The Krafft temperature (also known as Krafft point, or critical micelle temperature) is the minimum temperature at which surfactants form micelles. It is named after German chemist Friedrich Krafft. 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. Visually the effect of going below the Krafft point is similar to that of going above the cloud point, with the solution becoming cloudy or opaque due to the surfactant molecules undergoing flocculation.
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.
Moreover, since Krafft point is related to solid-liquid transition, better-packed polar heads within surfactant crystals increase Krafft temperature.