Question

why does micelle formation take place when soap is added to water? will micelle be formed in other solvents such as ethanol also?

Answer 1

Soap molecule has two ends. One end is hydrophilic and another end is hydrophobic. When soap is dissolved in water and clothes are put in the soapy solution, soap molecules converge in a typical fashion to make a structure; called micelle. The hydrophobic ends of different molecules surround a particle of grease and make the micelle; which is a spherical structure. In this, the hydrophilic end is outside the sphere and hydrophobic end is towards the centre of the sphere. This is why micelle formation takes place when soap is added to water.
Since ethanol is not as polar as soap, so micelles will not be formed in other solvents such as ethanol.

Answer 2

A soap is a sodium or potassium salt of long chain fatty acids. A soap molecule has two parts namely the non-polar (hydrocarbon) hydrophobic chain and polar (ionic) hydrophilic head, COO-Na+.

The long hydrocarbon chain being hydrophobic is insoluble in water but solublein oil and grease. On the other hand, the ionic part of soap being hydrophilic is soluble in water but insoluble in oil and grease.

When soap is added to water, soap molecules arrange them in a cluster to keep the non-polar hydrophobic tail are towards the interior of the cluster and the polar hydrophilic head towards water. Since the dirt present on clothes is organic in nature and insoluble in water, the hydrophobic ends of the clusters attach themselves to the dirt. This cluster formation in which the dirt is entrapped is the micelle. The micelles stay in solution as a colloid and does not come together to precipitate because of ion-ion repulsion.

Micelle formation does not occur in alcohol because the alkyl chain of soap becomes soluble in alcohol.