Counterions binding to micelle

Fundamental to our picture of counterion binding to micelles is a knowledge of whether counterions retain their hydration spheres or not. Mukeijee242 concluded from partial molar volume data that (for simple ions) it is the interaction of the hydrated ion with the micelle that is important and the same conclusion is drawn in a... 

Specific ion electrodes have been used extensively to investigate counterion binding to micelles [49-51], but recently electrodes have been developed which are sensitive to monomeric surfactant, and can be used to study the aggregation of surfactants near the cmc [182]. 

Counterions bound to the micelles are only electrostatically attracted by the oppositely charged micelle surface. - For instance, alkali metal counterions bind to micelles with a... 

Many theoretical discussions have centered on the degree of counterion binding to micelles, because an understanding of the specific binding of counterions to micelles is a prerequisite for an understanding not only... 

Table 4.2. Degree of Counterion Binding to Micelles (m/n) of Sodium Dodecyl Sulfate"...

Typical radii for spherical micelles (related to the length of a typical surfactant tail) are around 5 nm. Aggregation numbers N (surfactant monomers per micelle) are typically 40-100. The fractional counterion binding of micelles [3 generally lies... 

The effects of dilution of the micellar surface charge on the rate of alkaline hydrolysis of a betaine ester surfactant have been investigated for a mixture of decyl betainate and a nonionic surfactant with a similar CMC. It was shown that the relation between micellar composition and the hydrolysis rate essentially parallels the relation between micellar composition and counterion binding to mixed micelles made up of ionic and nonionic surfactants [20]. 

Reaction (B) is clearly an extension of Reaction (A), with the former admitting the possibility of counterion binding to the micelle. Additional refinements can be introduced into this reaction. The micelle still carries a net charge of — z, which means that zM+ ions must be present in solution to assure electroneutrality. This may be included in the representation of micellization by writing... 

Gan, L.-H. Reaction of hydroxide ion with 1,3,5-trinitrobenzene in cationic micelles evidence of variable counterion binding to micellar head groups. Can. J. Chem. 1985, 65(3), 598-601. 

Calculations usirig this value afford a partition coefficient for 5.2 of 96 and a micellar second-order rate constant of 0.21 M" s" . This partition coefficient is higher than the corresponding values for SDS micelles and CTAB micelles given in Table 5.2. This trend is in agreement with literature data, that indicate that Cu(DS)2 micelles are able to solubilize 1.5 times as much benzene as SDS micelles . Most likely this enhanced solubilisation is a result of the higher counterion binding of Cu(DS)2... 

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). 

The symbols, IE or M A indicate that counterion binding was calculated using the ion exchange or mass action models and ST that the micelle was assumed to be saturated with counterion. 

This calculation is for spherical micelles, but a similar calculation could be used to obtain estimates of salt concentrations for ionic wormlike micelles. Such salt concentrations for wormlike micelles are expected to be increased in comparison to spherical micelles. In fact, the addition of counterions or a sufficient increase in surfactant concentration often leads to a transition from spherical micelles to wormlike micelles. As the free counterion concentration in solution increases, so does the counterion binding. As a result, electrostatic repulsion between the charged head-groups is increasingly shielded and the mean cross-sectional (effective) headgroup... 

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