Why do we care about surfactant precipitation? Because precipitated surfactant is surfactant that isn’t changing the properties of a surface or interface. We almost always want to eliminate precipitation from our surfactant systems. The only exception is when we are looking to remove the surfactant from solution, as might occur in waste water treatment or a separation process.

For ionic surfactants there is no property that is more important than the Krafft temperature of the surfactant. The Krafft temperature is the minimum temperature at which the solubility of the surfactant monomer becomes high enough for the surfactant to start to form micelles.  In general, surfactants are inactive below their Krafft temperature. Mixtures of surfactants can be used to lower the Krafft point of a surfactant formulation. This is a key approach to improving the performance of a formulation in both accelerated aging chambers and in freeze-thaw cycles. In our short course we will show you how to use this information to avoid a nasty surprise that surfactants have been known to spring on the unwary technologist: Salt shock! It’s not toxic shock, but it’s close!

Articles

• Modification of Krafft Temperature or Solubility of Surfactants Using Surfactant Mixtures

  Krafft temperatures of anionic/anionic, anionic/nonionic, and anionic/anionic/nonionic surfactant mixtures were studied at various surfactant compositions and salt concentrations. The Krafft temperature of a binary mixture of anionic surfactants can be less than that of any single surfactant. Addition of a nonionic surfactant to an anionic surfactant can cause further Krafft temperature depression. Addition of a monovalent counterion to each system decreases the critical micelle concentration (CMC), but increases the Krafft temperature of anionic surfactants or their mixtures. A simple model based on the pseudo-phase separation approach can successfully predict the mixture Krafft temperatures based on pure surfactant behavior and interactions in mixed micelles. Predictions from the model are very sensitive to the solubility products and CMC parameters.

• Kinetics of Precipitation of Surfactants I - Anionic Surfactants with Calcium and Cationic Surfactants

  The question is often asked, "How do we know if surfactant precipitation is going to occur?" A closely related question is, "How can we know if it is going to happen in the time frame of my application?" These two papers answer both questions.

• Kinetics of Precipitation of Surfactants II - Anionic Surfactant Mixtures

  Continuing from part one, precipitation kinetics were measured for calcium-induced precipitation of mixtures of two anionic surfactants.

• Kinetics of Precipitation III: Atomic Force Microscopy of Precipitate Crystals

  The overall time required for precipitation to occur can increase dramatically for surfactant mixtures compared to single components. Atomic force microscopy (AFM) has been used in this study to examine the precipitated surfactant crystals, which gives insight into both the thermodynamics and kinetics of surfactant precipitation.

Recommended Reading

• Surfactant Solubility, R.G. Laughlin, in Handbook of Detergents, G. Broze, Ed., Marcel Dekker, New York, Ch. 4, 2000

• Anionic-Cationic Surfactant Mixtures, A. Mehreteab, in Handbook of Detergents, G. Broze, Ed., Marcel Dekker, New York, Ch. 5, 2000.

• Precipitation of Surfactant Mixtures, J.F. Scamehorn, and J.H. Harwell, in Mixed Surfactant Systems, K. Ogino and M. Abe, Eds., Marcel Dekker, New York, Ch. 10, 1993

• An Overview of Phenomena in Mixed Surfactant Systems, J.F. Scamehorn, in Phenomena in Mixed Surfactant Systems, J.F. Scamehorn, Ed., American Chemical Society, Washington, Chapter 1, 1986