Driving force, of micellization

DRIVING FORCES OF MICELLE FORMATION AND THERMODYNAMIC MODELS... 

The driving force of micelle formation is the elimination of the contact between the alkyl chains and water. The larger a spherical micelle, the more efficient this is since the volume-to-area ratio increases. Decreasing the micelle size always leads to an increased hydrocarbon-water contact. However, if the spherical micelle was made so large that no surfactant molecule could reach from the micelle surface to the centre, one would either have to create a void or some surfactant molecules would lose contact with the surface. Both of these options are unfavourable. 

In addition to the degree of hydrophilicity of the solubilizates, their size and structure, the size of the host microregions, or the occurrence of specific processes must be taken into account in order to rationalize the driving forces of the solubilization process and of the solubilization site within water-containing reversed micelles [25,138,139],... 

In the case of Kryptofix 221D, a cryptand able to complex the alkali metal cations [141-143], it has been observed that it is solubilized mainly in the palisade layer of the AOT-reversed micelles. And from an analysis of the enthalpy of transfer of this solubilizate from the organic to the micellar phase it has been established that the driving force of the solubilization is the complexation of the sodium counterion. In addition, the enthalpy... 

The heavy-end portions (usually called heavy fractions) of bitumen (e.g. asphaltenes, preasphaltenes) can exist both in a random oriented particle aggregate form or in an ordered micelle form, peptized with resin molecules (16.17). In their natural state, asphaltenes exists in an oil-external (Winsor s terminology) or reversed micelle. The polar groups are oriented toward the center, which can be water, silica (or clay), or metals (V, Ni, Fe, etc.). The driving force of the polar groups... 

Thus, in the case B, the repulsion between the alkyl chain and water has been removed. Instead, the alkyl-alkyl attraction (B) is the driving force for micelle formation. The surfactant molecule forms a micellar aggregate at a concentration higher than CMC because it moves from the water phase to the micelle phase (lower energy). The micelle reaches an equilibrium after a certain number of monomers have formed a micelle. This means that there are both attractive and opposing forces involved in... 

The table reveals that the adsorption free energy is dominated by the cmc term. Thus, the dominating driving force of adsorption is of the same origin as for the micellization, i.e. it depends on the... 

Critical micelle concentration in aqueous solutions was determined by fluorescence using pyrene as a probe. The driving force for micelle formation is the strong hydro-phobic interactions between [(R)-3-hydroxybutyrate] block. It was previously determined by this group that terpolymers with longer PHB blocks have much lower critical micelle concentrations because of PHB block aggregation in aqueous solution. Testing results are provided in Table 2. 

The central core is predominantly hydrocarbon. The expulsion of the hydrophobic tails of the surfactant molecules from the polar medium is an important driving force behind micellization. The amphipathic molecules aggregate with their hydrocarbon tails pointing together toward the center of the sphere and their polar heads in the water at its surface. 

One consequence of roughness at the surface of the micellar core is an increased contact between water and hydrocarbon. Figure 8.3b seems unrealistic because the water-hydrocarbon contact is scarcely less than in the bulk solution, a situation that apparently undermines an important part of the driving force for micellization. Figure 8.3c minimizes this effect without eliminating it. At the same time it allows for some water entrapment, which accounts for that part of the micellar hydration that was unexplained by the hydration of ions and charged groups. 

The functionalization of the reverse micelles will create a novel application in bioseparation processes in the analytical and medical sciences. It is therefore important to reveal the recognition mechanism of proteins at the liquid-liquid interface in reversed micellar solutions. DNA is also successfully extracted in a few hours by reversed micelles formed by cationic surfactants in isooctane. The driving force of the DNA transfer is the electrostatic interaction between the cationic surfactants and the negatively charged DNA. Another important factor is the hydrophobicity of the cationic surfactants. Doublechain type cationic surfactants are found to be one of the best surfactants ensuring the efficient extraction of DNA. These results have shown that reverse micellar solutions will become a useful tool not only for protein separation, but also for DNA separation. 

In highly polar nonaqueous solvents, such as formamide, iV-methylformamide, and /V,/V-di methyIformamide, from the limited data available, it appears that the driving force for micellization is again mainly entropic, i.e., the tendency of the lyophobic group to transfer from the solvent environment to the interior of the micelle (McDonald, 1970). 

A comparison of all the results reported here reveals that all the models reviewed predict qualitatively similar micellar behavior. Although the magnitudes of quantities such as the cmc or average aggregation number may be quite diflFerent for the different models (e.g., h t-i completely phase separates in Larson s model but forms well-behaved micelles in the simulations of Desplat and Care [31], because of the head-solvent attraction they used), each model predicts similar trends in these properties. This confirms the assumption that the solvophobic effect (i.e., the dislike of the solvophobic tail beads for the solvent) is the major driving force for micellization but also indicates that other forces are present that control specific micellar properties. 

In all models (as for mean-field theories), the reductimi of interfacial area upon micelUzation is considered to be the driving force for micellization. Thus, the interfacial tension is an important parameter. The interfacial free energy per chain of a spherical micelle (Eq. 8) can be written as ... 

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