Question

Write short notes on Surfactants and micelles.

Answer 1

Surfactants are interfacially active compounds. They consist of a polar head group and a non-polar hydrocarbon chain (see figure 1). The polar part of the molecule can interact strongly with polar solvents, like water, and is therefore also called the hydrophilic part. The non-polar part, on the other hand, can form strong interactions with non-polar solvents, like oil, and is therefore also called lipophilic or hydrophobic part.

Surfactants can be classified according to the charge of their polar head group:

anionic surfactants have a negatively charged head group

cationic surfactants have a positively charged head group

zwitterionic surfactants have a zwitterionic head group (positive and negative charge)

nonionic surfactants have an uncharged polar head group

Surfactants adsorb preferably at interfaces where they find the energetically most favourable conditions due to their two-part structure. At a water surface, for example, the surfactants orient themselves in such a way that the head group resides in the water and the hydrocarbon chain points to the gaseous phase (see figure 2). Thus surfactants can mediate between two phases as they can form strong interactions with both of them. The interfacial tension consequently decreases. The addition of surfactants hence facilitates the mixing of non-polar and polar phases, which is used in the detergent industry.

Answer 2

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In the bulk aqueous phase, surfactants form aggregates, such as micelles, where the hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the surrounding liquid. Surfactants reduce the surface tension of water by adsorbing at the liquid-air interface