Micellar structural characteristics

Numerous books and reviews have been published on this subject (e.g. Fendler and Fendler, 1975 Mittal, 1977). Therefore, the structural characteristics of micelles will be presented only to the extent that is necessary for the subsequent discussions. These surfactants form micelles at concentrations above the cmc (critical micelle concentration). Such micelles have average radii of 12-30 A and contain 20-100 surfactant molecules. The hydrophobic part of the aggregate forms the core of the micelle while the polar head groups are located at the micellar surface. Micelles at concentrations close to their cmc are assumed to possess spherical and ellipsoidal structures (Tanford, 1973, 1978). A schematic representation of a spherical ionic micelle is shown in Fig. 1. 

As an extension of the perspective of micelle formation by amphiphihc block copolymers the following part will focus on two other types of polymers. The micellar structures that will discussed are (i) micelles and inverse micelles based on a hyperbranched polymers and (ii) polysoaps, that are copolymers composed of hy-drophihc and amphiphihc or hydrophobic monomers. Whereas the first class of polymers is stiU very new and only few examples exist of the synthesis and appH-cation of such stracture in catalysis, the synthesis and aggregation characteristics of polysoaps has already been intensively discussed in the hterature. 

The characteristics of the water pool of reverse micelles has been explored by H, 23Na, 13C, 3IP-NMR spectroscopy. Since the initial association process in RMs is not totally understood, and because of the low CMC, aggregation studies from NMR are rather scarce. Direct determination of a CMC in the diethyl hexyl phosphate /water/benzene system (at Wo = 3.5) was possible because the chemical shift of 31P in phosphate groups is very sensitive to hydration effects. The structure and state of water in RMs and particularly at low water content has received considerable attention. The proton chemical shifts have been explored in AOT/water/heptane, methanol, chloroform, isooctane and cyclohexanone. The water behavior in small reverse micelles is close to that of the corresponding bulk ionic solution. Until now, the effect of a solute on micellar structure was not well... 

Many ionomers are used in polymer blends as a compatibilizer, because the ionomers have at least two different properties, that is, hydrophobic host-polymer matrix and hydrophilic ionic groups and/or aggregates. In addition, the solution properties of mixtures including the ionomers have also been investigated to clarify the details of characteristics not only in blending but also in micellar structures. For example, Bosse and Eisenberg recently reported on the kinetics of coil overlap in ionomer blends in DMSO by means of H NMR.47,48... 

An alternative method to produce cheeses of characteristics more similar to those produced by the conventional method wonld be to nse a UF retentate obtained at low concentration factor [79]. Kosikowski [84], for instance, suggested that 1.8x is the limit for making good quality Cheddar cheese. In a related work on raw milk, Hinrichs [76] reported a transition from Newtonian to non-Newtonian flow behavior above a concentration factor of 2 in the UF, which corresponds to more than 8% fat and 3.33% protein in the retentate. Similarly, Erdem [83,85] found that because of concentration by UF to two- and threefold using 10 kDa MWCO polysnl-fone membrane, the protein system in skim milk becomes more agglomerated via hydrophobic bonds, which results to reorganization of the protein system into a more compact micellar structure. 

We must conclude from this set of typical data that the crystallite properties of the gel and that of the lamellae formed in dilute solution are the same. Hence, barring an unusual set of coincidences the basic crystallite characteristic of the gel should be of lamellar form for the linear polyethylenes when crystallized from homogeneous solutions. Thus we have at least one system where the crystaUization elation phenomenon does not involve a fringed micellar structure. 

Although micelles are stable in time at fixed conditions, their characteristics depend for a given system on the thermodynamic quality of the solvent and on temperature. For this reason it is impossible to study that system under different conditions, for example in a different solvent, at a different temperature or at various concentrations. The idea of Prochazka and Baloch [227] and of Tuzar et al. [228] to circumvent this problem was to stabilize micellar structures by cross-linking of the micellar core, either by UV or fast electron irradiation. The unimer remaining in the system after cross-linking can easily be removed by fractional precipitation or dialysis. 

The most important property all the surface-active molecules have in common is the self-assembly characteristics. It was mentioned that ordinary surfactants (soaps, etc.) when dissolved in water form self-assembly micellar structures. The phospholipids are molecules like surfactants they also have a hydrophilic head and generally have two hydrophobic alkyl chains. These molecules are the main components of the membranes of cells. The lung flnid also consists mainly of lipids of this kind. In fact, usually the membranes of cells are made up of two layers of phospholipids, with the tails turned inward, in an attempt to avoid water. 

Sdf Assembly Measurements in Solution The critical micelle concentration (CMC) is the characteristic solution concentration at wiiich solubilized single molecules aggregate into micellar structures. A number of parameters, including osmotic pressure, conductivity, turbidity and surfiice tension e q>erience dramatic changes at the CMC. One method of determining the CMC involves surfiice tension measurements using a Wilhelnty plate. The CMC of various F-terminated cellulose ester oligomers and of xylans were determined in THF and water, reflectively. 

It is quite evident that both micellar structures can be obtained by the same amphiphilic diblock copolymer system only by changing the structural parameters of the AB copolymer. The relative lengths of the two blocks and the total of the copolymer not only dictate the morphology of the resulting micelles ( hairy vs. crew-cut ) but also determine the rest of the micellar characteristics (Aagg,... 

At relatively low polymer adsorption levels, oscillatory force profiles similar to those observed for micellar solutions are also seen in polyelectrolyte-containing systems. As with micellar structuring, these oscillations originate from an inhomogenous density distribution of polymer (or polymer/surfactant complexes) within the film. Furthermore, the characteristic length-scale of the oscillatory forces indicate that this structuring is controlled by electrostatic interactions. To date, no complete theory describing this phenomenon exists ... 

Surfactants consists of molecules with a hydrophobic and a hydrophilic part. Due to this amphiphilic nature these molecules adsorb from aqueous solutions onto interfaces, as is expressed in their name [1], The molecules are densely packed in these adsorbed monolayers. In the aqueous bulk phase the surfactant molecules assemble above a characteristic concentration, called cmc, into micellar structures, which can be understood as interfaces in the bulk solution. The driving force for the adsorption and the aggregation is the same for both processes and is given by the hydrophobic interaction [2]. 

Structural characteristics of micellar solutions, cubic phases,... 

When dispersed in aqneous media, the hydrophobic component constitutes the core while the hydrophilic block corona serves to stabilize the miceUar structure. In contrast to low molar mass surfactants, amphiphilic block copolymers have highly tunable composition and assembly characteristics. The constraints between the core-forming blocks, the interaction between chains in the corona, and the surface energy between the solvent and the core dictate the shape and size of the micellar structures [43]. An important property of amphiphilic block copolymers is the ability to enhance solubility of hydrophobic substances. As with low molar mass surfactants, the hydro-phobic core of the micellar structures formed by amphiphilic block copolymers can serve as a compatible microenvironment for sequestration of water-insoluble compounds. 

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