Difficulty: Intermediate | Prerequisites: Adsorption basics (Part 1 of Surface Chemistry notes), solutions and solubility
Source: Surface Chemistry 1, General Chemistry (University of Minnesota Twin Cities)
Colloids sit between true solutions and suspensions: their particles are too large to dissolve completely but too small to settle out under gravity. They appear everywhere, from milk and fog to blood and paint. Understanding colloids means understanding how particle size, surface charge, and the interaction between dispersed phase and medium determine stability, optical behaviour, and practical applications. This section builds on adsorption, since the charge that stabilises colloidal particles comes from preferential adsorption of ions.
Colloidal solutions contain particles sized 1 to 1000 nm, dispersed in a medium. They are classified by the physical states of their components, by how much the dispersed phase likes the medium (lyophilic vs lyophobic), and by the type of particles involved (multimolecular, macromolecular, or associated colloids like micelles). Colloids can be prepared by chemical reactions, electrical disintegration, or peptization, and purified by dialysis or ultra-filtration.
Colloidal solution (colloid)
A heterogeneous system intermediate between a true solution and a suspension, consisting of a dispersed phase and a dispersion medium. Particle size ranges from 1 to 1000 nm.
Think of it as particles small enough to stay suspended but large enough to scatter light.
Dispersed phase (DP)
The phase that is distributed or dispersed through the medium.
In simple terms, this is the stuff floating around in tiny particles.
Dispersion medium (DM)
The continuous phase in which colloidal particles are dispersed.
Think of it as the background liquid, gas, or solid that holds the particles.
Lyophilic colloid
A colloid in which the dispersed phase has great affinity for the dispersion medium ("liquid-loving"). Examples: glue, gelatin, starch, protein in a suitable liquid. Reversible and stable.
Lyophobic colloid
A colloid in which the dispersed phase has no affinity for the dispersion medium ("liquid-hating"). Examples: metal sols (gold, silver). Irreversible and requires special methods to prepare. Needs a stabilising agent.
Multimolecular colloids
Colloids formed when a large number of atoms or molecules of a substance aggregate to reach colloidal size (1 to 1000 nm). Examples: gold sol, sulphur sol.
Macromolecular colloids
Colloids in which the individual molecules (polymers) are large enough to fall in the colloidal size range on their own. Examples: starch, cellulose, proteins, enzymes, polythene.
Associated colloids (micelles)
Substances that behave as normal strong electrolytes at low concentrations but form colloidal-sized aggregates (micelles) at higher concentrations. Examples: soaps, detergents.
Critical micelle concentration (CMC)
The minimum concentration above which micelles begin to form.
Kraft temperature (T_K)
The minimum temperature above which micelle formation can occur.
Colloidal solutions are intermediate between true solutions and suspensions.
A colloidal solution = dispersed phase + dispersion medium.
Particle size of colloids: 1 to 1000 nm.
Example: gold sol contains gold particles (dispersed phase) in water (dispersion medium).
This is the most commonly tested classification:
Dispersed Phase | Dispersion Medium | Type of Colloid | Examples |
|---|---|---|---|
Solid | Solid | Solid sol | Coloured glasses, gem stones |
Solid | Liquid | Sol | Paints, cell fluids |
Solid | Gas | Aerosol | Smoke, dust |
Liquid | Solid | Gel | Cheese, jellies |
Liquid | Liquid | Emulsion | Milk, hair cream, butter |
Liquid | Gas | Aerosol | Fog, mist, cloud, insecticide sprays |
Gas | Solid | Solid sol | Pumice stone, foam rubber |
Gas | Liquid | Foam | Froth, whipped cream, soap lather |
Feature | Lyophilic ("liquid-loving") | Lyophobic ("liquid-hating") |
|---|---|---|
Affinity of DP for DM | Great affinity | No affinity |
Examples | Glue, gelatin, starch, protein | Gold sol, metal sols (Ag, Au) |
Reversibility | Reversible: if DM is separated from DP, the sol can be reconstituted by simply mixing | Irreversible: cannot be prepared just by mixing DP and DM; requires special methods |
Stability | Stable; cannot be precipitated easily | Unstable; needs a stabilising agent |
Multimolecular colloids: on dissolution, a large number of atoms or molecules aggregate together to form species in the colloidal size range. Example: gold sol, sulphur (S8) sol.
Macromolecular colloids: macromolecules (polymers) in suitable solvents naturally fall in the colloidal size range. Example: starch, cellulose, proteins, enzymes, polythene.
Associated colloids (micelles): substances that behave as normal strong electrolytes at low concentration, but at higher concentration aggregate into colloidal-sized clusters called micelles. Example: soap, detergents.
Micelles form only above the Kraft temperature (T_K) and above the critical micelle concentration (CMC).
Mechanism of micelle formation (using sodium stearate as example):
Soap is sodium or potassium stearate. Sodium stearate has two parts:
Hydrophobic part (water-repelling): the long hydrocarbon chain (C₁₇H₃₅), non-polar tail
Hydrophilic part (water-loving): the –COO⁻ group, polar/ionic head
At low concentrations, the C₁₇H₃₅COO⁻ ions sit on the water surface with their COO⁻ groups in the water and their hydrocarbon chains pointing away.
At the CMC, the anions are pulled into the bulk of the solution and aggregate into spherical clusters. Their hydrocarbon chains point inward (toward the centre), and their COO⁻ groups point outward (on the surface of the sphere). This spherical aggregate is an ionic micelle.
Detergent equivalent: sodium lauryl sulphate (CH₃(CH₂)₁₁SO₄⁻ Na⁺).
Cleaning action of soap:
Soap particles form micelles around oil droplets on a dirty surface. The hydrophobic tails dissolve into the oil droplet, while the hydrophilic heads project outward into the water. Because the hydrophilic part is on the outside, the entire oil droplet surrounded by stearate ions can now interact with water (due to polarity) and be pulled away from the dirty surface.
Chemical methods:
Double decomposition: As₂O₃ + 3H₂S → As₂S₃ (sol) + 3H₂O (arsenic sulphide sol)
Oxidation: SO₂ + 2H₂S → 3S + H₂O (sulphur sol)
Reduction: 2AuCl₃ + 3HCHO + 3H₂O → 2Au (gold sol) + HCOOH + 6HCl
Hydrolysis: FeCl₃ + 3H₂O → Fe(OH)₃ (ferric hydroxide sol) + 3HCl
Bredig's arc method (electrical disintegration):
An electric arc is struck between metal electrodes (gold, silver, platinum) immersed in deionised water with a trace of alkali. The metal vapourises and then condenses to form colloidal-sized particles. This process involves both dispersion and condensation.
Peptization:
The process of converting a precipitate into a colloidal sol by shaking it with the dispersion medium in the presence of a small amount of electrolyte (the peptizing agent).
During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface, developing a positive or negative charge that causes it to break up into smaller, colloidal-sized particles.
In equation form: precipitate + electrolyte → colloidal solution
Dialysis:
The process of removing dissolved substances from a colloidal solution by means of diffusion through a semipermeable membrane. A bag of semipermeable membrane containing the colloidal solution is suspended in a vessel through which fresh water flows continuously. Ions and small molecules diffuse out through the membrane; colloidal particles are too large to pass and remain behind.
Electro-dialysis:
Dialysis can be made faster by applying an electric field, which accelerates the movement of ions through the membrane.
Ultra-filtration:
Ordinary filter paper has very large pore sizes and lets both impurities and colloidal particles through. Ultrafilter paper is designed with pores small enough to allow only the true solution (ions and small molecules) to pass through. The pure colloid is left on the filter paper.
Soap and detergents work through micelle formation: the hydrophobic tail grabs oil or grease, and the hydrophilic head lets water carry the whole package away.
Dialysis machines used in kidney treatment work on the same principle as colloidal dialysis, separating waste products from blood through a semipermeable membrane.
Students often think lyophilic and lyophobic refer to water specifically. They refer to whatever liquid is the dispersion medium. When the DM is water, the terms become hydrophilic and hydrophobic.
Students sometimes believe that all colloids are liquids. Colloids can have solid, liquid, or gaseous dispersed phases and media (the table of eight types makes this clear).
The term "associated colloid" can be confusing. These substances are not always colloidal. They behave as colloids only above the CMC. Below it, they are ordinary electrolytes.
Students sometimes confuse peptization with dissolving. Peptization is about breaking a precipitate into colloidal-sized particles by adsorbing ions, not about dissolving it into individual molecules.
⚠️ The classification table (DP, DM, type, examples) is extremely high-yield. Know all eight combinations.
⚠️ Lyophilic vs lyophobic comparison (affinity, stability, reversibility) is a frequent short-answer or MCQ.
⚠️ The mechanism of micelle formation and the cleaning action of soap is a classic long-answer question. Be ready to describe the structure (hydrophobic tail inward, hydrophilic head outward).
⚠️ Peptization often appears as a one-mark definition. The key phrase: converting a precipitate into a colloidal sol using a peptizing agent.
Fill in the blank: Colloidal particles range in size from ________ to ________ nm. (1 to 1000)
True or False: A sol is a colloid with a solid dispersed phase in a gas medium. (False. A sol has a solid DP in a liquid DM. Solid in gas is an aerosol.)
Fill in the blank: Micelles form only above the ________ and the ________. (Kraft temperature and CMC)
True or False: Lyophobic colloids are more stable than lyophilic colloids. (False. Lyophilic colloids are more stable.)
Fill in the blank: The process of converting a precipitate into a colloidal sol is called ________. (Peptization)
Q: Distinguish between lyophilic and lyophobic colloids, with examples.
A: Lyophilic colloids have great affinity between the dispersed phase and dispersion medium (e.g. starch, gelatin in water). They are reversible, stable, and self-forming. Lyophobic colloids have no affinity between DP and DM (e.g. gold sol, metal sols). They are irreversible, unstable, and require special preparation methods and stabilising agents.
Q: What are micelles? What conditions are necessary for their formation?
A: Micelles are aggregates formed by associated colloids (such as soaps) above a certain concentration called the critical micelle concentration (CMC) and above the Kraft temperature (T_K). Below CMC, the substance behaves as a normal electrolyte.
Q: Explain the cleaning action of soap.
A: Soap molecules form micelles around oil or grease droplets. The hydrophobic hydrocarbon tails dissolve into the oil, while the hydrophilic COO⁻ heads remain on the outside, facing the water. This allows the oil droplet (now coated with charged heads) to be pulled into the water and washed away from the dirty surface.
Q: What is peptization? Name the electrolyte used to peptize ferric hydroxide precipitate.
A: Peptization is the process of converting a precipitate into a colloidal sol by shaking it with a dispersion medium in the presence of a small amount of electrolyte (the peptizing agent). For ferric hydroxide, FeCl₃ is used as the peptizing agent; the precipitate adsorbs Fe³⁺ ions and acquires a positive charge.
Q: How does dialysis purify a colloidal solution?
A: The colloidal solution is placed in a bag of semipermeable membrane, which is suspended in flowing water. Small ions and molecules diffuse through the membrane into the water, while colloidal particles (too large to pass) remain inside. The result is a purified colloidal solution.
Colloids connect back to adsorption: the charge on colloidal particles originates from preferential adsorption of ions, and coagulation is triggered by neutralising that charge with electrolytes.
This material also ties into emulsions (covered in Part 3), which are a specific type of colloid (liquid in liquid). The same principles of dispersed phase, dispersion medium, and stabilisation apply.
The concept of micelles is important in biochemistry as well: cell membranes are organised on similar principles, with hydrophobic and hydrophilic regions determining structure.
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