Colloidal Dispersions

The term "colloid" is derived from the Greek word kolla, meaning "glue," and refers to a system where tiny particles are dispersed within a continuous medium. Colloidal dispersions represent a crucial and diverse category in chemistry, existing between true solutions and coarse suspensions. They are fundamentally biphasic systems, consisting of a dispersed phase (the particulate matter) scattered throughout a continuous phase (the dispersion medium, usually a liquid).

📏 Classification of Dispersed Systems by Particle Size

Dispersed systems are categorized based on the diameter of the dispersed particles. Colloidal dispersions fall squarely in the middle, exhibiting properties of both extremes.

Type of Dispersion	Particle Diameter	Key Characteristics	Examples
Molecular	Less than $1 \text{ nm}$	Invisible even with an electron microscope; passes all filters; rapid diffusion; particles do not settle.	Ordinary ions, Glucose, Salt solutions
Colloidal	$1 \text{ nm}$ to 0.5 um	Visible only with an ultramicroscope; passes filter paper but not semipermeable membranes; settles slowly via ultracentrifugation; gradual diffusion.	Colloidal silver sols, Natural polymers (Gums, Proteins), Synthetic polymers
Coarse	Greater than 0.5u m	Visible under a standard microscope; held back by all filters; settles quickly due to gravity; does not diffuse.	Emulsions, Suspensions, Red blood cells

🌈 General Characteristics of Colloidal Dispersions

The unique size range of colloids gives them distinctive properties that are exploited in various industries, particularly pharmacy.

1. Specific Surface Area

When compared to an equal volume of larger particles, colloidal particles have an immense specific surface area (surface area per unit weight or volume).

Impact: This large surface area dramatically enhances surface phenomena such as adsorption and catalysis. For example, Platinum is only effective as a catalyst in its colloidal form because the large surface area can efficiently adsorb reactant molecules.

2. Color and Particle Morphology

The color of a colloidal dispersion is dependent on two factors:

Particle Size: As particle size decreases, it scatters shorter wavelengths of light. For example, a red gold solution (spherical particles) changes its color to blue as the particle size increases (often becoming disc-shaped).
Particle Shape: The form of the particle (e.g., spherical, rod, flake, thread, or ellipsoidal) is determined by the production method and the affinity for the dispersion medium. Morphology affects the system’s dynamics, including sedimentation, osmotic pressure, and flow.

3. Surface Charge and Stability

Colloidal particles carry a net surface charge, which may be positive or negative.

Mechanism: The surface of the particle is charged, while the particle's interior is neutral. The charge arises from the selective adsorption of ions or the ionization of surface groups.
Stability: This surface charge is crucial for the stability of colloids because the resulting electrostatic repulsion between similarly charged particles prevents them from aggregating and settling.
Examples:

Positive (+) Charge: Gelatin, Metal ions (e.g., metal sols).
Negative (-) Charge: Acacia, Tragacanth (natural gums).

💉 Pharmaceutical Applications of Colloidal Systems

The unique properties of colloids—specifically their small size, large surface area, and stability—make them invaluable in medicine and drug formulation.

Application	Description and Benefit
Enhanced Absorption	Due to their small size, colloidal drugs are often readily absorbed, leading to better bioavailability and action. E.g., Colloidal sulfur is used for treating sulfur deficiency because its small particles are quickly absorbed.
Targeted Drug Delivery	Colloidal systems are engineered as carriers to transport drugs directly to a specific organ or site, reducing systemic toxicity.
Drug Carriers	Systems like liposomes, niosomes, nanoparticles, and microemulsions are drug-entrapped colloidal carriers often targeted toward organs like the liver and spleen.
Therapeutic Agents	Certain elements historically used in medicine are effective in their colloidal form due to high surface activity: Colloidal Silver (germicidal), Colloidal Copper (anticancer), Colloidal Mercury (anti-syphilis).
Excipients and Vehicles	Colloidal systems are used as stabilizing agents, thickening agents, and vehicles in the formulation of various dosage forms due to their inherent stability characteristics.