Emulsifying agents (emulsifiers) are surface-active ingredients which adsorb at the newly formed oil–water interface during emulsion preparation, and they protect the newly formed droplets against immediate recoalescence. Surfactants can act as emulsifiers, which lower surface tension between liquids or between a solid and liquid. Surfactants can keep droplets from getting large enough for components to be able to separate based on density.
Structurally surfactants have two parts in their chemical structure, a hydrophilic head (water-loving) and a lipophilic tail (water-hating). They can form a protective barrier around the dispersed droplet by adsorbing on the oil-water interface and stabilizing the emulsion by decreasing the interfacial tension of the system. Some surfactants impart a charge on the surface of the drop, which reduces the physical contact between the droplets and decreases the coalescence potential of the droplet, which in turn increases the stability of the emulsion.
Based on the nature of the internal and/external phase, emulsions can be classified into different types, oil-in-water emulsion, water-in-oil emulsion, and multiple emulsions (including oil-in-water-in oil and water-in-oil-in-water emulsions).
In water-in-oil (W/O) emulsions, the water is the dispersed (discontinuous) phase, and the oil is the dispersion medium (continuous phase). In these emulsions, lipophilic surfactants, such as calcium palmitate, sorbitan esters (Spans), cholesterol, and wool fats, are used, as they enable the formation of W/O emulsions with the oil phase as the external, continuous phase. The W/O emulsions are used mainly for external applications.
In oil-in-water (O/W) emulsions, the dispersed (discontinuous) phase is oil, and the dispersion medium (continuous phase) is water. In these emulsions, hydrophilic surfactants, such as sodium lauryl sulfate, triethanolamine stearate, sodium oleate, and glyceryl monostearate are used. The surfactants are present in the external, continuous phase and help stabilize the interface with the dispersed phase globules.
In multiple emulsions, the drops of the dispersed phase themselves contain smaller dispersed droplets of a miscible liquid. They are often prepared according to a two-step process and commonly stabilized using a combination of hydrophilic and hydrophobic surfactants.
Every dispersed globule in the double emulsion makes a vesicular structure with single or multiple watery compartments divided from the aqueous phase via an oil phase layer. There are two important types of multiple emulsions, O/W/O and W/O/W double emulsions. In cosmetics, they can protect an active ingredient from degradation and release it at a controlled rate.
There is no way we can do without surfactants in our daily lives. Whether washing dishes, cleansing your face or doing a load of laundry, surfactants make cleaners work better.
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