Krafft temperature

The Krafft temperature, or critical micelle temperature, is the minimum temperature at which surfactants form micelles. Below the Krafft temperature, there is no value for the critical micelle concentration (CMC), i.e., micelles cannot form. The Krafft temperature is a point of phase change below which the surfactant remains in crystalline form, even in aqueous solution.

Surfactants in such a crystalline state will only solubilize and form micelles if another surfactant assists it in overcoming the forces that keep it crystallized, or if the temperature increases, thus causing entropy to have a stronger force and encouraging the crystalline structure to break apart.

Structural Effects
Surfactants are usually composed of a hydrocarbon chain and a polar head group.

Increasing the length of the hydrocarbon chain increases the Krafft temperature because it improves Van der Waals forces.