Micelle formation cooperativity

A consequence of the cooperativity is that, as schematized in Fig. 2.23, the size distribution curve has a deep minimum the appearance of a pronounced minimum in the size distribution curve is a good criterion for the formation of proper micelles. However, aggregation of amphiphilic compounds is of course not restricted to micelle formation and as indicated in Fig. 2.23 different types of size distribution curves... 

The CMC has its most clear-cut interpretation within the (pseudo) phase separation model of micelle formation. Although the micelles and the surrounding solution form a single phase, the amphiphile association shows a cooperativity that makes an analogy with a phase transition useful. Within this model, the CMC is the concentration at which the system enters a two phase region the two pseudophases formed being the aqueous system and the micelles. 

The strong cooperativity of amphiphile association into micelles is well established and for long-chain surfactants it is often a good approximation to consider micelle formation as analogous to a phase separation. Even if the concentration dependence of many physico-chemical properties, within experimental accuracy, is in concor-... 

As discussed, the major determinant of the distribution coefficient between the aqueous phase and the oil phase of relatively nonpolar lipids such as cholesterol is the concentration of lipolytic products in the aqueous phase, since the paraffin chains of the lipolytic products, forming the lipid core of the micelle, are responsible for its solvent capacity (23,73). Micelle formation and the associated appearance of new solvent properties result from a cooperative effect of bile acids and lipolytic products (Fig. 16). Bile acids alone aggregate to form micelles, but these micelles have extremely poor solvent properties. Lipolytic products alone form very large liquid crystalline aggregates which probably have excellent solvent properties for lipids. When mixed, micelles of intermediate size are formed. The size is sufficiently small to permit rapid diffusion, and the lipolytic products in the center of the micelle provide a core of liquid hydrocarbon with useful solvent properties. 

Figure 28.10 Micelle formation Is cooperative. When the surfactant concentration is below the cmc, most surfactant molecules are monomers. When the surfactant concentration is above the cmc, most molecules are in M-mer micelles. ...

From our experiments we cannot completely exclude the possibility of micelle formation of CHP in water. We found however little evidence for such micelles. The viscosity and light absorption of CHP solutions for example, are normal. We conclude therefore that no aggregates are formed in PMA-CHP solution that contain more than one PMA molecule. Our results can be quite well explained by the molecular model proposed by Lovrien [12]. With our results however a more refined mechanism can be formulated. Upon increasing the concentration of chrysophenine in an aqueous solution of PMA, the binding to the macromolecular surface increases. It reaches a critical level around a CHP concentration in solution of 0.001 M. A further increase of the CHP concentration leads to a level of bound dye that forces the polymer globule to open up to some extent. Thus new sites for dye binding become available, etc. Potentiometric titration is a more sensitive technique for recording the start of this process than is viscosity. However either technique demonstrates the cooperative nature of the phenomenon. 

Two nucleation processes important to many people (including some surface scientists ) occur in the formation of gallstones in human bile and kidney stones in urine. Cholesterol crystallization in bile causes the formation of gallstones. Cryotransmission microscopy (Chapter VIII) studies of human bile reveal vesicles, micelles, and potential early crystallites indicating that the cholesterol crystallization in bile is not cooperative and the true nucleation time may be much shorter than that found by standard clinical analysis by light microscopy [75]. Kidney stones often form from crystals of calcium oxalates in urine. Inhibitors can prevent nucleation and influence the solid phase and intercrystallite interactions [76, 77]. Citrate, for example, is an important physiological inhibitor to the formation of calcium renal stones. Electrokinetic studies (see Section V-6) have shown the effect of various inhibitors on the surface potential and colloidal stability of micrometer-sized dispersions of calcium oxalate crystals formed in synthetic urine [78, 79]. 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

Rasi, S., Mavelli, F., and Luisi, P. L. (2003). Cooperative micelle binding and matrix effect in oleate vesicle formation. J. Phys. Chem. B, 107, 14068-76. 

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