Nonaqueous microemulsions surfactants

Microemulsions are thermodynamically stable isotropically clear dispersions composed of a polar solvent, oil, and a surfactant(s). Labrafil and Gelucire 44/14 are all-in-one self-emulsifying surfactants which are in many oral products throughout the world. Microemulsions have much potential for drug-delivery since very hydrophobic molecules can be solubilized and formulated for oral administration (Tenjarla, 1999). All of the commercial products are actually nonaqueous microemulsions, also known as microemulsion preconcentrates or self-emulsifying drug delivery systems (SEDDS), since the polar solvent is not water. Upon contact with aqueous media, such as gastrointestinal fluids, a SEDDS formulation spontaneously forms a fine dispersion or aqueous microemulsion. 

The first reports on nonaqueous microemulsions, isotropic solutions containing a hydrophilic and a lipophilic component, stabilized by a surfactant, were made by Palit and McBain in 1946 [116] and by Winsor in 1948 [117]. They both used glycols as polar solvents. The microemulsion regions were only observed visually so no structural information could be obtained. 

Nonaqueous microemulsions with nonionic surfactants have been studied. The C12E4 surfactant was found to stabilize microemulsions of formamide and dodecane [138], The ternary phase diagrams were studied at different temperatures and the solubilization of hydrocarbon was shown to be very temperature dependent (Figure 6.7). It was also observed that the temperature intervals of the three-phase regions are dependent on the hydrocarbon used larger aliphatic hydrocarbons... 

As a brief conclusion it can be noted that many nonaqueous microemulsions reported do not seem to contain an organized structure, being simply molecular solutions. Since the degree of organization already in many aqueous microemulsion is low, in particular for quaternary systems containing ionic surfactant and cosurfactant, this is not really surprising. 

Sarkar and coworkers [15] formed nonaqueous microemulsions using N,NJV-trimethyl-A -propyl ammonium bis(trifluoromethanesulfonyl) imide ([Nj JITf N]) substituted for water with cyclohexane by the aid of nonionic surfactant TX-lOO. The ternary phase diagram of [N3 ][Tf2N]/TX-100/cyclohexane system at 25 °C is... 

Zheng et al. investigated how organic solvents (cyclohexane, p-xylene, toluene, and benzene) worked in the formation of [bmimjBF -based oil-in-IL nonaqueous microemulsion systems [12]. The added molecular solvents provided a nonpolar environment that resulted in the aggregation of the hydrophobic tails of the surfactant TX-lOO, so that the molecular solvents formed droplets dispersed in the continuous [bmim]BF phase. Results of 2D H-NMR confirmed that the solvophobic interaction between the molecular solvents and the hydrophobic tails of TX-lOO was the driving force in the formation of those oil-in-IL microemulsions. 

Nonionic surfactants such as TX-lOO, polyoxyethylene sorbitan monooleate (Tween-80), or anionic sodium bis(2-ethyUiexyl) sulfosuccinate (AOT) are generally regarded as essential components in the formulation of microemulsions. Nevertheless, very recently, Xu et al. made remarkable progress in the development of novel IL-based nonaqueous microemulsions by producing a siufactant-free IL-based nonaqueous microemulsion for the first time [13]. The microemulsion consisted of [bmim]BF, ethanol, and toluene, without the involvement of any traditional surfactants. The phase behavior of the [bmim]BFyethanol/toluene system was similar to... 

As described previously, most IL-based nonaqueous microemulsions consist of a hydrophilic IL, a surfactant, and a nonpolar solvent. Although these microemulsions have been successfully constructed and applied to different fields, the problem of volatility of the environmentally unfriendly organic solvents cannot be avoided. One strategy, to replace both the internal (dispersed) and external (continuous) phases with two immiscible ILs, is theoretically possible, but it may not be easy to find two immiscible ILs because all the ILs are organic salts. 

NONAQUEOUS MICROEMULSIONS USING ILS AS BOTH SURFACTANT AND POLAR SOLVENT... 

Cheng et al. highlighted a major contribution towards the formulation of nonaqueous microemulsions with two types of ionic liquids (Cheng et al., 2007 b). For the first time they reported ionic liquid in ionic liquid (IL/IL) structures. For this purpose, the hydrophobic ionic liquid [bmim] [PFg] and the hydrophilic protic ionic liquid propylammonium formate (PAF) were used as apolar and polar phase, respectively. Moreover, these microemulsions contained the anionic surfactant sodium bis (2-ethylhexyl) sulfosuccinate (AOT). One... 

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

Micro emulsions can be formulated with carbon dioxide in supercritical state instead of a hydrocarbon as nonaqueous solvent. Fluorinated surfactants are commonly used to prepare such microemulsions. Water-in-carbon dioxide microemulsions can be made and the droplet size has been found to be similar to the size of the droplets of water-in-hydrocarbon micro emulsions with similar composition [21]. Such a microemulsion was used for conversion of benzyl chloride to benzyl bromide using KBr as hydrophilic nucleophile. The yield was an order of magnitude higher in the carbon dioxide microemulsion than in a conventional microemulsion at similar conditions, a fact that has been ascribed to low interfacial viscosity [22]. The big advantage with these micro emulsions is the environmental friendliness and the ease of work-up associated with carbon dioxide as solvent. 

Microemulsions should be formed near or at the phase inversion temperature (PIT) or HLB temperature for a given nonionic surfactant, since the solubilization of oil (or water) in an aqueous (or nonaqueous) solution of nonionic surfactant shows a maximum at this temperature. 

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