Difficulty: Intermediate | Prerequisites: Parts 1 and 2 of Surface Chemistry notes (adsorption, colloid basics)
Source: Surface Chemistry 1, General Chemistry (University of Minnesota Twin Cities)
This final section of the surface chemistry unit covers what colloidal particles actually do once they are in solution: how they scatter light, move, carry charge, clump together, and how we use those behaviours in practice. It also covers emulsions (liquid-in-liquid colloids) and real-world applications ranging from smoke removal in factories to the formation of river deltas. You need the colloid classification and preparation material from Part 2 to follow this comfortably.
Colloidal particles scatter light (Tyndall effect), move randomly (Brownian motion), and carry electric charge (confirmed by electrophoresis). They can be destabilised by adding electrolytes (coagulation), and the Hardy-Schulze law tells you which ions are most effective at doing so. Emulsions are liquid-in-liquid colloids stabilised by emulsifying agents. Colloid science explains everyday phenomena from soap cleaning to delta formation at river mouths.
Tyndall effect
The scattering of light by colloidal particles, which illuminates the path of a beam passing through a colloidal solution.
Think of it as the reason you can see a beam of sunlight in a dusty room: the particles scatter the light sideways.
Brownian motion
The continuous, random, zig-zag motion of colloidal particles, caused by unbalanced bombardment from molecules of the dispersion medium.
In simple terms, the colloidal particles get bumped around unevenly by the liquid molecules and jiggle in place.
Zeta potential (electrokinetic potential)
The potential difference between the fixed layer of ions adsorbed directly on the colloidal particle and the diffuse layer of ions surrounding it.
Electrophoresis (cataphoresis)
The movement of colloidal particles under an applied electric field. Positively charged particles move toward the cathode; negatively charged particles move toward the anode. This experiment confirms that colloidal particles carry charge.
Coagulation (precipitation)
The process by which colloidal particles aggregate and settle out of solution. It happens when the stabilising charge on the particles is neutralised.
Hardy-Schulze law
The coagulating (precipitating) power of an ion on a colloid of opposite charge increases with the charge of the ion.
In simple terms, the higher the charge on the ion, the better it is at clumping colloid particles together.
Coagulation value
The minimum concentration of an electrolyte (in millimoles per litre) required to cause precipitation of a sol.
Gold number
The number of milligrams of a lyophilic (hydrophilic) colloid that will just prevent the precipitation of 10 mL of a gold sol on addition of 1 mL of 10% NaCl solution. A lower gold number means stronger protective power.
Emulsion
A colloidal system in which both the dispersed phase and the dispersion medium are liquids. A dispersion of finely divided liquid droplets in another liquid.
Emulsifying agent (emulsifier)
A substance added to stabilise an emulsion and prevent the two liquids from separating. Examples: soap, detergents, casein (protein) in milk.
De-emulsification
The process of breaking an emulsion into its constituent liquids, by heating, freezing, or centrifuging.
Scattering of light by colloidal particles is called the Tyndall effect.
It illuminates the path of a light beam in a colloidal solution (a true solution does not show this; a suspension scatters too much and appears cloudy).
Conditions for the Tyndall effect:
The colloidal particles must be approximately similar in size to the wavelength of light used.
The difference in refractive indices of the dispersed phase and dispersion medium should be large.
As the size of colloidal particles changes, the colour of the colloidal solution changes (because different particle sizes scatter different wavelengths).
Continuous random (zig-zag) motion of colloidal particles.
Caused by unbalanced bombardment of the particles by molecules of the dispersion medium.
This motion stabilises the colloidal solution: it does not allow particles to settle under gravity.
Charge on colloidal particles:
Colloidal particles always carry an electric charge, and the nature of the charge is the same for all particles in a given colloidal solution.
Positively charged sols | Negatively charged sols |
|---|---|
Hydrated metallic oxides (Al₂O₃·xH₂O, CrO₃·xH₂O, Fe₂O₃·xH₂O) | Metals (copper, silver, gold sols) |
Basic dye stuffs (methylene blue sol) | Metallic sulphides (As₂S₃, Sb₂S₃, CdS sols) |
Haemoglobin (blood) | Acid dye stuffs (eosin, congo red sols) |
Oxides (TiO₂ sol) | Sols of starch, gum, gelatin, clay, charcoal |
Preferential adsorption of ions:
The charge on colloidal particles originates from selective adsorption of ions common to the particles from the dispersion medium.
Example: Fe(OH)₃ + Fe³⁺ → Fe(OH)₃ / Fe³⁺ (positively charged)
AgNO₃ + KCl (excess KCl) → AgCl / Cl⁻ (negatively charged)
AgNO₃ (excess) + KCl → AgCl / Ag⁺ (positively charged)
The ion adsorbed depends on which reactant is in excess.
Zeta potential:
The potential difference between the fixed layer (ions adsorbed directly on the particle) and the diffuse layer (ions in the surrounding liquid). It is a measure of colloidal stability.
Electrophoresis:
When an electric field is applied to a colloidal solution, the charged particles move. Positively charged particles migrate toward the cathode; negatively charged ones toward the anode. This confirms the presence of charge on colloidal particles.
Coagulation (precipitation):
The settling of colloidal particles is called coagulation. The stability of lyophobic sols comes from the charge on the particles. If the charge is removed, particles aggregate and settle under gravity.
Methods of coagulation:
By electrophoresis
By boiling
By addition of electrolyte
By mixing two oppositely charged sols
Hardy-Schulze law:
The precipitating (coagulating) effect of an ion on a colloid of opposite charge increases with the charge of that ion.
For negatively charged Fe(OH)₃ sol, the order of coagulation power of cations is: Al³⁺ > Ba²⁺ > Na⁺
For negatively charged sol generally: [Fe(CN)₆]⁴⁻ > SO₄²⁻ > Cl⁻
Coagulation value:
The minimum concentration of an electrolyte (in millimoles per litre) required to cause precipitation of a sol.
Coagulation power is inversely proportional to coagulation value: Coagulation Power ∝ 1 / Coagulation Value
Protection of colloids:
Lyophilic sols are more stable than lyophobic sols because they are extensively solvated by the dispersion medium. Lyophilic sols can protect lyophobic sols from coagulation by forming a protective layer around the lyophobic particles.
Gold number:
The number of milligrams of a lyophilic colloid that will just prevent the coagulation of 10 mL of a gold sol on addition of 1 mL of 10% NaCl solution.
Protective power ∝ 1 / Gold Number
A smaller gold number means a stronger protective colloid.
Emulsions are liquid-liquid colloidal systems: a dispersion of finely divided liquid droplets in another liquid.
Two types of emulsions:
Type | Dispersed Phase | Dispersion Medium | Examples |
|---|---|---|---|
Oil in water (O/W) | Oil | Water | Milk, cream, soap, gums |
Water in oil (W/O) | Water | Oil | Heavy metal salts of fatty acids |
Emulsifying agents:
Water and oil are immiscible and tend to separate into two layers. To stabilise the emulsion and keep it homogeneous, an emulsifying agent is added.
Examples: soap, detergents (emulsifiers), casein (protein) in milk
De-emulsification:
Emulsions can be broken into their constituent liquids by heating, freezing, or centrifuging.
Exam-relevant question: Why does leather harden after tanning?
Animal skin has positively charged colloidal particles. Tannin consists of negatively charged colloidal particles. When combined, mutual coagulation occurs and the leather hardens.
Why are medicines more effective in colloidal state?
Colloidal particles have a large surface area per unit mass, so they are easily assimilated in the body.
Can a substance act as both a colloid and a crystalloid?
Yes. When particle size is between 1 and 1000 nm, the substance is a colloid. When smaller than 1 nm, it behaves as a crystalloid. Example: NaCl in water is a crystalloid; NaCl in benzene is a colloid.
Electrical precipitation of smoke (Cottrell precipitator):
Smoke is a colloidal solution of solid particles (carbon, arsenic compounds, dust) in air. A Cottrell precipitator is a chamber containing plates with a charge opposite to that carried by the smoke particles. When smoke passes through, the particles lose their charge on contact with the plates and get precipitated. The clean gas exits.
Coagulation of blood:
Blood is a colloidal particle that can be precipitated (coagulated) by using alum, which acts as an electrolyte.
Formation of deltas:
River water carries colloidal clay and sand particles (negatively charged). When this water meets sea water (which contains dissolved salts, i.e. electrolytes), the colloidal particles are coagulated and precipitate at the junction, forming a delta.
Why is a finely divided substance a more effective adsorbent?
A finely divided substance has a greater surface area, so more adsorption can occur.
The Cottrell precipitator is used in factories and power plants to remove smoke and particulate matter from exhaust gases, reducing air pollution.
Alum is applied to cuts to stop bleeding: it coagulates the blood (a colloidal system) by acting as an electrolyte.
Delta formation at river mouths is a geological consequence of colloidal coagulation. The Ganges delta, one of the world's largest, forms this way.
Medicines in colloidal form (e.g. colloidal silver, argyrol) are more effective because their large surface area allows rapid absorption in the body.
Students often think the Tyndall effect proves something is a colloid. It does not prove it on its own. The effect requires that particle size is comparable to the wavelength of light and that the refractive indices of DP and DM differ significantly. Some colloidal solutions with very small particles may show a weak Tyndall effect.
Students sometimes confuse Brownian motion with the motion of molecules. Brownian motion refers to the movement of colloidal particles (which are much larger than molecules), caused by uneven bombardment from molecules of the dispersion medium.
A common error is thinking that higher coagulation value means stronger coagulating power. The relationship is inverse: coagulation power is proportional to 1/coagulation value. A lower coagulation value means the electrolyte is more effective.
Students often assume that a lower gold number means a weaker protective colloid. The opposite is true: a lower gold number indicates stronger protective power.
⚠️ The Tyndall effect and Brownian motion are classic one-mark definitions. Know what causes each and what it confirms about colloidal particles.
⚠️ The Hardy-Schulze law appears as both a definition question and an application question (ordering ions by coagulating power for a given sol). Practise ranking ions by charge for positively and negatively charged sols.
⚠️ "Why does leather harden after tanning?" and "Why are medicines more effective in colloidal state?" are frequently asked short-answer questions. Have one-sentence answers ready.
⚠️ The gold number definition is tested almost verbatim. Memorise the exact wording: milligrams of lyophilic colloid that just prevent precipitation of 10 mL of gold sol on addition of 1 mL of 10% NaCl.
⚠️ Emulsion types (O/W vs W/O) with examples, and the role of emulsifying agents, are high-yield for both MCQs and short answers.
⚠️ Delta formation is a favourite application question linking colloidal chemistry to geography. The key idea: river clay (colloidal, charged) meets sea water (electrolyte), causing coagulation.
Fill in the blank: The scattering of light by colloidal particles is called the ________ effect. (Tyndall)
True or False: Brownian motion causes colloidal particles to settle under gravity. (False. Brownian motion prevents settling and stabilises the sol.)
Fill in the blank: According to the Hardy-Schulze law, coagulating power increases with the ________ of the ion. (charge)
True or False: A higher gold number indicates stronger protective power. (False. A lower gold number means stronger protective power.)
Fill in the blank: An emulsion of oil droplets dispersed in water is called an ________ type emulsion. (oil in water, or O/W)
Q: What is the Tyndall effect? State two conditions necessary for it to occur.
A: The Tyndall effect is the scattering of light by colloidal particles, making the path of the light beam visible. Conditions: (i) the size of colloidal particles should be approximately similar to the wavelength of the light used, and (ii) the difference in refractive indices of the dispersed phase and dispersion medium should be large.
Q: Explain how charge on colloidal particles originates. Give an example.
A: The charge originates from preferential adsorption of ions common to the particles from the dispersion medium. For example, when Fe(OH)₃ precipitate is peptized with FeCl₃, the particles adsorb Fe³⁺ ions and acquire a positive charge.
Q: State the Hardy-Schulze law. For Fe(OH)₃ sol (positively charged), arrange the following in decreasing order of coagulating power: Cl⁻, SO₄²⁻, [Fe(CN)₆]⁴⁻.
A: The Hardy-Schulze law states that the coagulating power of an ion increases with the charge of that ion (for ions of opposite charge to the sol). For a positively charged sol, the coagulating anion with the highest charge is most effective: [Fe(CN)₆]⁴⁻ > SO₄²⁻ > Cl⁻.
Q: What is the gold number? Why is a lower gold number considered better?
A: The gold number is the number of milligrams of a lyophilic colloid that will just prevent the coagulation of 10 mL of a gold sol on addition of 1 mL of 10% NaCl solution. A lower gold number means less of the protective colloid is needed, so it has stronger protective power.
Q: Distinguish between oil-in-water and water-in-oil emulsions, with examples.
A: In an oil-in-water (O/W) emulsion, oil is the dispersed phase and water is the dispersion medium (examples: milk, cream, soap). In a water-in-oil (W/O) emulsion, water is the dispersed phase and oil is the dispersion medium (example: heavy metal salts of fatty acids).
Q: Explain the formation of deltas at river mouths.
A: River water carries colloidal clay particles that are negatively charged. When river water meets sea water, which contains dissolved salts (electrolytes), the electrolyte ions neutralise the charge on the colloidal clay. The particles coagulate and precipitate at the junction of river and sea, forming a delta over time.
The electrical properties of colloids (charge, electrophoresis, zeta potential) connect directly back to adsorption: the charge originates from preferential adsorption of ions, which you covered in Part 1.
Coagulation by electrolytes ties into ionic chemistry and electrochemistry. The Hardy-Schulze law is essentially a statement about how ionic charge governs interactions, which also appears in discussions of lattice energy and solubility.
Emulsions connect to biochemistry: cell membranes, lipid bilayers, and drug delivery systems all rely on the same hydrophobic/hydrophilic principles that govern emulsion stability.
Tyndall effect, light scattering, Brownian motion, random motion, colloidal stability, electrical properties, charge on colloids, preferential adsorption, zeta potential, electrokinetic potential, electrophoresis, cataphoresis, coagulation, precipitation, flocculation, Hardy-Schulze law, coagulation value, coagulation power, gold number, protective colloid, lyophilic protection, emulsion, oil in water, water in oil, O/W emulsion, W/O emulsion, emulsifying agent, emulsifier, de-emulsification, Cottrell precipitator, smoke precipitation, blood coagulation, alum, delta formation, river delta, colloidal clay, medicines in colloidal state, surface area