Nonaqueous microemulsions

Friberg S E and Liang Y-C 1987 Nonaqueous microemulsions Microemulsions Structure and Dynamics ed S E Friberg and Bothorel (Booa Raton, FL Chemioal Rubber Company) pp 79-91... 

Silica Gels. The acid-catalyzed alkoxide sol-gel process produces gels (17). Frib-erg and coworkers (40-50) pioneered the extension of this process to silica synthesis in microemulsions both aqueous and nonaqueous microemulsions were used. For aqueous microemulsions, experiments were conducted mostly with the SDS/ pentanol/water/acid system. A representative flow diagram is shown in Figure 2.2.9. The nonaqueous microemulsion systems utilized included CTAB/decanol/ decane/formamide and AOT/decane/glycerol (44-46,49,50). The experimental approach followed the sequence nonaqueous microemulsion preparation, water addition, and then TEOS addition. 

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. 

Martino, A. and Kaler, E.W. (1990) Phase-behavior and microstructure of nonaqueous microemulsions. /. Phys. Chem., 94, 1627-1631. 

Das, K.R Ceglie, A., Lindman, B. and Friberg, S. (1987) Fourier-transform NMR self-diffusion studies of a nonaqueous microemulsion system. /. Colloid Interface Sci. 116, 390 400. 

In the aqueous synthesis of multicomponent oxides, a common approach is to first prepare a coprecipitate that can then be calcined to produce the desired complex oxide as the final product. When the component metal ions exhibit widely different aqueous solubilities, effecting coprecipitation becomes a challenge. Chhabra et al. [109] overcome this difficulty in the case of the ferrite BaFei20i9 by using a nonaqueous microemulsion system based... 

Zech O, Harrar A, Kunz W (2011) Nonaqueous microemulsions containing ionic liquids - properties and applications. In Kokorin A (ed) Ionic liquids theory, properties, new approaches. Rijeka, Croatia, pp 245-270 (open access)... 

Microemulsions with ILs acted as catalytic activity enhancer for oxidases. Zhou et al. [95] reported a water-m-[bmim][PFJ microemulsion system stabilized by TX-lOO that enhances the catalytic activity of hgnin peroxidase (LiP) and laccase. Optimum molar ratios of water to TX-lOO were 8 and >20 for UP and laccase, respectively. Compared to pure or water-saturated [bmim][PFJ, the derived microemulsion evidenced enhanced catalytic activity. Use O/W for oU-in-water ivation effect of [bmim][PFJ on LiP and laccase. Xue et al. [96] reported timable enzyme (laccase) activity in a microemulsion system, water/AOT+TX-100/[bmim][PFJ. The solution of IL [bmim][Cl] and polar organic solvent formamide (FA) were used to form a nonaqueous microemulsion as [bmim][Cl]-FA/TX-100/cyclohexane (Rg. 10.8) at 25 0.1°C, reported by Wei et al. [97]. By means of electrical conductivity, dynamic hght scattering (DLS) and UV-Vis spectroscopy measurements, microstructures, internal phases, and size regime were explored of the aforesaid microemulsion system. UV-Vis studies using CoCl indicated metal salt dissolution by microemulsion. 

In general, one can say that a new type of nonaqueous microemulsions can be formed, consisting of very small IL droplets stabilized by the other more surfactantlike IL. By increasing the amount of IL, a transition to a bicontinuous phase range can be identified by conductivity measurements as well as a transition to oil-in-IL microemulsion droplets. 

Aqueous microemulsions with the IL as apolar phase are much more interesting than nonaqueous microemulsions. The reason is that both water and the IL are considered green solvents. The IL most commonly used in the preparation of aqueous IL microemulsions is l-butyl-3-methylimidazohum hexafluorophos-phate ([C mim][PF ]). The first aqueous IL microemulsion, water/TX-100/ [C mim][PFJ, was reported for Gao et al. showing water in [C mim][PFJ (w/IL), bicontinuous, and [C mim][PF ] in water (IL/w) subregions (see Fig. 13.1) [52]. These microregions were identified by cyclic voltammetry method using... 

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... 

ILs are different from the conventional water solvent, as they consist of cations and anions, and thus, the structure and properties of IL-based nonaqueous microemulsions are unlike those of aqueous microemulsions. In this chapter, we outline recent advances in IL-based nonaqueous microemulsions, with a focus on the structure and properties of these systems. 

BUTYL-3-METHYLIMIDAZOLIUM TETRAFLUOROBORATE ([bmim]BF )-BASED NONAQUEOUS MICROEMULSION SYSTEMS... 

Compared to the many different types of ILs that have developed so far, a limited variety of those ILs are nsed in nonaqueous microemnlsions, and most of them are imidazohnm-based ILs. Among others, l-butyl-3-methyhmidazohum tetrafluorob-orate ([bmimJBF has been extensively used in many mieroemulsion studies since the pioneering work on IL-based microemulsions by Han et al. [5] probably, it is one of the most classical ILs to date. As a result, studies on [bmim]BF -based nonaqueous microemulsions are better characterized than those on other IL-based microemulsions. 

Zheng et al. reported a [bmim]BFyTX-100/toluene microemulsion system that was similar to the IL-based nonaqueous microemulsions mentioned earlier [10]. Cyclic voltammetry experiments were successfully introduced to verify the identification of microregions, in addition to conductivity measurements. 

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... 

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