As we know oil and water are two immiscible liquids, but milk is an example of a mixture in which oil (fats) particles are suspended in water. These types of mixtures are known as emulsions. In this article we will discuss various emulsions, properties of emulsions, what is emulsification and its mechanism and much more related to emulsification. 

What is Emulsion? 

A mixture of two or more liquids that are normally immiscible is called Emulsion. Generally, students get confused between the terms - emulsion and colloid. The term emulsion is specifically used for mixtures in which dispersed phase and dispersion medium both are liquids. Although emulsion is a type of colloid. But it does not mean that all colloids are emulsions. In colloidal solution it is not necessary that both dispersed phase and dispersion medium will always be liquids. For example, aerosol (Examples – Clouds, Fog etc.) is also a colloidal solution in which dispersion medium is gas and dispersed phase is liquid. The term emulsion comes from Latin word emulgere which means to “to milk out”. 

Examples of Emulsions – Milk, mayonnaise, hand creams (lotions), Latex, cutting fluid, vinaigrettes etc. are examples of emulsions. 

What is Emulsification?

The process of formation of emulsions is called emulsification. In this process, one immiscible liquid is dispersed in another immiscible liquid. Thus, we can say to emulsify two immiscible liquids is called emulsification. For example, in oil – water cutting fluid emulsion used for metalworking is formed by emulsifying oil in the water medium.

Mechanism of Emulsification

Many different chemical and physical processes and mechanisms can be involved in the process of emulsification. Mechanism of emulsification can be based on following three theories –

  • Surface tension theory – According to surface tension theory, emulsification takes place by reduction of interfacial tension between dispersed phase and dispersion medium. 

  • Repulsion theory – According to repulsion theory repulsion force between the particles of the dispersed phase cause them to remain dispersed in the dispersion medium. The emulsifying agent makes a film over one phase which makes globules of that phase and these globules repel each other. 

  • Viscosity modifications – Some emulsifying agents increase the viscosity of the medium. Due to increase in the viscosity of the medium, globules of dispersed phase remain dispersed in dispersion medium. 

Properties of Emulsions 

Properties of emulsions are listed below –

  • Emulsions contain both a dispersed phase and a dispersion medium.

  • The boundary between the dispersion phase and dispersed medium is called “interface”. 

  • They have cloudy appearance.

  • They show various colors depending on the dilution. Such an emulsion appears white if it scatters the light equally. If it is diluted it will appear blue while if it is concentrated, then it will appear yellow. 

  • It shows the Tyndall effect. 

  • Particle size of dispersed phase in emulsions may vary.

  • Generally, emulsions are inherently unstable, exposure to energy and power ultrasound is needed to form a stable emulsion.

  • Emulsion particles form dynamic inhomogeneous structures on a small length scale. 

  • Both the phases of emulsion may get separated if they are kept undisturbed for a longer period of time or in absence of an emulsifying agent. 

Types of Emulsions 

Broadly, Emulsions can be divided into two types –

  • Simple Emulsions 

  • Complex Emulsions 

Simple Emulsions – Simple emulsions are those emulsions which are formed by either dispersing oil in water or water in oil. Simple emulsions can be divided into following two types –

  • Water in oil emulsion 

  • Oil in water emulsion 

Water in oil emulsion – If dispersed phase is water and dispersion medium is oil in the emulsion, then these types of emulsions are called water in oil emulsions. It is also called W/O types of emulsions. 

In these types of emulsions, water is an internal phase and oil is an external phase. Cold cream, butter etc. are examples of water in oil emulsions. 

Oil in water emulsion – If dispersed phase is oil and dispersion medium is water in the emulsion, then these types of emulsions are called oil in water emulsions. It is also called O/W types of emulsions.

Complex Emulsions – Complex emulsions are also called multiple emulsions. In these types of emulsions, a complex system exists in which both oil in water and water in oil emulsion exist together and are stabilized by surfactants. These can be divided into following types –

  • Water - in – oil – in - water emulsion

  • Oil – in – water – in – oil emulsion  

Water - in – oil – in - water emulsion – These are also called W/O/W emulsion. In these types of emulsions oil droplets enclosing water droplets are dispersed in water. These are actually double emulsions of O/W emulsion and W/O emulsion. 

Oil – in – water – in – oil emulsion - These are also called O/W/O emulsion. In these types of emulsions water droplets enclosing oil droplets are dispersed in the oil phase. These are actually double emulsions of O/W emulsion and W/O emulsion.

What is an Emulsifier?

Emulsifiers are also known as emulsifying agents or emulgent. The substance which is used in the emulsion to stabilize it by increasing its kinetic energy is called an emulsifying agent. These have more or less solubility either in oil or water as emulsifiers are those compounds which have polar or hydrophilic means water soluble part and a non – polar or hydrophobic (or lipophilic part) means water insoluble part. In oil - in - water emulsion hydrophilic emulsifiers are used while in water – in – oil emulsion, lipophilic or hydrophobic emulsifiers are used. 

Examples of Emulsifying Agents – Lecithin, soy lecithin, sodium phosphates, monoglycerides, diglycerides, sodium stearoyl lactylate etc. are examples of emulsifying agents. These are generally used as food emulsifiers. 

Instability of Emulsions 

Stability of emulsion refers to ability of emulsion to retain its properties. Following four types of instabilities occur in emulsions –

  • Flocculation – It is caused by aggregation of particles of dispersed phase. It is generally found in oil in water emulsions.

  • Coalescence – It is caused by increasing the average particle size of the discontinuous phase. It can be seen in fog. 

  • Creaming – It is caused by the influence of buoyancy or centripetal force. It can be found in dairy products when they are left for a long time. 

  • Ostwald Ripening – In this, change of an inhomogeneous structure over time takes place. It is generally found in water – in – oil emulsions. 

Uses of Emulsions

Emulsions are used in various ways in many fields. Emulsions are used extensively in the pharmaceutical field.  Few of its uses are listed below –

  • Oil in water emulsions are used in food industries. For example, vinaigrettes are made by suspending oil in vinegar or something acidic. 

  • Cutting fluid is a type of coolant or lubricant which is also an emulsion and used in metalworking processes. 

  • Mayonnaise, various sauces etc. are oil in water emulsions which are stabilized by lecithin. 

  • Margarine is also an emulsion which is used in the flavoring, baking and cooking.

  • Creams, ointments, balms etc. pharmaceutical products are emulsions only. Emulsions are frequently used in pharmaceutics. 

  • Many hair creams and gels are emulsions.

  • Emulsions are used in personal hygiene also. 

  • Emulsions are used in vaccines.

  • Nano - emulsions are used as disinfectants for surfaces. 

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