Micelle counterion binding

The same thermodynamic quantities needed for mixed micelle formation (already discussed) are also needed for mixed admicelle formation. Luckily, the monomer-admicelle equilibrium data can be fairly easily and unambiguously obtained (e.g., see Chapter 15). This should be combined with calorimetric data for a more complete thermodynamic picture of the mixed admicelle. As with micelles, counterion bindings on mixed admicelles also need to be obtained in order to account for electrostatic forces properly. Only one study has measured counterion binding on single-component admicelles (3 .), with none reported for mixed admicelles. 

It is hard to overstate the observation that the admicelle has properties similar to a micelle. Counterion binding on the admicelle is nearly identical to that on the micelle ... 

Micellization is a second-order or continuous type phase transition. Therefore, one observes continuous changes over the course of micelle fonnation. Many experimental teclmiques are particularly well suited for examining properties of micelles and micellar solutions. Important micellar properties include micelle size and aggregation number, self-diffusion coefficient, molecular packing of surfactant in the micelle, extent of surfactant ionization and counterion binding affinity, micelle collision rates, and many others. 

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. 

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

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... 

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... 

Finally, as for micelle-forming surfactants, increasingly detailed experimental information on aggregation numbers, counterion binding, hydration, and... 

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]. 

Counterion Binding. The fractional counterion binding on charged mixed micelles is of fundamental interest because it gives an indication of surface charge density which is related to the mechanism of mixing nonidealities in ionic/nonionic micelles. It is also a necessary... 

The -fractional counterion binding on micelles composed o-f binary mixtures o-f similarly structured sur-factants o-f like charge varies monotonical ly between the values -for the two pure component micelles as the micellar composition is varied (15.40). For... 

The mass action model (MAM) for binary ionic or nonionic surfactants and the pseudo-phase separation model (PSM) which were developed earlier (I EC Fundamentals 1983, 22, 230 J. Phys. Chem. 1984, 88, 1642) have been extended. The new models include a micelle aggregation number and counterion binding parameter which depend on the mixed micelle composition. Thus, the models can describe mixtures of ionic/nonionic surfactants more realistically. These models generally predict no azeotropic micellization. For the PSM, calculated mixed erne s and especially monomer concentrations can differ significantly from those of the previous models. The results are used to estimate the Redlich-Kister parameters of monomer mixing in the mixed micelles from data on mixed erne s of Lange and Beck (1973), Funasaki and Hada (1979), and others. 

In Figure 3, the curves for the ionic/ionic systems are all below the curve for the nonionic/nonionic case. As the degree of counterion binding increases, i.e. as y9j and increase, the value of c /c2 decreases for a given value of a. The bottom curve in Figure 3 corresponds to the ionic/ionic system with Pi = 0.6, P2 = 0.0, and P = 1.0. This case is probably physically unrealistic, inasmuch as these parameters correspond to a system in which both surfactants 1 and 2 are ionic and contribute counterions to the system but surfactant 2 in the micelles acts as a nonionic surfactant and therefore has a zero degree of counterion binding. 

A mass action model (MAM) with monodisperse aggregation number N which depends on the micelle mole fraction x and the counterion binding parameter /3(x) has been developed for binary surfactants either ionic/ionic or nonionic/ionic. 

Counterion Binding. Counterion binding on mixed micelles is of crucial importance toward understanding the structure and electrostatic forces involved in micelle formiation involving ionic surfactants. Specific ion electrodes are effective at measuring counterion bindings... 

Addition of Kryptofix 222 and Kronenether to reverse micellar system induces no changes in the droplet size and an increase in the droplet-droplet interactions. The complexation of cations Na of AOT led to a decrease in counterion binding, and consequently repulsive interactions between polar head groups of AOT surfactant are increasing. This could induce a more flexible interface of reverse micelles. 

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... 

Equation (11) can be used to evaluate AG C from readily available CMC values. Note that setting m = 0 for ionic micelles is equivalent to reverting from Reaction (B) to (A) for a description of micellization. The AG c value calculated in this case describes the contribution of the surfactant alone without including the contribution of counterion binding. Since m = 0 for nonionics, the surfactant contribution alone is useful when comparisons between ionic and nonionic micelles are desired. 

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