Change of Phase Behaviour with Temperature

In Winsor s type 1 systems (r 1) the affinity of the surfactant for the water phase exceeds its affinity for the oil phase. Thus, the interface will be convex towards water. Davis used a triangular diagram to show that a type 1 nSOW system can be one or two phases. A system in the two-phase region will split into an oil phase containing dissolved surfactant monomers at CMC (critical micelle concentration in the oil phase) and an aqueous microemulsion—a water phase containing solubilised oil in normal surfactant micelles. 

In Winsor s type 3 systems (r = 1), the surfactant s affinity for the oil and the water phases is balanced. The interface will be flat. A type 3 nSOW system can have one, two or three phases depending on its composition. In the multiphase region the system can be (a) two phase—a water phase and an oleic microemulsion (b) two phase—an oil phase and an aqueous microemulsion (c) three phase—a water phase containing surfactant monomers at CMC, an oil phase containing surfactant at CMC and a surfactant phase . The surfactant phase may have a bicontinuous structure, being composed of cosolubilised oil and water separated from each other by an interfacial layer of surfactant. The surfactant phase is sometimes called the middle phase because its intermediate density causes it to appear between the oil and the water phases in a phase-separated type 3 nSOW system. 

One way of altering the surfactant affinity in an nSOW system is by changing the temperature. This change in temperature will change the surfactant s affinity for the two phases. At high temperature the nonionic surfactant becomes more 

With a similar system, Saito and Shinoda showed that the solubilisation of oil into an aqueous micellar solution phase increased rapidly as the type 3 region is approached. Around the type 3 region, as the temperature rose the system moved from two phases (0 and D, where D = surfactant phase), to three phases (O, W and D) and then back to two phases (W and D). With further rise in temperature, water solubilised in the oleic micellar phase was released to the water phase. Saito and Shinoda also showed that the interfacial tension changed during the transition from type 1 to type 2 phase behaviour. In the three-phase region the interfacial tension is ultra low this has important consequences for the drop sizes and stability of emulsions of type 3 systems. 

---