Winsor diagrams

Fig.1 Phase behavior types of surfactant-oil-water systems as Winsor Diagrams for difer-ent cases of the ratio R of interactions between the surfactant adsorbed at interface and the oil and water molecules...

Figure 7 Winsor diagrams, phase volume, and test tube aspect along a formulation scan. (From Ref. 41.)...

The second effect to be considered is the one due to the formulation variables (i.e., those that are able to change the Winsor diagram type). These should have some effect, as they are likely to induce a preferred or natural curvature through the molecular interactions, independently of the water-to-oil ratio. 

Winsor diagram types are very useful for explaining most transitions in phase behavior that are found in practice. However, the real ternary diagrams are often more complex, and on the other hand, the ternary approximation is not always sufficient. A quaternary approach (at least) is often required to describe the characteristics of a given system. 

In Winsor diagrams, it is assumed that the amphiphile is completely miscible in all proportions with both the water and oil phases. This is often untrue, in particular with Type I and II cases that exhibit lateral polyphasic regions near the AW or AO sides. On the other hand, many surfactants, particularly of the ionic type (e.g., soaps), produce complex liquid-crystal phases in equilibrium, even with a single solvent, and, of course, much more with two. 

First, it is obvious that none of the Winsor diagrams will match this behavior because the amphiphile mixture has a varying affinity that depends on the relative amounts of, for instance, a hydrophilic surfactant (S) and a lipophilic alcohol (A). A quaternary diagram is required to handle this four-component system (e.g., a regular tetreihedron with equal sides). At each vertex, one of the components is represented. 

Winsor reported that the phase behavior of SOW systems at equilibrium could exhibit essentially three types, so called Wl, Wll and Will, illustrated by the phase diagrams indicated in Fig. 1. In the Wl (respectively, Wll) case, the surfactant bears a stronger affinity for the water (respectively, oil) phase and most of it partitions into water (respectively, oil). As a consequence, the system exhibits a two-phase behavior in which a microemulsion is in equihb-rium with excess oil (respectively, water). 

Figure 1 shows changes in the system phase behavior as its HLB value is systematically adjusted. The left side of the diagram represents a two-phase system with micellar-solubilized oil in equilibrium with an excess oil phase (Winsor Type I) (Winsor 1954). The right side of the diagram represents a different two-phase system with reversed micellar-solubilized water. In-between these two systems a third phase coemerges which contains enriched surfactant with solubilized water and oil. This new thermodynamically stable phase is known as a Winsor Type HI middle phase microemulsion. 

FIGURE 2.23 Fractional flow diagram of a Winsor I microemulsion flood. 

FIGURE 2.26 Fractional flow diagram of Winsor I microemulsion flood at waterflood residual oil saturation,... 

At low temperature the amphiphile is more compatible with water than with oil. The phase diagram corresponding to this situation is shown in Figure 3.16 (al or a2). The tie line orientation is directly deduced from the partitioning of the amphiphile between water and oil because under the current conditions the surfactant is more compatible with water than with oil, the majority of the amphiphile is in the water phase and only a limited amount of amphiphile is present in the oil. Accordingly, the tie lines point in the direction of the oil vertex. The phase diagrams al and a2 of Figure 3.16 are referred to as Winsor I (WI). 

If the phase diagram exhibits a 3PT it is called a Winsor III (Will) system. In such a situation, the plait point curves do not merge but cross each other and stop at two terminal critical points (see Figure 3.17d or Figure 3.17e). 

Winsor behavior is not the only characteristic of water-oil-nonionic amphiphile systems. The lyotropic mesophases appearing on the water-amphiphile binary phase diagrams expand to some extent in the Gibbs triangle (Figure 3.19). 

The first representation is to select one formulation variable to be scanned (at all others constant) and to plot the variations of a property or of the phase behaviour versus this formulation variable. Figure 3.1(a) shows such a plot of interfacial tension (y) versus the salinity (S) of the aqueous phase together with the ranges in which different phases are formed for a system containing n-hexane as oil, an alkylbenzene sulphonate surfactant and sec-butanol as co-surfactant. The phase behaviour is indicated as 2 or 2 for two-phase systems in which the surfactant-rich phase is the lower water or the upper oil phase, corresponding to Winsor type I and II diagrams. Symbol 3 indicates the range of formulation for which a three-phase behaviour is attained. 

Another common case of bidimensional representation is a map of the phase behaviour versus two composition variables, while all formulation variables are kept constant. This is generally represented in a triangular diagram. If the three components are pure products, there are essentially the three Winsor s types of diagrams as in Fig. 1.2 (Chapter 1), the formation of which depends on the formulation variables. Winsor type III diagram correspond to the so-called optimum formulation situation in which there is a zone in which three phases coexist over some range of composition. 

Even more complex quaternary SOWA diagrams with three independent composition variables at constant formulation and T/p conditions have been proposed to be represented in an equilateral tetrahedron. Such diagrams maybe useful for some peculiar cases, but they are generally not amenable to simple interpretations and in most cases they are described by a series of bidimensional cuts, i.e. cuts through the tetrahedron, which are not amenable to Winsor s types as seen in Fig. 3.4, because the four types are arranged in a different way [17, 18]. 

Within certain industrial applications like gas and oil industry and ink and printing industry there is a need for cleaning when the remaining surface should be water-wet. A neutral microemulsion system based on a surfactant, a lactate ester as co-surfactant and an organic solvent like limonene is suggested by Harrison for this purpose. Butyl lactate is shown to enlarge the one-phase (Winsor IV) area in the phase diagram, for instance SDS and limonene in water [94, 95]. 

Fig. VI-18. Phase diagrams of water - hydrocarbon (oil) - nonionic surfactant system at three different temperatures. Winsor equilibria.

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