Micellar conductivity

Cardoso et al. [115] have shown that AOT concentration is related to percolation. The percolation phenomenon was followed by a steep increase in the micellar conductivity [115]. Huang and Lee [49] observed a drastic reduction in the recovery (around 40%) of the horse radish peroxidase when AOT concentration was at 5 mmol 1 and further increase in AOT concentration to 10 mmol 1- produced no recovery at all. 

The electrical conductance of a colloidal solution can be considered to be built up from contributions of the colloidal particles and the ions present in the solution. In a xnore schematic way the conductivity can be split up into a contribution of the sol particles and their counter ions (micellar conductivity) and a contribution of the rest of the liciuid (intermicellar conductivity). This second method is mostly used in the practice of conductance measurements of colloids, but it should be realised that it is less exact than the first onc In the first place it is rather arbitrary to denote a certain part of the positive ions (the particles are supposed to be negatively charged) as counter ions and the rest of them as belonging to the intcrmiccllar liquid. Moreover it neglects all interaction between the sol particles and the intermicellar ions, which is often not allowed. 

Van Paassen [57] describes the CMC of some polyether carboxylates with different fatty chains and EO degrees (Fig. 2). In an extensive study, Binana-Limbele et al. [59] investigated the micellar properties of the alkylpolyether carboxylates of the general formula CnH + OCF CH OCI COONa with n = 8, x = 5, and n = 12 and x = 5,1, and 9, by means of electrical conductivity (CMC, apparent micellar ionization degree) and time-resolved fluorescence probing (micelle aggregation number A7) as a function of temperature and surfactant concentration (Table 1). 

Solubilization studies on crude oil by ether carboxylates combined with nonionics have been conducted in the presence of varying amounts of CaCl2 [194]. Good solubility can be obtained when the micellar size is 100 nm and is strongly dependent on the CaCl2 concentration. 

The works of various investigators such as Gogarty and Tosch (1), Healy and Reed (2), and Davis and Jones (2), have shown that the micellar flooding process can be used effectively to mobilize residual oil in watered-out light oil reservoirs. Many field tests conducted in the U.S. have further proved its effectiveness. However, the economics of the process remain unattractive for implementing the process for tertiary oil recovery. 

Micellar catalysis, conducted in the absence of Lewis acid tends to inhibit the Diels-Alder reaction, relative to the reaction in water. The reason is that the local reaction medium in the Stern region is less favorable than bulk water. However, by combining Lewis-acid and micellar catalysis, enzyme-hke rate accelerations can be obtained (Table 7.5) in case the Lewis acid acts as the counterion for the miceUe. " ... 

At CMC, micelles (aggregates of SD" with some counterions, such as Na+) are formed, and some Na+ ions are bound to these, which is also observed from conductivity data. In fact, these data analyses have shown that approximately 70% Na+ ions are bound to SD" ions in the micelle. The surface charge was estimated from conductivity measurements (Birdi, 2002). Therefore, the concentration of Na+ will be higher than SD" ions after CMC. A large number of reports are found in the literature, in which the transition from the monomer phase (before CMC) to the micellar phase (after CMC) have been analyzed. 

Micellar and pre-micellar solutions of methanol in triolein were studied with three different surfactant systems using 2-octanol as a co-surfactant. Surfactants evaluated by viscosity, conductivity, density, refractive index and particle size data along with polarizing microscopic examinations were bis(2-ethylhexyl) sodium sulfosuccinate, triethylammonium linoleate and tetradecyldimethylammonium linoleate. Data show phase equilibria regions of liquid crystalline phases as well as micellar solutions. All systems were effective for solubilizing methanol in triolein. The order of effectiveness for water tolerance is Tetradecyldimethylammonium linoleate>... 

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). 

Surfactant Activity in Micellar Systems. The activities or concentrations of individual surfactant monomers in equilibrium with mixed micelles are the most important quantities predicted by micellar thermodynamic models. These variables often dictate practical performance of surfactant solutions. The monomer concentrations in mixed micellar systems have been measured by ultraf i Itration (I.), dialysis (2), a combination of conductivity and specific ion electrode measurements (3), a method using surface tension of mixtures at and above the CMC <4), gel filtration (5), conductivity (6), specific ion electrode measurements (7), NMR <8), chromatograph c separation of surfactants with a hydrophilic substrate (9> and by application of the Bibbs-Duhem equation to CMC data (iO). Surfactant specific electrodes have been used to measure anionic surfactant activities in single surfactant systems (11.12) and might be useful in mixed systems. ... 

Ionic micelles will migrate in an electric field, and the ion atmosphere of the colloidal particle is dragged along with it. Interpretation of micellar mobility (conductivity experiments) must take this into account. The same is true, however, of the mobility of simple ions, but the situation is more involved here since the micelle and the ion atmosphere have comparable dimensions. We see in Chapter 12 how particle and double-layer dimensions affect the interpretation of mobility experiments. 

One potential application of the work on oriented nematic phases of rodlike molecules is to solutions containing cylindrical micelles. Orientation could be achieved by a shear field or perhaps by an electric field. Gotz and Heckman (9) confirmed the existence of anisotropic electrical conductivity for a concentrated surfactant solution in a shear field. They used their results to show that the solution contained cylindrical rather than platelike micelles. Of course, the magnitude of the electrical conductivity in an aqueous micellar solution should be quite different from that in the nematic phase of an organic material. So the conditions for and types of electrohydrodynamic instabilities could be different as well. 

Winsor (17) describes how electrical conductivity varies during addition of an alcohol to an aqueous micellar solution containing some solubilized oil. Conductivity initially decreases as mixed (and probably larger) micelles containing both surfactant and alcohol are formed. When liquid crystal (presumably having a lamellar structure) starts to appear in equilibrium with the micellar solution, conductivity decreases even faster. As more alcohol is added, the aqueous solution disappears, only liquid crystal is present, and the conductivity reaches a minimum. Addition of still more alcohol results in the appearance of an oil-continuous micellar solution and an increase in conductivity. Eventually all liquid crystal disappears, the increase in conductivity ceases, and conductivity... 

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