Conductivity microemulsions

Vitamin B12 derivatives are also effective catalysts for the electroreductive cyclization of bromoalkenes in conductive microemulsions,299 300 or for ring-expansion reactions in cyclic a-(bromomethyl)-(3-keto esters in DMF.301 Vitamin Bi2 attached to an epoxy-polymer has been used in electrosynthesis of valeronitrile by reductive coupling of iodoethane and acrylonitrile.302... 

The use of electrochemical methods for the destruction of aromatic organo-chlorine wastes has been reviewed [157]. Rusling, Zhang and associates [166, 167] have examined a stable, conductive, bicontinuous surfactant/soil/water microemulsion as a medium for the catalytic reduction of different pollutants. In soils contaminated with Arochlor 1260, 94% dechlorination was achieved by [Zn(pc)] (H2pc=phthalocyanine) as a mediator with a current efficiency of 50% during a 12-h electrolysis. Conductive microemulsions have also been employed for the destruction of aliphatic halides and DDT in the presence of [Co(bpy)3]2+ (bpy=2,2 -bipyridine) [168] or metal phthalocyanine tetrasulfonates [169]. 

Rusling et al. performed electrochemically and light mediated radical additions of alkyl iodides to cyclohexenone in conductive microemulsions catalyzed by 20 mol% of 247 in 14—81% yield [303]. Radical allylations of alkyl bromides 249 with allyl sulfides, sulfones, or phosphates catalyzed by 5 mol% of cobalt (iminate) complex 250 in the presence of zinc as reducing agent proceeded in 52-85% yield [304],... 

Kantaria, S., G.D. Rees, and M.J. Lawrence, Formulation of electrically conducting microemulsion-based organogels. International Journal of Pharmaceutics, 2003.250(1) 65-83. 

Gao J, Rusling JF, Zhou D (1996) Carbon-carbon bond formation by electrochemical catalysis in conductive microemulsions. J Org Chem 61 5972-5877... 

These fascinating bicontinuous or sponge phases have attracted considerable theoretical interest. Percolation theory [112] is an important component of such models as it can be used to describe conductivity and other physical properties of microemulsions. Topological analysis [113] and geometric models [114] are useful, as are thermodynamic analyses [115-118] balancing curvature elasticity and entropy. Similar elastic modulus considerations enter into models of the properties and stability of droplet phases [119-121] and phase behavior of microemulsions in general [97, 122]. 

Giustini M ef a/1996 Microstructure and dynamics of the water-in-oil CTAB/n-pentanol/n-hexane/water microemulsions a spectroscopic and conductivity study J. Phys. Chem. 100 3190... 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

It has been reported that the percolation of conductance of water/AOT/n-heptane microemulsions is assisted by sodium cholate and retarded by sodium salicylate [282]. 

Electrical conductivity is an easily measured transport property, and percolation in electrical conductivity appears a sensitive probe for characterizing microstructural transformations. A variety of field (intensive) variables have been found to drive percolation in reverse microemulsions. Disperse phase volume fraction has been often reported as a driver of percolation in electrical conductivity in such microemulsions [17-20]. 

Lagues et al. [17] found that the percolation theory for hard spheres could be used to describe dramatic increases in electrical conductivity in reverse microemulsions as the volume fraction of water was increased. They also showed how certain scaling theoretical tools were applicable to the analysis of such percolation phenomena. Cazabat et al. [18] also examined percolation in reverse microemulsions with increasing disperse phase volume fraction. They reasoned the percolation came about as a result of formation of clusters of reverse microemulsion droplets. They envisioned increased transport as arising from a transformation of linear droplet clusters to tubular microstructures, to form wormlike reverse microemulsion tubules. 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

FIG. 2 Low-frequency conductivity at 45°C as a function of composition, a (weight fraction decane relative to decane and brine) for brine, decane, and AOT microemulsions exhibiting the phase behavior illustrated in Fig. 1. The breakpoint at a = 0.85 corresponds to the onset of percolation. This conductivity increases by two orders as a decreases from 0.85 to 0.7. (Reproduced by permission of the American Institute of Physics from Ref. 37.)... 

FIG. 5 Order parameter for disperse pseudophase water (percolating clusters versus isolated swollen micelles and nonpercolating clusters) derived from self-diffusion data for brine, decane, and AOT microemulsion system of single-phase region illustrated in Fig. 1. The a and arrow denote the onset of percolation in low-frequency conductivity and a breakpoint in water self-diffusion increase. The other arrow (b) indicates where AOT self-diffusion begins to increase. 

As described in the introduction, certain cosurfactants appear able to drive percolation transitions. Variations in the cosurfactant chemical potential, RT n (where is cosurfactant concentration or activity), holding other compositional features constant, provide the driving force for these percolation transitions. A water, toluene, and AOT microemulsion system using acrylamide as cosurfactant exhibited percolation type behavior for a variety of redox electron-transfer processes. The corresponding low-frequency electrical conductivity data for such a system is illustrated in Fig. 8, where the water, toluene, and AOT mole ratio (11.2 19.2 1.00) is held approximately constant, and the acrylamide concentration, is varied from 0 to 6% (w/w). At about = 1.2%, the arrow labeled in Fig. 8 indicates the onset of percolation in electrical conductivity. 

FIG. 8 Low-frequency conductivity (a) of water, toluene, and AOT reverse microemulsions at 25°C as a function of acrylamide (cosurfactant) concentration, (wt%). The Op and arrow at f = 1.2% shows the approximate onset of percolation in low-frequency conductivity. 

FIG. 9 Measured self-diffusion coefficients at 25°C for toluene (A), water ( ), acrylamide ( , and AOT ( ) in water, toluene, and AOT reverse microemulsions as a function of cosurfactant (acrylamide) concentration, f (wt%). The breakpoint at about 1.2% acrylamide approximately denotes, the onset of percolation in electrical conductivity. 

While the order parameters derived from the self-diffusion data provide quantitative estimates of the distribution of water among the competing chemical equilibria for the various pseudophase microstructures, the onset of electrical percolation, the onset of water self-diffusion increase, and the onset of surfactant self-diffusion increase provide experimental markers of the continuous transitions discussed here. The formation of irregular bicontinuous microstructures of low mean curvature occurs after the onset of conductivity increase and coincides with the onset of increase in surfactant self-diffusion. This onset of surfactant diffusion increase is not observed in the acrylamide-driven percolation. This combination of conductivity and self-diffusion yields the possibility of mapping pseudophase transitions within isotropic microemulsions domains. 

Both the structure and chain length of surfactants and cosurfactants have a striking influence on the structure of the micremulsion formed. The most systematic studies have been on the influence of the cosurfactant chain length and structure on the nature of the microemulsion region. Two main studies have been carried out to elucidate the difference obtained, namely electrical conductivity and NMR investigations. As we will see... 

Thus, in summary, self diffusion measurements by Lindman et a (29-34) have clearly indicated that the structure of microemulsions depends to a large extent on the chain length of the oosurfactant (alcohol), the surfactant and the type of system. With short chain alcohols (hydrophilic domains and the structure is best described by a bicontinuous solution with easily deformable and flexible interfaces. This picture is consistent with the percolative behaviour observed when the conductivity is measured as a function of water volume fraction (see above). With long chain alcohols (> Cg) on the other hand, well defined "cores" may be distinguished with a more pronounced separation into hydrophobic and hydrophilic regions. 

In this context numerous changes were made. The chapter Properties of Polymers was revised and a new section Correlations of Structure and Morphology with the Properties of Polymers was added. The chapter Characterization of Macromolecules was revised and enlarged. 15 examples have been deleted as they did no longer represent the state of the art and/or were of minor educational value. Several new experiments (plus background text) were added, as, for example controlled radical polymerization - enzymatic polymerization - microemulsions - polyelectrolytes as superabsorbants - hyperbranched polymers - new blockcopolymers - high impact polystyrene - electrical conducting polymers. 

Figure 6 is a plot of specific conductance against mole ratios of methanol to bis(2-ethylhexyl) sodium sulfosuccinate. Like the viscosity data, there are three regions. In the first region, a rapid rise in conductance occurs, which indicates the formation of a microemulsion. It is in this region that the swollen micellar solution and liquid crystalline phase of methanol in bis(2-ethylhexyl) sodium sulfosuccinate is breaking with the formation of microspheres that constitute the microemulsion (13). 

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