Classification of Colloids

Colloids are classified in three independent ways. (a) By physical state of the dispersed phase and dispersion medium: Table 11.4 (attached to this section) lists eight combinations -- for example solid-in-solid gives a 'solid sol' (coloured glasses, gem stones, porcelain, paper), solid-in-liquid gives a sol or gel (paints, cell fluids, gelatin, muddy water, starch solution), liquid-in-gas gives an aerosol (fog, mist, cloud, hair sprays), and gas-in-liquid gives a foam (froth, whipped cream, soap lather). (b) By interaction/affinity between the phases: a colloidal solution is lyophilic if the dispersed phase has a strong affinity for the dispersion medium (or hydrophilic specifically when the medium is water) -- lyophilic sols are reversible, since evaporating and then remixing with the medium reforms the sol, and they are stable and hard to coagulate. A colloid is lyophobic if the dispersed phase has little or no affinity for the medium (hydrophobic when the medium is water) -- common examples are Ag, Au, and hydroxides like Al(OH)3 and Fe(OH)3, and once a lyophobic sol is precipitated or coagulated it has little or no tendency to revert back to the colloidal state. Table 11.5 (attached to this section) gives the full point-by-point comparison. (c) By molecular size of the dispersed-phase particles: multimolecular colloids are aggregates of many atoms or small molecules, each individual particle smaller than about 10^3 pm, held together by van der Waals forces -- e.g. a gold sol (an aggregate of many gold atoms) or S8 sulfur molecules; macromolecular colloids are made of single molecules that are themselves large enough to be of colloidal size -- e.g. starch, cellulose, proteins, polythene, nylon and other plastics; and associated colloids (or micelles) are substances -- soap and detergent are the examples given -- that behave as normal electrolytes at low concentration but associate together into colloidal-sized aggregates called mic …