Question

Explain how chemical structures, temperature and electrolyte affect for both ionic and non ionic Critical Micelle Concentration.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 ). I need the critical parts on how all those interaction work. Thank in advance.

Answer 1

Micellisation: Attainment of minimum free energy state is the main reason for the formation of micelle. The hydrophobic regions at the surfactants are removed and highly ordered structure of the water molecules around this region of the molecule is lost. This increases the entropy of the system and this is the main driving force for the formation of micelle.

Following are the factors and the way the affect the CMC (Criticle Micelle Concentration):

A) Chemicas structure:

1) Structure of the hydrophobic group : a) Increase in length of the hydrocarbon chain causes Decrease in CMC. b) Hydrophilic groups : Non ionic surfactants have generally low CMC values and higher aggregation number than their ionic counter part with same hydrocarbon chain.Increase in ethylene oxide chain length of a non ionic surfactant makes the molecule more hydrophilic and CMC value increases.

B) Addition of an electrolyte:

i) Addition of an electrolyte decreases the CMC and increases the micellar size. The added elctrolyte reuces the repulsive forces between charged head groups at surface and this enhance the micelle formation.

ii) High concentration of an electrolyte may make the micelles of Ionic surfactant Non-Spherical.

C) Temperature:

i) Cloud point : The characterisytic temperature at which an aqueous solution of non-ionic surfactants became turbid is known as the Cloud temperature or cloud point.

ii) At temperature below the cloud point CMC decreases( i.e. micelle size increases)

iii) In Ionic surfactant case temperature is less important factor.

iv) Kraft temperature: This is the minimum temperature at which CMC is achieved. This is the minimum temperature of the solution at which aggregation is possible. Kraft temperature is depenedent of concentration of counter ions which are generally in their salt form.

4) Type of counter ion:

i) Micellaar size increases i.e. CMC decreases for cationic surfactant as the counter ion is changed in the order Cl- < Be- < I- .(i.e. with increase in counter anionic size)

ii) Micellar size increases i.e. CMC decreases for anionic surfactant as the counter ion is changed in the order Na+ < K+ < Cs+ (i.e. With increase in counter cationic size)

ii)