Experience might lead us to think that as temperature increases so does solubility or rate of solubilization. But nonionic ethoxylated surfactants exhibit a reverse solubility referred to as cloud point. As the temperature of an aqueous solution containing an ethoxylated surfactant is increased the solution becomes hazy as phase separation occurs.
Why do surfactants get hazy?
Ethoxylated surfactants are soluble in water due to their ability to participate in a special dipole interaction called hydrogen bonding. Hydrogen bonds are typically stronger normal dipole interactions and are responsible for the unique properties of water. As the temperature of a solution of these surfactants in water increased, the kinetic energy of the system is increased and molecular motion increases. The intensity of the molecular motions eventually overcomes the intermolecular hydrogen bonding forces decreasing solubility. As the degree of ethoxylation increases, so does the cloud point (Table 1).
Cloud point and cosmetic chemists
Cleaning and wetting properties are often optimized just below the cloud point. Cloud points can also be utilized in the Phase Inversion Temperature (PIT) method of emulsification. We’ll talk about this method of formulating in another post.