: 220
Colloidal particles: 1-1000 nm diameter. Cannot be seen by ordinary microscope but scatter light (Tyndall effect). Pass through filter paper but not semipermeable membrane (dialysis). Eight types based on dispersed phase/medium combinations. Most important for JEE: sols (solid in liquid) and emulsions (liquid in liquid). Lyophilic colloids: strong affinity for solvent, self-stabilising, reversible, can be reconstituted. Examples: starch, gum, gelatin, proteins. Viscosity increases. Need large amounts of electrolyte for coagulation (protective action). Lyophobic colloids: little affinity for solvent, require stabiliser, irreversible. Examples: metals (Au, Ag), metal hydroxides (Fe(OH)3), metal sulphides (As2S3). Small amounts of electrolyte coagulate them. Associated colloids (micelles): surfactants that aggregate above critical micelle concentration (CMC) and above Kraft temperature. In water: hydrophilic heads out, hydrophobic tails in (spherical or lamellar micelles). In non-polar solvents: reversed micelles. Soap: sodium salt of long-chain fatty acid. Above CMC: forms micelles that solubilise grease (cleaning action).