Small Volumes and Large Surfaces: The World of Colloids

Abstract

What do the following have in common: fog, water-based paints, milk and mayonnaise? They are all colloidal states. It was the British physicist Thomas Graham who, in 1861, invented the generic term ‘colloid’ from the Greek word kolla for glue. He intended thereby to highlight a characteristic which he attributed to a whole class of special chemical products. At the time, Graham was studying the diffusion properties of various substances in solution, and hoped to distinguish those capable of going through a wall of parchment from those which were not. Whereas the former invariably exhibited a crystalline form in the pure state (he used sugars and mineral salts), those that could not penetrate the wall, such as albumen and gelatin, had a rather glutinous appearance which, in his view, was specific to them. His proposed classification of chemical substances into two main categories, the ‘crystalloids’ and the ‘colloids’, was based upon these diffusion criteria.