Association of micelles

It is now well established that formation of hard or stiff gels is the result of association of micelles into cubic phases. The notation hard gel follows Hvidt and co-workers (Almgren et al 1995 Hvidt et al. 1994) and refers to a micellar solution with a dynamic elastic shear modulus G > 103Pa. The correlation between the formation of a cubic phase and the onset of plastic flow (i.e. formation of a gel with a finite yield stress) was first made for PS-PI solutions in... 

Vanin et al. studied order proflles of the perdeuterated laurate chain in chiral and achiral phases by means of deuterium NMR. The quadrupole splittings measured for the sequence of CD2 moieties are all quite similar for the nematic and chiral nematic phase the conclusion was that chiral distortions of the micellar shape must be very small. Their proposal for the mechanism behind the phase chirality is analogous to the one for polypeptides an asymmetric point charge distribution which in this case was speculated to be created by the chiral dopant on the micellar surface [19]. Radley and Tracey prepared samples from a racemic detergent and observed high fluidity, whereas those consisting of some enantiomeric excess showed a significantly increased viscosity [30]. But this is no proof for the association of micelles to necklaces. 

According to the original mass action model, micellization is a reversible association of micelles [20,116) ... 

This character, called amphiphilic, produces two characteristic sets of behavior, adsorption on the interfaces and auto-association in the form of micelles that extend into the oily surroundings as illustrated in Figure 9.8. 

Other properties of association colloids that have been studied include calorimetric measurements of the heat of micelle formation (about 6 kcal/mol for a nonionic species, see Ref. 188) and the effect of high pressure (which decreases the aggregation number [189], but may raise the CMC [190]). Fast relaxation methods (rapid flow mixing, pressure-jump, temperature-jump) tend to reveal two relaxation times t and f2, the interpretation of which has been subject to much disagreement—see Ref. 191. A fast process of fi - 1 msec may represent the rate of addition to or dissociation from a micelle of individual monomer units, and a slow process of ti < 100 msec may represent the rate of total dissociation of a micelle (192 see also Refs. 193-195). 

In otlier words, tire micelle surface is not densely packed witli headgroups, but also comprises intennediate and end of chain segments of tire tailgroups. Such segments reasonably interact witli water, consistent witli dynamical measurements. Given tliat tire lifetime of individual surfactants in micelles is of tire order of microseconds and tliat of micelles is of tire order of milliseconds, it is clear tliat tire dynamical equilibria associated witli micellar stmctures is one tliat brings most segments of surfactant into contact witli water. The core of nonnal micelles probably remains fairly dry , however. 

FIGURE 19 6 Space filling model of a micelle formed by association of car boxylate ions derived from a long chain carboxylic acid The hydrocarbon chains tend to be on the inside and the carboxylate ions on the surface where they are in contact with water mole cules and metal cations... 

Figure 6.9 represents schematically the formation of a micelle by the association of n surfactant molecules. The cutaway view of the spherical micelle shows the hydrocarbon interior of these particles. Incidentally, it is this sort of reversible... 

Historically, the absorption of lipid-soluble nutrients has been considered to be carrier-independent, with solutes diffusing into enterocytes down concentration gradients. This is true for some lipid-soluble components of plants (e.g. the hydroxytyrosol in olive oil Manna et al., 2000). However, transporters have been reported for several lipid-soluble nutrients. For example, absorption of cholesterol is partly dependent on a carrier-mediated process that is inhibited by tea polyphenols (Dawson and Rudel, 1999) and other phytochemicals (Park et al., 2002). A portion of the decreased absorption caused by tea polyphenols may be due to precipitation of the cholesterol associated with micelles (Ikeda et al., 1992). Alternatively, plant stanols and other phytochemicals may compete with cholesterol for transporter sites (Plat and Mensink, 2002). It is likely that transporters for other lipid-soluble nutrients are also affected by phytochemicals, although this has not been adequately investigated. 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

The concept of micelles consists of aggregation of amphiphilic molecules that contain polar and non-polar moieties, which associate in a manner that minimizes hydrophobic and lipophilic interactions. However, a cascade molecule consisting of an internal lipophilic framework and a external hydrophilic surface would effectively be a unimolecular micelle [59] capable of hosting molecular guest(s). 

In dilute aqueous solutions, copolymers having hydrophobic and hydrophilic parts may form polymeric micelles, i.e. stable particles with a core-shell structure. The association of the hydrophobic parts of the block copoly-... 

Colloids are introduced in the second half of the chapter. The various classifications of colloid types are discussed, together with ways of forming, sustaining and destroying colloids, i.e. colloid stability. Finally, association colloids ( micelles ) are discussed. 

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