Phase Inversion Temperature PIT Principle

Phase inversion in emulsions can be one of two types Transitional inversion induced by changing factors that affect the HLB of the system, e.g. temperature and/or electrolyte concentration, and catastrophic inversion, which is induced by increasing the volume fraction of the disperse phase. 

The diameter decreases and the rate constant increases as inversion is approached. To apply the phase inversion principle one uses the transitional inversion method demonstrated by Shinoda and co-workers [11, 12] when using ethoxylate-type nonionic surfactants. These surfactants are highly dependent on temperature, becoming lipophilic with increasing temperature due to the dehydration of the poly(ethylene oxide) chain. When an O/W emulsion prepared using a nonionic surfactant of the ethoxylate type is heated, then, at a critical temperature (the PIT), the emulsion inverts to a W/O emulsion. At the PIT the droplet size reaches a minimum and the interfadal tension also reaches a minimum. However, the small droplets are unstable and they coalesce very rapidly. By rapid cooling of the emulsion that is prepared at a temperature near the PIT, very stable, small emulsion droplets can be produced. 

At low temperature, over the Winsor I region, 0/W macroemulsions can be formed and are quite stable. On increasing the temperature, the O/W emulsion stability decreases and the macroemulsion finally resolves when the system reaches the Winsor III phase region (both O/W and W/O emulsions are unstable). At higher temperature, over the Winsor II region, W/O emulsions become stable. 

The minimum in y can be explained in terms of the change in curvature H of the interfadal region, as the system changes from O/W to W/O. 

For O/W systems and normal micelles, the monolayer curves towards the oil and H is given a positive value. For a W/O emulsions and inverse micelles, the mono-layer curves towards the water and H is assigned a negative value. At the inversion point (HLB temperature) Hbecomes zero and y reaches a minimum. 

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FIGU RE 4.4 Illustration of the preparation of a finely divided O/W emulsion from a coarse W/O emulsion using a heating-cooling cycle and the phase inversion temperature (PIT) principle. (From Forster [36]. Reprinted with permission of Blackwell Publishing Ltd.)... 

Production of nano-emulsions using (a) High-pressure homogenizers and (b) the phase inversion temperature (PIT) principle. 

Figure 12.6 Principle of the phase inversion temperature (PIT) method (Repr uced by permission of Dekker from ref 22)...

Figure 8.8 Principle of the PIT method an o/w-emulsion changes into a w/o-emulsion above a certain temperature. In the phase inversion range a microemulsion develops, which becomes a blue o/w-emulsion after cooling down. (From Ref. [48], reprinted with permission of Elsevier.)...

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