Emulsion

Emulsion is a biphasic liquid disperse system where the dispersed phase is composed of small globules of a liquid distributed throughout in an immiscible vehicle. In emulsion terminology, the dispersed phase is referred to as the discontinuous or internal phase, and the dispersion medium is the continuous or external phase. The diameter of the dispersed phase globules generally ranges from about 0.1 to 10 μm2. Emulsions are thermodynamically unstable and are usually stabilized by the presence of an emulsifier. The process of formation of an emulsion is termed emulsification.

Table of Contents

In this article we will see classification of emulsions, emulsifying agents, methods of preparation and stability problems.

Classification of emulsion

Emulsions can be classified into several types based on the nature of the dispersed phase and the dispersion medium. Here are the main types of emulsions with examples.

Oil-in-Water (O/W) Emulsion: In this type of emulsion, oil droplets are dispersed in a water phase. It is often found in moisturizing lotions and liquid foundations. Milk is a notable example of an O/W emulsion, where fat globules (dispersed phase) are present in water (dispersing medium).

Water-in-Oil (W/O) Emulsion: In a water-in-oil emulsion, water droplets are dispersed in an oil phase. This type of emulsion is commonly used in skincare products like creams and lotions. Butter and margarine are examples of W/O emulsions.

Multiple Emulsions: These are more complex systems that involve a combination of the two basic types of emulsions. They include,

Water-in-Oil-in-Water (W/O/W) Emulsion: Here, an initial W/O emulsion is dispersed in a water phase.
Oil-in-Water-in-Oil (O/W/O) Emulsion: In this case, an initial O/W emulsion is dispersed in an oil phase.

Emulsifying agents

Emulsifying agents, also known as emulsifiers or emulgents, are substances that help stabilize emulsions. They are composed of both oil-soluble hydrophobic (nonpolar) and water-soluble hydrophilic (polar) portions. These agents act as a bridge between the internal (dispersed) and external (continuous) phases of an emulsion, preventing the droplets of the internal phase from merging together.

Emulsifiers work by reducing surface tension and forming a protective layer around the dispersed droplets, which prevents them from coalescing or merging together. This helps maintain the stability of the emulsion.

Here are some examples of emulsifying agents.

Lecithin: It’s a natural emulsifier found in egg yolks and soybeans.
Soy Lecithin: It’s derived from soybeans and is commonly used in food products.
Diacetyl Tartaric Acid Ester of Monoglyceride (DATEM): It’s a synthetic emulsifier often used in bread making.
Mustard: Mustard contains a natural emulsifier that helps in the formation of stable emulsions.
Sodium Stearoyl Lactylate: It’s a versatile, FDA approved food additive used as an emulsifier, dough strengthener, and more.
Sodium Phosphates: These are used in a variety of food products for their emulsifying properties.
Mono- and Diglycerides: These are commonly used in food products as emulsifiers3.
Cellulose: It’s used in food and pharmaceutical industries as an emulsifier.

Tests for identification of type of emulsion

There are several tests that can be conducted to identify the type of an emulsion. Here are some of them.

Dilution Test: All emulsions can be easily diluted with their continuous phase or dispersion medium. If an emulsion is easily diluted with water, it indicates that the emulsion is oil-in-water (O/W). If it’s easily diluted with oil, it indicates that the emulsion is water-in-oil (W/O).
Conductivity Test: When current is passed to an emulsion connected to a voltage bulb, the bulbs glow if it is an O/W emulsion since water is a good conductor of electricity. If the bulb does not glow, it is a W/O emulsion because oil is a non-conductor of electricity.
Dye Test: The water-soluble dye will dissolve in the aqueous phase whereas the oil-soluble dye will dissolve in the oil phase. For example, Amaranth for O/W Emulsion, Scarlet/Sudan for W/O Emulsion.
Fluorescent Test: Oils give fluorescence under UV light, while water does not. Therefore, O/W emulsion shows a spotty pattern while W/O emulsion fluoresces.
Cobalt Chloride Test: A filter paper soaked in cobalt chloride (CoCl) solution and allowed to dry, turns blue to pink on evaporation to O/W emulsion.
Filter Paper Test: O/W emulsion should spread out rapidly when dropped onto filter paper, in contrast, W/O will migrate slowly.

These tests help in identifying the type of emulsion and are crucial in many industries, including food, cosmetics, and pharmaceuticals.

Methods of preparation of emulsion

Emulsions can be prepared using various methods. Here are some of the common methods.

Dry Gum Method

In this method, the oil is first triturated with gum with a little amount of water to form the primary emulsion. The trituration is continued till a characteristic ‘clicking’ sound is heard and thick white cream is formed. Once the primary emulsion is formed, the remaining quantity of water is slowly added to form the final emulsion.

Wet Gum Method

As the name implies, in this method first gum and water are triturated together to form a mucilage. The required quantity of oil is then added gradually in small proportions with thorough trituration to form the primary emulsion. Once the primary emulsion has been formed remaining quantity of water is added to make the final emulsion.

Bottle Method

This method is employed for preparing emulsions containing volatile and other non-viscous oils. Both dry gum and wet gum methods can be employed for the preparation. As volatile oils have a low viscosity as compared to fixed oils, they require a comparatively large quantity of gum for emulsification. In this method, oil or water is first shaken thoroughly and vigorously with the calculated amount of gum. Once this has emulsified completely, the second liquid (either oil or water) is then added all at once and the bottle is again shaken vigorously to form the primary emulsion. More water is added in small portions with constant agitation after each addition to produce the final volume.

Electrical Method

In this method, a colloid mill or a similar device is usually used to pass through a mixture of two liquids along with an emulsifier. Shearing forces are used to break down one of the liquids in the emulsion into small globules due to the presence of the emulsifier.

Nascent Soap Method

This method involves the use of nascent soap as the emulsifying agent.

The choice of method depends on the nature of the materials to be emulsified, the required stability of the emulsion, and the scale of production.

Stability problems involved in emulsions

Emulsions can face several stability problems. Here are some of them along with methods to overcome these issues.

Coalescence: This occurs when droplets merge to form larger ones. To prevent coalescence, emulsifiers are used to reduce the droplet sizes of the emulsions and enhance the emulsion stability.
Sedimentation: This happens when droplets settle at the bottom of the container over time. Regular shaking or stirring can prevent sedimentation.
Creaming: This is the upward movement of droplets. Anti-creaming agents can be used to prevent this.
Phase Inversion: This is a process where an oil-in-water (O/W) emulsion changes to a water-in-oil (W/O) emulsion or vice versa. The use of suitable emulsifying agents can prevent phase inversion.
Chemical Instability: The drug must remain chemically stable over the intended shelf-life of the product.
Microbial Growth: Microorganisms can grow in emulsions, leading to spoilage. The use of preservatives can prevent microbial growth.

Methods to overcome stability problems

Selection of Suitable Emulsifying Agents: The choice of emulsifying agent can greatly affect the stability of the emulsion.
Control of Droplet Size: The size of the droplets in the emulsion can be controlled through methods like homogenization.
pH Adjustment: Adjusting the emulsion’s pH can affect the droplets’ charge and the system’s stability.
Temperature Control: The stability of the emulsion can be affected by temperature changes.
Use of Stabilizers: Stabilizers can enhance the kinetic stability of emulsions.
Use of Multiple Emulsions: Multiple emulsions can be used to improve the stability of the emulsion.

Applications of emulsions

Drug Delivery: Emulsions, including macroemulsions and microemulsions, are well-documented carriers for both hydrophilic and lipophilic drugs. They can be used to deliver drugs in a controlled and targeted manner.
Taste Masking: Oils and drugs with objectionable taste or texture can be made more palatable for oral administration by formulating them into emulsions.
Nutrient Administration: Emulsions can be used for the administration of nutrients.
Diagnostic Agents: Emulsions can be used as vehicles for diagnostic agents.
Topical Applications: Topical creams and lotions are popular forms of emulsions for external use.
Multi-Drug Therapy: Complex emulsions can address the challenges of multi-drug therapy and polypharmacy. Microfluidic technology and complex emulsion fabrication can create highly efficient drug delivery vehicles.
Blood Plasma Substitutes: Oil-in-water (O/W) microemulsion systems with hydrophobic fluorocarbons (as oils) are used to produce short-time blood plasma substitutes to maintain the supply of oxygen in living systems.

Summary

Emulsions are mixtures of two immiscible liquids, typically oil and water, stabilized by an emulsifying agent. They can be classified as oil-in-water (O/W), where oil droplets are dispersed in water, or water-in-oil (W/O), where water droplets are dispersed in oil. More complex systems, like multiple emulsions (W/O/W or O/W/O), also exist. Emulsions have diverse applications in the pharmaceutical industry, including drug delivery, taste masking, nutrient administration, and as vehicles for diagnostic agents.

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