Micelle structural properties

Mortensen K 1998 Structural properties of self-assembled polymeric micelles Curr. Opin. Colloid Interface Sol. 3 12-19... 

Molecules that possess both hydrophilic and hydrophobic structures may associate in aqueous media to form dynamic aggregates, commonly known as micelles. The properties of micellar structures have been discussed in great detail [66-69], but thejr main pharmaceutical application lies in their ability to provide enhanced solubility to compounds lacking sufficient aqueous solubility [70], The ability of a micelle to solubilize compounds of limited aqueous solubility can be understood from consideration of the schematic drawing of Fig. 10a. Above the critical micelle concentration, these molecules orient themselves with the polar ends in interfacing with the aqueous solution and the nonpolar ends at the interior. A hydrophobic core is formed at the interior of the micelle, and hydrophobic solute molecules enter and occupy this region. 

DR. GUILLERMO FERRAUDI (University of Notre Dame) From your talk it appears that an important aspect of micelles is the modified reactivity imparted to excited states or chemical intermediates. If micelles are to be used to exploit this phenomenon, the structure of the micelle should be carefully defined. Can you tell us something more about the structural properties of micelles For example, if triton X-100 is used to form micelles, a variation in conditions yields micelles with different shapes, different dimensions, etc. 

The purpose of this article is to review studies carried out on hemes incorporated inside the micellar cavity, and examine the effect of micellar interaction on the electronic and structural properties of the heme. A comparison of these results with those on the metalloproteins is clearly in order to assess their suitability as models. The article begins with a general introduction to micellar properties, the incorporation of hemes in the micellar cavity, and then discusses results on hemes inside the micelles with different oxidation and spin states, and stereochemistry. The experimental techniques used in the studies on these aqueous detergent micelles are mostly NMR and optical spectroscopy. The present article has therefore a strong emphasis on NMR spectroscopy, since this technique has been used very extensively and purposefully for studies on hemes inside micellar cavities. 

Both ferric and ferrous hemes have been encapsulated in micelles with different spin states, and coordination geometries and oxidation states have been stabilized. Their electronic and structural properties are reviewed below. 

Ding K, Alemduroglu EE, Boersch M, Berger R, Herrmarm A. Engineering the structural properties of DNA block copolymer micelles by molecular recognition. Angew Chem Int Ed 2007 46 1172-1175. 

Principal micelle characteristics. The structure of the casein micelles has attracted the attention of scientists for a considerable time. Knowledge of micelle structure is important because the stability and behaviour of the micelles are central to many dairy processing operations, e.g. cheese manufacture, stability of sterilized, sweetened-condensed and reconstituted milks and frozen products. Without knowledge of the structure and properties of the casein micelle, attempts to solve many technological problems faced by the dairy industry will be empirical and not generally applicable. From the academic viewpoint, the casein micelle presents an interesting and complex problem in protein quaternary structure. 

Recently, carbohydrate amphiphiles have been tested in the asymmetric hydrogenation of (Z)-methyl a-acetamidocinnamate in water (98). With a rhodium(I)-BPPM complex, 50% of the reactant was converted in 5 min, and enantioselectivities up to 96% were observed. A comparison of amphiphiles with alkyl chains of different lengths showed that micelle-forming properties, hydrophilic-lipophilic balance, and the structure caused by hydrogen bonding in the head group may be responsible for these effects. 

Huibers, P.D.T., V.S. Lobanow, A.R. Katrizky, D.O. Shah, and M. Karelson. 1996. Prediction of critical micelle concentration using a quantitative structure-property relationsliip approach. 1. Nonionic surfactants. Langmuir, 12, 1462-1470. 

In dilute aqueous solutions, polyelectrolyte block copolymers self-assemble into micelles consisting of a hydrophobic core and a polyelectrolyte shell. The study of their structural properties is expected to provide a basic understanding of the properties of dense polyelectrolyte layers, electro-steric stabilization mechanisms, and actuator functions based on variations in the electrostatic interactions. 

Figure 19.2. Structural properties of the casein micelle and p-lactoglobulin.

It is also of interest to study the self-assembly of these inverted unimolecular dendritic micelles into vesicles in water at pH = 1 and at the air-water interface. In these vesicles, the dendrimers adopt a distorted conformation, far from spherical. These dendritic amphiphiles form a novel class of vesicle-forming surfactants [71] and introduce new perspectives towards a better understanding of the structure-property relation of amphiphiles on the one hand and dendrimers on the other hand. 

The surfactant AOT forms reverse micelles in non-polar fluids without addition of a cosurfactant, and thus it is possible to study simple, water/AOT/oil, three component systems. To determine micelle structure and behavior in water/AOT/oil systems, investigators have studied a wide range of properties including conductivity (15), light (JL ), and neutron (12) scattering, as well as solution phase behavior (1 ). From information of this type one can begin to build both microscopic models and thermodynamic... 

In the studies described here, we examine in more detail the properties of these surfactant aggregates solubilized in supercritical ethane and propane. We present the results of solubility measurements of AOT in pure ethane and propane and of conductance and density measurements of supercritical fluid reverse micelle solutions. The effect of temperature and pressure on phase behavior of ternary mixtures consisting of AOT/water/supercritical ethane or propane are also examined. We report that the phase behavior of these systems is dependent on fluid pressure in contrast to liquid systems where similar changes in pressure have little or no effect. We have focused our attention on the reverse micelle region where mixtures containing 80 to 100% by weight alkane were examined. The new evidence supports and extends our initial findings related to reverse micelle structures in supercritical fluids. We report properties of these systems which may be important in the field of enhanced oil recovery. 

Huibers, P.D.T., Lobanov, V.S., Katritzky, A.R., Shah, D.O. and Karelson, M. (1996). Prediction of Critical Micelle Concentration Using a Quantitative Structure-Property Relationship Approach. 1. Nonionic Surfactants. Langmuir, 12,1462-1470. 

Sandra, S. and Dalgleish, D. G. (2005). Effects of ultra-high-pressure homogenization and heating on structural properties of casein micelles in reconstituted skim milk powder. Int. Dairy ]. 15,1095-1104. 

The properties of a variety of surfactants (36-41), novel cationic phases (42-44), mixed micelles (45-49), microemulsions (50), vesicles (51-55), liposomes (56, 57), and synthetic polymers (58-65) have all be screened by LSER. Micelle structural modifications by differing head groups (66) and spacers (67,68), chain lengths (69), and counterions (70), as well as the use of deuterated water buffers (71) and the addition of cyclodextrins (72) and organic solvents (73) to the micellar medium, have also been characterized by LSER studies. 

This last observation deserves further consideration. Because any molecule exhibits a measurable volume, the parameter V is expected to be the most important variable in the MLR (Table 15.1) in comparison with other parameters, especially those representing specific interactions, properties that some of the solutes in the set might lack. If a three-compartment model can be invoked for the micelle structure (inner core, interface, and surface) as opposed to the Hartley model ( oil droplet, hydrophobic core encased by a hydrophilic region), remarkable differences in cavitation energy between the aqueous bulk and the micelle interface as well as between the aqueous bulk and the micelle inner core are anticipated. Thus, the parameter V coefficient in the MLR with the entire set of solutes is expected to be prominent as well. More importantly, the parameter V coefficient reflects an average behavior, that is, it is indicative of cavitation energy differences between a given micelle... 

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