Cosurfactants, in microemulsion

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]. 

Nonionics can function as cosurfactants in microemulsions. This cosurfactant role, which can also be performed by short-chain alcohols or glycol ethers, can make it possible to form microemulsions or other phases from anionic surfactant systems that would otherwise be unreachable in a pure anionic system. By one definition, a microemulsion system is one that achieves a zero or close-to-zero curvature [108]. This is easily achievable in nonionic systems, but very difficult in pure anionic surfactant systems due to the head group repulsions. Mixing the nonionic with the anionic makes these types of systems, which sometimes have greater cleaning ability, easily accomplished. 

Alcohols are often used as cosurfactants in microemulsions, and insight has been obtained from the electrochemistry of ferrocene alcohols (4). Oxidations of FcOHCio, FcOHCu, and FcOHCig [34] were nearly reversible and controlled by diffusion in microemulsions of DDAB, CTAC, or SDS. In micellar solutions, electrode reactions were more complex and reflected strong adsorption of the ferrocene alcohols onto the electrode. 

Uses Excipient, solubilizer, bioavailability enhancer, cosurfactant, vehicle for pharmaceuticals (orals, nasals, creams, aerosols, emulsions, soft gel capsules, hard shell capsules), cosmetics, veterinary preps. amphiphilic agent improving drug delivery coemulsifier in topical emulsions lipid phase or cosurfactant in microemulsions... 

Uses Excipient, bioavailability enhancer, coemulsifier for self-microemulsifying drug delivery systems carrier, solubilizer for liqs. and capsules oil phase or cosurfactant in microemulsions penetrant in topical preps. food additive Reguiatory FDA 21CFR 172.856 worldwide food additive status E477, JSFA... 

The above example highlights the important role of a polymerizable surfactant or cosurfactant in microemulsion polymerization. Gan and Chew [38] were thus able to produce transparent solid polymers by fully polymerizing 54 % MMA, 34 % AA, 10 % H2O, and 2 % SDS or with other lower water concentrations. On further replacement of SDS by a polymerizable surfactant, sodium acrylamidoundecanoate [37], the resulting transparent solid polymers were still compatible with the similar amount of water. This prompted further studies on copolymerization in microemulsions of MMA, AA, and sodium acrylamidostearate (NaAAS), all the three components being readily polymerizable and amenable to terpolymerization in microemulsions. 

It should be pointed out that although the preceding discussion was concerned with the use of alcohols as cosurfactants in microemulsion formation, many other types of material can also be used to the same end. Especially important are primary amines (commonly used with cationic surfactants) and thiols. 

The diastereoselection of the Diels Alder reaction of methyl acrylate with cyclopentadiene was investigated [74] in microemulsions prepared with isooctane oil, CTAB as surfactant and 1-butanol as cosurfactant, and the results were compared with those found in pure solvents and water (Table 6.12). In emulsions rich in 1-butanol and formamide (entries 1 and 4) the reaction was slow (72 h) and the diastereoselectivity was practically the same as that... 

Another example of chemical-potential-driven percolation is in the recent report on the use of simple poly(oxyethylene)alkyl ethers, C, ), as cosurfactants in reverse water, alkane, and AOT microemulsions [27]. While studying temperature-driven percolation, Nazario et al. also examined the effects of added C, ) as cosurfactants, and found that these cosurfactants decreased the temperature threshold for percolation. Based on these collective observations one can conclude that linear alcohols as cosurfactants tend to stiffen the surfactant interface, and that amides and poly(oxyethylene) alkyl ethers as cosurfactants tend to make this interface more flexible and enhance clustering, leading to more facile percolation. 

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. 

An additional point is that relatively high concentrations of surfactant, oil and cosurfactant are often used in microemulsions. Thus the volume of the microemulsion pseudophase is large and droplet-bound reactants are therefore diluted. Generally speaking, rate enhancements increase in the sequence microemulsions < micelles < vesicles simply because of a decrease in the volume of the micellar or droplet pseudophase. 

Solubilisation can best be illustrated by considering the phase diagrams of non-ionic surfactants containing poly(oxyethylene oxide) head groups. Such surfactants do not generally need a cosurfactant for microemulsion formation. At low temperatures, the ethoxylated surfactant is soluble in water... 

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. 

Thus it can be concluded that the structure of microemulsions depends on the structure of surfactant and cosurfactant. Moreover, this structure also determines the amount of solubilisation of oil and or water in microemulsions. 

Winsor [15] classified the phase equilibria of microemulsions into four types, now called Winsor I-IV microemulsions, illustrated in Fig. 15.5. Types I and II are two-phase systems where a surfactant rich phase, the microemulsion, is in equilibrium with an excess organic or aqueous phase, respectively. Type III is a three-phase system in which a W/O or an O/W microemulsion is in equilibrium with an excess of both the aqueous and the organic phase. Finally, type IV is a single isotropic phase. In many cases, the properties of the system components require the presence of a surfactant and a cosurfactant in the organic phase in order to achieve the formation of reverse micelles one example is the mixture of sodium dodecylsulfate and pentanol. 

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. 

Equation (8) constitutes the basic thermodynamic equation for the calculation of the radius of the globules. Of course, explicit expressions, in terms of the radius of the globules and volume fraction, are needed fort, C and af before such a calculation can be carried out. Expressions for Af will be provided in another section of the paper, but it is difficult to derive expressions fory and C. One may, however, note that y (and also C) depends on the radius for the following two reasons (1) its value depends upon the concentrations of surfactant and cosurfactant in the bulk phases, which, because the system is closed, depend upon the amounts adsorbed on the area of the internal interface of the microemulsion (2) in addition to the above mass balance effect, there is a curvature effect on y (this point is examined later in the paper). 

Safari, M., Kermasha, S. 1994. Interesterification of butterfat by commercial microbial lipases in a cosurfactant-free microemulsion system. J. Am. Oil Chem. Soc. 71, 969-973. 

Polymerization of styrene in microemulsions has produced porous solid materials with interesting morphology and thermal properties. The morphology, porosity and thermal properties are affected by the type and concentration of surfactant and cosurfactant. The polymers obtained from anionic microemulsions exhibit Tg higher than normal polystyrene, whereas the polymers from nonionic microemulsions exhibit a lower Tg. This is due to the role of electrostatic interactions between the SDS ions and polystyrene. Transport properties of the polymers obtained from microemulsions were also determine. Gas phase permeability and diffusion coefficients of different gases in the polymers are reported. The polymers exhibit some ionic conductivity. 

The degree of retardation is dependent on the micellar structure of the system. Hoefner and Fogler (75) described one such microemulsion system containing cetylpyridinium chloride and butanol as the surfactant-cosurfactant in a 35 65 weight ratio. In this system, dodecane was used as the... 

We report here on the use of alkyl sulfones as novel unconventional cosurfactants in CTAB-stabilized microemulsions. Sulfones, being fully oxidized at sulfur, have good stability to oxidants such as hypohalite. 

The microemulsions studied show a single resonance for the alpha-carbon appearing midway between those of its expected model environments. The diffusion of cosurfactant within microemulsions is very rapid on an NMR time scale as has been confirmed in a series of studies (27,28,29). In this case, the observed chemical shift is a weighted average of those of the different environments in which the cosurfactant is located. For our system, the observed shift could be represented as... 

Pattarino, F., Marengo, E., Trotta, M. and Gasco, M.R. (2000) Combined use of lecithin and decyl polyglucoside in microemulsions domain of existence and cosurfactant effect. /. Disp. Sci. Technol, 21, 345-363. 

Recently, MEEKC has been applied for the determination of catechins. In a similar fashion to MEKC, MEEKC uses a pseudo-stationary phase for separation of analytes. Instead of having micelles, surfactant-stabilized oil droplets in microemulsion solution serve as the pseudo-stationary phase for the partitioning of analytes. The formation of a microemulsion involves an organic solvent, SDS as a surfactant, and an alcohol as a cosurfactant. Pomponio et al. first developed a MEEKC method for the analysis of six catechins (C, EC, GC, ECG, EGC, and EGCG). This involved the use of heptane (1.36%, w/v), SDS (2.31%, w/v), and 1-butanol (9.72%, w/v), which serve, respectively, for the three functions mentioned above. The medium was a 50-mM phosphate buffer (86.61%, w/v) at pH 2.5. Electro-osmotic force (EOF) was almost absent, and analytes, due to their interactions with SDS-coated oil droplets. 

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