Micelle cores

Micellar medium has received great attention because it solubilizes, concentrates and orientates the reactants within the micelle core and in this way accelerates the reaction and favors the regio- and stereoselectivity of the process [68], In addition the micellar medium is cheap, can be reused, is more versatile than cyclodextrins and more robust than enzymes. With regard to Diels Alder reactions, we may distinguish between (i) those in which one or both reagents are surfactants which make up the micellar medium, and (ii) those that are carried out in a micellar medium prepared by a suitable surfactant. 

Kang N, Perron ME, Prudhomme RE et al (2005) Stereocomplex block copolymer micelles core-shell nanostructures with enhanced stability. Nano Lett 5 315-319... 

Surfactants are well known as stabilizers in the preparation of metal nanoparticles for catalysis in water. Micelles constitute interesting nanoreactors for the synthesis of controlled-size nanoparticles from metal salts due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained which can be easily used as catalysts. 

Block copolymers in a selective solvent, ie., a good solvent for one block but a precipitant for the other, behave like typical amphiphiles. The copolymer molecules aggregate reversibly to form micelles in a manner analogous to the aggregation of classical surfactants. Our block copolymers are very amphiphilic in the sense described above and form well-defined micelles in a wide range of selective solvents. In solvents for polystyrene, the polystyrene block is located in the micelle corona, while the modified block is hidden in the micelle core. 

Recently, Menger (1979) proposed an alternative model of the micellar structure. According to his reef model , micelles possess rugged, dynamic surfaces and water molecules penetrate close to the micelle core. 

FIGURE 1.2. Formation of nanoparticles of metal oxide by reverse micelle method. A solution of inverse micelles is first formed by adding a long-chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminum alkoxy amine adduct results in hydrolysis of the aluminum alkoxide adduct and formation of nano-sized particles of aluminum oxyhydroxide after drying. These particles are shown in the SEM picture above. 

PIPAAm-PBMA block copolymers form a micellar structures by selfassociation of the hydrophobic PBMA segments in water, a good solvent for PlPAAm chains below the LCST but a nonsolvent for the PBMA chains. This amphiphilic system produces stable and monodispersed micelles from polymer/A-ethylacetamide (good solvent for the both polymer blocks) solutions dialyzed against water. Hydrophobic dmgs can be physically incorporated into the iimer micelle cores with PBMA chains by hydrophobic interactions between the hydrophobic segments and dmgs. 

Figure 1 Schematic structures of micelle and liposome, their formation and loading with a contrast agent, (a) A micelle is formed spontaneously in aqueous media from an amphiphilic compound (1) that consists of distinct hydrophilic (2) and hydrophobic (3) moieties. Hydrophobic moieties form the micelle core (4). Contrast agent (asterisk gamma- or MR-active metal-loaded chelating group, or heavy element, such as iodine or bromine) can be directly coupled to the hydrophobic moiety within the micelle core (5), or incorporated into the micelle as an individual monomeric (6) or polymeric (7) amphiphilic unit, (b) A liposome can be prepared from individual phospholipid molecules (1) that consists of a bilayered membrane (2) and internal aqueous compartment (3). Contrast agent (asterisk) can be entrapped in the inner water space of the liposome as a soluble entity (4) or incorporated into the liposome membrane as a part of monomeric (5) or polymeric (6) amphiphilic unit (similar to that in case of micelle). Additionally, liposomes can be sterically protected by amphiphilic derivatization with PEG or PEG-like polymer (7) [1].

Using N-terminus modified polylysine, we developed a synthesis for an amphiphilic polychelator, A,a-(DTPA-polylysyl)glutaryl phosphatidyl ethanolamine (DTPA-PL-NGPE). This polychelator was incorporated into the liposomal membrane and micelle core during liposome or micelle preparation. This system sharply increased the number of chelated Gd atoms attached to a single lipid anchor. This increased the number of bound reporter metal atoms per vesicle and decreased the dosage of an administered... 

The Winsor II microemulsion is the configuration that has attracted most attention in solvent extraction from aqueous feeds, as it does not affect the structure of the aqueous phase the organic extracting phase, on the other hand, is now a W/0 microemulsion instead of a single phase. The main reason for the interest in W/0 microemulsions is that the presence of the aqueous microphase in the extracting phase may enhance the extraction of hydrophilic solutes by solubilizing them in the reverse micellar cores. However, this is not always the case and it seems to vary with the characteristics of the system and the type of solute. Furthermore, in many instances the mechanism of extraction enhancement is not simply solubilization into the reverse micellar cores. Four solubilization sites are possible in a reverse micelle, as illustrated in Fig. 15.6 [19]. An important point is that the term solubilization does not apply only to solute transfer into the reverse micelle cores, but also to insertion into the micellar boundary region called the palisade. The problem faced by researchers is that the exact location of the solute in the microemulsion phase is difficult to determine with most of the available analytical tools, and thus it has to be inferred. 

Bimetallic colloids (Pd-Au, Pd-Pt and Pd-Zn) stabilized into the micelle cores of PS-b-P4VP were investigated to determine the influence of the second metal (18). In this case, during the preparation of the catalyst, the addition of both metals salts occurs simultaneously, followed by reduction of metal compounds. 

Here Fcmc is the free energy due to deformation of chains in the micelle core... 

Micelles are colloidal dispersions that form spontaneously, under certain concentrations, from amphiphilic or surface-active agents (surfactants), molecules of which consist of two distinct regions with opposite afL nities toward a given solvent such as water (Torchilin, 2007). Micelles form when the concentration of these amphiphiles is above the critical micelle concentration (CMC). They consist of an inner core of assembled hydrophobic segments and an outer hydrophilic shell serving as a stabilizing interface between the hydrophobic core and the external aqueous environment. Micelles solubilize molecules of poorly soluble nonpolar pharmaceuticals within the micelle core, while polar molecules could be adsorbed on the micelle surface, and substances with intermediate polarity distributed along surfactant molecules in intermediate positions. 

As with normal hydrocarbon-based surfactants, polymeric micelles have a core-shell structure in aqueous systems (Jones and Leroux, 1999). The shell is responsible for micelle stabilization and interactions with plasma proteins and cell membranes. It usually consists of chains of hydrophilic nonbiodegradable, biocompatible polymers such as PEO. The biodistribution of the carrier is mainly dictated by the nature of the hydrophilic shell (Yokoyama, 1998). PEO forms a dense brush around the micelle core preventing interaction between the micelle and proteins, for example, opsonins, which promote rapid circulatory clearance by the mononuclear phagocyte system (MPS) (Papisov, 1995). Other polymers such as pdty(sopropylacrylamide) (PNIPA) (Cammas etal., 1997 Chung etal., 1999) and poly(alkylacrylicacid) (Chen etal., 1995 Kwon and Kataoka, 1995 Kohorietal., 1998) can impart additional temperature or pH-sensitivity to the micelles, and may eventually be used to confer bioadhesive properties (Inoue et al., 1998). 

Nystatin Micelle-Water Partition Coefficients and Pluronic Micelle Core Polarities as a Function of Pluronic and Temperature... 

Effect of Micelle Core Structure on Core Properties and Drug Encapsulation in Micelles Derived from PEO-f -P(6-HHA)... 

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