Binding counterion

The somewhat unexpected fact that the different types of methods give approximately the same values of 0 is discussed in Sect. 6. Quantitatively, there are significant differences though, so in comparisons between different amphiphile ions, counterions etc., the adherence to a single method is necessary. Having outlined the generalities of counterion binding, we will with selected examples discuss some of its features on a molecular level as revealed mainly by spectroscopic studies. 

Counterion specificity has been observed to be markedly more pronounced for cationic surfactants than for anionic ones. This can certainly be mainly referred to a weaker hydration of typical counter-anions. From the variation of CMC with counterion and from ion activity measurements it can be inferred that the binding to -N(CH3)3 and -NH3 headgroups follows the sequence P NOj Br CP. (As an example a recent study223-1 of decylammonium salts shows the CMC to decrease from 0.064 M for the chloride to 0.038 M for the iodide). The counterion specific effects on micellar shape for -N(CH3)3 surfactants were discussed above. For cationic (as well as some anionic) amphiphiles, a marked counterion specificity is also indicated in the phase diagrams8 but systematic studies of the counterion dependence have not yet been reported. 

Because of the possibility of charge-transfer interactions between polar head and halide ion, ion specific interactions can be expected to be particularly marked for alkylpyridinium halides. From the CMCs counterion dependence3, as well as from counterion dissociation studies, binding is found to follow the sequence I- Br CP. The size of hexadecylpyridinium micelles is very sensitive to the anion of added salt, aggregation being promoted according to the sequence227 F CP Br NOj l.  

The behavior of hydrophobic counterions is, not unexpectedly, different from that of small hydrophilic counterions. Increasing counterion binding with increasing counterion size can be deduced both for anionic27 and cationic30 surfactants. Fur- 

The life-time of a monomer in a micelle may be of the order of a microsecond and in view of the accepted dynamic state of a micelle this implies that less extensive motions occur on a shorter time-scale. Aniansson229 has recently examined the dynamic protrusion of methylene groups from the hydrocarbon core of a micelle 

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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. 

Here (log cmc) is tire log cmc in tire absence of added electrolyte, is related to tire degree of counterion binding and electrostatic screening and c- is tire ionic strengtli (concentration) of inert electrolyte. Effects of added salt on cmc are illustrated in table C2.3.7. 

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... 

The aromatic shifts that are induced by 5.1c, 5.If and S.lg on the H-NMR spectrum of SDS, CTAB and Zn(DS)2 have been determined. Zn(DS)2 is used as a model system for Cu(DS)2, which is paramagnetic. The cjkcs and counterion binding for Cu(DS)2 and Zn(DS)2 are similar and it has been demonstrated in Chapter 2 that Zn(II) ions are also capable of coordinating to 5.1, albeit somewhat less efficiently than copper ions. Figure 5.7 shows the results of the shift measurements. For comparison purposes also the data for chalcone (5.4) have been added. This compound has almost no tendency to coordinate to transition-metal ions in aqueous solutions. From Figure 5.7 a number of conclusions can be drawn. (1) The shifts induced by 5.1c on the NMR signals of SDS and CTAB... 

The increase of counterion binding with the charge on the polyion has... 

Begala, A. J. Strauss, U. P. (1972). Dilatometric studies of counterion binding by polycarboxylates. Journal of Physical Chemistry, 76, 254-60. 

Manning, G. S. (1979). Counterion binding in polyelectrolyte theory. Accounts of Chemical Research, 12, 443-9. 

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. 

The effect of the counterion binding reactions 23 and 24 on the surface chemistry will be treated in terms of ratios p and n, defined as ... 

When counterion binding reaction II is extremely rapid, the concentration dependence of the relaxation time is given by (11)... 

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. 

The symmetry is square-planar D4 , with respect to the ligands, but in practice the Cu(II) is subjected to an essentially octahedral field. When the ligand is ethylene diamine (not substituted), the four nitrogen atoms are about 2 A from the Cu(II) ion while the counterions bind 2.2-2.3 A above and below. In this description the electronic spectra have three characteristic bands relative... 

A polyelectrolyte solution contains the salt of a polyion, a polymer comprised of repeating ionized units. In dilute solutions, a substantial fraction of sodium ions are bound to polyacrylate at concentrations where sodium acetate exhibits only dissoci-atedions. Thus counterion binding plays a central role in polyelectrolyte solutions [1], Close approach of counterions to polyions results in mutual perturbation of the hydration layers and the description of the electrical potential around polyions is different to both the Debye-Huckel treatment for soluble ions and the Gouy-Chapman model for a surface charge distribution, with Manning condensation of ions around the polyelectrolyte. 

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... 

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