Article by: Kelly Dobos

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.

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3 comments

  1. all

    hi ,thanks for you kind info.
    cloud point extraction is come after cloud points?
    best regards
    inran

  2. Kelly
    Kelly

    Hi Ken,
    Thanks for the information, I haven’t seen this phenomena mentioned anywhere in the literature I have regarding cloud point. Do you have a reference that I can review and add to the piece? I’d love to include more information on the topic. – Kelly

  3. Ken klein

    Actually your answer regarding cloud point is only part of the answer! In addition to kinetic energy causing vibration which breaks hydrogen bonding, the shape of the ethoxylate changes from a helix where the oxygens are readily available for bonding to an open structure where stearic hinderance plays a role and the opportunity for hydrogen bonding is vastly decreased. Actually, as Bob Lochhead explains it (he and I used to teach a course in advanced emulsions) this stearic effect is more responsible for the cloud point effect.

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