Microemulsions ionic surfactants

Whereas an ethoxylated alcohol with dodecyl tails (e.g. C12E5) forms middle-phase microemulsions, ionic surfactants with dodecyl tails, such as sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB), are too hydrophilic for formation of middle-phase microemulsions. Simply increasing the length of the hydrocarbon tail to compensate for the high hydrophilicity of the ionic head-groups favours the formation of viscous liquid crystal line phases rather than fluid microemulsion phases (36, 37). However, increasing the hydrophobicity by adding double tails to the surfactant, as for example with didodecyldimethylam-monium bromide surfactant (DDAB), suppresses some of the tendency to form liquid crystals, and allows for formation of oil-rich microemulsions (38). However, this surfactant is too hydrophobic, and is far from the... 

Since salt must be added to Aerosol OT surfactant in order to form balanced middle-phase microemulsions (ionic surfactants are salt-sensitive), and salt also induces a lipophilic shift in ethoxylated alcohol mixtures (nonionics are less salt-sensitive than ionics), the salt concentration (e) is an important tuning parameter in addition to 5. As an example, consider the system of C)2E5/AOT/decane/water/NaCl, where 8 is the amount of AOT in the C12E5 plus AOT mixture. 

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... 

With ionic surfactants for which V/1 <0.7, microemulsion formation needs the presence of a cosurfactant. The latter has the effect of increasing V without affecting 1 (if the chain length of the cosurfactant does not exceed that of the surfactant). These cosurfactant molecules act as "padding" separating the head groups. 

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... 

RD.I. Eletcher and D.I. Horsup Droplet Dynamics in Water-in-Oil Microemulsions and Macroemulsions Stabilized by Non-Ionic Surfactants. J. Chem. Soc. Earaday Trans. I 88, 855 (1992). 

Consequently, the SDS microemulsion system is the best model for indirect measurement of log Pow. However, this is valid only for neutral solutes. We reported that the relationship between MI and log Pow for ionic solutes is different from that for neutral solutes (49). This would be caused by the ionic interaction between ionic solutes and the ionic microemulsion as well as ionic surfactant monomer in the aqueous phase. Kibbey et al. used pH 10 buffer for neutral and weak basic compounds and pH 3 buffer for weak acidic compounds (53). Although their purpose was to avoid measuring electrophoretic mobility in the aqueous phase, this approach is also helpful for measuring log Pow indirectly. 

Quantum Dot Synthesis Using Ionic Surfactant-Based Microemulsions. . . 213... 

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

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. 

The ultrafiltration of the microemulsion is a very useful operation for separating water and oil in these mixtures [117-120]. Because of the limited availability of solvent stable membranes, most of the work pubHshed so far was performed using ceramic membranes, which show a high adsorption of surfactant at the membrane surface and comparably low rejection rates of reverse micelles. Using electro ultrafiltration, where the concentration polarisation phenomenon of the reverse micelles (using the ionic surfactant AOT) at the membrane surface is depressed by asymmetric high voltage electrical fields, the rejection rates can be increased,but not to economical values [121,122]. 

Riess demonstrated recently that poly(styrene-b-oxirane) copolymers could act as non-ionic surfactants and lead to water/ toluene microemulsions (29, 30). Using isopropanol as cosurfactant, both 0/W and W/0 microemulsions are obtained (3l). This is a very important conclusion, since PO based diblock copolymers give rise only to 0/W microemulsions under the same experimental conditions (8, 31,). In this respect the "branched structure" of the PO hydrophilic component could favor a decrease in the packing density of the inverse micelle forming molecular and explain the different behavior of the linear and star-shaped PS/PO block copolymers in the W/0 microemulsification process. 

Markowitz et al. developed a different approach, again in an attempt to overcome some of the inherent difficulties that arise when imprinted bulk materials are used as catalysts [82], Here, the authors used a template-directed method to imprint an a-chymotrypsin TSA at the surface of silica nanoparticles, prepared with a number of organically modified silanes as functional monomers. Silica particle formation was performed in a microemulsion, where a mixture of a non-ionic surfactant and... 

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... 

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]. 

Fig. 12. Effect of addition of ionic surfactant to a microemulsion based on a nonionic surfactant on the rate of synthesis of decyl sulfonate from decyl bromide and sodium sulfite (from [60])...

The development of transparent polymer electrolyte membrane from the bi-continuous-microemulsion polymerization of 4-vinylbenzene sulfonic acid Hthium salt (VBSIi), acrylonitrile and a polymerizable non-ionic surfactant, co-methoxypoly(ethylene oxide)4o-undecyl-a-methacrylate (Ci-PEO-Cn-MA-40) was reported in 1999 [94,95]. The ionic conductivities of the polymer electro-... 

Fig. 8 SEM micrograph of the polymerized microemulsion solid, that contains the polymerizable non-ionic surfactant Ci-PEO-C,-MA-40 [96], after ethanol extraction...

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. 

The most simple representation, found in Figure 3, demonstrates the solubilization dependence on the amount of ionic surfactant dissolved in water. The amount of ionic surfactant in water is the critical condition to obtain O/W microemulsions. 

It should be noted that high concentrations of ionic species can alter the phase stability of microemulsions based upon ionic surfactant systems. Nonionic surfactant systems are much less susceptible to this effect. The curvature of the interfacial film of the microemulsion droplet is determined by a balance between the electrostatic interactions of the head groups and repulsive interactions of the surfactant tail group. Addition of ionic solutes can upset this delicate balance and induce phase separation. By changing the structure of the surfactant or through the addition of cosurfactants one can restore this balance and thus allow the dissolution of high concentrations of ionic species. 

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