Colloids

Colloids are part of everyday life -- milk, butter, jelly, whipped cream and mayonnaise are all colloids, and colloid chemistry underlies the understanding of cement, bricks, pottery, porcelain, glass, enamels, oils, lacquers, rubber, celluloid and other plastics, leather, paper, textiles, filaments, crayons, inks and road-construction materials, as well as everyday processes like cooking, washing, dyeing, painting, ore flotation, water purification, sewage disposal, smoke prevention, photography and pharmacy. A colloid is a heterogeneous mixture: the component present in the largest proportion is called the dispersion medium, and the other component(s), present in smaller proportion, are called the dispersed phase. The particles of the dispersed phase are larger than an individual molecule but still too small to be seen with the naked eye. This sits between two extremes: dissolving salt in water gives a true, homogeneous solution (solute particle diameters roughly 0.1 to 2 nm, the size of a typical ion or small molecule -- transparent, possibly coloured, and never separating on standing), while ground coffee or tea leaves mixed with milk forms a suspension (particles large enough to eventually settle out). Colloidal dispersions occupy the wide middle ground between these two: their dispersed-phase particles have diameters roughly in the range 2 to 500 nm -- l …