Winsor III Region

The Kabalnov-Wennerstrom theory predicts that the sharpness of the transition from unstable to stable emulsions is controlled by the value of When the surfactants are compared in a homologous series, k increases and ao decreases with the surfactant chain length. Accordingly, the sharpness of transition from unstable to stable systems is expected to increase with the surfactant chain length. 

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Kabalnov A, Weers J. Macremulsion stability within the Winsor III region theory versus experiment. Langmuir 1996 12 1931-1935. 

Figure 7 Interfacial tension of the planar interface between the microemulsion and the excess phase as a ftinction of the salt concentration for systems composed of AOT (sodium diethylhexylsulfosuccinate), brine, and linear alkanes of varying chain length. Points are experimental data obtained for Cg (O, ), Cio ( , ), and C12 (O, ) linear alkanes making up the oil phase. Open symbols refer to Winsor II systems corresponding filled symbols indicate the Winsor III region where the theory is no longer valid. The lines were calculated according to Eqs. (60) and (61), the fixed parameters listed in Table 2, and suitably chosen values of K/kT — 0.8 (Cg systems), 0.39 (Cio systems), and 1.2 (C12 systems). (Experimental data from Ref 46.)...

A Kalbanov, J Weers. Macroemulsion stability within the Winsor III region Theory versus experiment. Langmuir 12 1931—1935, 1996. 

This equation is based on neglecting the entropy of mixing term for the micelles, and is applicable only to spherical micelles (i.e. not over the Winsor III region). Although it is close in form to the empirical expansion, eqn. (7.10), it fails to account for the finite interfacial tension at the balance point, i.e. the Oq term. Experiment shows that the coefficient e is indeed close to k T for typical microemulsion systems, while the values of Oo vary considerably. Normally, Oo decreases with the surfactant chain length from 0.1 to 10 mN m (see Figure 7.6). 

Note that in the Winsor III equilibrium there are three phases in equilibrium and, therefore, three interfacial tensions. The a value we have been discussing so far in this section is the interfacial tension between the upper (oil) and lower (water) phases, denoted in Figure 7.7. The interfacial tensions between the middle (surfactant) phase and upper and lower phases also strongly depend on the spontaneous curvature because of the critical phenomena at the three-phase-body end points (Figure 7.7). The wetting phenomena over the Winsor III region are very peculiar. In the balanced point of long-chain surfactants, the middle phase does not... 

Figure 7.7 Interfacial tensions in an H20—oil—CiEj system over the Winsor III region as a function of temperature. Here is the interfacial tension between upper and lower phases, a c is the interfacial tension between the middle and lower phases and Ocb is the interfacial tension between the middle and upper phases. Od> vanishes at the upper end-point T and vanishes at the lower end-point T/ because of the critical phenomena. Except for the very vicinity of the end-points, Occ + Oct, > a b and the middle phase does not wet the interface between the upper and lower phases...

There are two more subtle features of the nonionic macroemulsion stability to be discussed. Firstly, within the Winsor III region, the stability of macroemulsions is very temperature sensitive. Although exactly in the balanced state, the macroemulsions are very unstable and break within minutes, the system becomes stable only several tenths of a degree away from the balanced point, while still being within the Winsor III region. Secondly, the macroemulsion stability pattern is not completely symmetric. W/O emulsions reach maximum stability at ca. 20 C above the balanced point, after which the stability starts to decrease. On the other... 

Triphasic Top oil phase, middle bicontinuous phase (mixed regions of both w/o and o/w) and bottom water phase (Winsor III)... 

If more and more surfactant is added to a Winsor III system, the surfactant-rich phase swells at the expense of the excess oil and water phases. From a certain point, a single surfactant-rich phase is found. Upon increasing the amount of surfactant even further, a first-order transition to a lamellar phase may be observed. In a special case, it has been shown that the coexistence region between the (bicontinuous) microemulsion phase and a lamellar phase was extended into the region where the surfactant-rich phase coexists with excess oil and water, leading to a four-phase equilibrium water-lamellar phase-microemulsion phase-oil [58]. In Ref. 46, even a three-phase equilibrium, water-lamellar phase-oil, was observed, the bicontinuous microemulsion phase apparently being absent. 

FIG. 7 Idealized phase diagram for microemulsion-forming water/oil/surfactant system under well-balanced conditions. (Redrawn from A. Kabalnov, B. Lindman, U. Olsson, L. Piculell, K. Thuresson, and H Wennerstrom, Colloid. Polym. Sci. 1996, 274, 297.) The Winsor I and Winsor II microemulsions are on the right-hand and left-hand sides, respectively, of the central microemulsion region, whereas the Winsor III microemulsion is located at the downward tip. 

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. 

Microemulsions in equilibrium with excess oil or water are classified according to the scheme introduced by Winsor. These two-phase regions are sketched in Fig. 3.18. A Winsor I microemulsion is an oil-in-water system with excess oil, a Winsor II microemulsion is a water-in-oil system with excess water and a Winsor III microemulsion is a middle phase system, with an excess of both water and oil. Winsor III microemulsions are thus used in tertiary oil recovery. Transitions between these phases formed using non-ionic surfactants can be controlled by variation of the HLB through temperature, whereas for ionic surfactants transitions can be driven by changing salt concentration. The temperature at which a microemulsion contains equal amounts of water and oil is termed the phase inversion temperature (PIT). 

The hydrophilic-lipophilic deviation (HLD) is a dimensionless representation of SAD, given by HLD = SAD/RT. Either SAD or HLD values can be used to determine composition regions for which macroemulsions or microemulsions are likely to be stable, break or invert. Negative SAD or HLD values refer to Winsor type 1 systems (O/W), positive SAD or HLD values refer to Winsor type II systems (W/O) and SAD = HLD = 0 refers to Winsor type III systems (most of the surfactant is in a middle phase with oil and water). Much of the use of SAD and HLD has been in developing surfactant formulations. 

From S/A= 75 25 to 55 45 the diagram exhibits a three-pha.se behavior region which is not very different iiom the Winsor type III diagram, with the... 

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