Catastrophic inversion

This section is concerned with the factors affecting the drop sizes in nSOW systems before and after catastrophic inversions. There are a number of different drop types present at different stages of a catastrophic invetsion. Therefore, in order to help the reader s understanding of this section, the drop types and drop notations used are described below. 

Drops present before catastrophic inversion. When the catastrophic inversion is brought about by the addition of the aqueous phase to the oil phase (high HLB), two drop types can be present before phase inversion  

When the catastrophic inversion is brought about by adding the oil phase to the water phase (low HLB), the drop types present before inversion can be one of  

Unstable oil drops containing surfactant micelles, in a continuous aqueous phase i.e. 0 /W). 

Phase Inversion and Drop Formation in Agitated Liquid-Liquid Dispersions 

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N. Zambrano, E. Tyrode, I. Mira, L. Marquez, M.-P. Rodriguez, and J.L. Salager Emulsion Catastrophic Inversion from Abnormal to Normal Morphology. 1. Effect of the Water-to-Oil Ratio Rate of Change on the Dynamic Inversion Frontier. Ind. Eng. Chem. Res. 42, 50 (2003). 

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. 

A third class is phase inversion. Here, emulsions are made by starting with an emulsion in which the ultimate continuous phase is the dispersed phase and vice versa. Then by adding more and more dispersed phase, one can induce the emulsion to suddenly invert (catastrophic inversion). Alternatively, one can choose the surfactant system such that, for example, by a temperature change, the surfactant system changes from favouring the initial emulsion to favouring an inverted emulsion. This is called transitional phase inversion. 

Brooks, B.W. Richmond, H.N. Phase inversion in non-ionic surfactant-oil-water systems. II. Drop size studies in catastrophic inversion with turbulent mixing. Chem. Eng. Sci. 1994, 49, 1065-1075. 

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. 

Catastrophic inversion is illustrated in Figure 10.33, which shows the variation of viscosity and conductivity with the oil volume fraction (f. As can be seen, inversion occurs at a critical maximum packing fraction. 

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

It was mentioned earlier that the B and C regions often exhibit multiple emulsions, which is actually the simultaneous occurrence of both emulsion types. There ts some evidence that multiple emulsions start occurring before the catastrophic inversion takes place, and some researchers have proposed a competitive kinetic model in which one of the emulsions could be more stable and thus would prevail (87,89,110). This is consistent with the fact that the variables susceptible to influence the breaking-coalescence mechanisms do influence the location of the inversion tine and hysteresis region. 

The crossing of the vertical branches of the inversion line results in a completely different phenomenon called catastrophic inversion (172,197) because it can be modeled as a cusp catastrophe transition as pointed out by Dickinson (201) and fiirther discussed by others (195—203). 

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). 

Brooks, B. W., and H. N. Richmond (1994c). Phase invasion in non-ionic surfactant-oil-water systems in. The effect of oil phase viscosity on catastrophic inversion and the relationship between the drop size present before and after catastrophic invCT-sion, Chem. Eng. ScL, 49, 1843-1853. 

Catastrophic inversion. Inversions across A" /C and A /B" boundaries involve a catastrophic change from W/O to O/W and from O/W to W/0, respectively. Inversions of this type are induced by altering the system s WOR. 

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. 

Drops present after catastrophic inversion. After catastrophic inversion has taken place the resulting emulsion consists of stable oil drops in a continuous water phase containing surfactant micelles i.e. 0/Wm) when the initial continuous phase is oil. When the initial continuous phase is aqueous the resulting emulsion consists of stable water drops in a continuous oil phase containing surfactant micelles i.e. W/0 ). 

Ostwald first modelled catastrophic inversions as being caused by the complete coalescence of the dispersed phase at the close packed condition (corresponding to a dispersed phase fraction of 0.74 in Ostwald s uniform hard sphere model). Other studies, e.g. Marzall, have shown that catastrophic inversions (though these inversions were not called catastrophic inversions by that author) can occur over a wide range of WOR. It has been suggested that this may be due to the formation of double emulsion drops (0/W /0), boosting the actual volume of the dispersed phase. 

Very few studies of dynamic factors affecting catastrophic inversions are found in the literature. Virtually all studies have been concerned with the movement of inversion boundaries with either changes in the system composition, or changes in the system s dynamics. EIP studies of catastrophic inversions in nSOW systems, however, have not been concerned with the system dynamics. Those studies that have looked at the effect of agitation conditions on catastrophic inversion boundaries have been concerned with oil-water systems with no surfactant present. " ... 

In oil-water systems with no surfactant present the formation of OAV/O and W/O/W drops may be limited. However, some authors have noted the presence of these drop types. Delayed catastrophic inversion can occur in oil-water disper-... 

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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