Nonionic surfactants microemulsions

The ultralow interfacial tension can be produced by using a combination of two surfactants, one predominantly water soluble (such as sodium dodecyl sulfate) and the other predominantly oil soluble (such as a medium-chain alcohol, e.g., pentanol or hexanol). In some cases, one surfactant may be sufficient to produce the microemulsion, e.g., Aerosol OT (dioctyl sulfosuccinate), which can produce a W/O microemulsions. Nonionic surfactants, such as alcohol ethoxylates, can also produce O/W microemulsions, within a narrow temperature range. As the temperature of the system increases, the interfacial tension decreases, reaching a very low value near the phase inversion temperature. At such temperatures, an O/W microemulsion may be produced. 

Many solutions of common nonionic surfactants and water separate into two phases when heated above a certain temperature (the cloud point), and some investigators call the phase of greater surfactant concentration, a microemulsion. Thus, there is not even universal agreement that a microemulsion must contain oil. 

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. " ... 

Surfactant Solutions New Methods of Investigation, edited by Raoul Zana Nonionic Surfactants Physical Chemistry, edited by Martin J. Schick Microemulsion Systems, edited by Henri L Rosano and Marc Clausse Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C. C. Gray... 

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,... 

Ethoxylated fatty esters, emulsifiers, detergents, and dispersants, 8 710t Ethoxylated nonionic surfactants, microemulsions based on, 16 428 Ethoxylated surfactants, 24 142, 148 Ethoxylates, 24 149-151 Ethoxylation, fatty amines, 2 523 2-Ethoxypyridine, 21 104 Ethoxyquin, 10 854 13 42t, 51 2-Ethyl-1-butanol... 

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. 

L.T. Lee, D. Langevin, J. Meunier, K. Wong, and B. Cabane Film Bending Elasticity in Microemulsions Made with Nonionic Surfactants. Prog. Colloid Polymer Sci. 81, 209 (1990). 

Poprawski J, Catt M, Marquez L, Marti MJ, Salager JL, Aubry JM (2003) Application of hydrophilic-lipophilic deviation formulation concept to microemulsions containing pine oil and nonionic surfactants. Polym Int 52 629-632... 

Binks BP, Hetcher PDl, Taylor DJF (1998) Microemulsions Stabilized by lonic/Non-ionic Sm-factant Mixtures. Effect of Partitioning of the Nonionic Surfactant into the Oil. Langmuir 14 5324-5326... 

Dye-Doped Silica Nanoparticle Synthesis Using Nonionic Surfactant-Based Microemulsion Systems... 

In another communication using w/o microemulsions containing a nonionic surfactant, it is shown that TEOS hydrolysis and siUca-particle growth occur at the same rate, indicating the growth of siUca particles is rate-controlled by the hydrolysis of TEOS [54], The rate of TEOS hydrolysis also depends on the surfactant concentration, which controls the molecular contact between hydroxyl ions and TEOS in solution. Because of the reaction-controlled growth mechanism, the silica-particle size distribution remains virtually same over the growth period. 

Different shapes of CdS nanomaterials such as quasi-nanospheres, nanoshuttles and nanotubes have been prepared in the w/o microemulsion system of nonionic surfactant (Tween-80, polyetheneoxy(20)octadecyl ether, polyoxyethylele(9)dodecyl ether (C12E9) or Tx-100)/ -pentanol/aqueous so-lution/cyclohexane [191]. Another kind of QDs, PbS, have also been prepared in nonionic CnEg/water/cyclohexane microemuisions [192]. 

The choice of surfactant, which is mostly constrained by the choice of the oil and the resulting phase behaviour of the microemulsion, can have different effects on the enzyme stability and activity. In general we have to differentiate between ionic and nonionic surfactant types ... 

In contrast to nonionic surfactants, ionic surfactants build up a high zeta-po-tential at the water-oil interface which can also can influence the enzyme activity. Most investigated systems used AOT as the surfactant because its phase behaviour is well understood. However, AOT is often not very suitable, because it can totally inhibit enzymes (e.g. the formate dehydrogenase from Candida bodinii). The usage of lipases in AOT-based microemulsions is generally unfavourable as AOT is an ester that is hydrolysed itself. 

Microemulsions are dynamic systems in which droplets continually collide, coalesce, and reform in the nanosecond to millisecond time scale. These droplet interactions result in a continuous exchange of solubilizates. The composition of the microemulsion phase determines the exchange rate through its effect on the elasticity of the surfactant film surrounding the aqueous microdomains. Compared with nonionic surfactant-based microemulsions, AOT reverse micelles have a more rigid... 

Water-in-fluorocarbon emulsions, stabilised with fluorinated nonionic surfactants, were investigated by small angle neutron scattering (SANS) spectroscopy [8,99]. The results indicated that the continuous oil phase comprised an inverse micellar solution, or water-in-oil microemulsion, with a water content of 5 to 10%. However, there was no evidence of a liquid crystalline layer at the w/o interface. A subsequent study using small angle x-ray scattering (SAXS) spectroscopy gave similar results [100]. 

We saw in Section 8.6 that phase diagrams are an effective way of representing the complex behavior of surfactant systems. Let us take a look at microemulsions in terms of phase diagrams. It turns out that nonionic surfactants form microemulsions at certain temperatures without requiring cosurfactants. Since only three components are present, these have somewhat simpler phase diagrams this kind of system offers a convenient place to begin. 

What are the most important factors influencing the type of microemulsion Here again we have to distinguish between nonionic and ionic surfactants. For nonionic surfactants, often alkylethylene glycols, temperature is the dominating parameter for the structure of a microemulsion. For ionic surfactants, mostly SDS or CTAB, the salt concentration dominates... 

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). 

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