Hydrophilic anion reactions, micellar effects

Micellar effects upon reactions of hydrophilic anions... 

Table II. Micellar Effects on Reactions of Hydrophilic Anions...

Specific-ion electrodes are expensive, temperamental and seem to have a depressingly short life when exposed to aqueous surfactants. They are also not sensitive to some mechanistically interesting ions. Other methods do not have these shortcomings, but they too are not applicable to all ions. Most workers have followed the approach developed by Romsted who noted that counterions bind specifically to ionic micelles, and that qualitatively the binding parallels that to ion exchange resins (Romsted 1977, 1984). In considering the development of Romsted s ideas it will be useful to note that many micellar reactions involving hydrophilic ions are carried out in solutions which contain a mixture of anions for example, there will be the chemically inert counterion of the surfactant plus the added reactive ion. Competition between these ions for the micelle is of key importance and merits detailed consideration. In some cases the solution also contains buffers and the effect of buffer ions has to be considered (Quina et al., 1980). 

Little is known about the structures of these kinetically effective complexes, or even about the aggregates of the amphiphile. Both hydrophobic and coulombic interactions are important because these aggregates are much less effective than micelles at assisting reactions of hydrophilic nucleophilic anions. These observations are consistent with the view that the aggregates are much smaller than micelles. It is probable that the structures and aggregation numbers of these aggregates depend on the nature of the solutes which bind to them and Piszkiewicz (1977) has suggested that such interactions play a role in micellar kinetics. 

These effects are noted especially in region D of the plot in Fig. 11.12a the results for three samples of NaDS are shown in Fig. 11.12b. In this work the nickel concentration at the micellar surface was estimated by titration with murexide, a chromophoric anion. This interaction has been studied in more detail the reaction between Ni " and murexide (below) results in a pronounced colour change making the reaction easy to follow. Murexide being hydrophilic is not solubilized by the micellar species and thus can be used as an indicator of the appearance of micellar surface . 

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