Question

What is CMC and Krafft’s temperature?

Answer 1

Hint: Surfactants are usually composed of a hydrocarbon chain (hydrophobic in nature) and a polar head group. Krafft’s temperature and CMC are both related to each other since for a temperature below the Krafft’s temperature, the micelles will not form.

Complete step-by-step solution:
> In order to understand CMC and Krafft’s temperature, we first need to understand surfactants and how they function. Surfactants function by decreasing the surface tension between two liquids, a gas and a liquid, or a liquid and a solid. Surfactants are used as detergents, wetting agents, emulsifiers, foaming agents, and as dispersants. So how do the surfactants lower the surface tension? Since a surfactant contains both an oil soluble component and a water-soluble component, the water-insoluble hydrophobic group extends out of the bulk water phase, into the air or into the oil phase such that the water-soluble head group remains in the water phase. In the bulk aqueous phase, aggregates called micelles are formed. In the micellar structure, the hydrophobic tails of the surfactant form the core of the micelle and the hydrophilic heads are in contact with the surrounding liquid.
> These aggregates can be spherical or cylindrical. The chemical structure of the surfactants determines the shape of the aggregate. Now that we have explained how the surfactants function, let us look into the types of surfactants. Generally surfactants are classified in accordance with their polar head group.
> Non-ionic surfactant: no charged groups in its head.
> Ionic surfactant: the head carries a net positive or negative charge. If the charge is negative, the surfactant is called anionic; on the other hand if the charge is positive, it is called cationic.
> Zwitterionic: the surfactant contains a head with two oppositely charged groups.

Critical micelle concentration (CMC):
Micelles can be formed only when the concentration of the surfactant is more than the critical micelle concentration (CMC). Its value is dependent upon temperature, pressure, concentration of any other surfactant present for a particular surfactant in a given medium.

Krafft Temperature:
When the solubility of a surfactant becomes equal to its critical micelle concentration, then the temperature at which this takes place is called Krafft’s temperature, named after the German chemist Friedrich Krafft. If the temperature is below the Krafft temperature, the maximum solubility of the surfactant will be lower than the critical micelle concentration, due to which micelles will not form. This happens because below the Krafft temperature the surfactant remains in crystalline form, even in aqueous solution but at Krafft’s temperature a phase change of the surfactant takes place. If we go below the Krafft point, the solution will become muddy due to the surfactant molecules undergoing flocculation.

Note: If the length of the hydrocarbon chain increases then Krafft temperature also increases because Van der Waals forces become strong. Krafft point is related to solid-liquid transition; therefore better-packed polar heads within surfactant crystals increase Krafft temperature.