Reversed phases microemulsions

Figure 5.17 Schematic synthesis of silica nanoparticles by the Stober method (top) and via reverse-phase microemulsion (bottom). The scale bars represent 1000 nm and 500 nm, respectively. Adapted from ref. 51 with permission from the Royal Society of Chemistry.

Figure C2.3.10. Ternary phase diagram of surfactant, oil and water illustrating tire (regular) and (reverse) L2 microemulsion domains.

High-pressure FT-IR spectroscopy has been used to clarify (1) the rotational isomerism of molecules, (2) characteristics of water and the water-head group, and (3) RSO3 Na4- interactions in reverse micellar aggregates in supercritical ethane. This work demonstrates interesting pressure, temperature, and salt effects on an enzyme-catalyzed esterification and/or maintenance of a one-phase microemulsion in supercritical fluids from practical and theoretical points of view (Ikushima, 1997). 

Figure 1 shows changes in the system phase behavior as its HLB value is systematically adjusted. The left side of the diagram represents a two-phase system with micellar-solubilized oil in equilibrium with an excess oil phase (Winsor Type I) (Winsor 1954). The right side of the diagram represents a different two-phase system with reversed micellar-solubilized water. In-between these two systems a third phase coemerges which contains enriched surfactant with solubilized water and oil. This new thermodynamically stable phase is known as a Winsor Type HI middle phase microemulsion. 

Liquid-liquid methods include solvent extraction with immiscible liquid-liquid systems in which a suitable ligand is dissolved in an organic phase and contacted with a metal ion containing an aqueous phase and liquid membranes. Separations can also be achieved with pseudo-phase systems such as micelles, microemulsions, and vesicles. Such separations can be solid-liquid or liquid-liquid and include separations with normal- and reversed-phase silica, and polymeric supports where the mobile phase contains the organized molecular assembly (OMA) of micelles, microemulsions, or vesicles. Separation of metal ions using the pseudo-phase systems is stiU in its infancy and a brief account will be provided here. 

Microemulsions based on non-ionic surfactants of alcohol ethoxylate type are sensitive to temperature changes and those based on ionic surfactants are sensitive to variations in the electrolyte concentration. Such variations may cause a one-phase microemulsion to form a two- or a three-phase system in which a microemulsion phase coexists with one or two excess phases. As a work-up approach the concept is particularly useful for microemulsions based on non-ionic surfactants because the transitions obtained by temperature variations are reversible. 

The size of w/c microemulsion droplets has been measured by neutron scattering for a di-chain hybrid surfactant (C7Hi5)-(C7Fi5)CHS04 Na [32], 667 g/mol PFPE-C00"NH4 [33], and for a partially fluorinated di-chain sodium sulfo-succinate surfactant [34]. For the PFPE-COO NH4 surfactant, the droplet radius increases from 20 A to 36 A for W o values of 14 and 35, respectively. For the di-chain sodium sulfosuccinate surfactant, droplet radius varied linearly from 12 to 36 A as Wo increased from 5 to 30. This linear relationship has also been shown for AOT reverse micelles in organic solvents [7]. In each of these studies for a one-phase microemulsion, droplet size and Wq were found to be only a weak function of pressure, unless the pressure is reduced to the phase boundary where droplets aggregate. This trend was calculated theoretically [6,23] and has been measured in AOT w/o microemulsions in supercritical propane [35,36]. 

Systems containing equal amounts of oil and water often can be classified according to the four Winsor types [34]. These classical configurations are known as type I (normal micelles in equilibrium with excess oil, often designated 2, which means two phases with the surfactant in the lower phase), type II (reverse micelles in equilibrium with excess water, 2), type III (middle phase microemulsion with excess water and excess oil phases, 3), and type IV (one-phase microemulsion, 1). 

Figure 7 Schematic illustration of middle-phase microemulsion formation in surfactant-brine-oil systems. ( ) Oil-swollen micelles (microdroplets of oil) (O) reverse micelles (microdroplets of water).

In systems with reverse micelles/microemulsion, the transfer of extracted species occurs both through the macroscopic interface between dispersed and continuous phases and through the large microscopic interface between water pools and extractant hydrophilic groups in the cores of the micelles. The transfer of water with hydrophilic species is possible due to reverse micelles forming at the interface and sucking the aqueous phase. 

Ex(30) values show a good, often linear, correlation with a large number of other solvent sensitive processes, such as reaction rates and shifts of chemical equilibria. The betaine dye (Scheme 3) and specially designed derivatives are useful molecular probes in the study of micellar interfaces, microemulsions and phospholipid bUayers, of rigid rod-Uke isocyanide polymers, and the retention behaviour in reversed-phase chromatography. In addition to its solvatochromic behaviour, the dye is sensitive to temperature ( thermosolvatochromism ) and pressure changes ( piezosolvatochromism ) and also to the presence of electrol)d es ( halosolvatochromism ). 

Figure 11.15 Phase diagram of the 0.1 M aqueous NaCl/CnEOfdecane system as a function of temperature. Decane was added to 3 wt% C12EO4 aqueous solution, and the weight percent of decane in the system is plotted horizontally, (a) Phase diagram over a wide range of temperature, (b) Detailed phase diagram around the HLB temperature. W oil-swollen micellar solution (0/W-type microemulsion) Om, water-swollen reverse micellar solution (W/O-type microemulsion) D, surfactant phase (middle-phase microemulsion) L3, bicontinuous surfactant phase LC, lamellar liquid crystal Wand O, excess water and oil phases, respectively. I, II, and III indicate one-, two- and three-phase regions (Reproduced by permission of the American Chemical Society from ref 40)...

A number of other reports have been made relating to the use of surfactants, reverse micelles, microemulsions, membranes and polyelectrolytes for the synthesis of noble metal nanoparticles [157-159]. This type of synthetic method generally involves a two-phase system with a surfactant causing the formation of a microemulsion or micelle, and maintains a favorable microenvironment together with extraction of metal ions from aqueous phase to organic phase. In such cases, the surfactant not only acts as a stabilizer but also plays an important role in controlling crystal growth. 

Microemulsions or reverse micelles are composed of enzyme-containing, surfactant-stabiHzed aqueous microdroplets in a continuous organic phase. Such systems may be considered as a kind of immobilization in enzymatic synthesis reactions. 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

On a microscopic scale, a microemulsion is a heterogeneous system and, depending on the relative amounts of the constituents, three main types of structures can be distinguished [69] oil in water (OAV, direct micellar structure), water in oil (W/O, reverse micellar structure) and a bicontinuous structure (B) (Figure 6.1). By adding oil in water, OAV dispersion evolves smoothly to a W/O dispersion via bicontinuous phases. 

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