Microemulsions ionic strength

The influence of pH, ionic strength, and protein concentration on the extraction of a-lactalbumin and 3-lactoglobulin from an aqueous solution with water/AOT/isooctane microemulsions and their separation has been reported [168],... 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

Effect of Ionic Strength. Both yE systems were examined for ionic strength effects. Microemulsion compositions were prepared at 70% water, with a cyanide concentration of 0.032 M with respect to the water content. Potassium bromide was used to vary the ionic strength of the reaction mixtures. Ionic strength in the CTAB yE was varied from 0.04 to 0.34. Since the Brij yE tolerated a much higher salt concentration without phase separation, ionic strength in that system was varied between 0.04 and 1.80. As will be seen, the Brij system exhibits a salt effect, while the CTAB yE does not. Rate constants obtained for reaction (1) in the Brij yE were therefore corrected to take into account the effect of ionic strength in that system (vide infra). 

The rate constants for the reaction of N-dodecyl-3-carbamoyl-pyridinlum ion with cyanide in both cationic and nonionic o/w microemulsions have been measured as a function of phase volume. Added salt has no effect in the cationic system, but the rate constants in the nonionic system depend upon ionic strength as would be expected for a reaction between two ions. In order to compare the two microemulsions, the ionic strength in the reaction region has been estimated using thicknesses of 2-4A. The former produces values of the effective surface potential which yield... 

Optimum conditions. Initially it was found that the BSA solution scattering intensity was enhanced the most in the B-R buffer solution. So we choose pH 1.62 to control solution pH. It was found that the highest sensitivity was obtained in the presence of the SDS microemulsion. The optimum reagent amounts of buffer solution, SDS microemulsion and M4MRASP were 2.0 mL, 0.03 mL and 2.5 mL. Finally, 0.02 mol/LofNaCl was chosen to control the ionic strength of the solution. 

This may be quite important in processes in which the ionic strength is determinant such as sol-gel transitions or chemical reactions in microemulsions [137-141]. Double-tailed surfactants such as dioctyl sulfosuccinate or diallQ lmethylammonium salts are likely to produce either vesicles (with excess water) or inverse W/O microemulsions with a polar core [142,143] that is used as a nanoreactor for a score of processes such as esterification or hydrolysis [144] in which enzymes are immobilized in an organogel [145]. Organogels can be made so that their structure depends on the composition of the microemulsion [146-148]. 

Up to now we have considered systems where changes in the microemulsion structure are achieved by changing the temperature. However, similar transformations can be induced for systems containing ionic surfactant by varying the salinity, i.e., the ionic strength of the solution. One such system that was studied in some detail was made up of 46.8% NaCl brine, 47.2% toluene, 2% SDS, and 4% butanol [86]. Here a Winsor I -> Winsor III transition occurs at 5.4% salinity and a Winsor III Winsor II transition at 7.4%. A situation basically similar to the one discussed above for the same phase transitions is observed the viscosity data as a function of salinity are given in Fig. 3. An increase in viscosity occurs upon... 

In contrast to microemulsions, ordinary emulsions are thermodynamieally unstable, but they can be stable in a practical sense if the energy barrier to flocculation is snfficiently high. As with other colloidal dispersions, this energy barrier may be electrical in nature if the drops are charged, if water is the continuous phase, and if the ionic strength is not too high (cf. the discussion of DLVO theory in Chapter 3). Typically, stabihty is provided by adsorbed surfactants and polymers. However, it can also stem from small solid particles that are not completely wet by either phase and thus accumulate at the drop surfaces (Aveyard et al., 2003). 

Fig. 4.12. Variation of interfacial tensions between the various phases O (oil), W (water) and M (microemulsion, containing surfactant), in the three-phase zone (P1P2), when a parameter governing chemical composition is varied (e.g., ionic strength of the aqueous solution)...

Inaeasing precursor salt concentration increases the ion occupancy number per reverse micelle, which promotes intramicellar nucleation and growth and leads to higher uptake. On the other hand, high-ionic strength inside water pools of reverse micelles impacts the stability of the (w/o) microemulsions [63,64]. Moreover, the stabihty of the (w/o) microemulsions depends on the surfactant counterion, which results from ion exchange between the solubilized metal salt and sodium. Since the focus of this work was on nanoparticle uptake by (w/o) microemulsious, the concentrations of the metal precursors were limited to values which maintained stable reverse-micellar structures. 

Whereas Eq. 1 is almost perfectly confirmed by the experimental data of Antonietti et al. and Wu, the present data do not follow Eq. 1 and show much more scatter than literature results (Fig. 14.1). Variation of the ionic strength of the continuous phase, of the reaction temperature, of the total monomer content, and of the concentration of crosslinking agent has essentially no effect on the resulting microemulsion. 

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