When we look around, many everyday materials — milk, fog, paints, and even blood — belong to a fascinating class known as colloidal dispersions. These systems lie between true solutions and coarse suspensions, displaying unique physical and chemical behaviors. Colloids play a vital role in pharmaceuticals, food science, and cosmetics, influencing texture, stability, and appearance.
This article explores the classification, properties, and behavior of colloidal dispersions in detail, offering a comprehensive look at how these tiny particles shape the world we interact with every day.
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Classification of Dispersed Systems
A dispersed system consists of two phases:
Dispersed phase – the substance distributed as fine particles.
Dispersion medium – the continuous phase in which the particles are distributed.
Based on particle size, dispersed systems are broadly classified into three categories:
True solutions – with particle sizes less than 1 nm (e.g., salt in water).
Colloidal dispersions – with particle sizes between 1 nm and 1000 nm (e.g., milk, gelatin solution).
Coarse suspensions – with particles larger than 1000 nm (e.g., sand in water).
Colloidal dispersions occupy the middle ground — small enough to remain uniformly distributed, yet large enough to exhibit visible effects like light scattering.
General Characteristics of Colloidal Systems
Colloidal dispersions exhibit distinct physical and chemical characteristics that set them apart from true solutions and suspensions.
Heterogeneous nature: Though colloids appear homogeneous to the naked eye, they are actually heterogeneous under a microscope.
Non-diffusibility: Colloidal particles do not pass through semi-permeable membranes.
Tyndall effect: When light passes through a colloidal solution, it is scattered by the particles, making the path of light visible.
Stability: Colloids are stable systems where particles do not settle down easily due to Brownian motion and surface charge.
These features make colloids ideal for applications requiring smooth dispersion, long shelf life, and consistent performance.
Size and Shape of Colloidal Particles
Colloidal particles range between 1 nanometer to 1 micrometer in diameter. Their size and shape strongly influence their behavior, appearance, and stability.
Spherical particles (like micelles and latex) are common in emulsions and suspensions.
Rod-shaped or fibrous colloids occur in materials like asbestos or cellulose fibers.
Lamellar (plate-like) colloids appear in clays and some layered silicates.
The smaller the particle size, the greater the surface area — leading to enhanced reactivity, adsorption, and interaction with the surrounding medium.
Classification of Colloids
Colloids can be classified based on several criteria:
1. Physical State of the Phases
Depending on whether the dispersed phase and dispersion medium are solid, liquid, or gas, colloids can take various forms such as:
Sol: Solid dispersed in a liquid (e.g., gold sol, paint).
Gel: Liquid dispersed in a solid (e.g., jelly, cheese).
Emulsion: Liquid dispersed in another liquid (e.g., milk, cream).
Foam: Gas dispersed in a liquid or solid (e.g., shaving cream, sponge).
Aerosol: Solid or liquid particles dispersed in gas (e.g., fog, smoke).
2. Nature of Interaction
Lyophilic colloids (solvent-loving) – easily formed and stable, e.g., gelatin, starch.
Lyophobic colloids (solvent-hating) – require special methods for preparation and are less stable, e.g., gold sol, sulphur sol.
3. Type of Particles and Composition
Multimolecular colloids – aggregates of smaller molecules (e.g., sulphur sol).
Macromolecular colloids – large molecules forming colloidal dimensions (e.g., starch, proteins).
Associated colloids (micelles) – behave as normal molecules at low concentration but form colloids at higher concentration (e.g., soaps, detergents).
Optical, Kinetic, and Electrical Properties of Colloids
Colloids exhibit several characteristic physical properties that help in their identification and study.
Optical Properties
The Tyndall effect is a hallmark of colloids, where light is scattered by the dispersed particles. This phenomenon distinguishes colloids from true solutions and forms the basis for instruments like the ultramicroscope.
Kinetic Properties
Due to their small size and constant bombardment by solvent molecules, colloidal particles show Brownian motion — a continuous, zigzag movement that helps prevent sedimentation and contributes to stability.
Electrical Properties
Colloidal particles often carry an electrical charge (positive or negative) which keeps them apart through electrostatic repulsion. This charge is determined by adsorption of ions on the particle surface. When an electric field is applied, particles move toward the oppositely charged electrode — a phenomenon known as electrophoresis.
Influence of Electrolytes and Other Phenomena
Effect of Electrolytes
The addition of electrolytes can neutralize the surface charge of colloidal particles, leading to coagulation or precipitation. The degree of coagulation depends on the valency of ions (as described by the Schulze–Hardy rule).
Coacervation
Coacervation refers to the separation of a colloidal system into two distinct liquid phases — one rich in colloid (coacervate) and the other poor in colloid. This process is crucial in microencapsulation and the formulation of controlled-release drug systems.
Peptization
Conversely, peptization is the process of converting a precipitate back into a colloidal sol by adding a suitable electrolyte, known as a peptizing agent. This occurs due to adsorption of ions on the solid surface, restoring charge and dispersion.
Protective Action
Certain lyophilic colloids act as protective agents for lyophobic colloids, preventing their coagulation. For instance, gelatin can protect gold sol from precipitation when electrolytes are added — a principle widely applied in pharmaceutical and food formulations to enhance stability.