B.Sc. 2nd Year Chemistry – Detailed and Exam-Oriented Notes
A colloidal system is a heterogeneous system in which particles of one substance are dispersed throughout another substance, with the dispersed particles having dimensions intermediate between those of true solutions and coarse suspensions.
The substance present in the form of small particles is called the dispersed phase, while the substance in which these particles are distributed is called the dispersion medium.
Colloidal systems are common in nature and everyday life. Milk, fog, clouds, smoke, paints, gelatin, blood and many biological materials exhibit colloidal characteristics.
The particle size of the dispersed phase is an important basis for distinguishing true solutions, colloids and suspensions.
| System | Approximate particle size | General characteristics |
|---|---|---|
| True solution | Less than about 1 nm | Homogeneous; particles not visible by ordinary methods |
| Colloidal system | About 1–1000 nm | Intermediate particle size; shows Tyndall effect |
| Suspension | Generally greater than about 1000 nm | Large particles; may settle on standing |
The boundaries are approximate because colloidal behavior depends not only on particle size but also on the nature of the particles and the medium.
The dispersed phase consists of particles distributed throughout the dispersion medium.
The dispersion medium is the continuous phase in which the dispersed particles are distributed.
For example, in milk, very small fat droplets are dispersed in an aqueous medium containing proteins, lactose and mineral salts.
| Dispersed phase | Dispersion medium | Type | Example |
|---|---|---|---|
| Solid | Gas | Aerosol | Smoke |
| Liquid | Gas | Aerosol | Fog |
| Solid | Liquid | Sol | Paint |
| Liquid | Liquid | Emulsion | Milk |
| Gas | Liquid | Foam | Soap foam |
| Solid | Solid | Solid sol | Colored glass |
| Gas | Solid | Solid foam | Some porous materials |
| Liquid | Solid | Gel | Gelatin |
Lyophilic means "solvent-loving." In these colloids, the dispersed phase has appreciable attraction for the dispersion medium.
Examples include starch, gelatin and gum in water.
Lyophobic means "solvent-hating." The dispersed phase has relatively little affinity for the dispersion medium.
Examples include sols of metals such as gold and platinum.
| Property | Lyophilic colloids | Lyophobic colloids |
|---|---|---|
| Affinity for medium | High | Low |
| Stability | Generally high | Generally lower |
| Reversibility | Usually reversible | Usually irreversible |
| Electrolyte sensitivity | Lower | Higher |
| Examples | Starch, gelatin | Gold sol, sulfur sol |
Certain substances behave as ordinary electrolytes at low concentrations but form colloidal-sized aggregates above a particular concentration.
Such substances are called association colloids. Soaps and synthetic detergents are important examples.
The concentration above which surfactant molecules begin to aggregate into micelles is called the critical micelle concentration (CMC).
The aggregates formed are called micelles.
Colloids can be prepared mainly by two broad approaches: dispersion methods and condensation methods.
In dispersion methods, larger particles are broken down into colloidal-sized particles.
The substance is mechanically broken down into smaller particles using suitable grinding or colloid mills.
This method is commonly used for preparing colloidal sols of metals such as gold, silver and platinum.
An electric arc is produced between electrodes of the metal under suitable conditions. The intense heat vaporizes the metal and the vapor condenses in the dispersion medium to form colloidal particles.
High-frequency ultrasonic waves can break larger particles into colloidal dimensions.
In condensation methods, small molecules or ions combine to form particles of colloidal dimensions.
Metal sols may be prepared by reducing suitable metal salts.
Oxidation reactions can be used to produce colloidal sulfur and other systems.
Hydrolysis of suitable salts can produce colloidal hydroxides.
A suitable reaction between two electrolytes can produce a sparingly soluble substance in colloidal form.
Under suitable conditions, silver iodide can form a colloidal sol.
Colloidal sols prepared by chemical methods often contain dissolved electrolytes and other impurities. These can be removed using purification techniques.
Dialysis is based on the difference in ability of colloidal particles and small ions to pass through a semipermeable membrane.
Small ions and molecules pass through the membrane, whereas colloidal particles are retained.
Electrodialysis is an improved form of dialysis in which an electric field is applied to accelerate the movement of ionic impurities.
In ultrafiltration, a special membrane allows small molecules and ions to pass while retaining colloidal particles.
The Tyndall effect is the scattering of light by colloidal particles, making the path of a light beam visible through the colloidal system.
The phenomenon occurs because colloidal particles are large enough to scatter visible light.
Individual colloidal particles are generally too small to be seen by an ordinary microscope. An ultramicroscope can detect the scattered light from colloidal particles.
Brownian movement is the continuous, random motion of colloidal particles suspended in a dispersion medium.
It results from unequal collisions of rapidly moving molecules of the dispersion medium with the colloidal particles.
Brownian movement helps prevent colloidal particles from settling under gravity and therefore contributes to colloidal stability.
Colloidal particles generally carry an electrical charge. The charge may arise through selective adsorption of ions, ionization of surface groups or other interfacial processes.
The presence of similarly charged particles produces electrostatic repulsion and contributes to the stability of many colloidal sols.
At the interface between a charged colloidal particle and the dispersion medium, counter-ions are attracted toward the particle. This arrangement produces an electrical double layer.
The double layer is commonly described in terms of a relatively strongly associated layer near the surface and a more diffuse region containing counter-ions.
Electrophoresis is the movement of charged colloidal particles toward the electrode of opposite charge when an electric field is applied.
Electrophoresis provides evidence that colloidal particles carry electrical charge.
Electro-osmosis is the movement of the dispersion medium relative to a stationary charged surface when an electric field is applied.
It is considered complementary to electrophoresis.
| Electrophoresis | Electro-osmosis |
|---|---|
| Colloidal particles move. | Dispersion medium moves. |
| Occurs under an electric field. | Occurs under an electric field. |
| Used to study particle charge. | Used to study movement of liquid relative to charged surfaces. |
Colloidal systems may remain stable for long periods because several factors prevent aggregation of the dispersed particles.
If the repulsive forces between particles are sufficiently reduced, the particles may aggregate and the colloid may become unstable.
Coagulation is the process in which colloidal particles aggregate and lose their colloidal stability.
When an electrolyte is added to a colloidal sol, its ions can reduce the effective electrical repulsion between colloidal particles. Aggregation can then occur.
The Hardy-Schulze rule describes the relationship between the coagulating power of an electrolyte and the valency of the ion responsible for neutralizing the charge on the colloidal particles.
In general, for oppositely charged ions, a higher valency ion has a greater coagulating effect than a lower valency ion.
For a negatively charged sol, cations are the effective counter-ions; for a positively charged sol, anions are the effective counter-ions.
Some lyophilic colloids can protect lyophobic colloids from coagulation by forming a protective layer around their particles.
Such substances are called protective colloids.
Examples include gelatin, starch and gum.
The protective action of a colloid toward a gold sol can be expressed using the concept of gold number.
The gold number is related to the minimum amount of protective colloid required to prevent a specified color change or coagulation of a standard gold sol under defined conditions.
A smaller gold number indicates greater protective power.
A gel is a colloidal system in which a liquid is immobilized within a three-dimensional solid or semi-solid network.
Examples include gelatin gel, agar gel and silica gel.
Syneresis is the contraction of a gel accompanied by separation or expulsion of some of the liquid.
Imbibition is the absorption of a liquid by a solid or semi-solid material, often accompanied by swelling.
An emulsion is a colloidal system in which one liquid is dispersed in another immiscible liquid.
Oil droplets are dispersed in water.
Example: Milk is commonly described as an oil-in-water type emulsion, with fat droplets dispersed in an aqueous phase.
Water droplets are dispersed in oil.
Example: Butter is commonly considered a water-in-oil type system.
| Type | Dispersed phase | Dispersion medium | Example |
|---|---|---|---|
| Oil-in-water | Oil | Water | Milk |
| Water-in-oil | Water | Oil | Butter |
An emulsifying agent helps stabilize an emulsion by reducing interfacial tension and/or forming a protective film around dispersed droplets.
Examples include soaps, detergents, proteins and certain natural gums.
The emulsifying agent prevents or slows the coalescence of droplets and thereby improves emulsion stability.
Dialysis is important not only in colloid chemistry but also in biological and medical applications.
The basic principle is the selective movement of small solutes through a semipermeable membrane while larger colloidal particles are retained.
The principle is related to the operation of certain membrane-based purification systems used in biomedical fields.
Many foods are colloidal systems, including milk, butter, cream, mayonnaise and ice cream.
Colloidal systems are used in drug delivery, pharmaceutical formulations and biological preparations.
Coagulation and flocculation are used to remove colloidal impurities from drinking water and wastewater.
Many paints and inks contain finely dispersed solid particles.
Creams, lotions and several cosmetic preparations are based on colloidal or emulsion systems.
Colloidal chemistry is important in rubber processing, textiles, paper manufacture, ceramics and surface coatings.
Colloidal behavior is fundamental to many biological systems because proteins, nucleic acids and other macromolecules can exist as colloidal-sized dispersed particles.
| Property | True solution | Colloid | Suspension |
|---|---|---|---|
| Particle size | < 1 nm | Approximately 1–1000 nm | Generally > 1000 nm |
| Nature | Homogeneous | Heterogeneous at microscopic level | Heterogeneous |
| Settling | No | Usually no rapid settling | Often settles on standing |
| Tyndall effect | Generally absent | Present | May scatter strongly |
| Filtration | Passes ordinary filter paper | Generally passes ordinary filter paper | Usually retained |
| Term | Meaning |
|---|---|
| Dispersed phase | Phase distributed as colloidal particles |
| Dispersion medium | Continuous phase in which particles are dispersed |
| Tyndall effect | Scattering of light by colloidal particles |
| Brownian movement | Random motion of colloidal particles |
| Electrophoresis | Movement of charged colloidal particles in an electric field |
| Electro-osmosis | Movement of dispersion medium relative to a charged surface |
| Coagulation | Aggregation and destabilization of colloidal particles |
| Dialysis | Removal of small ions through a semipermeable membrane |
| Micelle | Aggregate formed by association of surfactant molecules |
| Gel | Colloidal system with liquid immobilized in a solid network |
Colloidal chemistry deals with systems containing particles intermediate in size between true solutions and suspensions.
A colloidal system consists of a dispersed phase and a dispersion medium. Colloids can be classified according to the physical state of these two components and according to their affinity for the dispersion medium.
Important properties of colloids include the Tyndall effect, Brownian movement, electrical charge, electrophoresis and electro-osmosis.
Colloidal sols can be prepared by dispersion and condensation methods and purified by dialysis, electrodialysis and ultrafiltration.
Colloidal systems may undergo coagulation when their stability is disturbed. Electrolytes play an important role in coagulation, and the Hardy-Schulze rule relates coagulating power to the valency of the effective counter-ion.
Emulsions, gels, micelles and protective colloids are important examples of colloidal systems with extensive applications in biological, pharmaceutical, food and industrial chemistry.