Surfactants microemulsion-forming

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102]. 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

Liquid Crystal Third Phase. In addition to micelles and microemulsion droplets, surfactants may form Hquid crystals. A Hquid crystal is a separate phase, which comes out of solution, not like the micelles or microemulsion droplets, which are microscopic entities within the solution. 

The phase inversion temperature (PIT) method is helpful when ethoxylated nonionic surfactants are used to obtain an oil-and-water emulsion. Heating the emulsion inverts it to a water-and-oil emulsion at a critical temperature. When the droplet size and interfacial tension reach a minimum, and upon cooling while stirring, it turns to a stable oil-and-water microemulsion form. " ... 

Liu, J., Han, B., Li, G., Zhang, X., He, J. and Liu, Z. (2001) Investigation of nonionic surfactant Dynol-604 based reverse microemulsions formed in supercritical carbon dioxide. Langmuir,... 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

The packing ratio also explains the nature of microemulsion formed by using nonionic surfactants. If v/a 1 increases with increase of temperature (as a result of reduction of a ), one would expect the solubilisation of hydrocarbons in nonionic surfactact to increase with temperature as observed, until v/a l reaches the value of 1 where phase inversion would be expected. At higher temperatures, va l > 1 and water in oil microemulsions would be expected and the solubilisation of water would decrease as the temperature rises again as expected. 

The influence of surfactant structure on the nature of the microemulsion formed can also be predicted from the thermodynamic theory by Overbeek (17,18). According to this theory, the most stable microemulsion would be that in which the phase with the smaller volume fraction forms the droplets, since the osmotic term increases with increasing i. For w/o microemulsion prepared using an ionic surfactant, the hard sphere volume is only slightly larger than the water volume, since the hydrocarbon tails of the surfactant may interpenetrate to a certain extent, when two droplets come close together. For an oil in water microemulsion, on the other hand, the double layer may extend to a considerable extent, depending on the electrolyte concentration... 

Further information on the dependence of structure of microemulsions formed on the alcohol chain length was obtained from measurement of self diffusion coefficient of all the constitutents using NMR techniques (29-34). For microemulsions consisting of water, hydrocarbon, an anionic surfactant and a short chain alcohol and C ) the self diffusion... 

Recently, the phase equilibria of a microemulsion were reported. The phase behavior of a microemulsion formed with food-grade surfactant sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) was studied. Critical microemulsion concentration (cpc) was deduced from the dependence of the pressure of cloud points on the concentration of... 

Finally, surfactants that break down into non-surface active products in a controlled way may find use in speciahzed applications, such as in the biomedical field. For instance, cleavable surfactants that form vesicles or microemulsions can be of interest for drug dehvery, provided the metabolites are nontoxic. 

Khomane et al. prepared dodecanethiol-capped CdS QDs of 4 nm size by using a Winsor II microemulsion system [242], which are soluble in solvents such as n-heptane, toluene, n-hexane, thus demonstrating the dual role of the anionic surfactant, viz., forming the microemulsion and facilitating the extraction of oppositely charged ions from the aqueous to the organic phase. 

Microemulsions form spontaneously in much the same way as structural elements, such as surfactant micelles, rearrange themselves following the addition of the cosurfactant. Because the water may be incorporated into the hydrophilic structures of reverse micelles, when examined by x-ray analysis spherical droplets with diameters of 6-80 nm have been reported. 

Chapter 8 discusses self-assembly of surfactants to form micelles, models of micelliza-tion, use of micelles in catalysis and solubilization, and oil-in-water and water-in-oil microemulsions. 

In contrast to the conventional emulsions or macroemulsions described earlier are the disperse systems currently termeraiicroemulsions. The term was Lrst introduced by Schulman in 1959 to describe a visually transparent or translucent thermodynamically stable system, with much smaller droplet diameter (6-80 nm) than conventional emulsions. In addition to the aqueous phase, oily phase, and surfactant, they have a high proportion of a cosurfactant, such as an alkanol of 4-8 carbons or a nonionic surfactant. Whereas microemulsions have found applications in oral use (as described in the next chapter), parenteral use of microemulsions has been less common owing to toxicity concerns (e.g., hemolysis) arising from the high surfactant and cosolvent levels. In one example, microemulsions composed of PEG/ethanol/water/medium-chain triglycerides/Solutol HS15/soy phosphatidylcholine have been safely infused into rats at up to 0.5 mL/kg. On dilution into water, the microemulsion forms a o/w emulsion of 60-190 nm droplet size (Man Corswant et al., 1998). 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

A second group of researchers has noted a parallelism between emulsions and microemulsions formed with surfactants possessing a poly(ethylene oxide) head group [6,7], The emulsions were of the oil in water (O/W) kind at low tempera-... 

The inner core of microemulsion can dissolve water to form water core called water pool [1], Little water pools of microemulsion surrounded by the single molecule interface formed by surfactants and co-surfactants can form particles whose sizes are from tens to hundreds of A [2], In this paper, a microemulsion-based method, and the formation conditions and growth mechanism of nanosized nickel sulfide particles are reported. This research is the base of application of ultrafme NiS particles on catalysis and optics. 

The chapter by Fulton and Smith (Chapter 5) shows that ionic surfactants can form microemulsions with ethane and water under conditions that might be encountered in miscible floods with light hydrocarbons. These microemulsions correspond to the single-phase regions of the model diagrams in Figure 11. 

These are transparent isotropic structured fluids composed of two immiscible phases that are stabilized by surfactants. Often a co-surfactant and a co-solvent are present in the formulation. Microemulsions form spontaneously and are thermodynamically stable. Their transparency is due to the small droplet size (<100 nm) in microemulsions (Flanagan and Singh, 2006 Garti and Aserin, 2007). 

Salinity is essential for all chemical processes. It directly affects polymer viscosity, and it determines the type of microemulsion a surfactant can form. Salinity effects in waterflooding, in both sandstone and carbonate reservoirs, have recently drawn research interest. This chapter briefly discusses sahnity and ion exchange. At the end of this chapter, the sahnity effects on waterflooding in sandstone and carbonate reservoirs are summarized. 

Microemulsions are clear (transparent and translucent are also used in the literature), thermodynamically stable, isotropic liquid mixtures of oil, water, and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt(s) and/or other ingredients, and the oil may actually be a complex mixture of different hydrocarbons and olehns. In contrast to ordinary emulsions, microemulsions form upon simple mixing of the components and do not require high shear conditions generally used in the formation of ordinary emulsions. Microemulsions tend to appear clear due to the small size of the disperse phase. However, clear appearance (transparency) may not be a fundamental property. Sometimes microemulsion may not look clear to the naked eye in the case where dark viscous oil exists. The solution may not be purely transparent because it contains aggregates of micelles. Quite often, we still use these terms, even in this book. Probably we should simply use the term homogeneous solution. 

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