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DrX Whiz Niraj
Biphasic Liquids: Emulsions
Complete exam notes on Emulsions. Learn Classification, Identification Tests, Preparation Methods, Emulsifying Agents, and Stability Problems (Cracking/Creaming).
Definition: An Emulsion is a thermodynamically unstable biphasic liquid dosage form consisting of two immiscible liquids (usually oil and water). One liquid is dispersed as minute droplets into the other liquid, stabilized by the addition of a third substance called an Emulsifying Agent (Emulgent).
Oil-in-Water (O/W) Emulsion
Oil is the dispersed phase (droplets) and Water is the continuous phase (medium).
- Generally preferred for internal (oral) use to mask the bitter taste of oils.
- Non-greasy and easily washable with water.
- Example: Milk, Vanishing creams.
Water-in-Oil (W/O) Emulsion
Water is the dispersed phase (droplets) and Oil is the continuous phase (medium).
- Generally preferred for external use to prevent evaporation of moisture from the skin.
- Greasy and not easily washable.
- Example: Butter, Cold creams.
Since O/W and W/O emulsions look physically similar (milky white), the following tests are performed to identify the continuous phase.
| Test Name | Procedure & Principle | O/W Emulsion Result | W/O Emulsion Result |
|---|---|---|---|
| 1. Dilution Test | Add a few drops of water to the emulsion. If water is the continuous phase, it will mix perfectly. | Remains stable (Dilutes easily). | Breaks / Separates into layers. |
| 2. Conductivity Test | Water is a good conductor of electricity, oil is not. Pass electric current through the emulsion. | Bulb glows (Continuous phase is water). | Bulb does NOT glow. |
| 3. Fluorescence Test | Some oils fluoresce under UV light. | Spotty fluorescence (Oil is in droplets). | Continuous fluorescence (Oil is the medium). |
Emulsifying agents (Emulgents) reduce the interfacial tension between oil and water and form a protective mechanical film around the droplets to prevent them from coalescing (joining together).
- Natural Agents: Acacia, Tragacanth (Form O/W emulsions). Wool fat, Beeswax (Form W/O emulsions).
- Synthetic Surfactants: Tweens (High HLB, O/W) and Spans (Low HLB, W/O).
- Finely Divided Solids: Bentonite, Magnesium hydroxide.
Methods of Preparation (The 4:2:1 Rule)
To prepare a stable emulsion on a small scale, a Primary Emulsion must first be formed using the correct ratio of Oil : Water : Gum.
- Fixed Oils (Almond, Castor, Arachis oil) = 4 : 2 : 1
- Mineral Oils (Liquid Paraffin) = 3 : 2 : 1
- Volatile Oils (Peppermint, Cinnamon oil) = 2 : 2 : 1
Dry Gum Method: Oil and Gum are triturated first, then water is added all at once.
Wet Gum Method: Water and Gum are triturated first to form mucilage, then oil is added dropwise.
A stable emulsion should remain uniformly dispersed. However, being thermodynamically unstable, they exhibit the following physical instabilities over time:
| Instability Type | Description | Is it Reversible? | How to Overcome? |
|---|---|---|---|
| 1. Creaming | The upward (or downward) movement of dispersed droplets to form a concentrated layer at the surface (or bottom). It follows Stokes' Law. | Yes (By gentle shaking) | Increase the viscosity of the continuous phase or reduce particle size using a homogenizer. |
| 2. Cracking (Breaking) | The complete and permanent separation of the emulsion into two distinct layers of oil and water. The protective emulsifier film is destroyed. | No (Irreversible) | Use appropriate emulsifying agent quantity, avoid extreme temperatures, and avoid adding incompatible electrolytes. |
| 3. Phase Inversion | An O/W emulsion suddenly turns into a W/O emulsion, or vice versa. | Sometimes | Keep the dispersed phase volume strictly below 74%. Avoid sudden temperature changes. |
Viva Question Alert!
Question: What causes Phase Inversion?
Answer: The most common reason for an emulsion to invert is when the concentration of the dispersed phase exceeds 74% of the total volume. The droplets get packed so tightly that they break the continuous phase and become the medium themselves. Another reason is the addition of an electrolyte (like CaCl₂) that reacts with an O/W soap (Sodium stearate) to form a W/O soap (Calcium stearate).