Transitional inversion

The results of Shinoda et al. for emulsion stability across the phase transition show that for 0/W emulsions the drainage rate was low at temperatures about 20-40 C below the PIT and also low for W/O emulsions at temperatures about 20-40 °C above the PIT. This led to Shinoda s observation that the HLB numbers of surfactants whose HLB temperatures in an oil-water system are 25-70 C higher than the emulsion s storage temperature are the required HLB numbers for emulsification of that system . Shinoda et al. also showed that the stability of emulsions falls to a minimum in type 3 systems. Many other studies have shown stability maxima either side of a stability minimum in the three-phase region, although there has been much debate as to the relevance of the measurement of stability used in some studies.  

Shinoda also studied emulsions that were produced at temperatures below the PIT, at the PIT and above the PIT with rapid cooling to aid stability. Drop sizes of emulsions produced at the PIT were retained in the final cooled emulsion. Shinoda noted that emulsions with the finest drops were produced by emulsifying 2-4 C below the PIT and then cooling. Shinoda termed this emulsification method emulsification by the PIT method . The study also showed that emulsification by the inversion method , i.e. emulsification above the PIT as a W/O emulsion and then cooling, did not result in such small drops. 

Friberg et a/. performed similar experiments to Shinoda and found that, at temperatures above the PIT, there was no surfactant phase present and there was no reduction in drop sizes. However, at temperatures below the PIT when the system was three phase and the emulsion volume contained 20% and 50% surfactant phase, there was a large reduction in the drop sizes. Hence, it was concluded that as the surfactant phase separate on cooling it will produce extremely small drops. These extremely small drops skew the average droplet size to smaller values.  

Parkinson and Sherman looked at the drop sizes at the PIT of the systems stabilised by Tween-Span mixtures and found only small differences in the drop sizes of emulsions made at the PIT with those produced at other temperatures. The results of these workers may again be explained by noting that conditions for transitional inversion points, in systems stabilised by these surfactants, may not exist if the disperse phase fraction is 80% as the surfactant phase may not become continuous. Hence, the inversion observed by these workers may have been a catastrophic inversion. 

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Miiiana-Perez, M., Gutron, C., Zundel, C., Anderez, J.M. and Salager, J.L. (1999) Miniemulsion formation by transitional inversion. Journal of Dispersion Science and Technology, 20, 893-905. 

Rgure 1.6. (a) Schematic formulation-composition map. SAD is the surfactant affinity difference it is positive for a lipophilic surfactant and negative for a hydrophilic one. The gray zones are abnormal, (b) Schematic representation of the proposed mechanism for a transitional inversion, (c) Schematic representation of the proposed mechanism for a catastrophic phase inversion. 

B.W. Brooks and H.N. Richmond Phase Inversion in Non-Ionic Surfactant-Oil-Water Systems, I. The Effect of Transitional Inversion on Emulsion Drop Size. Chem. Eng. Sci. 49, 1053 (1994). 

J.L. Salager Evolution of Emulsion Properties Along a Transitional Inversion Produced by a Temperature Variation. In Proceedings of the 3rd Word Congress on Emulsions l-F-094, Lyon, France (2001). 

S. Sajjadi, F. Jahanzad, and M. Yianneskis Catastrophic Phase Inversion of Abnormal Emulsions in the Vicinity of the Locus of Transitional Inversion. Colloid and Surfaces A Physicochem. Eng. Aspects 240,149 (2004). 

Figure 15.24 schematically shows a state diagram of the system. Compositions left of the nodal curve will be a B-in-A emulsion, when more A is added, (catastrophic) inversion will take place at the modal line. However, in a specific area where the affinity of the surfactant system towards both phases is approximately equal, transitional inversion may take place. 

Brooks, B.W. Richmond, H.N. Phase inversion in non-ionic surfactant-oil-water systems. I. The effect of transitional inversion on emulsion drop sizes. Chem. Eng. Sci. 1994, 49, 1053-1064. 

The phase inversion of emulsions can be one of two types (i) transitional inversion, which is induced by changing facers which affect the HLB of the system (e.g., temperature and/or electrolyte concentration) and (ii) catastrophic inversion, which is induced by increasing the volume fraction of the disperse phase. 

Transitional inversion can also be induced by changing the HLB number of the surfactant at constant temperature, using surfactant mixtures. This is illustrated in Figure 14.2, which shows the average droplet diameter and rate constant for... 

To apply the phase inversion principle, the transitional inversion method should be used, as demonstrated by Shinoda and coworkers [11, 12] when using nonionic surfactants of the ethoxylate type. These surfactants are highly dependent on temperature, becoming lipophilic with increasing temperature due to dehydration of the poly(ethylene oxide) (PEO) chain. When an O/W emulsion that has been prepared using a nonionic surfactant of the ethoxylate type is heated, at a critical temperature - the PIT - the emulsion will invert to a W/O emulsion. At the PIT, the droplet size reaches a minimum and the interfacial tension also reaches a minimum, but the small droplets are unstable and coalesce very rapidly. Rapid cooling of an emulsion that has been prepared close to the PIT results in very stable and small emulsion droplets. 

Fig. 27. Experimental (left) and simulated (right) central transition inversion recovery spectra of 47% enriched poly crystalline cristoballite for the temperatures given, recorded as a function of the length t of the relaxation delay.40 All simulations assume a six-site motion between six equally probable sites on a circle orthogonal to the Si-Si axis between adjacent Si04 tetrahedra, with rate constants log( ) = 3.50 at T= 298 K, log(Jfc) = 3.95 at T= 473K and log(fc) = 5.80 at T= 528 K.

In systems that are impact-modified systems, it could be observed that the critical volume concentration phenomenon also depends on the properties of the matrix polymer. Publications have shown that the size of the particles of the dispersed phase and the concentration at which the so called brittlc-to-tough transition occurs, arc influenced by many factors. It is therefore not surprising that topological theories in their present fonu, which do not take into account many of these factors, cannot give any help to predict the properties of unknown systems. Recently, a phase transition ( inversion ) in polymer blending has been experimentally observed [78],... 

PIT method, which was introduced by Shinoda, is an anulsiiication technique for preparation of microemulsions with low energy. Advantages like being low cost have made the PIT method more attractive in recent years. The PIT concept is based on one type of phase inversion in anulsions (transitional inversion) induced by changing tanperature, which affects the HLB of the system. 

On the other hand, transitional inversion is used as well to attain extremely small drop emulsions, som imes referred to as miniemulsions or gel emulsions, because of the high viscosity resulting frtnn the exceedingly small drop size, even at a low internal phase ratio (111-115). 

The two different ways of crossing the inversion line are associated with quite different behaviors. The first one, which is known as transitional inversion, is produced by changing formulation at a constant water-to-oil ratio, i.e., along a vertical path in the bidimensional map. Such a crossing takes place in the A region in the central zone of the map. The experimental evidence indicates that, in this kind of dynamic process, the inversion takes place at the very moment the standard inversion line is crossed, i.e., essentially at SAD = 0, whatever the direction of change [from A to A or vice versa as indicated with white arrows in Fig. 12 (left)]. The horizontal branches of the standard and dynamic inversion lines are thus identical. The term... 

The butterfly catastrophe model explains why the transitional inversion is not really an inversion but a surfactant transfer from one phase to the other, while the catastrophic inversion is a nonreversible hysteresis type instability. This approach, which is out of the scope of this chapter, is well documented elsewhere (197). 

M Mifiana-Perez, C Gutron, C Zundel, JM Anderez, JL Salager. Miniemulsion formation by transitional inversion. J Dispers Sci Technol 20 893—905, 1999. 

The question now arises as to what is meant by a resonant intermediate state. Spectral lines are generally Lorentzian in shape thus, as we move away from the line centre, the intensity drops quite rapidly but remains non-zero for a considerable distance from the line centre. Excitation in the wings of a line is still possible if the laser intensity is high enough. However, at high intensities, coherent excitation processes become important, and two or more photons can be absorbed simultaneously through virtual states (i.e. in the absence of real/resonant intermediate states). Theory shows that such transitions result from the collective effect of all allowed transitions, inversely weighted by the difference in... 

The horizontal branch of the inversion line has been associated with the so-called "transitional" inversion, while the vertical branches correspond to the catastrophic" inversion, a labeling whose origin will become evident later on. 

Transitional inversion. The inversion across SAD = 0 is continuous from W/0 to MOW (microemulsion-oil-water) to O/W and is induced by altering the surfactant s affinity, which alters the nSOW system phase behaviour. 

Shinoda et u/. s PIT work and Marzairs - - EIP work are studies of dynamic inversion. PIT inversions (induced by change in temperature altering the surfactant affinity) can now be seen to be transitional inversions, while EIP inversions (induced by adding a dispersed water phase to a continuous oil phase) are catastrophic inversions. 

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