Microemulsions percolation phenomena

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. 

A description of the percolation phenomenon in ionic microemulsions in terms of the macroscopic DCF will be carried out based on the static lattice site percolation (SLSP) model [152]. In this model the statistical ensemble of various... 

Another approach to determining the viscoelastic properties of dense microemulsions at high frequencies is to conduct ultrasonic absorption experiments. In such experiments it has been found that the percolation process is correlated to a shift of the ultrasonic dynamics from a single relaxation time to a distribution of relaxation times [121]. Other experiments showed an increase in the hypersonic velocity for samples at and beyond the percolation threshold. The complex longitudinal modulus deduced from such experiments is also correlated with the occurrence of the percolation phenomenon, which suggests that the velocity dispersion is clearly correlated with structural transformations [122]. 

In order to understand why the nanoparticles are monodisperse, the microemulsion systems have to be well characterized. Important parameters are the radius of the inner water cores, which depends on R the possible change in the microemulsion due to the presence of reactants in the inner water cores a possible percolation phenomenon within the microemulsion the localization and site of solvation of the precursor ion the distribution of reactants in the inner water cores and the rate of exchange of the content of the aqueous droplets. 

A similar observation of increasing A gx when the temperature is increased toward the haze point was made in a comprehensive study by Lang et al. [29]. The results were later correlated with the percolation phenomenon of microemulsions as revealed by conductivity measurements [33]. Surprisingly high values of the second-order exchange rate constant... 

Upon further addition of acrylamide, the interaction potential becomes so attractive that transient clusters form. Above a threshold volume fraction, a large increase in the electrical conductivity is observed, which is an indication of a percolation phenomenon [25] (Fig. 2). The percolation threshold decreases with increasing AM/H2O ratio, i.e., with increasing attractive interactions, in good agreement with theoretical analyses [26] and data obtained for other microemulsions containing alcohols as cosurfactants [27-29]. As shown in Sec. III.C, this percolating structure has an effect on the formation of polymer latex particles and the polymerization mechanism. 

Nagy [242] has discussed in detail the parameters to be optimized and the conditions to be established in W/O microemulsions for synthesis of metals and metal borides. Parameters that demand attention for standardization of a microemulsion system are the (a) water pool size (dependent on the w value), (b) changes brought about by dissolved reactants, (c) a possible percolation phenomenon in the system, (d) site of solvation of the precursor ions, (e) distribution of the reactants in the droplets and (f) exchange rate among the reactants. 

C. Boned, Z. Saidi, P. Xans, and J. Peyrelasse 1994 Percolation phenomenon in ternary microemulsions The effect of pressure, Phys. Rev. E 49, 5295-5302. 

Q Percolating Phenomenon in Microemulsions Effect of External Entity... 

When we reach a certain volume of the disperse phase, the conductivity abruptly increases to give values of up to four orders of magnitude, which is greater than typical conductivity of water in oil microemulsions. This increase remains invariable after reaching the maximum value that is much higher than that for the microemulsion present before this transition occurs. Similar behavior is observed for the fixed composition of the microemulsion when either water or temperature or volume fraction is increased. This phenomenon is known as electric percolation, [21-32] the moment at which an abrupt transition occurs from poor electric conductor, system (10" Q" cm" ) to the system with fluid electric circulation (10" Q" cm" ). The mechanism proposed to explain the electric percolation phenomenon is based on the formation of channels exchanging matter between the dispersed water droplets and the continuous phase, as shown in Scheme 3.2. 

Garcia-Rio, L., Herves, P, Leis, J. R., and Mujeto, J. C. (1997). Influence of crow ethers and macrocyclic kryptands upon the percolation phenomenon in AOT/isooctane/ HjO microemulsions. Langmuir 13, 6083-6088. 

Mehta, S.K., Dewan, R.K., and Kiran Bala. 1994 Percolation phenomenon and the study of conductivity, viscosity, and ultrasonic velocity in microemulsions, Phys. Rev. E 50 4759-4762. 

The core of a micelle is an exclusion region where substances that are incompatible with the solvent can enter spontaneously in a process called solubilization (4,7). Because of solubilization, micelles become swollen atid may attain the size of a small droplet, i.e., 1000 A or 0.1 nn. If the surfk tant concentration increases well above the CMC, many micelles are formed. If another phase is present, c.g., oil if the solvent is water, and provided that the physicochemical formulation is appropriate, the micelles would solubilize large amounts of this phase and become swollen until ih start interacting in a phenomenon called percolation. Such packed swollen micelle structures that could solubilize large amounts of both oil and water have been called microcmul-sions because they were first thought to be extremely small droplet emulsions (8-11). Actually this is a misnomer for at least two reasons. First, a microemulsion is before all a single-phase system, that is thermodynamically stable. Second, many microemulsions cannot be considered as dispersions of very small droplets, but rather as percolated or bicontinuous structures (12) in which there is no internal or external phase, and no possibility of dilution as in normal emulsions. 

The ion conductance in a microemulsion depends on its type. In an o/w (Winsor I) microemulsion, the conductance is almost like that of an aqueous medium in a w/o (Winsor II) microemulsion, it is very low, whereas in the bicontinuous (Winsor III) condition, the conductance can be conspicuously large. Depending on the composition and temperature, a dramatic increase in conductance may occur this phenomenon is called percolation. 

Abstract Faradaic electron transfer in reverse microemulsions of water, AOT, and toluene is strongly influenced by cosurfactants such as primary amides. Cosurfactant concentration, as a field variable, drives redox electron transfer processes from a low-flux to a high-flux state. Thresholds in this electron-transport phenomenon correlate with percolation thresholds in electrical conductivity in the same microemulsions and are inversely proportional to the interfacial activity of the cosurfactants. The critical exponents derived from the scaling analyses of low-frequency conductivity and dielectric spectra suggest that this percolation is close to static percolation limits, implying that percolative transport is along the extended fractal clusters of swollen micellar droplets. and NMR spectra show that surfactant packing... 

Z. Saidi, J.L. Daridon, and C. Boned 1995 The influence of pressure on the phase diagram, the phenomenon of percolation and interactions in a ternary microemulsion, J. Phys. D Appl. Phys. 28, 2108-2112. 

Conductivity percolation is a phenomenon that has been reported in hundreds of studies of microemulsions, especially... 

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