Winsor-type emulsions

A microemulsion that has a high water content and is stable while in contact with a bulk oil phase, and in laboratory tube or bottle tests tends to be situated at the bottom of the tube, underneath the oil phase. For chlorinated organic liquids, which are denser than water, the oil is at the bottom phase rather than the top. See Microemulsion, Winsor-Type Emulsions. 

A special kind of stabilized emulsion in which the dispersed droplets are extremely small (<100 nm) and the emulsion is thermodynamically stable. These emulsions are transparent and can form spontaneously. In some usage a lower size-limit of about 10 nm is implied in addition to the upper limit see also Micellar Emulsion. In some usage the term microemulsion is reserved for a Winsor type IV system (water, oil, and surfactants all in a single phase). See also Winsor Type Emulsions. 

A microemulsion that has high oil and water content and is stable while in contact with either bulk oil or bulk water phases. This stability can be caused by a bi-continu-ous structure in which both oil and water phases are simultaneously continuous. In laboratory tube or bottle tests involving samples containing unemulsified oil and water, a middle-phase microemulsion tends to situate between the two phases. See also Winsor Type Emulsions. 

Several types of phase behaviour occur in microemulsions they are denoted as Nelson type IT, type II+, and type III. These designations refer to equilibrium phase behaviours and distinguish, for example, the number of phases that can be in equilibrium and the nature of the continuous phase. Winsor-type emulsions are similarly identified, but with different type numbers. 

Winsor-Type Emulsions. Several categories of microemulsions that refer to equilibrium phase behaviors and distinguish, for example, the number of phases that may be in equilibrium and the nature of the continuous phase. See reference [27], They are denoted as Winsor Type I (oil-in-water), Type II (water-in-oil). Type III (most of the surfactant is in a middle phase with oil and water), and Type IV (water, oil, and surfactant are aU present in a single phase). The Winsor Type III system is sometimes referred to as a middle-phase microemulsion, and the Type IV system is often referred to simply as a microemulsion. An advantage of the Winsor category system is that it is independent of the density of the oil phase and may lead to less ambiguity than do the lower-phase or upper-phase microemulsion type terminology. Nelson-type emulsions are similarly identified, but with different type numbers. 

The type III phase environment may contain a maximum of three phases. When this is the case, the emulsion present corresponds to Winsor type III, in which a microemulsion is in equilibrium with pure oil and pure brine phases. However, type II(-) behavior and type II(+) behavior may also be observed under certain conditions. In practice, type II(-) or II(+) behavior occurs when all of the brine or oil can be incorporated into the microemulsion or when insufficient surfactant is present to produce a measurable microemulsion. 

The second intent to numerically characterize the formulation concept was the so-called phase-inversion temperature (PIT), originally introduced by Shinoda in 1964 as the temperature at which a nonionic surfactant switched its dominant affinity from the aqueous phase to the oil phase. This inversion occurred in a so-called phase transition process that was both easy to determine experimentally and simple to understand as far as the associated phenomenology was concerned [107,108]. Later, it was related to the temperature at which the emulsion inversion takes place [57,58,109], which is the same in most cases, with scarce exceptions and with no real significance [110]. It was finally fine-tuned again and renamed the HLB temperature [111], as a way of stating that it was the temperature at which the hydrophilic and lipophilic tendencies of the surfactant were balanced (i.e., whenever a Winsor Type III system is occurring). 

All types of micro emulsions were obtained in salinity scans with mixtures of Aerosol MA (sodium dihexyl sulphosuccinate) and twin-tailed (Guerbet and Exxon type) alcohol ethoxy and propoxy sulphates for perchloroethylene (PCE), carbon tetrachloride, 1,2-dichlorobenzene and trichloroethylene [59] at 25°C. At lower temperatures, however, stable macro emulsions are formed. Chloroform, 1,2-dichloroethane and other chlorinated hydrocarbons were found to be too polar for those anionic surfactants. Extremely hydrophilic and temperature-insensitive surfactants are necessary for effective solubilisation of chlorinated hydrocarbons yielding Winsor III systems. N-methyl-N-D-glucalkaneamide surfactants showed good performance for DNAPL solubilisation even at 16°C [56]. 

The introduction of micro emulsions in the scientific literature is normally ascribed to Schulman - although such systems had appeared in the patent literature before -and he and his co-workers produced a considerable fraction of the early work regarding their preparation and properties [1-8]. Other major contributors in the early period of microemulsions were Winsor [9, 10], Friberg [11-14] and Shinoda [15-22] it can also be mentioned that Ekwall [23, 24], although not using the term microemulsion, made pioneering work on similar types of systems. 

Under certain conditions, the oil or water droplets in emulsions can be made small enough (<100 nm) that the emulsions appear transparent. Such dispersions are called microemulsions. Three types of microemulsions can be formed, namely oil-in-water, water-in-oil, and middle-phase microemulsions. The latter microemulsions occur when the Winsor s ratio / = 1, and when the SAD = 0. All microemulsions are thermodynamically stable, which implies that they form spontaneously at certain concentrations of oil, water and surfactant, and the formation is limited only by the diffusion of the molecules. It has been reported (17) that the change in free energy of dispersions shows a minimum at an equilibrium droplet size in the range of 100-1000 A for... 

To form stable MLO-based EMEs, the following important requirements need to be fulfilled (1) the system should be biphasic (Winsor-II-type system), i.e., the inverted type microemulsion systems should coexist with excess water, (2) the presence of an efficient stabilizer covering the outer surface of the kinetically stabifized droplets containing the internal W/O microemulsion system is needed, and (3) this internal microemulsion sfiucture should be preserved after the dispersing procedure. It should here be pointed out that microemulsions are, at a certain temperature, known to be sensitive systems with respect to any changes in their composition that may lead to the loss of their thermodynamic stability. Such a loss would in turn cause the formation of normal emulsions or, in a worst case scenario, a phase separation into oleic- and aqueous-rich phases. 

Winsor developed a classitication scheme for oil-water-amphiphile emulsions in 1948, dividing behavior into four general types (1). Winsor I and II are two-phase systems. Winsor I involves micelles in equilibrium with an excess oil phase, whereas Winsor II comprises reverse micelles in equilibrium with excess water. Winsor III includes three phases wherein most of the surfactant is found in a middle phase in equilibrium with both excess oil and water. Finally, Winsor IV is a single-phase system. 

To stabilise an emulsion, the surfactant must be present at a concentration above the CMC hence, we shall be mainly concerned with systems of this sort. The phase in which the surfactant forms micelles is dependent on the surfactant s affinity for oil and water. The surfactant s affinity is controlled by a number of factors. Winsof first addressed the problem of describing surfactant affinity. He introduced the concept of interaction energies between surfactant molecules adsorbed at the interface and the oil and water phases. Salager identified different types of interactions. The ratio of the total interaction energies (per unit area of interface) of the surfactant for the oil and water phases is known as Winsor r (symbols are defined below) ... 

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...

If the surfactant concentration is high enough, the system exhibits so-called Winsor IV monophasic behavior in the vicinity of HLD = 0. This means that when the formulation is changed, the emulsified system starts as a two-phase emulsion, then becomes a single-phase microemulsion, and finally ends up in the other type of two-phase emulsion. 

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