Critical micelle concentration dodecyl sulfate system

Lewis acid catalysis in micellar systems [33] was first found in the model reaction in Table 14-6 [34]. While the reaction proceeded sluggishly in the presence of 0.2 eq. Yb(OTf)3 in water, remarkable enhancement of the reactivity was observed when the reaction was carried out in the presence of 0.2 eq. Yb(OTf)3 in an aqueous solution of sodium dodecyl sulfate (SDS, 0.2 eq., 35 mM), and the corresponding aldol adduct was obtained in 50% yield. In the absence of the Lewis acid and the surfactant (water-promoted conditions), only 20% yield of the aldol adduct was isolated after 48 h, while 33% yield of the aldol adduct was obtained after 48 h in the absence of the Lewis acid in an aqueous solution of SDS. The amount of the surfactant also influenced the reactivity, and the yield was improved when Sc(OTf)3 was used as a Lewis acid catalyst. Judging from the critical micelle concentration, micelles would be formed in these reactions, and it is noteworthy that the Lewis acid-catalyzed reactions proceeded smoothly in micellar systems [35]. It was also found that the surfactants influenced the yield, and that Triton X-100 was effective in the aldol reaction (but required long reaction time), while only a trace amount of the adduct was detected when using cetyltri-methylammonium bromide (CTAB) as a surfactant. 

Surfactant concentration (varied after polymerization) greatly affects the viscosity of associating polymer systems. Iliopoulos et al. studied the interactions between sodium dodecyl sulfate (SDS) and hydrophobically modified polyfsodium acrylate) with 1 or 3 mole percent of octadecyl side groups [85]. A viscosity maximum occurred at a surfactant concentration close to or lower than the critical micelle concentration (CMC). Viscosity increases of up to 5 orders of magnitude were observed. Glass et al. observed similar behavior with hydrophobically modified HEC polymers. [100] The low-shear viscosity of hydrophobically modified HEC showed a maximum at the CMC of sodium oleate. HEUR thickeners showed the same type of behavior with both anionic (SDS) and nonionic surfactants. At the critical micelle concentration, the micelles can effectively cross-link the associating polymer if more than one hydrophobe from different polymer chains is incorporated into a micelle. Above the CMC, the number of micelles per polymer-bound hydrophobe increases, and the micelles can no longer effectively cross-link the polymer. As a result, viscosity diminishes. 

Ionic surfactants will form micelles in a solution if their concentrations are above their critical micelle concentration (CMC). The ionic surfactant, for example sodium dodecyl sulfate (SDS), is prepared as the salt in which Na" " is the counterion and DS", representing dodecyl sulfate (Ci2S04 ), is the surfactant ion (here an anion). A micelle of such a surfactant will have numerous ionic headgroups, DS", around the periphery of individual micelles. If there are other metallic ions present in the system, e.g. Ca " ", Cu " ", Zn " ", Cd ", etc., there will be an exchange between these ions and Na" " as the counterions for the DS" ions. Thus heavy metals, and other metals, present in solution will be bound to the headgroups of the micelle formed from the ionic surfactant. By using an appropriate membrane/filter, one can concentrate the ionic micelles and their bound heavy metallic counterions. 

Calculate (in nm ) the area occupied by a surfactant molecule (actually each adsorbed dodecyl sulfate ion) at the critical micelle concentration. Do we have an ideal gas film at CMC for this system Comment on the answer ... 

Later, Yoshikawa and Matsubara [40] further studied a non-linear system and proposed a mechanism for the periodic behaviour that involved the formation of inverted micelles that suddenly moved to the oil phase after the concentration of adsorbed surfactants reached a critical value. They extended the experiment to a water/oil/water three-phase system in a U-shaped glass tube that gave spontaneous and stable oscillatory behaviour over a long period [41]. Since then, various characteristics of non-linear behaviour have been investigated and several mechaiusms for the non-linear behaviour have been proposed by many research groups including ours[2,5,10,42-48] however, the mechanism at a molecular level has not been clarified yet and no consensus has been achieved. The difficulty in the explanation seems to come from not only the complexity and diversity of the systems, but also limitations of the observation methods that enable us to monitor dynamic molecular behaviour at liquid/liquid interfaces with sufficient interfacial selectivity and time resolution. In this section, the TR-QELS method has been applied to the investigation of W/NB—sodium dodecyl sulfate (SDS) two-phase system [10]. 

Micellar media are formed from tensioactive molecules in aqueous solution. Mi-cellization is a manifestation of the strong self-association of water and water-like solvents [95]. Micelles are known to increase the solubilization of weakly polar substances in water and, as a consequence, their presence determines the magnitude of hydrophobic interactions. Micelles aggregate spontaneously in aqueous solution beyond a critical concentration which is a function of pressure [96]. As a result, pressure may induce an extra kinetic effect on the rate of organic reactions carried out in aqueous micellar systems. Representative ionic micelles are sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB). Recent examples demonstrate the beneficial effect of the presence of surfactants in Lewis acid-catalyzed reactions, a kind of biactivation [97]. 

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