Cloud point (surfactant)
- Also known as
- cloud point, clouding temperature
The cloud point of a surfactant is the temperature at which a solution of a nonionic surfactant, such as an alcohol ethoxylate, turns cloudy on heating as the surfactant becomes less water-soluble and separates into a second phase.
What is the cloud point of a surfactant?
Most substances dissolve better as temperature rises. Many nonionic surfactants do the opposite. Their head groups, often chains of ethylene oxide units, are held in solution by hydrogen bonds with water. As the solution warms, those bonds weaken, the surfactant becomes less water-soluble, and at a certain temperature the solution separates into two phases and turns visibly cloudy. That temperature is the cloud point. The standard test method for nonionic surfactants is ASTM D2024.
This is a different property from the cloud point of crude oil, which is another name for its wax appearance temperature.
What affects the cloud point
- Head group size. Surfactants with longer ethylene oxide chains are more water-soluble and have higher cloud points.
- Salts. Dissolved salts generally lower the cloud point, so a surfactant that is clear in fresh water can separate in brine; see total dissolved solids.
- Other ingredients. Anionic surfactants and some solvents raise the cloud point; other additives lower it.
Why it matters
- Formulation stability. A product stored above its cloud point may separate.
- Performance. Nonionic surfactants often clean well and foam little near their cloud point, a property used in some industrial and machine cleaners.
- Oilfield and hot applications. In hot, salty reservoirs and processes, a surfactant used above its cloud point may separate from the water and lose effectiveness, so cloud point is checked at the actual temperature and salinity.
Frequently asked questions
Heat weakens the hydrogen bonds between water and the surfactant’s head groups, so the surfactant becomes less soluble and separates into a surfactant-rich phase.
Generally not in the same way. Ionic surfactants usually become more soluble with temperature, and instead have a Krafft point, below which they come out of solution as crystals.
Sources
- ASTM D2024, Standard Test Method for Cloud Point of Nonionic Surfactants, ASTM International
- Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012