Microemulsions with double-tailed surfactant

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102]. 

This may be quite important in processes in which the ionic strength is determinant such as sol-gel transitions or chemical reactions in microemulsions [137-141]. Double-tailed surfactants such as dioctyl sulfosuccinate or diallQ lmethylammonium salts are likely to produce either vesicles (with excess water) or inverse W/O microemulsions with a polar core [142,143] that is used as a nanoreactor for a score of processes such as esterification or hydrolysis [144] in which enzymes are immobilized in an organogel [145]. Organogels can be made so that their structure depends on the composition of the microemulsion [146-148]. 

Further studies have demonstrated that PFPE-based surfactants can form microemulsions (with water cores) in supercritical CO2 (21). At higher water loadings, the CO2 was saturated with water and micelles began to solubilize water, which demonstrated bulk-like properties using spectroscopic probes. Although the PFPE-ammonium carboxylate surfactant was able to aggregate in CO2 at low water concentrations, a double-tailed surfactant, Mn(PFPE)2, was not soluble in CO2 without water. However, in the presence of water, Mn(PFPE)2-based micelles formed and the water core was able to ionize the manganese. 

The actual structure also depends on the surfactant molecular structure. For instance, dual-tail amphiphiles such as sulfosuccinate surfactants are more likely to produce W/O-type miniemulsions and microemulsions with water core islands. If too much water is present, because of the inability of this surfactant to accommodate its branched double tails in an oily core, it would result in more complex structures such as vesicles, in which a surfactant bilayer closes on itself, as shown in Fig. 4. 

In a 1994 work, the effect of the surfactant on lipase-catalyzed hydrolysis of palm oil in microemulsion was further investigated [62]. Three surfactants were used one anionic, one nonionic, and one cationic. As shown in Fig. 10, all three compounds were double-tailed, with similar hydrophilic-lipophilic balance, giving large regions of L2 microemulsions with isooctane and water at 37°C. 

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... 

The rate of decomposition in sewage plants of this class of nonionic surfactants is much higher than for normal ethoxylates [28]. Nonionic dioxolane surfactants can also be made by reacting the remaining hydroxyl group with a reactive derivative of a monomethyl-PEG. When the dioxolane is made from a ketone, a double-tailed, easily degradable nonionic surfactant is obtained, and this surfactant has been used for formulation of microemulsions that decompose in weak acid [29]. 

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... 

Photoredox reactions at organized assemblies such as micelles and microemulsions provide a convenient approach for modeling life-sustaining processes. Micelles are spontaneously formed in solutions in the presence of surfactants above a certain critical concentration. In aqueous solutions, the hydrophobic tails of the surfactant form aggregates with the polar head facing toward the aqueous environment, as depicted in Fig. 9. The hydrophobic core in micelles is amorphous and exhibits properties similar to a liquid hydrocarbon. The polar heads are also randomly oriented, generating an electrical double layer around the micelle structure. In this respect, surface properties of micelles can be somewhat correlated with the polarized ITIES. The structure of micelles is in dynamic equilibrium, in which monomers are exchanged between bulk solution and the assembly. 

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