what are micelles?? and explain its formation?

When soap molecules are present in water, the molecules arrange themselves in the form of a cluster in such a manner that their hydrophobic ends are away from the water molecules, and their hydrophilic or ionic ends are towards the water molecules (as shown in the following figure). This is known as micelle formation and the cluster that is formed is called a micelle.

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To understand this question you should understand the principle of solubility first. There are generally two type of solvent- ionic and organic. And according to principle of solubility, “like dissolves like” that is ionic substance used to get dissolved in ionic and organic in organic. There are many substances that lie between ionic and organic. And, micelles are formed when a molecule having both lypophilic (hydrophobic) and lipophobic (hydrophilic) end go into water. In simple language you can tell hydrophobic (hydro means water and phobia- fear of something) molecules run away from water while hydrophilic (hydro means water and philia means love for something) goes towards water. You can see in the figure below that CH3 ends (hydrophobic end) of sodium stearate is running away and hiding in the circle whereas COO- end (hydrophilic end) is facing the water outside the circle.

Definition of micelle-

Micelle, in physical chemistry, a loosely bound aggregation of several tens or hundreds of atoms, ions (electrically charged atoms), or molecules, forming a colloidal particle—i.e., one of a number of ultramicroscopic particles dispersed through some continuous medium. Micelles are important in the chemistry of surfaces—e.g., the power of soap solutions to disperse organic compounds insoluble or only slightly soluble in water is explained as a property of micelles.

Formation of micelle-

Micelles are formed by specific molecules which have lyophilic as well as lyophobic ends. Ordinary soap which contains sodium stearate (C17H35COONa) forms micelle in water. The stearate ion has a long hydrocarbon end that is hydrophobic (because it is nonpolar) and a polar carboxyl group (COO-) that is hydrophilic.

 

 

CH3CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2COO-

Hydrophobic  Hydrophilic

 

When the concentration of sodium stearate is below its CMC, then it behaves as a normal electrolyte and ionizes to give Na+ and C17H35COO- ions. As the concentration exceeds the CMC, the hydrophobic end starts receding away from the solvent and approach each other. However, the polar COO- part interacts with water. This leads to the formation of a cluster having the dimensions of a colloid particles. In each cluster a large number of stearate groups clump together in a spherical manner such that the hydrocarbon parts interact with one another and the COO- groups remains projected in water.

 

 

For fig: cut and paste this address in your address bar-

 

 

http://www.tutorvista.com/content/chemistry/chemistry-iv/surface-chemistry/micelle-formation.php

 

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