Colloidal Dispersion, Classification of Dispersed System

Colloidal Dispersion: It is a heterogeneous mixture of particles dispersed in a liquid continuous phase. The size of dispersed solid particles is in between nanometer and micrometer, normally 1 nanometer to 1 micrometer. These particles are insoluble but individual particle is not visible to the naked eye. These particles are called colloidal particles or colloids.

Colloidal particles remain suspended in the liquid medium but do not sediment under the influence of gravity. They remain suspended in the liquid medium due to Brownian motion, electrostatic repulsion, or steric hindrance effect. They are found in milk, paints, and pharmaceutical products like Iron dextran injection, iron sorbitol injection, etc.

Classification of Dispersed Systems:

Classification of the dispersed phase is based on various criteria. The following are classifications of dispersed systems.

A. Based on the Nature of Dispersed Phase: There are two types of dispersed systems on the basis of dispersed phase.

• 1. Liquid Dispersion: Liquid droplets are dispersed in another liquid or gas medium. Liquid droplets are the dispersed phase (Internal Phase) and liquid or gas medium is the dispersion medium (Continuous phase or External phase). Examples are oil in water emulsion (O/W Emulsion) and water in oil emulsion (W/O Emulsion).

• 2. Solid Dispersion: Solid particles are dispersed in a liquid or gas medium. Solid particles are the dispersed phase (Internal Phase) and liquid or gas medium is the dispersion medium (Continuous phase or External phase). Examples are Suspensions.

B. Based on the Nature of Dispersion Medium:

• 1. Gas Dispersion: Gas is dispersed either in a liquid or another gas. Example foam. The foam contains gas dispersed in a liquid. Aerosol. Aerosol contains gas or liquid dispersed in a gas.

• 2. Liquid Dispersion: Liquid is dispersed either in liquid or gas. Example emulsion and mist (Liquid droplets dispersed in gas).

C. Based on Particle Size:

• 1. Colloidal dispersion: It contains particle sizes ranging from 1 nanometer to 1 micrometer.

• 2. Coarse Dispersion: It contains insoluble particles ranging from 1 micrometer to 1000 micrometer. These particles sediment under the influence of gravity. But they remain suspended for a longer duration.

D. Based on Nature Interaction Between Phases:

• 1. Lyophobic Dispersions: Dispersed phase and dispersion medium have little affinity with each other. Stabilizing agents like surfactants or colloidal protectants are used to prevent coalescence or coagulation.

• 2. Lyophilic Dispersions: Dispersed phase and dispersion medium have a strong affinity for each other. It does not require a stabilizing agent.

E. Based on Preparation Methods:

• 1. Mechanical Dispersion: Grinding, milling, or shearing force is used to reduce particle size and its dispersion in a medium.

• 2. Chemical Dispersion: A chemical reaction is carried out to produce finely divided particles. These chemical reactions may be the precipitation process or condensation process.

F. Based on Application:

• 1. Pharmaceutical Dispersions: Dispersion systems are used to deliver drugs to the body. Examples are suspension, emulsion, and liposomes.

• 2. Food and Beverages Dispersions: Several food products are available as dispersion. Examples mayonnaise, dairy products, salad dressings, etc.

• 3. Paints and Coating

• 4. Environmental Dispersion: Aerosol and colloids are used in air pollution and soil chemistry.

Alok Bains