Classification

The most fundamental classification uses the physical states (solid, liquid, gas) of both the dispersed phase and the dispersion medium. Eight combinations are possible (gas-in-gas gives a true mixture, not a colloid). Key types include: (a) Sol — solid dispersed in liquid (e.g., gold sol, paint, ink, starch solution). (b) Gel — liquid dispersed in solid (e.g., gelatin, butter, cheese, agar). (c) Emulsion — liquid dispersed in liquid (e.g., milk, mayonnaise, cold cream). (d) Foam — gas dispersed in liquid (e.g., whipped cream, soap froth). (e) Solid foam — gas dispersed in solid (e.g., pumice, styrofoam). (f) Aerosol — liquid or solid dispersed in gas: liquid aerosol = fog, clouds; solid aerosol = smoke, dust. (g) Solid sol — solid dispersed in solid (e.g., coloured gemstones, some alloys). The dispersion medium determines which prefix is used; for example, an aerosol always has gas as the dispersion medium, regardless of whether the dispersed phase is solid or liquid.

Lyophilic colloids (Greek: lyophilic = solvent-loving; hydrophilic when water is the solvent) are those in which the dispersed phase has a strong affinity for the dispersion medium. Examples include starch, proteins (gelatin, albumin), gum arabic, and rubber in water. Key characteristics: (1) Stability — lyophilic colloids are thermodynamically stable and form spontaneously; once coagulated they can be redispersed simply by adding solvent (they are reversible). (2) Viscosity — significantly higher than the dispersion medium due to solvation shells around particles. (3) Surface tension — lower than pure solvent. (4) Coagulation — requires large amounts of electrolyte (Hardy–Schulze rule applies less strongly). (5) Charge — particles may carry charge, but charge is not essential for stability; the solvation layer itself provides stabilization. The strong interaction between dispersed phase and solvent (solvation or hydration) creates a protective shell that prevents particle aggregation.

Lyophobic colloids (hydrophobic when water is the dispersion medium) are those in which the dispersed phase has little or no affinity for the dispersion medium. Examples include gold sol, silver sol, arsenious sulfide sol ($\ce{As2S3}$ sol), ferric hydroxide sol ($\ce{Fe(OH)3}$ sol), and sulfur sol. Key characteristics: (1) Instability — lyophobic colloids are thermodynamically unstable; they require stabilizing agents (electrolytes or surfactants) and do not form spontaneously. (2) Irreversibility — once coagulated (e.g., by adding electrolyte), they cannot be reconstituted simply by adding solvent. (3) Charge — stability depends critically on the surface charge of particles (zeta potential). Adding electrolytes neutralizes the charge → coagulation (Hardy–Schulze law: higher valency of oppositely charged ion → greater coagulating power). (4) Low viscosity compared to lyophilic sols. (5) Adsorption — particles adsorb ions from solution to acquire charge; e.g., $\ce{Fe(OH)3}$ sol adsorbs $\ce{Fe^{3+}}$ ions and becomes positively charged. Lyophobic colloids can be stabilized by adding lyophilic colloids, which form a protective film around the lyophobic particles (protective colloid action, measured by the gold number introduced by Zsigmondy).

A third classification focuses on how the colloidal particles themselves are constructed: (a) Multimolecular colloids — particles formed by aggregation of a large number of small molecules (atoms or molecules with diameter < 1 nm) held together by van der Waals forces, until the aggregate reaches colloidal size (1–1000 nm). Examples: gold sol (aggregates of Au atoms), sulfur sol (aggregates of $\ce{S8}$ molecules). These are typically lyophobic. (b) Macromolecular colloids — naturally occurring or synthetic polymers whose individual molecules are large enough (molecular mass 10³–10⁷ g/mol) to fall in the colloidal size range. Examples: starch, cellulose, proteins, nylon, polystyrene, natural rubber. These macromolecules form colloidal solutions directly in suitable solvents and are typically lyophilic. They are used in drug delivery, food technology, and materials science. (c) Associated colloids (micelles) — substances (surfactants/amphiphilic molecules) that behave as normal electrolytes at low concentration but, above the critical micelle concentration (CMC), associate to form aggregates called micelles of colloidal dimensions. Each micelle contains 50–100 monomer units. Examples: soap (sodium stearate, $\ce{C17H35COONa}$), detergents (sodium dodecyl sulfate). The hydrophobic tails cluster inward; hydrophilic heads face the aqueous medium, enabling solubilization of oils.

The stability of lyophobic colloids depends on the zeta potential (the electrokinetic potential at the slipping plane around a colloidal particle). When electrolytes are added, oppositely charged ions (counterions) migrate to the particle surface, neutralizing the charge and reducing the zeta potential below the critical value (~25–30 mV), causing rapid aggregation (coagulation or flocculation). The Hardy–Schulze law states: the coagulating power of an ion increases sharply with increasing valency of the ion having charge opposite to that of the colloidal particle. For a negatively charged sol (e.g., $\ce{As2S3}$ sol), coagulating power order: $\ce{Al^3+}$ > $\ce{Ba^2+}$ > $\ce{Na+}$. The minimum concentration of electrolyte required to cause coagulation in 2 hours is the coagulation value (or flocculation value); its reciprocal gives the coagulating power. Schulze empirically found that coagulating power is approximately proportional to the 6th power of the ionic valency (z⁶), though modern DLVO theory (Derjaguin–Landau–Verwey–Overbeek) provides a full quantitative treatment using van der Waals attraction and electrostatic double-layer repulsion.

• \ce{As2O3} — Arsenious oxide (arsenic trioxide) (mol): Starting material; a solid that dissolves slightly in water to form arsenious acid.
• \ce{H2S} — Hydrogen sulfide (mol): Reducing and sulfiding agent; passed through the solution to precipitate sulfide in colloidal form.
• \ce{As2S3} — Arsenious sulfide (colloidal) (—): Negatively charged lyophobic sol particles; particle diameter 1–1000 nm; stabilized by adsorbed HS⁻ ions.
• \ce{H2O} — Water (mol): Dispersion medium and by-product of the reaction.

Problem. A student prepares two colloidal solutions: (A) gelatin in water and (B) gold sol in water. She adds 1 mL of 1 M NaCl to each. Solution A remains stable, but Solution B coagulates immediately. (i) Identify each colloid type (lyophilic/lyophobic). (ii) Explain why A is stable but B coagulates. (iii) Using the Hardy–Schulze law, predict which electrolyte — NaCl, CaCl₂, or AlCl₃ — will coagulate Solution B most efficiently, and state why. Gold sol particles carry a negative charge.

Answer. Solution A (gelatin) is lyophilic and stable due to hydration shells; Solution B (gold sol) is lyophobic and coagulates because Na⁺ neutralizes its surface charge. AlCl₃ is the most effective coagulant because Al³⁺ has the highest valency among the three cations, and Hardy–Schulze law states coagulating power increases sharply with ionic valency.

• Tyndall Effect — The Tyndall effect (scattering of light by colloidal particles) is the primary optical property used to distinguish colloids from true solutions and depends directly on the colloidal particle size range established by the classification system.
• Adsorption — Surface adsorption of ions is the mechanism by which lyophobic colloidal particles acquire their stabilizing surface charge; understanding adsorption is essential to understanding why lyophobic colloids coagulate upon electrolyte addition.
• Emulsions and Emulsification — Emulsions are a specific subclass of colloids (liquid dispersed in liquid) and are stabilized by emulsifying agents (lyophilic colloids or surfactants) — directly applying the lyophilic/lyophobic distinction.
• Hardy–Schulze Law and Coagulation — The Hardy–Schulze law quantifies how electrolyte valency causes coagulation of lyophobic colloids, and is the primary tool for predicting colloidal stability based on the lyophobic classification.
• Surface Tension and Intermolecular Forces — The affinity between dispersed phase and dispersion medium (the basis of lyophilic vs lyophobic classification) is fundamentally determined by intermolecular forces such as hydrogen bonding, dipole–dipole interactions, and van der Waals forces.