Counterion binding with ionic micelles

The counterion binding with ionic micelles is generally described in terms of two alternative approaches the first one is the widely used pseudophase ion-exchange model (Chapter 3, Subsection 3.3.7) and the second one, less commonly used, is to write the counterion binding constant in terms of an ionic micellar surface potential (Q) (Chapter 3, Section 3.4). The value of K in Equation 6.16 is expected to remain independent of [CTACllj as long as the degree of association... 

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]. 

It is important to realize, however, that the determination of the substrate-micelle binding constant from solubility data relies entirely on data for saturated solutions and that, in the case of ionic surfactants, differences in the counterion interactions with the micelle and the micelle-substrate complex and activity coefficient effects may seriously complicate the results. In these respects, distribution studies with varying substrate and surfactant concentrations are certainly preferable. In view of the assumptions involved in the derivation and application of equations (10) and (11), the agreement between the K values obtained from kinetic data (equation 10) and those obtained from solubility measurements (equation 11) for several substrate-micelle interactions is certainly both remarkable and significant. 

Another way to study the aggregation process is to use an electrode specific to the surfactant s counterion. As depicted in Figure 1, there is considerable binding of counterions by an ionic micelle and this would be affected by association with a polymer. (See Fig. 11 and later discussion.) This method has been applied to studies of the PEO/SDS and PVA/SDS systems (17,21-23) and confirms results obtained by other methods. 

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. 

If one considers solely the consecutive equilibria, the concentration of monomer can only increase with increasing total amphiphile concentration even above the CMC. (Apart from the trivial decrease in the monomer concentration calculated on the total volume which may arise when the micelles occupy a substantial volume fraction). However, if one realizes that micelles are not only composed of amphiphile, the result may be different. Thus counterion binding helps to stabilize the micelles and for ionic surfactants it can be predicted that the monomer activity may decrease with increasing surfactant concentration above the CMC. Good evidence for a decreasing monomer concentration above the CMC has been provided in the kinetic investigations of Aniansson et al.104), and recently Cutler et al.46) demonstrated, from amphiphile specific electrode studies, that the activity of dodecylsulfate ions decreases quite appreciably above the CMC for sodium dodecylsulfate solutions (Fig. 2.14). 

Figure 1 shows schematically the monomeric amphipathic particle, in this case an ionic one, with its polar head and its hydrophobic tail which is curled up in the aqueous medium. This is in equilibrium with a micelle formed by many monomers, all oriented with their heads outward toward the water and their tails randomly intertwined in the interior. A microdroplet of oil with an ionic hydrophilic surface is thus formed. The cooperative action of the many charged polar heads binds tightly a substantial fraction of the counterions thus effectively reducing the surface charge. 

For the solution without NaCl the occupancy of the Stem layer, r2/Ti rises from 0.15 to 0.73 and then exhibits a tendency to level off. The latter value is consonant with data of other authors, who have obtained values of r2/Fi up to 0.70 to 0.90 for various ionic surfactants pronounced evidences for counterion binding have been obtained also in experiments with solutions containing surfactant micelles." ° As could be expected, both Fj and F2 are higher for the solution with NaCl. These results imply that the counterion adsorption (binding) should be always taken into account. 

Much less is known about micellar charge and counterion binding in the case of bile salts. Based on the result of ionic self-diffusion measurements [20,163,173], conductance studies [17,18,187], Na, and Ca activity coefficients [16,19,144,188,189] and NMR studies with Na, Rb and Cs [190], a number of generalities can be made. Below the operational CMC, all bile salts behave as fully dissociated 1 1 electrolytes, yet interionic effects between cations and bile salt anions decrease the equivalent conductance of very dilute solutions [17,18,187]. With the onset of micelle formation, counterions become bound to a small degree values at this concentration are about < 0.07-0.13 and are not greatly influenced by the species of monovalent alkali cations [163,190]. At concentrations above the CMC, values remain relatively constant to 100 mM in the case of C and this... 

When nonsurfactant solutes (electrolytes, etc.) are added to the micellar reaction mixture, the results can be quite unpredictable. It is often found that the presence of excess surfactant counterions (common ions), when added to a system in which an ionic reactant is involved, retards the catalytic activity of the micelle, with larger ions being more effective in that respect. The effect can probably be attributed to an increase in ion pairing at the micelle surface and a reduction of its attractiveness to charged reactants. In contrast, the addition of neutral electrolyte has been found to enhance micellar catalysis in some instances. It has been proposed that the retardation effect of excess common counterions is due to a competition between the excess ions and the reactive substrate most closely associated with the micelle for the available positions or binding sites on or in the micelle. The enhancing effect, however, has been attributed to the more general effects of added electrolyte on... 

In NaLS solutions above the CMC, one molecule of this dye appears to be associated with one molecule of surfactant [214]. Interaction between other acid dyes, e.g. Cl Acid Blue 120 (XVI) has been observed above and below the CMC of non-ionic nonylphenyl and octylphenyl ethers. At low concentrations the dye surfactant ratio is around 1 2 to 1 3 and at higher concentrations larger complexes may be formed with dye surfactant ratios of between 1 10 and 1 30 [215]. Similar conclusions have been reached on the interaction of three nonionic dyes with NaLS [216]. Detailed kinetic studies of dye-surfactant interactions have been published [217] in which the process of surface adsorption and subsequent incorporation of the dye in the interior of the micelle has been investigated, the latter process being referred to as absorption. Using anionic micelles, James et al [217] found that neutral dyes were absorbed more rapidly than positively charged dyes increased counterion binding, however, increases the rate of absorption of the latter. 

Such a micelle acts like an oil drop in an aqueous solution and allows solubilization of organic compounds present in the aqueous solution into the hydrophobic interior. If the surfactant is ionic, i.e. either cationic or anionic, then oppositely charged ionic species, namely counterions from the solution, will be adsorbed on the surface of the micelle or bind with the charged micellar surface. Such surfrice binding will effectively Increase the micelle dimension. A unique number of surfrictant molecules between 50 and 100 aggregate to form a micelle. 

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