressure at a constant temperature. Physisorbed layers are generally considered to be multilayers, meaning that multiple layers of adsorbate molecules can accumulate on the surface, unlike chemisorption which is typically limited to a single molecular layer.