Micelles or associated collids

There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibits colloidal behavior due to the formation of aggregates. The aggregated particles thus formed are called micellies. these are also known as associated colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Examples of such type of substances are soap and detergents. For soap, the CMC is 10^-4 to   10^-5 mol/L. These collids have  both lyophobic and lyophilic parts. Micelle may contain as manyas 100 molecules or more. 

Mechanism of Micelles formation:

It can be explain by taking an example of soap. Soap is sodium salt of higher fatty acid and may repersented as RCOONa, e.g sodium stearate viz. CH₃(CH₂)₁₆COO^-Na⁺  or sodium palmitate viz. 

CH₃(CH₂)₁₄COO^-Na⁺  . When we dissolve it in water, it dissociates in to RCOO^-  and Na⁺ ions. The RCOO^- ions consists two parts i.e., non-polar long hydrocarbon chain (R) called the tail, which is hydrophobic (water repelling) and the polar  COO^-  called the head, which is hydrophilic i.e water loving. The RCOO^-  ions are therefore present on the surface of water with its COO^-  group in water  and hydrocarbon chains R staying away from water and remain at the surface.

However, at higher concentration, these ions do not remain on the surface but pulled into the bulk of the solution. As a result, at higher concentration, the RCOO^-  ions form an aggregate of spherical shape with their hydrocarbon chains pointing toward the centre and COO^-  part outwards on the surface of the sphere. An aggregate thus formed is known as "ionic micelle".

(http://eng.thesaurus.rusnano.com/upload/iblock/383/micelle1.jpg)