Micelles, formation reactions

As shown by the above review of the common methods used for the determination of the CMC, a series of measurements of some property of the surfactant solution as a function of surfactant concentration is usually involved. This is followed by the detection of some characteristic point - which is called the CMC. Methodical differences may originate from the choice of the characteristic point, the kind of plot on which this point is chosen, the kind of data which are plotted and the effect of the dye. The CMC is not a very sharply defined point above which properties are qualitatively different from those below. In fact, all properties of a solution in the CMC region vary in a continuous manner and so do all of their derivatives. A micelle is by definition a reversible aggregate of a large but not infinite number of monomers. The micelle formation reaction must obey the laws of chemical equilibrium and, as such, the concentration dependence of the degree of micellization has to change gradually. Consequently, all properties of the... 

The CMC for sodium dodecylbenzenesulfonate is about lO Af at 25°C. Calculate K for the preceding reaction, assuming that it is the only process that occurs in micelle formation. Calculate enough points to make your own quantitative plot corresponding to Fig. Xni-13. Include in your graph a plot of (Na )(R ). Note It is worthwhile to invest the time for a little reflection on how to proceed before launching into your calculation ... 

Likewise, Grieco, while working with amphiphile-like reactants, observed an enhanced preference for endo-adduct in aqueous solutions, which he attributed to orientational effects within the micelles that were presumed to be present in the reaction mixture ". Although under the conditions used by Grieco, the presence of aggregates cannot be excluded, other studies have clearly demonstrated that micelle formation is not the reason for the improved selectivities . Micellar a peg tes even tend to diminish the preference for endo adduct. ... 

Complex formation takes place in an organic solvent or in a water/monomer mixture by reaction of the macroligand with a metal compound (e.g. a Cu(I)-ha-lide). It is supposed that the conditions in the reaction mixture are comparable to those in conventional emulsion polymerization, where monomer droplets stabilized by surfactant molecules coexist with monomer swollen micelles [64]. Reaction sites are presumably the hydrophobic core of the micelles and the monomer droplets as well. Initial results of the micellar-catalyzed ATRP of methyl methacry-... 

Inspection of Table 3.6 together with Scheme 3.11 reveals a few general trends. First of all, the effect seems to be connected to micelle formation. The data of Table 3.6 together with other results of detailed studies [132-133,136-139] show that the largest effect of the surfactants on the reaction rate can be observed around the critical micellar concentration (c.m.c.) of the amphiphiles. Accordingly, non-ionic surfactants (Brij, Tween) with very... 

Kinetics and mechanisms of complex formation have been reviewed, with particular attention to the inherent Fe +aq + L vs. FeOH +aq + HL proton ambiguity. Table 11 contains a selection of rate constants and activation volumes for complex formation reactions from Fe " "aq and from FeOH +aq, illustrating the mechanistic difference between 4 for the former and 4 for the latter. Further kinetic details and discussion may be obtained from earlier publications and from those on reaction with azide, with cysteine, " with octane-and nonane-2,4-diones, with 2-acetylcyclopentanone, with fulvic acid, and with acethydroxamate and with desferrioxamine. For the last two systems the various component forward and reverse reactions were studied, with values given for k and K A/7 and A5, A/7° and A5 ° AF and AF°. Activation volumes are reported and consequences of the proton ambiguity discussed in relation to the reaction with azide. For the reactions of FeOH " aq with the salicylate and oxalate complexes d5-[Co(en)2(NH3)(sal)] ", [Co(tetraen)(sal)] " (tetraen = tetraethylenepentamine), and [Co(NH3)5(C204H)] both formation and dissociation are retarded in anionic micelles. 

JafvertC.T. and J.K. Heath (1991). Sediment-and saturated-soil-associated reactions involving an anionic surfactant (dodecylsulfate). 1. Precipitation and micelle formation. Environmental Science and Technology 25 1031-1039. 

We conclude this section with a brief discussion of the relatively large, positive values of AS°,C, which we have seen are primarily responsible for the spontaneous formation of micelles. At first glance it may be surprising that AS for Reaction (A) is positive after all, the number of independent kinetic units decreases in this representation of the micellization process. Since such a decrease results in a negative AS value, it is apparent that Reaction (A) is incomplete as a description of micelle formation. What is not shown in Reaction (A) is the aqueous medium and what happens to the water as micelles form. The water must experience an increase in entropy to account for the observed positive values for AS °,c. 

In spite of the fact that the concentration of surfactants in the outer solution is assumed to be smaller than the critical micelle concentration, inside the network, micelles are supposed to be formed. The reason for this assumption is, first of all, intensive adsorption of surfactants on the network as a result of the ion exchange reaction. Moreover, in Refs. [38, 39], it was shown that critical concentration of micelles formation c c" within a polyelectrolyte network is much less than that in the solution of surfactant c° . Indeed, when a micelle is formed in solution immobilization of counter ions of surfactant molecules takes place, because these counter ions tend to neutralize the charge of micelles (see Fig. 13), whereas there is no immobilization of counter ions when the micelles are formed in the network the charge of micelles is neutralized by initially immobilized network charges which do not contribute to the translational entropy (Fig. 13). 

Trialkylamines are used as additives in the telomerization of butadiene and water in a two-phase system (103). The catalyst comprises a palladium salt and tppms or tppts. The amines may build cationic surfactants under catalytic conditions and be capable of micelle formation. The products include up to five telomerization products (alcohols, alkenes, and ethers), and thus the reaction is nonselective. 

Table 7. Proposed reaction scheme for topological transformation during micelle formation in nonpolar solvents. AOT in C6H12 [Ber. Bunsenges. Physikal. Chem. 79, 667 (1975)]... 

Since phosphates and sulfates with long chain alkyl substituents form micelles at concentrations above their CMC, the hydrolysis of these esters can be subject to micellar catalysis thereby providing a simplified system in which micelle formation and structure are not alfected by the presence of a foreign solubilizate. The hydrolysis of such surfactants must be considered, however, in investigations of their effects on reaction rates. Fortunately, the rate constants for the neutral hydrolysis of esters such as sodium dodecyl sulfate are extremely slow at 90° = 296 days at pH = 8-63), and the acid-catalyzed hydrolysis of the same ester is some three orders of magnitude faster and thus is still negligible in most cases (Kurz, 1962). 

The effects of macromolecules other than surfactants on the rates of organic reactions have been investigated extensively (Morawetz, 1965). In many cases, substrate specificity, bifunctional catalysis, competitive inhibition, and saturation (Michaelis-Menten) kinetics have been observed, and therefore these systems also serve as models for enzyme-catalyzed reactions and, in these and other respects, resemble micellar systems. Indeed, in some macromolecular systems micelle formation is very probable or is known to occur, and in others mixed micellar systems are likely. Recent books and reviews should be consulted for a more detailed description of macromolecular systems and for their applicability as models for enzymatic catalysis and other complex interactions (Morawetz, 1965 Bruice and Benkovic, 1966 Davydova et al., 1968 Winsor, 1968 Jencks, 1969 Overberger and Salamone, 1969). 

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