Micelle dynamic processes

Figure 3. Dynamic equilibrium of asphaltene micelle inversion process.

By covalent linkage of different types of molecules it is possible to obtain materials with novel properties that are different from those of the parent compounds. Examples of such materials are block-copolymers, soaps, or lipids which can self-assemble into periodic geometries with long-range order. Due to their amphiphilic character, these molecules tend to micellize and to phase-separate on the nanometer scale. By this self-assembly process the fabrication of new na-noscopic devices is possible, such as the micellization of diblock-co-polymers for the organization of nanometer-sized particles of metals or semiconductors [72 - 74]. The micelle formation is a dynamic process, which depends on a number of factors like solvent, temperature, and concentration. Synthesis of micelles which are independent of all of these factors via appropriately functionalized dendrimers which form unimolecular micelles is a straightforward strategy. In... 

When benzophenone itself is dissolved in detergent solutions, the photochemical processes are affected by the structure of the micelle 38 43). Scheme XIII represents the dynamical processes. 

Ultrasonic relaxation spectroscopy (URS) is nothing but a special treatment of data from ultrasonic absorption measurements. Micelle dynamics involves characteristic relaxation processes, namely micelle-monomer exchange and micelle formation-breakdown. Ultrasonics can provide information about the kinetics of the latter, the fast relaxation process also, theoretical expressions for the relaxation time and relaxation strength such as those derived by Teubner [76] provide self-consistent estimates of both. 

There continues to be extensive interest in latexes and micellar systems. The structure of acrylic latex particles has been investigated by non-radiative energy transfer by labelling the co-monomers with fluorescent acceptor-donor systems. Phase separations could also be measured in this way. Excimer fluorescence has been used to measure the critical micelle temperature in diblock copolymers of polystyrene with ethylene-propylene and the results agree well with dynamic light scattering measurements. Fluorescence anisotropy has been used to measure adsorption isotherms of labelled polymers to silica as well as segmental relaxation processes in solutions of acrylic polymers. In the latter case unusual interactions were indicated between the polymers and chlorinated hydrocarbon solvents. Fluorescence analysis of hydrophobically modifled cellulose have shown the operation of slow dynamic processes while fluorescence... 

Effects of ionizing radiation on lipid molecules have been understood by studying model systems which are simpler than the real biological membranes, such as PUFA micelles and liposomes. The formation of lipid oxidative modifications of PUFAs appears as a dynamic process initiated by hydroxyl free radicals generated by water radiolysis, amplified by a propagating-chain mechanism involving alkyl and peroxyl free radicals, and leading not only to hydroperoxides but also to a lot of other lipidic oxidized end-products. Kinetic data, such... 

The advantage of the microemulsion-based route is that it is a soft technique, i.e. it does not require extreme conditions of pressure and temperature. But it is the dynamic of micellar dispersions that makes themso relevant for this kind of purpose the droplets are indeed subject to Brownian motion and collide continuously, leading to the formation of short-lived dimers and to the exchange of the aqueous contents of the micelles. This dynamic process ensures a homogeneous repartition of the reactants among the aqueous droplets or water pools and thus the formation of very monodispersed particles [3]. 

A new insight into the dynamic processes in the bulk and at the surface of surfactant solutions can be seen in molecular dynamics simulations. Only now are computers sufficiently powerful that such simulations can be performed without too many simplifications. The state of the art of molecular dynamics was recently summarised by van Os Karabomi (1993), showing that complex processes such as micelle formation (Karabomi O Connell 1993), emulsion formation or solubilisation processes (Smit et al. 1993) can be simulated. Future improvements of computers and algorithms will provide a deep insight into even more complex processes connected with dynamics of interfacial phenomena, such as adsorption layer stracture and formation, effects of molecular interfacial and bulk interactions in mixed systems of surfactants and polymers. 

Models with increasing sophistication for the analysis of dynamic processes in supramolecular systems, notably micelles, as well as for the determination of other parameters have been developed over the past two decades. The basic conceptual framework has been described early on [59,60,95,96] and has been classifred into different cases which take into account the extent of quencher mobility and the mechanism of quenching [95]. Two of those cases lead to information about mobility and will be discussed. It is important to emphasize that this analysis is only applicable to self-assembled system such as micelles and vesicles it cannot be applied to host-guest complexes. This model assumes that the probe is exclusively bound to the supramolecular system and that no probe migration occurs during its excited state lifetime. The distribution of probe and quencher has been modeled by different statistical distributions, but in most cases, data are consistent with a Poisson distribution. The Poisson distribution implies that the quencher association/dissociation rate constants to/from the supramolecular system does not depend on how many... 

FIGURE 15.6. Micelle formation is a rapid and dynamic process involving continuous movement of surfactant molecules into and out of the micelle and, perhaps, submicellar aggregates. The residence time of a given molecule in a micelle is estimated to be between 10 and 10 s. 

Figure 2.6 Dynamic exchange in micelles. Slow process (ms to s) micelle formation...

Likewise, water content, surfactant concentration, and polarity of organic solvent can be adjusted to yield different sizes and shapes of the nonreactor pockets. However, particle synthesis and growth is a dynamic process, with rapid exchange of micelle contents occurring, and so only a rough control of resultant nanoparticle size and shape can be expected. 

The first meaningfnl stndies on the kinetics of dynamic processes within micellar solutions, which looked at dissociation rates via temperature-jump (T-jump) and related techniqnes, were carried out in the 1960s. The first notable attempt at a complete theory of micellization kinetics was the work of Kresheck et al. [77], who proposed a stepwise surfactant aggregation model based on a monodisperse system, where all micelles have the same aggregation nnmber, n. However, this model found... 

So far, all of the studies detailed have dealt primarily with nonequilibrium states, i.e micelle formation or breakdown stimulated by an external trigger, e.g., a jump in pressure, temperature, or concentration. It has of course been shown, however, that even at equilibrium, micelles are by their nature transient species with millisecond-scale lifetimes, and that dynamic processes exchange surfactant molecules between aggregates, interfaces, and bulk solutions. Studies of the kinetics of these processes in equilibrium solutions are much more scarce, mainly owing to experimental difficulties. 

As has been shown above, the kinetics of micelle formation, breakdown, and associated dynamic processes has been documented. However, much less is known about the kinetic processes involved with transformations between other aggregate structures. 

Membrane Proteins. - While solid-state NMR is undergoing enormous growth in the characterization of membrane proteins in oriented phospholipid bilayers and native membranes, liquid-state NMR has been routinely used for studies of peptides and small membrane proteins in detergent micelles. Hwang et al. demonstrate the usefulness of these NMR methods on PagP and show how key dynamic processes can be probed. ... 

Other shapes may be produced, e.g. rod-shaped and lameDar micelles. Since micelles play a vital role when considering adjuvants, their properties must be understood in some detail. As mentioned in Chapter 2, micelle formation is a dynamic process, i.e. a dynamic equilibrium is set up whereby surface active agent molecules are constantly leaving the micelles while others enter the micelles (the same apphes to the counter-ions). The dynamic process of micellization is described by two relaxation processes (1) A short relaxation time ti (of the order of s), which is the lifetime for a surfactant molecule in a micelle. (2)... 

A longer relaxation time Z2 (of the order of 10 -1 s), which is a measure of the micellization-dissolution process. Both ti and xi depend on the surfactant structure, its chain length, and these relaxation times determine some of the important factors in selecting adjuvants, such as the dynamic surface tension (discussed below). 

The value of the critical micelle concentration (CMC) is an important parameter in a wide variety of industrial applications involving adsorption of surfactant molecules at interfaces, such as foams, froths, emulsions, suspensions, and surface coatings. It is probably the simplest means of characterizing the colloid and surface behaviour of a surfactant solute, which in turn determines its industrial usefulness. Many industrial processes are also dynamic processes in that they involve a rapid increase in interfacial area, such as foaming, wetting, emulsification and solubilization. First, the available monomers adsorb on to the freshly created interface. Then, additional monomers must be provided by the breakup of micelles. Especially when the free monomer concentration (i.e. CMC) is low, the micellar breakup time or diffusion of monomers to the newly created interface can be rate-limiting steps in the supply of monomers, which is the case for many nonionic surfactant solutions (3). 

The continuous exchange of the surfactant molecules (as well as cosurfactant molecules in case of mixed micelles) constitutes a major dynamic process in micellar systems. The situation in microemulsions, although more complex, directly derives from that in simple micelles. For this reason, we will briefly recall here the main conclusions that have been estab-hshed concerning the micellar dynamics, reviewed in detail in Chapter 3. 

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