Surfactants emulsions/microemulsions

Typical approaches to this biphasic system have involved the immobilization of catalysts in the aqueous phase as colloids [53] or using water-soluble catalysts based on ligands such as the trisulfonated TPPTS [55, 56]. Particularly high reaction rates have been obtained with surfactant-stabilized microemulsions and emulsions that allow for intimate contact of all reagents with the catalyst during the reaction [57]. The emulsions separate readily into two phases by small pressure changes and the C02-phase is then vented to isolate the products. The catalyst RhCl(tppds)3 (tppds =... 

Various excipients have been used as solubilizers for BCS class II and class IV drugs. Cyclodextrins provide a prime example of the use of excipients as solubilizers, and have been discussed in detail in a separate chapter. Various surfactants have also been used to create emulsion-/microemulsion-type formulations. These have been discussed in a separate chapter as well. 

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). 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

Oil and water do not mix, but on addition of a suitable surfaetant a microemulsion can be formed depending on the relative concentrations of the three components. Microemulsions (i.e. surfactant/water/oil mixtures) can also be used as reaction media see references [859-862] for reviews. Microemulsions are isotropic and optically clear, thermodynamically stable, macroscopically homogeneous, but microscopically heterogeneous dispersions of oil-in-water (O/W) or water-in-oil (W/O), where oil is usually a hydrocarbon. The name microemulsion, introduced by Schulman et al. in 1959 [863], derives from the fact that oil droplets in O/W systems or water droplets in W/O systems are very small (ca. 10... 100 nm nanodroplets). Unlike conventional emulsions, microemulsion domains fluctuate in size and shape with spontaneous coalescence and breakup. The oil/water interface is covered with surfactant molecules and this area can amount to as much as 10 m per litre ( ) of microemulsion. 

A similar study was performed on the formation of nano crystalline HA in nonionic surfactant emulsions [163]. Instead of using NP-5/NP-9 surfactant, KB6ZA (nonionic surfactant which is a lauryl alcohol condensed with an average of 6 mol of oxyethylene oxide) was used together with petroleum ether as the oil phase to prepare HA powder in an 0/W emulsion system. One of the very apparent advantages of using 0/W emulsion over W/0 microemulsion is... 

Microemulsions. The structure of microemulsion systems has been reviewed (22). Both bicontinuous and droplet-type structures, among others, can occur in microemulsions. The droplet-type structure is conceptually more simple and is an extension of the emulsion structure that occurs at relatively high values of IFT. In this case, very small thermodynamically stable droplets occur, typically smaller than 10 nm (7). Each droplet is separated from the continuous phase by a monolayer of surfactant. Bicontinuous microemulsions are those in which oil and water layers in the microemulsion may be only a few molecules thick, separated by a monolayer of surfactant. Each layer may extend over a macroscopic distance, with many layers making up the microemulsion. 

The chapter has dealt with the stability and stabilisation of colloidal systems and covered topics such as their formation and aggregation. If the particle size of a colloidal particle determines its properties (such as viscosity or fate in the body), then maintenance of that particle size throughout the lifetime of the product is important. The emphasis in the section on stability is understandable. Various forms of emulsions, microemulsions and multiple emulsions have also been discussed, while other chapters deal with other important colloidal systems, such as protein and polymer micro- and nanospheres and phospholipid and surfactant vesicles. 

Surfactant aggregates (microemulsions, micelles, monolayers, vesicles, and liquid crystals) are recently the subject of extensive basic and applied research, because of their inherently interesting chemistry, as well as their diverse technical applications in such fields as petroleum, agriculture, pharmaceuticals, and detergents. Some of the important systems which these aggregates may model are enzyme catalysis, membrane transport, and drug delivery. More practical uses for them are enhanced tertiary oil recovery, emulsion polymerization, and solubilization and detoxification of pesticides and other toxic organic chemicals. 

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. 

Several classes of formulations of disperse systems are encountered in the chemical industry, including suspensions, emulsions, suspoemulsions (mixtures of suspensions and emulsions), nanoemulsions, multiple emulsions, microemulsions, latexes, pigment formulations, and ceramics. For the rational preparation of these multiphase systems it is necessary to understand the interaction forces that occur between the particles or droplets. Control of the long-term physical stability of these formulations requires the application of various surfactants and dispersants. It is also necessary to assess and predict the stability of these systems, and this requires the application of various physical techniques. 

Microemulsions. Unlike emulsions, microemulsions are transparent and thermodynamically stable colloidal systems, formed under certain concentrations of surfactant, water, and oil (Fig. 18.8). The transparency is because the droplet size of the microemulsions is small enough (<100 nm) that they do not reflect light. Because of its thermodynamic stability, microemulsions may have long shelf lives and spontaneously form with gentle agitation. However, microemulsions are not infinitely stable upon dilution because dilution... 

With recent theoretical and experimental advances in the understanding of colloid and interface science of SCF systems, it is becoming possible to design surfactants for microemulsions, emulsions, and latexes on a rational basis. The-... 

The enhanced oil recovery using solutions of surfactants or their mixtures has attained relatively little application. This is, first of all, due to the fact that surfactants adsorb from the solution on porous media of the reservoir, the specific surface of which may range from 150 to 3000 cmVcm, therefore, the use of emulsions, microemulsions and the so-called micellar-polymer flooding turned out to be more effective. In all of these processes, the flow... 

Depending on the properties of the amphiphilic molecules used, of the temperature and of the oil and water fractions, 0/W or W/0 microemulsions can be formed, but O/W are those with highest potential application to the delivery of bioactive compounds in food products. In comparison to emulsions, microemulsions require the use of relatively large amounts of surfactant, with the consequence of their loading capacity being significantly lower. ... 

Surfactant molecules commonly self-assemble in water (or in oil). Even single-surfactant systems can display a quite remarkably rich variety of structures when parameters such as water content or temperature are varied. In dilute solution they form an isotropic solution phase consisting of micellar aggregates. At more concentrated surfactant-solvent systems, several isotropic and anisotropic liquid crystalline phases will be formed [2]. The phase behavior becomes even more intricate if an oil (such as an alkane or fluorinated hydrocarbon) is added to a water-surfactant binary system and the more so if other components (such as another surfactant or an alcohol) are also included [3], In such systems, emulsions, microemulsions, and lyotropic mesophases with different geometries may be formed. Indeed, the ability to form such association colloids is the feature that singles out surfactants within the broader group of amphiphiles [4]. No wonder surfactants phase behavior and microstructures have been the subject of intense and profound investigation over the course of recent decades. 

Figure 3 Weight percentage of surfactants required for the emulsion-microemulsion transition versus the HLB value for various compositions of the monomer feed. (From Ref. 30.)...

The importance of the above mentioned parameters depends on the type of surfactant formulations such as emulsions, microemulsions, foams or foams with steam etc. It is a difficult task to generalize the significance of an individual parameter. In general, the following properties are suggested to consider before selecting a surfactant for EOR processes. 

Emulsions are defined as dispersions of one liquid in another, stabilized by an interfacial film of emulsifiers such as surfactants and lipids. Emulsion formulations include water in od and oil in water emulsions, multiple emulsions, microemulsions, microdroplets, and liposomes. Microdroplets are unilamellar phospholipid vesicles that consist of a spherical lipid layer with an oil phase inside. 

Dr. Miller s research interests center on equilibrium and dynamic phenomena in oil/water/surfactant systems, specifically interfacial stability and behavior of emulsions, microemulsions and foams and their application in areas such as detergency, enhanced oil recovery and environmental remediation. He is a Fellow of the American Institute of Chemical Engineers and a member of the American Chemical Society, American Oil Chemists Society, International Association of Colloid and Interface Scientists, and the Society of Petroleum Engineers. He has published numerous research papers and review articles on interfadal phenomena, served on the editorial boards of leading journals in the field, and given invited lectures at conferences, universities and industrial laboratories in many countries. 

SEDDs and SMEDDs are used for in situ formation of emulsions/microemulsions in the GI tract. Those systems include an oil solution of the drug, surfactants, cosolvents, and other water-soluble... 

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