Nonionic Surfactant, Water, and Oil

From the phase behavior of both binary mixtures (water-amphiphile and oil— amphiphile), it is now possible to account, at least qualitatively, for the three-component phase diagram as a function of temperature. The presence of a haze point on the oil-amphiphile phase diagram (critical point a) at temperature Ta shows that the surfactant is more compatible with the oil at high than at low temperature. The presence of a cloud point on the water-amphiphile phase diagram (the lower critical point ( ) at temperature Tjj shows that (at least in the neighborhood of the temperature domain) the amphiphile is less compatible with water at high than at low temperature. As a consequence (the other parameters being kept constant), the amphiphile behavior depends on temperature. 

15 Examples of water-ethoxylated nonionic amphiphile binary phase diagrams. (FromBroze, G., Comm. J. Com. Esp. Deterg., Barcelona, 20, 133,1989. With permission.) 

16 Evolution of water-ethoxylated nonionic amphiphile-oil ternary phase diagrams with temperature (temperature increasing from a to c). 

If the temperature at which the phase diagram is recorded is above Ta (the haze point), a critical point CPa is present near the oil vertex (although pure amphiphile and pure oil are miscible, the presence of a small amount of water recalls the lack of compatibility between amphiphile and oil). If the temperature is below Ta, no critical point appears in the three-component phase diagram (it would be positioned at a negative water concentration). 

In WI and WII representations, the critical points CPp and CP are called plait points. If the difference between the temperature T at which the phase diagram is recorded and the critical point of the binary mixture, 7p or Ta, increases, the distance from the plait point to the oil-amphiphile axis for CPp and the water-amphiphile axis for CP also increases. An important characteristic of a ternary system is the line that links the plait points as a function of temperature. The plait point curve is really the trace of the partitioning of the amphiphile between oil and water. The closer the plait point is to the oil, the more water soluble the amphiphile, and vice versa. 

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Figure 4. Generic phase prism of a nonionic surfactant, water, and oil system. Reprinted with permission from Kahlweit et at. [11],...

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