Winsor II Region

A faster emulsion breakage at the lowest surfactant concentration (4%) is probably due to the fact that the surfactant monolayer is no longer saturated. The 

The C12E5 concentration in oil is equal to 4% (squares), 15% (circles), 20% (triangles), and 30% (diamonds). Once the surfactant concentration exceeds the critical micelle concentration (ca 4% at 80 the emulsion breakage is consistent with the Arrhenius effect with an activation energy 42 49 kjT, which is fairly independent of the surfactant concentration. At 4% surfactant concentration the monolayer is not saturated and its bending modulus is very low, which produces a steeper stability decay 

CMC of C12E5 in heptane is reported to increase with temperature from 1 to 4 wt% between 30 and 80 C. At temperatures above 40 C the monolayer is no longer saturated, the bending modulus decreases, and the macroemulsion resolves faster, as has been discussed by others. 

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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. 

Figure 7.9 The PIT concept. As the temperature increases, the macroemulsion type changes from O/Wover the Winsor I region to W/O over the Winsor II region, through the emulsion breakage at the balanced point...

Over the Winsor II region, the W/O emulsion breakage was analyzed by the conventional Arrhenius plot ... 

Figure 7.24 Breakage of W/0 emulsions in Arrhenius coordinates in the Winsor II region.

The Winsor II microemulsion is the configuration that has attracted most attention in solvent extraction from aqueous feeds, as it does not affect the structure of the aqueous phase the organic extracting phase, on the other hand, is now a W/0 microemulsion instead of a single phase. The main reason for the interest in W/0 microemulsions is that the presence of the aqueous microphase in the extracting phase may enhance the extraction of hydrophilic solutes by solubilizing them in the reverse micellar cores. However, this is not always the case and it seems to vary with the characteristics of the system and the type of solute. Furthermore, in many instances the mechanism of extraction enhancement is not simply solubilization into the reverse micellar cores. Four solubilization sites are possible in a reverse micelle, as illustrated in Fig. 15.6 [19]. An important point is that the term solubilization does not apply only to solute transfer into the reverse micelle cores, but also to insertion into the micellar boundary region called the palisade. The problem faced by researchers is that the exact location of the solute in the microemulsion phase is difficult to determine with most of the available analytical tools, and thus it has to be inferred. 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

Here, the superscript n and n + 1 represent the previous and current trials, respectively. Based on this approach, we find C33m o = C33max2 = 0.03, at which the solubilization ratio is 2.7. This value is close to the test value of 2.8, so we can leave it for the moment and move to a point in the Winsor type II region. 

At lower surfactant concentrations (line b) regions of phase separation appear. In such a phase-separated state there is a sequence of equilibria between phases, commonly referred to as Winsor equilibria [13,23]. In point A two macroscopic phases are formed. These are a microemulsion of composition B and an aqueous solution containing dissolved surfactant and micelles with solubilized hydrocarbon. The volumes of these macroscopic phases can be estimated in the usual way by applying the lever rule with a correction for the densities of these phases. In such a state of separation into two macroscopic phases, the equilibrium between the microemulsion and the aqueous solution is referred to as the Winsor II (WII) equilibrium. 

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.)...

In the two-phase region, there is a microemulsion phase containing almost all amphiphile, which coexists with an oil-rich (20, Winsor I [59]) or with a water-rich (20, Winsor II [59]) phase [56],... 

The scheme outlined here has turned out to make up a convenient approach, first, to treating Winsor I and Winsor II microemulsion systems where an excess phase is present [38], in which case it is sufficient to consider merely the first interfacial term of Eq. (191), and, second, to the corresponding one-phase microemulsions elose to the two-phase region (i.e., on the border to what occasionally is referred to as emulsification failure) [47]. 

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. 

Between S/A = 55/45 and S/A = 50/50, the phase behavior changes to mostly lipophilic, and the Winsor Type II region invades the whole diagram. In this case, the surfactant is not hydrophilic enough to compensate for the alcohol effect. It is also probable that such a high amount of alcohol results in a consolute system in which most of the alcohol is miscible in the oil phase at equilibrium with an almost pure water phase. Note that a nonionic surfactant of the alkyl phenol type with even 7.5 EO groups as an average is certainly at least 10 times more soluble in a mixture of heptane and pentanol than in water [49,87]. 

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. 

If R > I. the opposite occurs. Micellar systems arc of the inverse type, and if there is enough surfactant to produce a microemulsion it will he the type encountered in Winsor type II diagram above the biphasic region. 

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