Emulsion - definition, properties, and examples

An emulsion is a mixture consisting of two or more liquids that normally do not mix well together or don't mix at all, but one is dispersed over another. The process of forming an emulsion by mixing  two or more different liquids is called emulsification. 

Emulsions are generally classified as colloids involving a mixture of two or more immiscible liquids. In an emulsion, one liquid (the dispersed phase) is dispersed in the other (the continuous phase). The dispersed phase is a liquid in form of tiny droplets, crystals or particles with ultramicroscopic size, scattered and distributed throughout another phase of liquid, called the continuous phase.

The word 'emulsion' is gotten from the Latin word for "to milk", as milk is an emulsion of fat and water together with other components. The most common example of an emulsion is a mixture of water and oil. If you pour some oil into a cup of water, instead of the two liquids mixing together, the oil will just float on top of the water. Even if you try to stir them together, you'll notice that the oil and water will quickly separate again, forming two layers (or phases). Now if you beat up an egg and mix it with the oil and water, you'll also notice that the oil and water aren't separating into layers, but stays together. How is this possible? An emulsion is a mixture in which the substances combined are not soluble with each other or are immiscible. But the substances will not mix or stay mixed with each other without the help of what we called an 'emulsifier'. In the water and oil emulsion, the egg is the emulsifier.

TYPES OF EMULSIONS

Two liquids can form different types of emulsions. As an example, oil and water can form, first, an oil-in-water emulsion, wherein the oil is the dispersed phase, and water is the dispersion medium. Second, they can form a water-in-oil emulsion, wherein water is the dispersed phase and oil is the external phase. Multiple emulsions are also possible, including a "water-in-oil-in-water" emulsion and an "oil-in-water-in-oil" emulsion. Thus, we have two types of emulsions.

1. Oil in water(O/W) emulsion: In this type of emulsion, the oil will be the dispersed phase and water will be the dispersion medium. The best example for o/w emulsion is Mayonnaise. Mayonnaise is an oil-in-water (O/W) emulsion with a dispersed oil phase and a continuous water phase containing egg, vinegar, salt, sugar, mustard and water. The oil-water interface is stabilized by egg yolk, which acts as an emulsifier. Milk is also a perfect example. In milk, the fat globules (which act as the dispersed phase) are suspended in water (which acts as the dispersion medium).

2.Water in Oil(W/O) emulsion:  In this type, water will be the dispersed phase and oil will be the dispersion medium. Margarine (a spread used for flavoring, baking and working) is an example of water in oil emulsion.

DEFINITION OF EMULSIFIERS/EMULSIFYING AGENTS

Emulsifiers (or emulsifying agents or emulgents) are substances that stabilizes an emulsion or helps to keep an emulsion from separating. Below are images of certain mixtures showing before and after emulsification.

mayonnaise before and after adding an emulsifying agent

oil and vinegar emulsion before and after adding an emulsifying agent

PROPERTIES OF EMULSIONS

• Emulsions are fluid systems of tiny liquid droplets dispersed in another liquid.

• Emulsions are composed of two phases of heterogeneous liquid mixtures, known as the dispersed phase and the continuous phase. A boundary exist between the phases called the 'interface.

• The dispersed phase contains tiny droplets/particles which may be amorphous liquid crystalline, or any mixture thereof, having a diameter ranging from approximately 10nm to 100µm. 

• Emulsions, being colloids, do exhibit the tyndall effects by scattering light rays passing through it. That is why emulsions do tend to have a cloudy or whitish appearance especially when the light is scattered in equal proportion. This is as a result of the many phase interfaces scattering light passing through the emulsion. The more concentrated the emulsion is, the more higher-frequency (low-wavelength) light will be scattered more.

• The physical properties of emulsions do differ from that of it components. For example, while water and oil are transparent, emulsions are usually opaque and it can sometimes be design to have a lustrous and pearlized appearance. Also, while water and oil (for example) are thin free flowing liquids, emulsions can be made as think creams, that do not flow (like ice creams).

• Some emulsions due to presence of water lead to corrosion of the containers in which they are stored. An example is the containers used in paints. Packing of paints used to be in coated tin containers which were easily corroded. Plastic containers are in vogue now, largely avoiding the tin containers and the problem of corrosion associated with it.

• Emulsions can be converted into separate liquids by process of heating, centrifuging, freezing etc. This process of reconverting an emulsion into it components liquids is know as dimulsification.

EXAMPLES OF EMULSIONS

Some common examples of emulsions in everyday life includes:

•  Milk
•  hair creams
•  ice creams
•  paintings
•  mixture of oil in water and vice versa
•  Ointments
•  margarine
•  yogurts
•  asphalts etc.

USES/APPLICATIONS OF EMULSIONS

1. In medicines/pharmaceuticals : Lots of pharmaceutical products like some special body creams, soaps, toothpaste etc are in form of emulsions used for treatments of external disorders.

         Pharmaceutical emulsions are also used to deliver drugs that possess a low aqueous solubility. They can also be used to mask the taste of therapeutic agents, in which the drug is dissolved in the internal phase of an oil-in-water emulsion. Then the external phase then be formulated to contain the appropriate sweetening and flavoring agents.

2. In food productions: Several food products and soft drinks are considered as emulsions. Examples are: Milk, ice creams, yogurts, margarine, butter, Vinaigrette etc.

mayonnaise serves as sauce for salad and other food products

3. In Sprays: Sprays like insecticides, pesticides, and fungicides employ the use of emulsions as a medium for delivery. They are usually supplied in form of 'emulsifiable concentrate' (a solution of the pesticide in an organic solvents). This method helps in both farmland and livestock agriculture.

4. In cosmetics : In cosmetics, emulsions are the delivery vehicle for many hair and skin conditioning agents. Anionic and nonionic emulsions are used to deliver various oils and waxes that provide moisturization, smoothness, and softness to hair and skin. Emulsions formed with cationic emulsifiers are themselves effective conditioning agents since their positive charge is attracted to the negative sites on the hair, thus allowing them to resist rinse off.

5. In paintings: Many paints and inks are based on emulsions. Such products may be true liquid-in-liquid emulsions or they may be dispersion. Dispersion are similar to emulsions except that the dispersed phase is usually divided finely into solid particles. The same surfactant technology used to formulate emulsions is used to create dispersion of pigments that are used in paints and inks. These dispersion are designed to dry quickly and form waterproof films, while not affecting the color. In this regard emulsions provide benefits over solvent containing systems because of reduced odor and flammability.