Colloids: Properties, Classification, and Pharmaceutical Applications

Colloids and Disperse Systems

Introduction to Disperse Systems

Disperse systems are mixtures where one substance is distributed throughout another. The three main types are solutions, colloids, and suspensions. Each type is characterized by the size of its dispersed particles and its physical properties.

• Solution: Homogeneous mixture with very small particles (ions or molecules).

• Colloid: Mixture with intermediate particle size, appearing homogeneous but actually consisting of two phases.

• Suspension: Heterogeneous mixture with large particles that may settle out over time.

Properties of Solutions

• Particles are ions or molecules, typically < 1 nm in size.

• Transparent and do not scatter light.

• Stable; do not separate on standing.

• Cannot be filtered by ordinary filters.

Properties of Suspensions

• Particles are > 1000 nm in size.

• Heterogeneous; particles may settle out (sedimentation).

• Can be filtered.

• Examples: blood, muddy water.

Properties of Colloids

• Particle size: 1–1000 nm.

• Appear homogeneous to the naked eye but are actually microheterogeneous.

• Cannot be filtered by ordinary filters.

• Exhibit unique properties such as the Tyndall effect and Brownian motion.

• Examples: milk, blood, paints, detergents, fog.

Colloidal Systems

Phases in Colloidal Systems

A colloidal system consists of two phases:

• Dispersed phase: The particles (1–1000 nm) distributed throughout the medium.

• Dispersion medium: The substance in which the particles are dispersed (often a solvent).

Note: The colloid is the whole system, not just the particles or the solvent alone.

Classification by Phase

Colloids can be classified based on the physical state of the dispersed phase and the dispersion medium.

Dispersion Medium	Dispersed Phase	Type of Colloid	Example
Solid	Solid	Solid sol	Ruby glass
Solid	Liquid	Solid emulsion/gel	Fruit cheese
Solid	Gas	Solid foam	Pumice
Liquid	Solid	Sol	Paints, cell fluids
Liquid	Liquid	Emulsion	Milk, oil in water
Liquid	Gas	Foam	Soap, whipped cream
Gas	Liquid	Aerosol	Fog, mist

Properties of Disperse Systems

	Heterogenous Dispersion (Suspensions)	Colloid Dispersion	True Solution
Pass through membranes	-	Semipermeable does not pass	+
Visibility of particles	Eye, optical microscope	Electron microscope	-
Sedimentation	+	Ultracentrifugation	-
Thermal motion	Small	Middle	High

Properties of Colloids

Tyndall Effect

The Tyndall Effect is the scattering of light by colloidal particles. When a beam of light passes through a colloidal solution, the path of the light becomes visible due to scattering by the particles. The intensity of scattered light is proportional to the concentration of colloidal particles.

• Used to distinguish colloids from true solutions.

• Example: Mixture of water and milk scatters light, while a true solution does not.

Brownian Motion

Brownian motion refers to the random movement of colloidal particles caused by collisions with molecules of the dispersion medium. This motion prevents the particles from settling out under gravity.

• Helps maintain colloidal stability.

• Observed under a microscope.

Classification of Colloids

Based on Interaction w

ith Dispersion Medium

• Lyophilic colloids (solvent loving): Particles have a strong affinity for the solvent, leading to stable and reversible sols. Example: gelatin in water.

• Lyophobic colloids (solvent hating): Particles do not have affinity for the solvent, resulting in unstable and irreversible sols. Example: gold sol.

• Association colloids (amphiphiles): Molecules with both hydrophilic and hydrophobic regions, such as surfactants, which can form micelles at higher concentrations.

Lyophilic vs. Lyophobic Colloids

Lyophilic	Lyophobic
Colloidal particles love their dispersed medium	Colloidal particles hate their dispersed medium
Sols of organic substances like gelatin, gum, starch, proteins	Sols of inorganic substances like As2S3, Fe(OH)3, Pt
Easy to prepare	Difficult to prepare
More stable	Unstable, need stabilizers
Viscosity higher than medium	Viscosity same as medium

Association/Amphiphilic Colloids

Amphiphiles (surface active agents) have both hydrophilic and hydrophobic regions. At low concentrations, they exist as individual molecules; at high concentrations, they aggregate to form micelles.

• Critical micelle concentration (CMC): The concentration at which micelles form.

Colloidal Stability

Stable vs. Unstable Colloids

Colloidal stability refers to the ability of particles to remain dispersed in the medium over time. Stable colloids do not aggregate or settle, while unstable colloids may undergo flocculation, coagulation, and sedimentation, leading to phase separation.

• Stability is influenced by particle charge, solvation, and the presence of stabilizing agents.

Pharmaceutical Applications of Colloids

Emulsions

Emulsions are colloids where liquid particles are dispersed in another liquid. They require emulsifying agents (amphiphiles) to stabilize the mixture.

• Example: Milk (oil in water emulsion), mayonnaise (egg yolk as emulsifier).

Gels

Gels are colloids of large molecules (lyophilic) that adsorb the dispersion medium, forming a semi-solid or rigid structure.

• Example: Gelatin, starch.

Sols

Hydrophobic colloids can be stabilized by wetting agents to improve their dispersibility.

Detergents

Detergents are association colloids (soaps) that help disperse oily dirt into water by forming micelles.

• General formula for soap: $\mathrm{C_{17}H_{35}COO^- Na^+}$

• Long hydrocarbon chain is oil-soluble; carboxylate group is water-soluble.

Soaps act as emulsifying agents, allowing greasy dirt to be suspended in water for cleaning.

Additional info: Colloidal systems are essential in pharmaceutical formulations for drug delivery, stability, and bioavailability. Understanding colloidal properties aids in designing effective medicines and therapeutic agents.