Question

Define what is Critical Micelle conentration and association of surfactant? Using your explanation explain how chemical structures, temperature and electrolyte affect for both ionic and non ionic CMC.Please explain as clearly as possible because I am having a tomorrow !(use the interaction between the hydrophobic and the hydrophilic tail to explain also )

Answer 1

It is the minimum concentration at of dispersed phase at which micelle formation starts.

. For soaps, the CMC is 10^–4 to 10^–3 mol l^–1. And its value is different for different micelles. micelle have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more.

Mechanism of micelle formation: Let us take the example of soap solutions. Soap is sodium or potassium salt of a higher fatty acid and may be represented as RCOO^–Na⁺ (e.g.,sodium stearate CH₃(CH₂)₁₆COO^–Na⁺, which is a major component of many bar soaps). When dissolved in water, it dissociates into RCOO^– and Na+ ions. The RCOO^– ions, however, consist of two parts — a long hydrocarbon chain R (also called non-polar tail) which is hydrophobic (water repelling), and a polar group COO^– (also called polar-ionic head), which is hydrophilic (water loving). The RCOO^– ions are, therefore, present on the surface with their COO^–groups in water and the hydrocarbon chains R staying away from it and remain at the surface. But at critical micelle concentration, the anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chains pointing towards the centre of the sphere with COO– part remaining outward on the surface of the sphere. An aggregate thus formed is known as ionic micelle. These micelles may contain as many as 100 such ions.

Cleansing action of soaps

It has been mentioned earlier that a micelle consists of a hydrophobic hydrocarbon – like central core. The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles. Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus soap helps in emulsification and washing away of oils and fats. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.

(a) Grease on cloth (b) Stearate ions arranging around the grease droplet and (c) Grease droplet surrounded by stearate ions (micelle formed)