Dispersed System : Classification Based on Particle Size

A dispersed system, also known as a dispersion system, is a type of mixture where one substance is finely distributed or dispersed within another substance. In such systems, the dispersed phase (the substance that is dispersed) is typically made up of smaller particles or droplets that are distributed throughout the continuous phase (the substance in which the dispersed phase is distributed). The properties of dispersed systems can vary widely depending on the nature of the dispersed and continuous phases, as well as the method of dispersion.

Introduction to Dispersed System:

Dispersed systems are ubiquitous in nature and play a crucial role in various industrial, biological, and environmental processes. From everyday products like milk and paint to complex biological systems like cells and tissues, dispersed systems are encountered in numerous forms and applications.

The study of dispersed systems, known as colloid science or colloidal chemistry, focuses on understanding the behavior, properties, and interactions of colloidal particles and their host medium. Colloidal systems exhibit unique properties due to the large surface area-to-volume ratio of the dispersed phase, leading to phenomena such as Brownian motion, stability against gravitational settling, and sensitivity to external forces like electric fields.

Dispersed systems find applications across diverse fields, including pharmaceuticals, food science, cosmetics, materials science, and environmental engineering. Understanding the principles governing dispersed systems is essential for optimizing processes, developing new technologies, and solving challenges in various industries.

1. Disperse Phase:

   • The dispersed phase refers to the substance or component that is finely distributed or dispersed throughout the dispersion medium.
   • It exists in the form of smaller particles, droplets, or molecules suspended within the dispersion medium.
   • The properties and behavior of the dispersed phase can vary widely depending on factors such as particle size, shape, composition, and surface chemistry.
   • Examples of dispersed phases include solid particles in a colloid, oil droplets in water, or gas bubbles in a liquid.

2. Disperse Medium (Continuous Phase):

   • The disperse medium, also known as the continuous phase, is the substance or medium in which the dispersed phase is distributed.
   • It serves as the continuous matrix or bulk phase that surrounds and supports the dispersed phase.
   • The disperse medium provides a medium for the dispersion of the dispersed phase and often determines the overall properties and behavior of the dispersed system.
   • The disperse medium can be a liquid, solid, or gas, depending on the specific system. For example, in a foam, the continuous phase is typically a liquid or gas, while in a colloidal gel, it may be a solid.

Dispersed systems can be classified based on the particle size of the dispersed phase.

• Molecular Dispersions:
• Colloidal Dispersions (Colloids):
• Coarse Dispersions:

Molecular Dispersions:

• Description: In molecular dispersions, the dispersed phase consists of individual molecules that are dissolved in the dispersion medium at the molecular level.
• Range of Disperse phase particle size: Less than 1 nm
• Characteristics of System:
• Invisible in Electronic Microscope
• Disperse phase can pass through ultrafilter and semipermeable membrane
• Undergo Rapid diffusion
• Example: Sugar dissolved in water, Oxygen molecules in air.

Colloidal Dispersions (Colloids):

• Description: Colloids have dispersed phase particles with sizes between 1 nanometer (nm) and 0.5 micrometer (μm). These particles are larger than individual molecules but smaller than those in coarse dispersions.
• Range of Disperse phase particle size: 1 nm to 0.5 µm
• Characteristics of System:
• Invisible in ordinary Microscope
• May visible to ultramicroscopy and electronic microscope.
• Disperse phase can pass through filter paper
• Can not pass through semipermeable membrane
• Undergo very slow diffusion
• Subtypes:
• Sol: Solid particles dispersed in a liquid medium (e.g., ink).
• Gel: Liquid dispersed in a solid medium (e.g., jelly).
• Emulsion: Liquid droplets dispersed in liquid (e.g., milk).
• Example: Milk is an emulsion, where fat globules are dispersed in water.

Coarse Dispersions:

• Description: Coarse dispersions have larger particles, typically in the range of 0.5 micrometer (μm) to 1000 micrometers (μm) or even larger.
• Range of Disperse phase particle size: Greater than 0.5 µm
• Characteristics of System:
• Visible in ordinary Microscope
• Disperse phase do not pass through filter paper
• Do not pass through semipermeable membrane
• Do not diffusion
• Subtypes:
• Suspension: Solid particles dispersed in a liquid medium (e.g., orange juice with pulp).
• Aerosol: Liquid or solid particles dispersed in a gas medium (e.g., fog or smoke).
• Example: Paint is a suspension, where pigment particles are dispersed in a liquid medium.

Together, the dispersed phase and the dispersion medium interact to determine the overall characteristics, stability, and behavior of the dispersed system. Understanding the properties and interactions of these components is essential for studying and manipulating dispersed systems in various applications across industries such as pharmaceuticals, food science, materials science, and environmental engineering.