Nature of Colloidal Dispersions

A colloidal dispersion consists of a dispersed phase (particles 1–1000 nm) distributed in a dispersion medium. This intermediate size range confers unique properties not seen in true solutions (<1 nm) or suspensions (>1000 nm). Colloids pass through ordinary filter paper (like true solutions) but not through ultrafiltration membranes. Suspensions settle on standing; colloids do not.

Classification

By interaction with solvent: lyophilic (solvent-loving, e.g., starch, gelatin — stable, reversible, self-stabilizing through a solvation shell) and lyophobic (solvent-hating, e.g., gold sol, $As_{2}S_{3}$ sol, Fe(OH)_{3} sol — unstable, irreversible, stabilized only by surface charge). By particle nature: multimolecular (aggregates of small molecules, e.g., $S_{8}$ sol), macromolecular (single large molecules, e.g., proteins, starch), and associated colloids/micelles (amphiphilic molecules above CMC, e.g., soap).

Four Key Colloidal Properties

1. Tyndall Effect: Colloidal particles scatter light. When a beam passes through a colloid, the path is visible (like dust in sunlight). True solutions do not scatter light. This test distinguishes colloids from true solutions. Cause: particle size comparable to wavelength of visible light (400–700 nm).

2. Brownian Motion: Colloidal particles exhibit continuous random zig-zag movement. Cause: unequal bombardment by dispersion medium molecules from all directions. This kinetic energy keeps particles suspended, preventing settling under gravity.

3. Electrophoresis: Charged colloidal particles migrate in an applied electric field toward the electrode of opposite sign. Fe(OH)_{3} sol (positive) migrates to cathode; $As_{2}S_{3}$ sol (negative) migrates to anode. This proves that colloidal particles carry electric charge.

4. Coagulation: Neutralizing the surface charge causes particles to aggregate and settle. Methods: adding electrolyte (most common), applying electric field (Cottrell precipitator), mixing oppositely charged sols, boiling. The Hardy-Schulze rule: coagulating power ∝ z^6, where z = valency of the ion with charge opposite to the colloid.

Coagulation and Hardy-Schulze Rule

For negatively charged $As_{2}S_{3}$ sol, cations coagulate it: $Al^{3+}$ (3+) > $Ba^{2+}$ (2+) > $Na^{+}$ (1+). For positively charged Fe(OH){3} sol, anions coagulate it: [Fe(CN){6}]^{4-} (4−) > $PO_{4}^{3-}$ (3−) > $SO_{4}^{2-}$ (2−) > $Cl^{-}$ (1−). This is because higher-valency ions more effectively compress the electric double layer around colloidal particles, reducing zeta potential to below the coagulation threshold (~15–20 mV).

Protective Colloids and Gold Number

Lyophilic colloids (e.g., gelatin) can protect lyophobic colloids (e.g., gold sol) from coagulation by adsorbing on their surface and forming a protective sheath. The gold number quantifies this: mass in mg of protective colloid that just prevents coagulation of 10 mL gold sol by 1 mL of 10% NaCl. Lower gold number = more efficient protection. Gelatin (0.005) > albumin (0.1) > starch (25).

Preparation and Purification

Chemical methods: hydrolysis of $FeCl_{3}$ → Fe(OH)_{3} sol; reduction of $AuCl_{3}$ with HCHO → gold sol. Physical: Bredig's arc method (electric arc between metal electrodes in cold water). Peptization: adding small amount of electrolyte to fresh precipitate disperses it into colloid. Purification: dialysis (semi-permeable membrane, slow), electrodialysis (with electric field, faster), ultrafiltration (fine-pore membrane under pressure, fastest).