Nonionic surfactant microemulsion formation

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. 

It has been shown that the addition of a small amount of the anionic surfactant sodium dodecyl sulfate (SDS) to a microemulsion based on nonionic surfactant increased the rate of decyl sulfonate formation from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2) [59,60]. Addition of minor amounts of the cationic surfactant tetradecyltrimethylammonium gave either a rate increase or a rate decrease depending on the surfactant counterion. A poorly polarizable counterion, such as acetate, accelerated the reaction. A large, polarizable counterion, such as bromide, on the other hand, gave a slight decrease in reaction rate. The reaction profiles for the different systems are shown in Fig. 12. More recent studies indicate that when chloride is used as surfactant counterion the reaction may at least partly proceed in two steps, first chloride substitutes bromide to give decyl chloride, which reacts with the sulfite ion to give the final product [61]. 

When a single chain anionic surfactant (such as sodium dodecyl sulfate, SDS) is used, it generally requires a cosurfactant for the formation of a microemulsion. A cosurfactant may not be needed to form a microemulsion if nonionic surfactant(s), certain types of cationic surfactants, or double-chain surfactants such as sodium l,4-bis(2-ethylhexyl)sulfosuccinate (Aerosol OT or simply AOT) are used. 

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... 

In addition, Shinoda and Friberg (53) have summarized their extensive studies on the formation of microemulsions using nonionic surfactants. They proposed the following conditions to form microemulsions with minimum amount of surfactants ... 

Solubilisation can also be illustrated by considering the phase diagrams of nonionic surfactants containing polyfethylene oxide) (PEO) head groups. Such surfactants do not generally need a cosurfactant for microemulsion formation. 

In some cases a single surfactant may be sufficient for lowering y far enough for microemulsion formation to become possible examples include Aerosol OT (sodium diethyl hexyl sulphosuccinate) and many nonionic surfactants. 

Liquid-crystalline phase or microemulsion formation between surfactant, water, and oily soil accompanies oily soil removal from hydrophobic fabrics such as polyester (Raney, 1987 Yatagai, 1990). It has been suggested (Miller, 1993) that maximum soil removal occurs not by solubilization into ordinary micelles, but into the liquid-crystal phases or microemulsions that develop above the cloud point of the POE nonionic. 

Not only is aggregation favored as R increases, but there is also a change in the nature of the solubilized water. Initially, water is tightly hydrogen-bonded to the oxyethylene groups of the nonionic surfactant Further water addition induces aggregation (dipole-dipole interactions), and a point is reached where unbound or free water molecules are present in the hydrophilic (polar) domain. The state of water in the polar domain is relevant to the formation of particles, because the initial hydrolysis of TEOS in the reverse micelle is expected to be favored as more free water molecules are available. Such an effect is expected because the hydrolysis of titanium alkoxides (naturally more reactive than TEOS toward water) is strongly inhibited in reverse microemulsions formulated with nonionic or anionic surfactants at low R values [10,16]. 

In another study, radiolabeled and fluorescent lipid nanocapsules were synthesized by using a phase inversion process that followed the formation of an o/w microemulsion containing triglycerides, lecithins, and a nonionic surfactant. Results of the experiment revealed that lipid nanocapsules were rapidly accumulated within cells through active and saturating mechanisms. Nanocapsules could bypass the endo-lysosomal compartment with only 10% of the cell-internalized fraction found in isolated lysosomes. When nanocapsules were loaded with paclitaxel, smallest lipid nano capsules (LNCs) also were found to trigger the best cell death activity. ... 

A great variety of surfactants have been used in microemulsion formation. These include common soaps, other anionic and cationic surfactants, nonionic surfactants of the polyethylene oxide type, and other structures. The hydrophobic part contains one or two linear or branched hydrocarbon chains containing about 8-18 carbon atoms. Quite often microemulsions require, in addition to oil, water, and surfactant, the presence of simple electrolytes, alcohols, and/or other weakly surface-active substances. 

Figure 19 Double-oil diffusion experiment with nonionic surfactant, (a) Self-diffusion coefficients and (b) diffusion coefficient ratio A" as a function of temperature in a water-rich microemulsion with nonionic surfactant. A transition from oil-in-water droplets to a bicontinuous microstructure occurs with increasing temperature (decreasing spontaneous curvature of the C12E5 surfactant film). The maximum in K indicates that an attractive interaction between the micelles is operating prior to the formation of a bicontinuous structure. Kq = 1.69 is the diffusion coefficient ratio in the pure oil mixture and is indicated as a broken line in (b). Note that the initial decrease of the self-diffusion coefficients shows that the droplets grow in size before the bicontinuous transition. The phase boundary at 25.7 C is indicated as a vertical broken line. (Data from Ref 43.)...

Despite the reasonable tolerance of nonionic surfactants, particularly in topical applications, microemulsions prepared from (phospho)lipids seem to be preferred over those prepared by synthetic surfactants from a toxicity point of view. As discussed by Shinoda et al. [13], lecithin in water-oil systems does not spontaneously form the zero mean curvature amphiphile layers required for the formation of balanced microemulsions but rather forms reverse structures. On decreasing the polarity of the aqueous phase by addition of a short-chain alcohol, e.g., propanol, lecithin was found to form microemulsions at low amphiphile concentrations over wide ranges of solvent composition. The structure of the microemulsions formed was investigated by NMR self-diffusion measurements, and it was found that with a decreasing propanol concentration there was a gradual transition from oil droplets in water, over a bicontinuous structure, to water droplets in oil [13]. 

To form a microemulsion three ingredients are necessary polar solvent (water), apolar solvent (oil), and surfactant. Since typical microemulsions only occur under rather selective circumstances it is in practice necessary to have an additional tuning variable that can be adjusted to obtain optimal conditions for microemulsion formation. In the early studies of Schulman et al. (3) the amount of cosurfactant was used to tune the systems in addition to the salt concentration. This introduces a fourth (cosurfactant) and sometimes a fifth (salt) component, making the ther-modynamic description nearly intractable. Below we illustrate the basic principles by staying with three-component systems, using the temperature as the tuning variable. This situation is most easily realized in practice with nonionic surfactants of the type, where E denotes an ethylene oxide unit. 

Kunieda, H., Ozawa, K., Aramaki, K., Nakano, A., and Solans, C. (1998) Formation of microemulsions in mixed ionic-nonionic surfactant systems. Langmuir, 14, 260-263. 

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