Surface potential, effective microemulsions

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. 

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... 

The high surface potentials and differential polarities of molecular assemblies such as micelles, vesicles and microemulsions suggest that they may be of use in effecting charge separation after a photochemical redox event either by preferential electrostatic repulsion of one of the products or by differential solubilities of the two products in the different phases. This area of research has been extensively reviewed325 330 and we give a brief overview of the use of these systems. 

There is limited evidence for the applicability of the pseudophase kinetic model to microemulsions. Amines can act as cosurfactants and stabilize o/w microemulsions, and these amines are effective nucleophiles towards 2,4-dinitrochlorobenzene, and the rate constants in the microemulsion droplets are not very different from those in water [156]. Reactions in which bromide ion acts as a nucleophile also have similar second-order rate constants in a microemulsion aggregate as in a micelle, suggesting that it should be possible to apply an ion exchange model to microemulsions [157]. However, Mackay and coworkers have treated ion binding in microemulsions in terms of the surface potential of the aggregates [153,154], following an approach which has been applied to ionic micelles in water [158]. 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

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