Surfactants find multitudes of usage in household detergents, personal care products, industrial cleaners and industrial processing.  One particular type, the nonionic surfactants, is subject to a formation of a new surfactant-rich phase usually at an elevated temperature called the cloud point.  The cloud point then, is the temperature at which a surfactant becomes insoluble in water as the sample is warmed.  This parameter is important, for example, when designing detergents for use in hot water.

Surfactant Phase Behavior

Surfactants may undergo several phase separations upon cooling and heating.  In some systems, phase separation occurs upon both cooling below and warming above the ambient temperatures.  Figure 1 is a phase plot of a detergent that begins as a single liquid phase at room temperature and separates into two phases at sub-ambient temperatures.  In this plot, the test began at point A, which represented a single liquid phase (low light scattering signal) at room temperature.  As the sample was cooled, the light signal remained low until phase separation occurred at point B, where light signal began to rise.  The light signal increased from B to C due to increasing amount of phase separation.  The sample was subsequently warmed from C to D, causing resolubilization.  Point D illustrates the temperature at which the sample returned completely to a single phase.

Figure 1:  Phase plot of a surfactant sample

Cloud Point Test of Shampoo and Detergents

The cloud point test of a detergent that displayed phase separation at temperatures above and below room temperature is shown in Figure 2.  In this case, point A was the initial temperature of the test, at which the sample was in a single liquid phase.  Point B is the clouding temperature upon cooling.  Point C is the temperature when the sample is clearing up during warming.  As the sample was warmed beyond room temperature, another phase separation occurred at point D.  Cooling of the sample at this stage leads to a return to a single phase at point E.

Figure 2:  Phase plot of a surfactant exhibiting multiple phase transitions

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