In pharmaceutical formulations, not every substance dissolves easily. Some remain as finely divided particles or droplets dispersed within another medium. These systems — known as coarse dispersions — include suspensions and emulsions, both of which are vital in designing stable, effective, and patient-friendly dosage forms.
This article delves into the fascinating world of coarse dispersions, examining their properties, formulation principles, and stability factors, while highlighting how science ensures that every bottle, cream, or lotion maintains its quality over time.
Download UNIT 3 – Coarse Dispersions Notes
Get simplified revision notes for this unit:
⬇️
Download Unit 3 Notes PDF
Understanding Coarse Dispersions
A coarse dispersion is a two-phase system where the dispersed particles have a size greater than 1 micron (1000 nm). These particles or droplets are large enough to scatter light and sometimes even be seen under a microscope. Unlike colloids, coarse dispersions tend to settle or separate over time, requiring careful formulation and stabilization.
The two major types of coarse dispersions are:
Suspensions – solid particles dispersed in a liquid medium.
Emulsions – liquid droplets dispersed in another immiscible liquid.
Suspensions: Solids in Motion
A suspension is a heterogeneous system consisting of insoluble solid particles distributed throughout a liquid phase. They are commonly used for oral, topical, and parenteral formulations where solubility is limited — such as antacids, antibiotics, and pediatric syrups.
Interfacial Properties of Suspended Particles
Particles in suspension possess a solid–liquid interface that determines their behavior. The key interfacial properties include:
Surface charge (zeta potential): Determines particle repulsion and stability.
Wettability: Influences how easily the solid disperses in the liquid.
Adsorption: Affects how surfactants or stabilizers attach to particle surfaces.
Controlling these properties helps prevent clumping and ensures uniform dispersion.
Settling in Suspensions
Over time, particles in a suspension tend to settle under gravity, a process influenced by size, shape, and density. The rate of settling is described by Stokes’ law, which states that smaller and lighter particles settle more slowly. Formulators often adjust viscosity or use suspending agents to slow down this process, improving uniformity and shelf life.
Flocculated vs. Deflocculated Suspensions
Flocculated suspensions contain loose aggregates (flocs) of particles that settle rapidly but form a non-caking sediment, which is easy to redisperse.
Deflocculated suspensions, on the other hand, contain individual particles that settle slowly but form a dense cake at the bottom, making redispersion difficult.
The goal in suspension formulation is to strike a balance — usually preferring a controlled flocculated system that combines easy redispersion with good physical stability.
Emulsions: Blending Oil and Water
An emulsion is a heterogeneous mixture of two immiscible liquids, one dispersed as droplets within the other, stabilized by an emulsifying agent. Depending on the phase distribution, emulsions can be:
Oil-in-water (O/W) – where oil droplets are dispersed in water (e.g., creams, lotions).
Water-in-oil (W/O) – where water droplets are dispersed in oil (e.g., cold creams, ointments).
Emulsions improve the bioavailability, texture, and aesthetic appeal of many pharmaceutical and cosmetic products.
Theories of Emulsification
Several theories explain how emulsifying agents stabilize these otherwise immiscible systems:
1. Surface Tension Theory
This theory suggests that emulsifiers reduce the interfacial tension between oil and water, allowing droplets to break and remain dispersed with less energy.
2. Oriented Wedge Theory
According to this concept, emulsifying agents form a monomolecular film around dispersed droplets. The film’s orientation (hydrophilic or lipophilic) determines whether the system becomes O/W or W/O.
3. Interfacial Film Theory
This theory focuses on the formation of a flexible, cohesive film around dispersed droplets, preventing coalescence and promoting stability. Natural gums, proteins, and synthetic surfactants act via this mechanism.
Microemulsions and Multiple Emulsions
Microemulsions
These are thermodynamically stable, transparent systems of oil, water, and surfactant (often with a cosurfactant). Droplet sizes are typically below 100 nm, giving them a clear appearance.
Microemulsions enhance drug solubility and absorption, making them useful for oral, topical, and parenteral delivery.
Multiple Emulsions
These are more complex systems such as W/O/W or O/W/O, where droplets contain smaller dispersed droplets within them. They are used for controlled drug release and protection of sensitive ingredients like vitamins or proteins.
Stability and Preservation of Emulsions
Like suspensions, emulsions are prone to instability. Common problems include:
Creaming: Upward movement of dispersed droplets due to density differences.
Coalescence: Fusion of droplets leading to phase separation.
Phase inversion: Change from O/W to W/O type or vice versa under certain conditions.
To maintain stability, formulators add emulsifiers, stabilizers, and viscosity enhancers. Proper storage temperature, pH control, and preservatives are also crucial to prevent microbial contamination and maintain product integrity throughout its shelf life.
Rheological Properties of Emulsions
Rheology plays a vital role in determining the texture, spreadability, and flow of emulsions.
Low-viscosity emulsions flow easily and are suitable for oral or injectable use.
High-viscosity emulsions (like creams) provide better coverage and are ideal for topical applications.
Understanding rheological behavior ensures that emulsions remain stable during handling, storage, and application.
Emulsion Formulation by the HLB Method
The Hydrophilic-Lipophilic Balance (HLB) system helps formulators choose the right surfactant or blend of surfactants.
Each emulsifying agent has an HLB value that reflects its affinity for water or oil.
Low HLB values (3–6) favor W/O emulsions.
High HLB values (8–18) favor O/W emulsions.
By calculating the required HLB of an oil phase and matching it with suitable surfactants, stable emulsions can be developed systematically — making the HLB method a cornerstone of modern emulsion formulation.