Unimer micelle spherical

Figure 1 Cartoon of the surfactant unimer distribution in aqueous solution and the spontaneous self-assembly of surfactant unimers into spherical micelles just above the cmc. The two sets of arrows represent the concept of dynamic equilibrium in which the exchange rates of unimer between the air/water interface and micelles are equal. The cross section of the spherical micelle shows the core region containing the tails, the interfacial region containing hydrated headgroups (and a fraction of the counterions for ionic micelles, not shown), and the surrounding aqueous region. Such iconic images of micelles are unrealistic because experiments show that micelles are fluids and the tails are almost randomly distributed, and headgroups move at near diffusion-controlled rates that do not define a smooth surface.

High-pressure static and dynamic light scattering were used to closely examine the behavior of block copolymers of poly(vinyl acetate) (PVAc) and poly(l,l,2,2-tetrahydroperfluoroalkyl acrylate (PTAN) as a function of C02 density (Buhler et al., 1998). The phase diagram for PVAc-6-TAN shows three distinct phases as a function of polymer concentration and C02 density at a fixed temperature of 45 °C (see Figure 9.1). The block copolymer forms a precipitated phase at low C02 densities, spherical micelles at intermediate C02 densities, and unimers, or free polymer chains in solution, at high densities. The micelles-to-unimer transition was found to be very... 

Pluronics, also known as poloxamers, are a class of synthetic block copolymers which consist of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO), arranged in an A-B-A triblock structure, thus giving PEO-PPO-PEO (Fig. 11.7) (Batrakova and Kabanov 2008). They can be found either as liquids, pastes or solids (Ruel-Gariepy and Leroux 2004). Due to their amphiphilic characteristics (presence of hydrophobic and hydrophilic components), pluronics possess surfactant properties which allow them to interact with hydrophobic surfaces and biological membranes (Batrakova and Kabanov 2008). Being amphiphilic also results in the ability of the individual block copolymers, known as unimers, to combine and form micelles in aqueous solutions. When the concentration of the block copolymers is below that of the critical micelle concentration (CMC), the unimers remain as molecular solutions in water. However, as the block copolymer concentration is increased above the CMC, the unimers will self-assemble and form micelles, which can take on spherical, rod-shaped or lamellar geometries. Their shapes depend on the length and concentration of the block copolymers (i.e. EO and PO), and the temperature (Kabanov et al. 2002). Micelles usually have a hydrophobie eore, in this case the PO chains, and a hydrophilic shell, the EO ehains. 

The hydrophilic-hydrophobic nature of amphiphilic molecules leads to their self-assembly into a variety of structures in aqueous solution, as will be discussed further in Section 4.10.2. Micelles are one of the main types of structure formed by the association of amphiphiles. They consist of a core of hydrophobic chains (often alkyl chains) shielded from contact with water by hydrophilic head groups, which may be ionic or nonionic. The hydrophilic units of surfactants form a micellar corona. Micelles can either be spherical or extended into an ellipsoidal or rod-like shape. This depends on the packing of the molecules, as discussed further in Section 4.10.1. In this section we consider spherical micelles, since these are usually the type formed at the critical micelle concentration. A spherical micelle is sketched in Fig. 4.14. Unassociated molecules coexisting with micelles are often called unimers, and this nomenclature is used here. 

Amphipathic molecules such as phospholipids placed in an aqueous solution can also form micelles, which are small spherical stmctures with the hydrophilic heads of the molecules on the surface and a dense core region consisting of the hydrophobic tails (Fig. 28.8). Polymeric micelles are typically 5-100 nm in diameter. They are spontaneously formed above a certain critical concentration, the critical micelle concentration (CMC), by surfactants or block copolymers composed of hydrophilic and hydrophobic regions. The stability of the micelles depends on the CMC. Micelles with high CMC are unstable they may dissociate into unimers and their content may precipitate out. By contrast, micelles with low CMC are more stable. As micelles must be stable upon dilution with a large volume of blood in vivo, their CMC must be very low for them to be effective dmg carriers. The... 

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