Why does Adsorption Occur?

Adsorption is driven by London dispersion forces (a component of the broader van der Waals forces) acting between the adsorbent's surface and the incoming molecules. These forces are short-range (they act only over molecular distances) and additive, meaning the total attractive force at a surface is the sum of very many individual atom-atom interactions across the whole surface. These same pulling interactions are also what makes the surface of a liquid behave like a stretched elastic film -- a measure of that elastic pull is called surface tension (covered in Chapter 10), defined as the energy needed to stretch or increase a liquid's surface by one unit of area. Because systems always tend toward minimum surface energy (i.e. minimum surface tension), a surface 'prefers' to reduce its excess free energy, and adsorbing molecules onto the surface is one way that free energy is lowered -- so the drive toward minimum surface free energy is itself what leads to adsorption. Two terms are defined here and used thr …