Oil/water/surfactant/cosurfactant

From the point of view of traditional thermodynamics, a microemulsion is a multicomponent mixture formed of oil, water, surfactant, cosurfactant, and electrolyte. There is, however, a major difference between a conventional mixture and a microemulsion. In the former case, the components are mixed on a molecular scale, while in the latter, oil or water is dispersed as globules on the order of 10-100 nm in diameter in water or oil. The surfactant and cosurfactant are mostly located at the interface between the two phases but are also distributed at equilibrium between the two media. In conventional mixtures, the sizes of the component species are fixed. In the case of microemulsions, the sizes of the globules are not given but are provided by the condition of thermodynamic equilibrium. 

Figure 15.5 Schematic representation of the pseudoternary phase diagram of oil/water/ surfactant/cosurfactant.

Miniemulsions are typically formed by subjecting the oil/water/surfactant/ cosurfactant system to a high shear field created by devices such as an ultra-sonifier, the Manton Gaulin homogenizer and the Microfluidizer. These rely tn mechanical shear and/or cavitation to break the oil phase into submicron size droplets. When monomer is used as the oil phase, free radical polymerizatim can subsequently be carried out by addition of an initiator (e.g. potassium persulfate, KPS). 

Fig. 10.5. Pseudoterna7 phase diagram of oil-water-surfactant-cosurfactant.

The selection of surfactants for producing OAV or W/O microemulsions is not simple and one needs to establish the phase diagram of the ternary system oil-water-surfactant-cosurfactant to arrive at the regions of formation of OAV or W/O microemulsions. A useful concept for selection of surfactants is based on the critical packing parameter (CPP) of the molecules. 

The formation of microemulsions usually involves a combination of oil, water, surfactant, and cosurfactant. The tendency toward a w/o (oil-in-water) or an o/w (water-in-oil) microemulsion is... 

Johnston et al. l also examined the solvatochromic shift of pyridine N-oxide in an ethane/CjEj (C = 10-13 E = 5) water-in-oil microemuision, also in equilibrium with a lower liquid phase. Contrary to the behavior exhibited by the AOT system, the nonionic microemulsions display a polar environment at low pressures, which becomes progressively less polar as pressure increases. At a pressure of only 50 bar, they reported that the probe s environment resembles that observed in bulk hexane. Added water increases the polarity somewhat, yet a cosurfactant (octanol) is required to produce an environment similar to that in bulk water. The polarity of the ethane/ water/surfactant/cosurfactant system remains essentially constant as pressure increases up to 350 bar. 

Microemulsions. In some systems the addition of a fourth component, a cosurfactant, to an oil-water-surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10 to 10" mN/m low interfacial tension allows spontaneous or nearly spontaneous emulsification to very small droplet sizes, ca. 10 nm or smaller. The droplets can be so small that they scatter little light the emulsions appear to be transparent and do not break on standing or centrifuging. Unlike coarse emulsions, these microemulsions are usually thought to be thermodynamically stable. The thermodynamic stability is frequently attributed to transient negative interfacial tensions, but this hypothesis and the question of whether microemulsions are really lyophilic or lyophobic dispersions are areas of some discussion in the literature (J 7). As a practical matter, microemulsions can be formed, have some special qualities, and can have important applications. 

This is best illustrated by what happens when the amount of salt is increased, for fixed amounts of oil, water, surfactant and cosurfactant (18-20). For relatively low amounts of salt, an oil in water microenulsion coexists with excess oil phase at sufficiently high salt content, a water in oil microemulsion is in equilibrium with excess water phase, while at intermediate salinities, a (middle phase) microemulsion coexists with both water and oil excess phases. 

As shown in Figure 3.9, the L2 phase is able to solubilize a very large amount of a hydrocarbon such as decane or hexadecane. In fact, a composition containing up to 75% decane and water/surfactant/cosurfactant proportions corresponding to the L2 phase is still clear, fluid and isotropic, forms spontaneously, and is thermodynamically stable. The structure of this microemulsion can be (to some extent) regarded as a dispersion of tiny water droplets (reverse micelles) in a continuous phase of the hydrocarbon. The surfactant and cosurfactant are mainly located at the water/oil interface. This type of system is often referred to as a w/o microemulsion. 

A microemulsion is a multicomponent (3-4 components) system, e.g., water in hydrocarbon (water/oil) or hydrocarbon in water (oil/water), surfactant, and cosurfactant, and generally it exists only in small concentration ranges. Nevertheless, the capacity for reactants and variability of solubilization properties are high and of practical interest [76]. On the basis of microemulsions Menger and co-workers developed a method for an economical environmental cleanup of chemical warfare contamination [77]. As an example of organometallic catalysis in a microemulsion, Beletskaya [78] performed palladium-catalyzed C—C coupling reactions in aqueous medium with a very high content of surfactant. 

Micro emulsions are thermodynamically stable, clear fluids, composed of oil, water, surfactant, and sometimes cosurfactant, that have been widely investigated during recent years because of their numerous practical applications. The chemical structure of smfactants may be of low molecular weight as well as being polymeric, with nonionic or ionic components (87-90). In the case of an oil-continuous (W/0)... 

Extensive studies have been reported by Kunieda s group regarding the formation of worm-like micelles and micellar transient networks in water-surfactant-cosurfactant systems. However, for applications, it is also relevant to know the effect of additives on systems containing worm-hke micelles. It is reported that oils induce a rod-sphere transition in surfactant micellar solutions, leading to a reduction in viscosity [32]. Kunieda s group studied the solubilization of different oils in wormlike micellar solutions [19, 33]. The amount of solubilized oil, its location within the micelle, and its effect on micellar shape and size demonstrated to strongly depend on the nature of the oil and its interactions with the surfactants. 

Parameters Affecting Phase Behavior. In general, the phase behavior at reservoir conditions for oil-water-surfactant systems without alcohol present is very sensitive to the oil composition. Polar components in the crude oil may act as cosurfactants. On the other hand, model oils like n-alkanes do not contain this type of material. Different papers using the Exxon surfactant product termed RL-3011 (dodecyl-o-xylene sulfonate) illustrate the importance of oil composition on the phase behavior regarding changes in reservoir parameters like temperature, pressure, and salinity [47-50], Conclusions from these papers may be summarized in the following way ... 

The miscibility of oil, water, and amphiphile (surfactant plus cosurfactant) depends on the overall composition which, in turn, depends on the system. Ternary (water/surfactant/oil), pseudo-ternary (water/amphiphile/oil) or explicitly quaternary (water/surfactant/cosurfactant/oil) phase diagrams are usually employed to describe the phase manifestation which is essential in the study of microemulsions. These phase diagrams help define the microemulsion areas. Samples from the best combinations, i.e., those that produce the largest volume of microemulsion can be subjected to further characterization by different methods, such as polarized light microscopy, differential scanning calorimetry, zeta sizer, rheometer, etc. 

The knowledge of phase manifestations of the pseudo-ternary (water/amphiphile/ oil) or explicitly quaternary (water/surfactant/cosurfactant/oil) mixtures has been systematized. According to Winsor [10], four types of microemulsion phases exist in equilibrium. These phases are commonly referred to as Winsor phases they are Winsor I with two phases, the lower o/w microemulsion phase in equilibrium with... 

The work carried out in the last 25 years in the synthesis of the poly(alkyl methacrylates) as VI improvers is in the area of solution polymerization and is available mainly in patents. A microemulsion [38], which is a thermodynamically stable, isotropic system of oil, water, surfactant, and/or cosurfactant, was selected as the medium for polymerization as it gives rise to high-molecular-weight and more stereoregular polymers with narrow polydispersity. The products were also synthesized in an emulsion medium, and the performance of the additives from the two media was compared. 

Eigure 6 illustrates how the three tensions among the top, middle, and bottom phases depend on temperature for a system of nonionic surfactant—oil—water (38), or on salinity for a representative system of anionic surfactant—cosurfactant—oil—water and electrolyte (39). As T approaches from lower temperatures, the composition of M approaches the composition of T, and the iaterfacial teasioa betweea them, goes to 2ero at T =. ... 

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

A microemulsion droplet is a multicomponent system containing oil, surfactant, cosurfactant, and probably water therefore there may be considerable variation in size and shape depending upon the overall composition. The packing constraints which dictate size and shape of normal micelles (Section 1) should be relaxed in microemulsions because of the presence of cosurfactant and oil. However, it is possible to draw analogies between the behavior of micelles and microemulsion droplets, at least in the more aqueous media. 

Microemulsions are microstructured mixtures of oil, water, emulsifiers, and other substances. Since their structures differ in many ways from that of ordinary emulsions, it will be described separately. Liquid crystals (LC) are substances that exhibit special melting characteristics. Further, some surfactant-water-cosurfactant mixtures may also exhibit LC (lyotropic crystal) properties. 

A microemulsion is defined as a thermodynamically stable and clear isotropic mixture of water-oil-surfactant-cosurfactant (in most systems, it is a mixture of short-chain alcohols). The cosurfactant is the fourth component, which effects the formation of very small aggregates or drops that make the microemulsion almost clear. 

Microemulsions are thermodynamically stable mixtures. The interfacial tension is almost zero. The size of drops is very small, and this makes the microemulsions look clear. It has been suggested that microemulsion may consists of bicontinuous structures, which sounds more plausible in these four-component microemulsion systems. It has also been suggested that microemulsion may be compared to swollen micelles (i.e., if one solubilizes oil in micelles). In such isotropic mixtures, short-range order exists between droplets. As found from extensive experiments, not all mixtures of water-oil-surfactant-cosurfactant produce a microemulsion. This has led to studies that have attempted to predict the molecular relationship. 

Microemulsions are thermodynamically stable, homogeneous, optically isotropic solutions comprised of a mixture of water, hydrocarbons and amphiphilic compoxmds. The microemulsions are usually four- or three-component systems consisting of surfactant and cosurfactant (termed as emulsifier), oil and water. The cosurfactants are either lower alkanols (like butanol, propanol and hexanol) or amines (Hke butylamine, hexylamine). Microemulsions are often called swollen micelles (Fig. 3) and swollen re-... 

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