Phase Behavior in Surfactant-Oil-Water Systems

Generally, water/oil/surfactant phase diagrams are not worked out in so much detail [47, 105] before particle synthesis and often, only the area important for synthesis is delineated for obvious practical reasons (partial phase diagrams, as in Fig. 3.3). We shall describe at the end of this Section, for the uninitiated, the basics of the phase diagrams and the procedures of setting them up. Preparation of microemulsions will be taken up in Section 3.3. 

It is logical to ask what may happen if the dispersed solubilizate phase (water or oil) is added to the water-oil-surfactant system to such a high extent that the core of the micelle cannot contain it any more. The answer is that at the initial stage of such extra addition the result will be a microemulsion coexisting with some excess solubilizate phase (e.g. W/O with excess W). When with further addition the ratio between the continuous and dispersed phases comes to, say, about 1 1, the two commonly form an intermediate microemulsion phase between the OAV and W/O types within a specific range of temperatures this is known as bicontinuous microemulsion . With further addition of the dispersed phase, there can be an inversion from one type to the other. We shall discuss below these points in more detail from a historical viewpoint. 

The questions of W/O and O/W microemulsions, their respective co-existence with W- and O-phases, the possible equilibrium situations and related issues have often been addressed by various authors on the basis of observations made more than half a century ago by Winsor [ 106] which are, even today, widely accepted descriptions of the equilibria. Winsor considered the system to consist of an oil 

The four main types of equilibria proposed by Winsor are  

Winsor has also pointed out the arbitrariness of this classification. Thus, a Type I system can also be called a Type IV system in equilibrium with excess oil, and a Type II system, with excess water. On the other hand, several pathways have been described by Winsor for increasing the content of the dispersed phase, and for eventual conversion of one type equilibrium situation to another. It has been shown that progressive conversion of Type I to Type II and the reverse is possible when an adequate proportion of surfactant is present. A description of the 

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