Introduction

Adsorption is introduced through two everyday observations. First, when a metal spoon is dipped into milk and then withdrawn, a thin film of milk particles clings to the spoon's surface -- rinsing removes it easily, but visibly it was there. Second, a cold water bottle taken out of a refrigerator and left on a table develops droplets of moisture on its outer surface as water vapour from the surrounding air condenses on it. In both cases, particles or molecules from outside (milk particles in the first case, water molecules in the second) accumulate on a solid surface without penetrating into the bulk of the solid. Every object exposed to the atmosphere behaves this way to some extent: water vapour and other gas molecules such as N2 and O2 present in air settle as an invisible multi-molecular film on exposed surfaces. This general phenomenon, where molecules from a gas, liquid or dissolved substance accumulate at the surface of another substance (usually a solid), is called adsorption. The rest of the chapter builds this everyday observation into a full account of why adsorption happens, how it is classified, what controls its extent, and how it underlies both catalysis and the wider behaviour of colloidal systems, which are examined in the second half of the chapter.