Effect of inorganic additives on solutions of nonionic surfactants IV: Krafft points.

Two polydisperse polyoxyethylated nonionic surfactants were found to possess Krafft points as well as cloud points. The significance of the Krafft point and the reason why it is rarely observed in nonionic surfactants are discussed. The values of the Krafft and cloud points of the two surfactants underwent only small changes as a function of surfactant concentration in the range of 0.5-7.5% (w/w). All electrolytes investigated, as well as urea, raised the Krafft points by between 1 and 4 degrees in the concentration range of 0.5-4 molal. Included were salting-in electrolytes, which raised the cloud points, as well as salting-out electrolytes, which lowered them. Electrolytes that salt out nonionic surfactants strongly may depress the cloud point to the Krafft point temperature, rendering a water-soluble surfactant insoluble in salt solutions at all temperatures.