Properties

When a beam of light passes through a true solution (e.g., NaCl in water), it passes through invisibly because dissolved ions are too small to scatter visible light. In a colloid, however, particles in the 1–1000 nm range are comparable in size to the wavelength of visible light (400–700 nm). These particles scatter the incident light in all directions — a phenomenon called the Tyndall effect, after John Tyndall (1820–1893). The scattered beam becomes visible as a bright cone or 'Tyndall cone' when viewed from the side. The intensity of scattering follows Rayleigh's law: I ∝ 1/λ⁴, meaning shorter wavelengths (blue, violet) scatter more than longer ones. This is why the sky is blue! Practical test: shine a laser pointer through milk (a colloid) versus sugar solution — the beam is visible only in milk.

In 1827, botanist Robert Brown observed under a microscope that pollen grains suspended in water moved in a continuous, random, zigzag path. This Brownian movement is caused by unequal bombardment of colloidal particles by the fast-moving molecules of the dispersion medium. Because colloidal particles are small enough that the statistical fluctuations in the number of solvent molecules hitting them from different sides are significant, they experience net random pushes. Brownian motion is significant because: (1) it keeps colloidal particles suspended against gravity (kinetic stabilisation), preventing sedimentation; (2) its quantitative analysis by Einstein in 1905 provided proof of the existence of atoms and allowed the first accurate estimate of Avogadro's number. The mean square displacement of a particle is given by: ⟨x²⟩ = 2Dt, where D is the diffusion coefficient and t is time.

Colloidal particles adsorb ions from the dispersion medium and acquire a net electric charge (all particles in a given sol acquire the same sign of charge). When electrodes are placed in a colloidal sol and a direct current is applied, the charged particles migrate toward the oppositely charged electrode — this is electrophoresis. For example, As₂S₃ sol particles are negatively charged and migrate to the anode (+); Fe(OH)₃ sol particles are positively charged and migrate to the cathode (−). The velocity of migration depends on the size and charge of the particle and the strength of the electric field. Electrophoresis demonstrates that colloidal particles are electrically charged. Modern applications include gel electrophoresis (separation of DNA and proteins), electrocoating of car bodies, and the purification of industrial colloids. The layer of adsorbed ions and associated counter-ions surrounding each particle is called the electrical double layer or Helmholtz–Gouy–Chapman layer, and the potential at its boundary is the zeta potential (ζ).

Colloidal particles are stable because they carry like charges — they repel each other and cannot aggregate. Coagulation (also called flocculation) is the process of destabilising a colloid so that particles clump together and settle. This can be achieved by: (1) adding an electrolyte, (2) mixing two oppositely charged sols, (3) boiling (reducing adsorption), or (4) electrophoresis (particles are discharged at the electrode). The most important principle governing electrolyte-induced coagulation is the Hardy–Schulze Rule, formulated by W.B. Hardy and H. Schulze: 'The greater the valency (charge) of the coagulating ion (the ion whose charge is opposite to that of the colloidal particle), the greater is its coagulating power.' The minimum concentration of electrolyte required to coagulate a sol is called the coagulating value or flocculation value. For a negatively charged sol like As₂S₃, the coagulating power of cations follows: Al³⁺ > Ba²⁺ > Na⁺. For a positively charged sol like Fe(OH)₃, the coagulating power of anions follows: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻. Higher-valency ions compress the electrical double layer more effectively, reducing the zeta potential until repulsion can no longer prevent aggregation. This principle is applied in water purification (adding alum, Al₂(SO₄)₃, to coagulate clay colloids) and in the formation of river deltas (seawater electrolytes coagulate the colloidal clay carried by rivers).

• I — Intensity of scattered light (W m⁻²): The power of light scattered per unit area by colloidal particles.
• \lambda — Wavelength of incident light (nm (nanometres)): The wavelength of the light beam entering the colloid. Shorter wavelengths (blue/violet) scatter far more than longer wavelengths (red).

Problem. A student has three colloidal sols: (A) As₂S₃ sol (negatively charged), (B) Fe(OH)₃ sol (positively charged), and (C) gold sol (negatively charged). She adds separate electrolytes — NaCl, MgSO₄, and AlCl₃ — to each sol. (i) Which electrolyte will most effectively coagulate As₂S₃ sol, and why? (ii) Which ion is responsible for coagulating Fe(OH)₃ sol when MgSO₄ is added, and why? (iii) If she mixes As₂S₃ sol and Fe(OH)₃ sol together, what happens?

Answer. (i) AlCl₃ (Al³⁺, valency 3, highest coagulating power by Hardy–Schulze Rule). (ii) SO₄²⁻ ions coagulate the positive Fe(OH)₃ sol. (iii) Mutual coagulation occurs — both sols precipitate.

• Adsorption — Colloidal particles acquire their surface charge through adsorption of ions from the dispersion medium; understanding adsorption is essential to explaining electrophoresis and coagulation.
• Types of Colloids and Classification — Knowing the classification of colloids (lyophilic, lyophobic, sols, gels, emulsions) provides the context for understanding why different colloids show varying stability and coagulation behaviour.
• Emulsions and Surfactants — Emulsions are a colloidal type stabilised by surfactant molecules; their stability and breaking (demulsification) directly relate to coagulation principles and electrostatic repulsion.
• Electrochemical Cells — Electrophoresis requires an applied electric field and involves ion migration, directly paralleling the movement of ions at electrodes in electrochemical cells.