Micelles hydrophilic core

Figure 11. Size- and shape-control of nanoparticles via salt reduction in (a) the hydrophobic core of a surfactant oil in water micelle and (b) the hydrophilic core of a water-in-oil reverse micelle.

Wooley and coworkers have cross-linked the micellar corona and obtained the so-called shell cross-linked knedellike micelles [83,84]. This strategy was further applied to a wide variety of block copolymer micelles. Armes and coworkers have used a similar approach for the preparation of shell cross-linked micelles with hydrophilic core and shell [85]. Many other related examples can be found in the literature. 

As far as micelles in organic media are concerned, two types of block copolymers can be considered—those with two hydrophobic blocks and those with one hydrophilic and one hydrophobic block. The latter form the so-called reverse micelles, which contain a hydrophilic core surrounded by a soluble hydrophobic corona. 

Recently, Mecking et al. reported the synthesis of inverse micelles based on a hy-perbranched polyglycerol polymer. Terminal -OH groups were modified with palmi-toyl chloride and gave a polymeric catalyst soluble in organic solvents with hydrophilic core to immobilize water-soluble guest molecules such as PdCl2 or Pd(OAc)2. 

An inverse (or reverse) micelle, which forms in a non-polar solvent, will have the hydrophilic head groups oriented toward the inside of the sphere, where a water pool is formed and a hydrophilic probe can become associated [10]. Some surfactants commonly employed to stabilize reverse micelles include sodium diisooctylsulfosuccinate (AOT), benzylhexadecyldimethylammonium chloride (BHDC), and dodecylammonium propionate (DAP). Ionic surfactants induce formation of a larger water pool than non-ionic surfactants, but the size of the hydrophilic core also depends on temperature and on the ratio of water to surfactant. 

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... 

Block copolymer micelles The core is a loading space for hydrophobic drugs, and the shell is a hydrophilic corona that makes the micelle water soluble, thereby allowing delivery of the poorly soluble contents For delivery of camptothecin, a topoisomerase I inhibitor that is effective against cancer but insoluble 27... 

Micelles are composed of amphiphilic block copolymers that self-assemble into spherical shapes of nanometer diameter due to energy minimization with the surrounding solvent. When exposed to a hydrophilic solvent the hydrophilic domains orient toward the solvent, while the hydrophobic domains orient toward the core and form a clump away from the solvent. In a similar manner, when amphiphilic molecules are exposed to a hydrophobic solvent they form micelles with a hydrophobic block on the surface and a hydrophilic block in the core. Micelles thus have a unique core-shell architecture composed of either hydrophobic or hydrophilic blocks depending on the chemical structures and the medium. The hydrophobic or hydrophilic core provides a reservoir for water-soluble or insoluble drugs and protects them from decomposition in order to maintain activity and stability. Stearic acid (SA)-grafted chitosan oligosaccharide (CSO-SA) formed... 

FIGURE 26.6 Micelle formation by hydrophobic interactions, (a) The hydrophobe gets dissolved in the hydrophobic core of the micelle, (b) The hydrophobe adjusts within the micelle so that the molecule orients as the micelle, (c) The hydrophobe gets coiled around the hydrophilic core of the micelle, (d) The tails encapsulating the hydrophobes. 

In addition, polymer micelles have been demonstrated to be more stable and also have a significantly lower cmc than surfactant micelles. Further discussion of surfactant micelles is beyond the scope of this review, and, instead, the reader is directed to a recent review article by Armes. In fact, the polymer building blocks need not be amphiphilic and such phase-separated nanostructures can be formed from completely hydrophobic or lipophilic diblock copolymers that contain two segments with differing solubility (such as polystyrene- -polyisoprene) and hence can undergo phase separation in selective solvents. One example of such completely hydrophobic phase-separated micelles are those reported by Wooley and coworkers, which can be obtained from toluene and acetone solutions of a [polystyrene-a/f-poly(maleic anhydride)]-fc-polyisoprene Iriblock. Conversely, inverse structures are also accessible and are known as reverse micelles. These can be formed by adding a nonsolvent for the hydrophilic block to afford the opposite of a conventional micelle, for which the hydrophilic core is surrounded by a hydrophobic shell in a hydrophobic surrounding media. There have been a handful of reports on the application of these reverse micelles, for example, as nanoreactors and for the extraction of water-soluble molecules. ... 

SYNTHESIS OF NOVEL SHELL CROSS-LINKED MICELLES WITH HYDROPHILIC CORES Butun V Billingham N C Armes S P Sussex,University (ACS,Div.of Polymer Chemistry)... 

The other possibility, at first examined by Wooley and co-workers [231,232] is to crosslink the corona of the micelles. These kinds of nanoparticles are designated by shell cross-linked knedel-Hke (SCK) micelles by these authors. Wooley et al. have applied this concept to a large variety of block copolymers, mainly hydrophobic-hydrophilic copolymers with PAA or quaternized PVP as the water-soluble block, which can be chemically cross-linked in their micellar form. A similar approach has been described by Armes and co-workers [233] for the synthesis of shell cross-linked micelles where core and shell are both hydrophilic. 

BUtUn, V., BiUingham, N.C. and Armes, S.P. (1999) Synthesis of novel shell cross-linked micelles with hydrophilic cores. ACS Polym. Prepr. (Div. Polym. Chem.), 40(2), 236-237. 

In addition to the swelling of micelles by oil to form microemulsions, oil-soluble surfactants can form inverted aggregates which solubilize water. These reverse micelles have a hydrophilic core, often containing trace amounts of water together with the surfactant head-groups, surrounded by the hydrophilic tails of the surfactant. Many reverse micelles can be swollen by addition of water to form sub-micron sized water droplets, or... 

Amphiphilic diblock copolymers undergo a self-assembly micellar process in solvents that are selective for one of the blocks [100]. By choosing selective conditions for each block, conventional micelles and so-called inverse micelles can be formed. Examples of the so-called schizophrenic micelles were reported [101]. In this case hydrophilic AB diblock copolymers can form micelles in an aqueous solution, in which the A block forms the inner core and inverted micelles (with the B block forming the iimer core) [102]. A diblock copolymer with two weak polybases, (poly-[2-(N-morphoUno)ethyl methacrylate-i)Iock-2- and (diethyl amino)ethyl methacrylate) (PMEMA-block-DEAEMA), forms stable micelles with DEAEMA cores by adjusting the pH value of the solution. The formation of inverted micelles (MEMA core) was achieved by a salting out effect by adding electrolytes to the aqueous solution. 

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