Bulk phase micelle formation

Studies have shown that surface micelles are formed at lower concentrations than the ones in the bulk phase. The formation of micelles in the bulk phase is... 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

Raghvan and Srinivasan developed a model, for bimolecular micellar catalysed reactions, which also predict constancy in /cobs values at high detergent concentration and may be used for evaluating the binding constants of reactants. They proposed the distribution of both reactant and nucleophile in aqueous and micellar phases. The product formation is assumed to result from decomposition of ternary complex involving substrate, nucleophile and micelle. After analyzing the data on the basis of this model, they concluded that almost all the nucleophile is present in the bulk phase. 

The primary mechanism for energy conservation is adsorption of surfactant molecules at various available interfaces. However, when, for instance, the water-air interface is saturated conservator may continue through other means (Figure 12.3). One such example is the crystallization or precipitation of the surfactant from solution, in other words, bulk phase separation. Another example is the formation of molecular aggregates or micelles that remain in solution as thermodynamically stable, dispersed species with properties distinct from those of an isotropic solution containing monomeric surfactant molecules (Myers, 1992). 

Additives are usually amphiphilic in nature, and thus are either ionic or neutral surfactants or even polymers. The role of surfactants in solvent extraction is ambiguous. Usually, they should be avoided as they lower the interfacial tension, which may lead to emulsion formation in an agitated extractor. However, every metal-loaded ion exchanger is amphiphilic, and can adsorb at the interface or aggregate in the bulk phase. This occurrence is well known with sodium or other metals [17], and above a critical surfactant concentration (cmc, critical micelle concentration) micellar aggregates are formed. A dimensionless geometric parameter is decisive for the structure of the associates, according to Fig. 10.6 ... 

Micelle solutions were originally characterized with a bulk aqueous phase where the hydrophobic carbon chains were turned inward to help stabilize the oil phase. Later, reverse micelles were also characterized, where the conditions were reversed. A bulk oil phase was used with the hydrophilic head groups turned inward to help stabilize the aqueous phase. Micelles require very stringent conditions, dictated by the molar proportions of oil, water, and surfactant. However, the formation of micelle solutions is driven by the differences in the polarity of the two groups any factor that affects the polarity, such as temperature,... 

Hydrophobically modified polybetaines combine the behavior of zwitterions and amphiphilic polymers. Due to the superposition of repulsive hydrophobic and attractive ionic interactions, they favor the formation of self-organized and (micro)phase-separated systems in solution, at interfaces as well as in the bulk phase. Thus, glasses with liquid-crystalline order, lyotropic mesophases, vesicles, monolayers, and micelles are formed. Particular efforts have been dedicated to hydrophobically modified polyphosphobetaines, as they can be considered as polymeric lipids [5,101,225-228]. One can emphasize that much of the research on polymeric phospholipids was not particularly focused on the betaine behavior, but rather on the understanding of the Upid membrane, and on biomimicking. So, often much was learnt about biology and the life sciences, but little on polybetaines as such. 

The presence of micelles can also result in the formation of different reaction products. A diazonium salt, in an aqueous micellar solution of sodium dodecyl sulfate, yielded the corresponding phenol from reaction with OH- in the bulk phase but the corresponding hydrocarbon from material solubilized in the micelles (Abe, 1983). 

The efficiency of a block copolymer is fimited by the formation of micelles in bulk phases and by the kinetic factors. Consequently, the block copolymer used as a compatibilizer should be designed by taking thermodynamic and kinetic parameters into account to achieve the desired effects. Thus the stmcture and transitions in copolymers and homopolymer/copolymer systems is of great interest. 

This inequality agrees with the results of direct measurements of the formation and disintegration rate of micelles for solutions of DSN, DACh and CTACh [115]. To the best of our knowledge the micellisation kinetics in solutions of DPO has not been studied so far by relaxation spectrometry of the bulk phase. 

The physical manifestation of one such mechanism is the crystalUzation or precipitation of the surfactant from solution—that is, bulk-phase separation. An alternative is the formation of molecular aggregates or micelles that remain in solution as thermodynamically stable, dispersed species with properties... 

There is one other effect that gives rise to an increase in entropy on formation of micelles. It is known that water molecules form clusters around hydrophobic molecules (Franks, 1975). Since these form a structure ice , that is, have an order, their entropies are lower than in the bulk. On the formation of micelles, the hydrocarbon tails are no longer in contact with water, that is, the molecides from the cluster are released into the water phase with an increase in entropy. Of course, there can be no structure formation at high temperatures, and this effect disappears there (Anacker, 1970). This effect is entered as ice in Table 4.1, where it is assumed that as it is zero in the melting of ice, in micellization the corresponding term must be very small. 

The coalescence of hydrocarbon chains allows the ordered hydration layers to be expelled into the bulk phase, resulting in a considerable net gain in entropy. Indeed, micelle formation is primarily an entropy-driven process the enthalpy of hydrocarbon association is comparatively weak and can even be endothermic (opposing association). As an example, dimethyl-n-dodecylamine oxide (illustrated in Fig. 2) undergoes or free energy change of micellization of AG = —6.2 kcal/mol (a fairly typical value), of which the enthalpic contribution AH = 4-1.1 kcal/mol and the entropic contribution — T A S = -7.9 kcal/mol. 

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