Physisorció

Physisorció, also known as physical adsorption, is a type of adsorption where the adsorbate molecules are held to the surface of the adsorbent by weak van der Waals forces. This is in contrast to chemisorption, where chemical bonds are formed between the adsorbate and adsorbent. Physisorció is a reversible process, meaning that the adsorbate can be easily removed from the adsorbent surface by increasing the temperature or decreasing the pressure.

The forces involved in physisorció are non-specific and arise from the temporary or permanent dipoles of the adsorbate molecules interacting with the surface. These forces are relatively weak, typically in the range of 20-40 kJ/mol. The extent of physisorció is generally dependent on temperature and pressure. At low temperatures and high pressures, physisorció is more favorable. As temperature increases or pressure decreases, the adsorbate molecules gain enough kinetic energy to overcome the weak van der Waals forces and desorb from the surface.

Physisorció is a crucial phenomenon in various fields, including gas separation, catalysis, and chromatography. For example, in gas separation, materials with high surface areas like activated carbon are used to physisorb specific gases from a mixture, allowing for their purification. In catalysis, physisorció can play a role in the initial interaction of reactants with the catalyst surface before chemisorption or reaction occurs. In chromatography, the separation of compounds is often based on differences in their physisorptive interactions with the stationary phase.