Anionic microemulsion system

Khomane et al. prepared dodecanethiol-capped CdS QDs of 4 nm size by using a Winsor II microemulsion system [242], which are soluble in solvents such as n-heptane, toluene, n-hexane, thus demonstrating the dual role of the anionic surfactant, viz., forming the microemulsion and facilitating the extraction of oppositely charged ions from the aqueous to the organic phase. 

In this paper, a molecular thermodynamic approach is developed to predict the structural and compositional characteristics of microemulsions. The theory can be applied not only to oil-in-water and water-in-cil droplet-type microemulsions but also to bicontinuous microemulsions. This treatment constitutes an extension of our earlier approaches to micelles, mixed micelles, and solubilization but also takes into account the self-association of alcohol in the oil phase and the excluded-volume interactions among the droplets. Illustrative results are presented for an anionic surfactant (SDS) pentanol cyclohexane water NaCl system. Microstructur al features including the droplet radius, the thickness of the surfactant layer at the interface, the number of molecules of various species in a droplet, the size and composition dispersions of the droplets, and the distribution of the surfactant, oil, alcohol, and water molecules in the various microdomains are calculated. Further, the model allows the identification of the transition from a two-phase droplet-type microemulsion system to a three-phase microemulsion system involving a bicontinuous microemulsion. The persistence length of the bicontinuous microemulsion is also predicted by the model. Finally, the model permits the calculation of the interfacial tension between a microemulsion and the coexisting phase. 

The double microemulsion-mediated process also provides a convenient method for preparing a metal-containing sihcate coating. The two microemulsion systems contained two common components anionic sirrfactant AOT and cyclohexane [134]. The difference was that the first microemulsion consisted of an aqueous solution of sodiirm metasihcate (0.2 M) and 10 wt% SDS as the co-surfactant, while the second microemulsion consisted of an aqueous solution of copper nitrate (0.1 M) and 10 wt% SDS. The copper-ion microemulsion was added to the silicate-ion microemulsion with constant stirring. After 8 h of gel-lation, and ageing for an additional 24 h, copper nitrate crystals were identified within the sihcate network. SUica-copper composite powders with various copper contents (4-20 wt%) and surface areas of 200-400 m /g were synthesized. 

Polymerization of styrene in each of the three types of microemulsions was performed using a water soluble initiator, potassium persulfate (K2S208), as well as an oil-soluble initiator, AIBN. As desired, solid polymeric materials were obtained instead of latex particles. In the anionic system, the cosolvent 2-pentanol or butyl cellosolve separates out during polymerization. Three phases are always obtmned after polymerization. The solid polymer was obtained in the middle with excess phases at the top and bottom. GC analysis of the upper phase indicates more than 80% 2-pentanol, while Karl-Fisher analysis indicated more than 94% water in the lower phase. Some of the initial microemulsion systems have either an excess organic phase on top or an excess water phase as the bottom layer. GC analysis showed the organic phase to be rich in 2-pentanol. However, the volume of the excess phase is much less in the initial system than in the polymerized system. 

In the nonionic system observed under EVM, the initial microemulsion showed no tendency of gelation until it reached 60 C. After reaching 60<>C, the system gels and starts to polymerize after 10-12 hours. As polymerization proceeds, the water separates out. After about 20-24 hours, the gel starts to become a solid with an excess emulsion phase formed at the bottom. The polymerization is essentially complete after 36 hours. Due to different modes of polymerization in the anionic and nonionic surfactant systems, the mechanical properties of the solid are different. The polymers obtsuned from anionic microemulsions are brittle, while those obtmned from nonionic microemulsions are ductile. 

In an effort to investigate the universality of this t rpe of polymer-microemulsion interaction, an anionic surfactant system was studied. Two sulfonate surfactants were chosen to enable variation of the hydrophile-lipophile characteristics of the surfactant couple and in turn of the mlcroemulslon. The... 

Table I shows the effect of various systems such as micelles, swollen micelles (achieved by adding hexanol to CTAB), microemulsion systems, vesicles formed from a double-chain CTAB surfactant, and reversed micelles with water cores formed with benzyl dimethylcetylammonium bromide in benzene. Hie active chromophore exists either as pyrene, pyrene sulfonic acid or pyrene tetrasulfonlc acid. Essentially the concept here is that the polar derivatives of pyrene will always locate pyrene at the surface of the micelle as these anionic species of pyrene complex with the positively charged surface. Dimethylaniline is used as an electron donor in each case, it can be seen that for pyrene, a continual decrease in the yield of the pyrene anion (ion yield of unity in the micelle) is observed on going from micelle to swollen micelle, to microemulsion, and no yield of ions is observed in a reversed micelle system. With pyrene tetrasulfonic acid the yield of ions over the different systems is fairly constant, even across to the reverse micellar system. However, the lifetime of the ions is extremely short in the reversed micellar system. An explanation for such behavior can be given as follows as we transverse across the...

Alcohol composition Just as electrolytes do, alcohols help to balance the physicochemical environment in order to keep the surfactant formulation close to optimal, according to the so -called/(A) or ( A) effects discussed in Chapter 3. Besides, even so some surfactant formulations do not contain alcohols, they are often added into microemulsion systems as co-solvents (particularly in those containing anionic surfactants) to improve the solubility of the main surfactants and prevent the formation of highly viscous meso-phases [114] such as liquid crystals, which are additionally known to stabilise the emulsions that may be formed. Alcohols can also change the surfactant partition coefficients which has a great effect on the oil recovery efficacy [ 110,111 ]. 

Mixtures of anionic and nonionic surfactants were proposed to provide temperature-insensitive systems [37], a suggestion that has considerable practical interest not only for microemulsion systems but also in emulsion polymerization and enhanced oil recovery It was recently shown that since both the anionic and nonionic surfactants can be selected, this double degree of freedom can be used to attain both temperature insensitivity and mixture composition insensitivity so that the formulation is a particularly robust one as far as the applications are concerned [39]. 

This brief survey begins in Sec. II with studies of the aggregation behavior of the anionic surfactant AOT (sodium bis-2-ethylhexyI sulfosuccinate) and of nonionic pol-y(ethylene oxide) alkyl ethers in supercritical fluid ethane and compressed liquid propane. One- and two-phase reverse micelle systems are formed in which the volume of the oil component greatly exceeds the volume of water. In Sec. Ill we continue with investigations into three-component systems of AOT, compressed liquid propane, and water. These microemulsion systems are of the classical Winsor type that contain water and oil in relatively equal amounts. We next examine the effect of the alkane carbon number of the oil on surfactant phase behavior in Sec. IV. Unusual reversals of phase behavior occur in alkanes lighter than hexane in both reverse micelle and Winsor systems. Unusual phase behavior, together with pressure-driven phase transitions, can be explained and modeled by a modest extension of existing theories of surfactant phase behavior. Finally, Sec. V describes efforts to create surfactants suitable for use in supercritical CO2, and applications of surfactants in supercritical fluids are covered in Sec. VI. 

Nonionics can function as cosurfactants in microemulsions. This cosurfactant role, which can also be performed by short-chain alcohols or glycol ethers, can make it possible to form microemulsions or other phases from anionic surfactant systems that would otherwise be unreachable in a pure anionic system. By one definition, a microemulsion system is one that achieves a zero or close-to-zero curvature [108]. This is easily achievable in nonionic systems, but very difficult in pure anionic surfactant systems due to the head group repulsions. Mixing the nonionic with the anionic makes these types of systems, which sometimes have greater cleaning ability, easily accomplished. 

Various types of microemulsion systems can form with neutral, anionic, cationic, and even zwitterionic surfactant molecules, with unique properties and characteristics. Some important applications of microemulsion are as follows ... 

The microemulsion system to estimate the CMC of free, glycine, and taurine derivative is MEEKC-SDS. This system employs SDS as surfactant, butanol as cosurfactant, octane as oil and an aqueous buffer at pH 7.50. At that pH, the BAs are in the anionic form. 

The nanoparticles have been synthesized in different microemulsion systems. Some of them are shown in Fig. 1. The anionic Aerosol-OT (AOT)/ heptane/water system is one of the best characterized microemulsions [8,9]. The system AOT/ -xylene/water [10] has also been used. The cationic ce-tyltrimethylammonium bromide (CTAB)/hexanol/water system contains hex-anol, which forms the organic phase and plays the role of cosurfactant [11]. The nonionic penta(ethylene glycol)-dodecylether (PEGDE)/hexane/water was studied by Eriberg and Eapczynska [12]. The reverse micellar droplets have a cylindrical shape in which the surfactant molecules are parallel to each other, forming a bilayer impregnated with water. Triton X-100... 

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