Emulsions and gels

Introduction to surface chemistry

Surface chemistry is a branch of chemistry that deals with the study of phenomena that occur at the surfaces or interfaces of substances. It is an important field because the surface of a substance often has different properties from its bulk. The study of surface chemistry includes various topics such as adsorption, catalysis, colloids, emulsions, and gels. Of these, emulsions and gels are essential as they have many applications in daily life and industry.

What are emulsions?

Emulsions are mixtures of two immiscible liquids, typically oil and water, where one liquid is dispersed in the other. They are unstable systems; therefore, they require the presence of an emulsifying agent to maintain stability. There are two types of emulsions:

• Oil in water (o/w) emulsions: Here, oil droplets are dispersed in a continuous water phase. Examples include milk and mayonnaise.
• Water-in-oil emulsions: These contain water droplets continuously dispersed throughout the oil phase. Examples include butter and margarine.

Visual example

The figure given below shows a simple representation of oil in water and water in oil emulsion:

Emulsifying agent

Emulsifying agents or emulsifiers are substances that stabilize emulsions. They are amphoteric molecules, meaning they contain both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. This allows them to position themselves at the interface of two immiscible liquids, reducing the surface tension and preventing dispersed droplets from merging together.

Examples of emulsifying agents

Some common emulsifiers include:

• Lecithin: Found in egg yolks, it is widely used in food products such as mayonnaise.
• Sodium stearoyl lactylate: Used in bakery products.
• Monoglycerides and diglycerides: Commonly used in processed foods.

Properties of emulsions

Emulsions have specific properties that distinguish them from simple mixtures:

• Appearance: Emulsions are usually opaque or sometimes translucent, caused by the scattering of light by the dispersed droplets.
• Stability: Without emulsifiers, emulsions tend to separate over time. However, with the right emulsifiers, they can remain stable for longer periods of time.
• Viscosity: Emulsions often have a higher viscosity than the individual components. This property is used to improve texture in many food and cosmetic applications.

Applications of emulsions

Emulsions are used in a variety of areas due to their specific properties:

• Food industry: Emulsions are important in products such as salad dressings, ice cream, and sauces, where texture and shelf stability are important.
• Pharmaceutical industry: Emulsions are used in lotions, creams, and ointments for drug delivery and application to the skin.
• Cosmetic industry: They are used in creams, lotions and shampoos to provide a specific sensory experience.
• Paints and coatings: Many paints are emulsions, which offer a balance between durability and ease of application.

What are gels?

Gels are a type of colloidal system where a solid network is dispersed throughout a liquid. They exhibit properties of both solids and liquids. Gels have a three-dimensional network that traps the liquid, giving them semi-solid characteristics. This network is formed by chemical or physical bonds between the molecules of the dispersed phase.

Features of gels

Gels have unique characteristics that make them useful in a number of areas:

• Elasticity: Gels have elastic properties, meaning they can be deformed under stress, but return to their original shape when the stress is removed.
• Viscosity: Gels have a higher viscosity than their liquid components, causing resistance to flow.
• Swelling: Many gels can absorb liquid, thereby swelling to a larger size. This property is useful in many medical and agricultural applications.
• Thixotropy: Some gels are thixotropic, meaning that they become less viscous when stirred or agitated and return to a gel-like state when relaxed.

Types of gels

Gels can be classified based on the type of interactions that hold the network together:

• Physical gels: These are formed through physical bonds such as hydrogen bonds, van der Waals forces, or the trapping of particles. An example of this is gelatin, which becomes a gel when cooled.
• Chemical gels: These are formed through covalent bonds between polymers. They are generally more stable and less reversible than physical gels.

Visual example

The figure below shows the structure of the gel network:

Applications of gels

Gels are ubiquitous in a variety of applications due to their unique properties:

• Medical applications: Gels are used in wound dressings, drug delivery systems, and ultrasound gels for diagnostic procedures.
• Personal care products: Gels are found in shampoos, hair gels, and skin care products due to their texture and spreadability.
• Food industry: Gels such as gelatin and agar are used in desserts and jellies for texture and flavour.
• Agriculture: Hydrogel materials are used to retain soil moisture and slowly deliver nutrients to plants.

Comparison between emulsions and gels

Although both emulsions and gels are colloidal systems, they have clear differences:

Property	Emulsion	Gels
Composition	Mixture of two immiscible liquids	Trapping of fluids in solid networks
Stability	Emulsifier is required	May be stabilized through physical or chemical bonds
Appearance	Opaque or translucent	Translucent or transparent
Stickiness	Medium to high	High
Example	Milk, mayonnaise, cream	Jelly, gelatin, hair gel

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