What is Emulsion
An Emulsion is a thermodynamically unstable system consisting of two immiscible liquid phases, one of which is dispersed/ distributed as a globules/droplet size of 0.1-100 μm (internal or dispersed phase) in the other Liquid medium(External/ continuous phase) stabilized by a third substance called an emulsifying agent.
The diameter of the dispersed phase globules is generally in the range of about 0.1 to 10 μm, although it can be as small as 0.01 μm or as large as 100 μm. It is a Biphasic Liquid Dosage Form.
The process of formation of an emulsion is termed emulsification.
The two well known examples of emulsions are:
1) Milk in which the particles of the liquid fat are dispersed in water.
2) Cod liver oil emulsion in which water is dispersed in the oil.
- For Pharmaceutical emulsion, globules size is 0.1 – 10 μ.
- globules/ droplets/ dispersed phase ⇒ Known as internal/ discontinue phase
- dispersion medium ⇒ known as external phase/ continuous phase
Types of emulsions
- Multiple emulsion is also known as -emulsion within-emulsion and denotes as w/o/w & o/w/o
- Systemic emulsion: orally/intravenous ⇒ are mostly o/w type emulsion.
A. Emulsions typically consist of a polar (aqueous) and a relatively nonpolar (an oil) liquid phase. Based on the nature of the internal and/external phase, emulsions can be classified into two types.
1. Oil-in-water emulsion
When the oil phase is dispersed as globules throughout an aqueous continuous phase, the system is referred to as an oil-in-water (o/w) emulsion. The emulsifier is present in the external, continuous phase and helps stabilize the interface with the dispersed phase globules.
Example: Hydrophilic emulsifier, such as sodium lauryl sulfate, triethanolamine stearate, sodium oleate, and glyceryl monostearate are used.
2. Water-in-oil emulsion
When the aqueous phase is dispersed, and the oil phase is the continuous phase, the emulsion is termed as water-in-oil (w/o) emulsion. A lipophilic emulsifier is used for preparing w/o emulsions. The w/o emulsions are used mainly for external applications.
Example of emulsifiers: calcium palmitate, sorbitan esters (Spans), cholesterol, and wool fats. Use of a lipophilic emulsifier enables the formation of w/o emulsions with the oil phase as the external, continuous phase.
B. Based on Globules Size
Depends on particle/globules size of internal phase, pharmaceutical emulsions can be classified as
1. Micro emulsion
Micro-emulsion contain globules of size about 0.01 µm. droplets of such dimension cannot refract light and as a result globules are invisible to the naked eye. So microemulsion are transparent in appearance.
2. Fine emulsion
Fine emulsion have a milky appearance and the globules size range from 0.25 to 25 µm (micrometre).
C. Multiple emulsions
Multiple emulsions are emulsions whose dispersed phase contains droplets of another emulsion. They can be considered as emulsions within emulsions.
Multiple emulsions can be classified as the following
- Water-in-oil-in-water (w1/o/w2) emulsion
- Oil-in-water-in-oil (o1/w/o2) emulsion.
Emulsifying a w/o emulsion using water-soluble surfactants (which stabilize an oily dispersed phase) can produce w/o/w emulsions with an external aqueous phase, which generally has a lower viscosity than the primary w/o emulsion. Oil-in-water-in-oil (o/w/o) type multiple emulsions on the other hand consist of very small droplets of oil dispersed in the water globules of a water-in-oil emulsion.
Application of Multiple emulsion- Sustained release of Drug– the drug that is incorporated in the innermost phase must cross to face boundaries before getting absorbed.
Identification of emulsions
Identification of emulsion is done by following methods
1. Dye Solubility test:
- Water-soluble dye -Amaranth and Methylene blue-form uniform tint in O/W emulsion.
- Sudan III & Scarlet Red (oil-soluble dye)-form uniform tint in W/O emulsion.
2. Dilution test
- When a dispersion medium (i.E. External/continuous phase) is added to an emulsion, NO phase separation occurs.
- When water is added to O/W emulsion- no phase separation; if the oil is added to O/W emulsion- phase separation takes place.
- When the oil is added to W/O emulsion- no phase separation occurs; if the water is added to O/W emulsion- phase separation takes place.
3. Conductivity test
This test is based on the ability of water to conduct electricity. If water is the continuous phase (i.e. O/W), then the emulsion conduct electricity. if oil is the continuous phase (i.e W/O emulsion), the emulsion fails to conduct electricity.
4. Creaming test
- (direction of creaming upward or downward)
- W/O emulsion: normally cream downwards as well is usually less dense than water
- O/W emulsion: normally cream upwards
5. Other tests:
- Cobalt chloride test filter paper test (blue to pink, indicating that the emulsion is o/w type.) &
- Fluorescence test (fluorescence under microscope, then it is w/o type and if it shows only spotty fluorescence, then it is o/w type)
Preparation of emulsion/ Formulation of emulsion
To Prepare emulsion, there is a need for emulsifying agents and Formulation additives (Antioxidants, Preservatives, Humectants, Colors and flavorings agents).
An emulsion is prepared by shaking strongly the mixture of the two immiscible liquids or by passing the mixture through a mill known as the homogenizer. The emulsions thus prepared from the pure liquids are usually not stable and the two liquids separate out on standing. To get a stable emulsion, small quantities of certain other substances are added during preparation. The substances thus added to stabilize the emulsions are called emulsifiers or emulsifying agents. The substances commonly used as emulsifying agents are soaps of various kinds, long-chain sulphonic acids or lyophilic colloids like proteins, gum, and agar.
An emulsion is prepared by the following methods
|Type of oil||Oil:Water:Gum ratio|
Stability of Emulsion
- Dispersed droplets tend to fuse themselves and finally separated into two layers ( Since Cohesive force is greater than adhesive force).
- Cohesive force:- attractive force between the same type of molecules in liquids
- adhesive force:– force between unlike molecules
- If Cohesive force is more ⇒Coalescence of droplets ⇒ Separation of Phase. So Emulsifiers are used to prevent Coalescence/ regrouping of globules.
Emulsifying agent & Bankroft’s Rule
- Emulsions are the thermodynamically unstable system. The stability of the emulsion system can be increased by using an appropriate emulsifying agent.
- Emulsifying agent Stabilizes emulsion by preventing/Reducing the coalescence of dispersed globules.
- They act as Bridge between the polar and nonpolar phases and decrease the interfacial tension. ( emulsifying agent are amphiphilic in nature, have polar & nonpolar group).
- Emulsifier should have a balance between its hydrophilic and hydrophobic groups, produce a stable emulsion, be stable itself, be chemically inert, be non-toxic and cause no irritation on the application, be odorless, tasteless & colorless and be expensive.
- Emulsifying agents are classified into the following three groups.
- Describe the relationship between the nature of the emulsifying agent and type of emulsion formed.
- If a surfactant/emulsifying agent is more soluble in water, then the aqueous phase becomes continuous phase ⇒ O/W emulsion ⇒ eg. Tween, acacia, bentonite
- If oil-soluble emulsifier⇒ Nonaqueous phase becomes continuous phase ⇒ W/O emulsion ⇒ eg. Span
HLB (Hydrophilic-Lipophilic Balance for surfactant):
- Established by Griffin; Provide a scale of surfactant hydrophilicity; An emulsifier is a molecule with ambiphilic properties; Value range from 0 to 20.
- The relative solubility of the emulsifying agent in one of the phase is expressed by HLB scale.
- Emulsifier with high HLB value (8 to 16) ⇒ favor O/W emulsion
- Emulsifier with love HLB value (3-8) ⇒favor W/O emulsion
Physically Instability of Emulsion
- Neighboring globals come closer to each other ⇒ forms Flocs
- Re-Dispersed on shaking
- Prevented by imparting charges on globules and using uniform-sized globules
- Occurs Due to the density difference
- Upward creaming-O/W emulsion
- Downward creaming-W/O emulsion
- If Creaming occurs, Drug is not uniformly distributed ⇒ variable in dosage ⇒ emulsion should be shaken for only before use
- Globules tend to fuse with each other and forms bigger globules and ultimately lead to the breaking of the emulsion
- Occurs due to destruction of emulsifier film around the globules due to insufficient agent or by temperature change or By the presence of micro-organism, or by creaming
- Complete separation of aqueous and oil phases
- Irreversible step
5. Phase inversion:
- Change of emulsion type from O/W to W/O and vice versa