Micelle disintegration

Micelles are a dynamic species, which rapidly break up and reform. In a microsecond time scale, single surfactant molecules retreat from and travel towards micelles, and within milliseconds the whole micelle disintegrates and reassembles . 

The formed mixed micelles can diffuse to the unstirred water layer that lines the epithelium, where the micelles disintegrate and lipid amphiphiles are ahsorhed. Bile salts are recycled hack into the lumen and continue to interact with lipid digestion. Thus at any given time during lipid digestion, a complex mixture of different colloid phases is present in the intestinal lumen (Rigler et al., 1986). 

The left-hand side of this equation characterizes the rate of surfactant adsorption on the wetted surface (g/cm s), and the right-hand side characterizes the rate of micelle disintegration per unit surface area. It is assumed that at C = Cc the limiting adsorption = FiC = constant is achieved. The difference C/m = Cm — Cc is equal to the micelle concentration and Cm is the concentration of micellar solution. The value dF/dS = r/2 represents the ratio... 

The equilibrium constant K for the formation of micelles as expressed by Equation 3.3 is independent of equilibrium constants Kg for micellar solubilization of different solubilizates, and rate constants k, for micellar-mediated reactions. In other words, the rates of formation and disintegration of micelle are independent of the corresponding rates of micellar intake and exit of a solubilizate kf k and k k where kf and k represent rate constants for micelle formation and micelle disintegration, respectively, and therefore kj /k = K (Scheme 3.1 and Scheme 3.2). In Equation 3.3,... 

In this section, a theory is developed to describe a spontaneous imbibition of surfactant solutions into hydrophobic capillaries, which takes into account the micelle disintegration and solution concentration reduction close to the moving meniscus as a result of adsorption, as well as the surface diffusion of surfactant molecules. The theory predictions are in good agreement with the experimental investigations on the spontaneous imbibition of the nonionic aqueous surfactant... 

The following factors must be considered when assessing the stability of the casein micelle The role of Ca++ is very significant in milk. More than 90% of the calcium content of skim milk is associated in some way or another with the casein micelle. The removal of Ca++ leads to reversible dissociation of P-casein without micellular disintegration. The addition of Ca++ leads to aggregation. The same reaction occurs between the individual caseins in the micelle, but not as much because there is no secondary structure in casein proteins. 

Pressure cycling high hydrostatic pressure (e.g., 500 Mpa) induces disintegration of casein micelles and reassociation on pressure reduction Hydrophobic casein particles, formed under pressure, reassociate into smaller and more irregularly shaped aggregates Dickinson 2006a... 

We have seen earlier in this chapter how the self-assembly of casein systems is sensitively affected by temperature. Another thermodynamic variable that can affect protein-protein interactions in aqueous media is the hydrostatic pressure. Static high-pressure treatment causes the disintegration of casein micelles due to the dismption of internal hydro-phobic interactions and the dissociation of colloidal calcium phosphate. This phenomenon has been used to modify the gelation ability of casein without acidification as a consequence of exposure of hydrophobic parts of the casein molecules into the aqueous medium from the interior of the native casein micelles (Dickinson, 2006). High-pressure treatment leads to a reduction in the casein concentration required for gelation under neutral conditions, especially in the presence of cosolutes such as sucrose (Abbasi and Dickinson, 2001, 2002, 2004 Keenan et al., 2001). 

As the pH of milk is reduced, the colloidal calcium phosphate (CCP) dissolves and is completely soluble at pH 4.9 (Chapter 5). pH adjustment, followed by dialysis against bulk milk, is a convenient and widely used technique for varying the CCP content of milk. As the concentration of CCP is reduced, the properties of the micelles are altered but they retain some of their structure even after removing 70% of the CCP. Removal of more than 70% of the CCP results in disintegration of the micelles into smaller particles (aggregates). 

Removal of colloidal calcium phosphate (CCP) results in disintegration of the micelles into particles of mass 3 x 106 Da. The properties of the CCP-free system are very different from those of the normal milk system, e.g. it is sensitive to and precipitated by relatively low concentrations of Ca2 +, it is more stable to high temperatures, e.g. 140°C, and is not coagulable by rennets. Many of these properties can be restored, at least partially, by increased concentrations of calcium. 

Association and shattering of micelles. Electron microscopy shows that the casein micelles aggregate initially, then disintegrate and finally aggregate into a three-dimensional network. 

Sometimes a monodisperse system can be derived from a polydisperse one. For example, free sub-micelles (produced on disintegration of whole micelles) are quasi-monodisperse, and can be analyzed by Guinier plots and other techniques applicable to monodisperse systems. 

The micelles of this size and shape (i.e., in sufficient close proximity) must be concentrated sufficiently to result in the formation of the mesophase lattice at the prevailing temperature. At some higher temperature, this lattice will break down to give the amorphous liquid by the disintegrating effect of the increased thermal motion. 

FIGURE 28.1 Chemical modification of the protein with a water-insoluble reagent in the reverse micelles of Aerosol OT in octane. The protein molecule is entrapped in the reverse micelle surrounded by a cover of hydrated surfactant molecules. The water-insoluble reagent is located in the bulk organic phase and can be incorporated into the micelle surface layer coming into contact with the reactive group in the protein. After completion of the reaction the reverse micelle system is disintegrated and the protein is precipitated by cold acetone. 

Formation of the tribofilm layer on friction surfaces occurs under the effect of the field in the electrochemical metal,-lubricant-metal2 system, owing to formation of electro-potential (emf), forming free copper tribofilm (Shpenkov, 1995a). Since the process of tribofilm formation takes place during the friction process, disintegration of the reverse micelles takes place in a tribochemical reaction, where a redox reaction occurs, and copper oxide reduces to free copper. 

Griffin et al. (1988) reported that when the colloidal calcium phosphate was depleted, by addition of a EDTA solution to a micellar dispersion, there was essentially no selective dissociation of the individual caseins. This difference from the results of Holt et al. (1986) could reflect a difference of methodology. The method of Griffin et al. (1988) could bring about an almost complete and therefore non-selective disintegration of some micelles in the immediate vicinity of the added EDTA while leaving others virtually intact. In the dialysis method of Holt et al. (1986), the free Ca2+ concentrations is never depressed and hence micelles dissociate only because of the solubi-lazation of the colloidal calcium phosphate. 

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