Critical micelle concentration , molecular self-assembly

Micelles are the simplest organised form of the self-assembly produced by amphiphilic molecules due to the so-called hydrophobic effect , firstly recognized by Tanford.NMR parameters experience dramatic effects as a result of the strong intermolecular interactions among the amphiphiles. In the case of isotropic liquid systems, NMR experiments can be easily performed and modelled, since many advances have been produced in the last two decades.Hence, information on critical micelle concentration (c.m.c.), molecular conformations and interactions, counterion binding, hydration can be obtained from chemical shifts, relaxation, and self-diffusion NMR measurements, also in mixed systems. 

In aqueous solutions of surfactants at concentrations above the critical micelle concentration (CMC), the molecules self-assemble to form micelles, vesicles, or other colloidal aggregates. These may vary in size and shape depending on solution conditions. In addition to surfactant molecular structure, the effects of concentration, pH, other additives, cosolvents, temperature, and shear affect the nanostructure of the micelles. The presence of TLMs or cylindrical, rodlike, or wormlike micelles at concentrations > CMCii are generally believed to be necessary for surfactant solutions to be drag reducing [Zakin et al., 2007]. 

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. 

It is well established that when an amphiphilic block copolymer is dissolved in a selective solvent at a fixed temperature, above a specific concentration called the critical micelle concentration (cmc), micellisation occurs. Below the cmc, only molecularly dissolved copolymer chains (unimers) are present in the solution, while above the cmc multimolecular micelles are in thermodynamic equilibrium with the unimers. This process is in analogy to classical low molecular weight surfactants, differing in that the cmc is much lower in the case of block copolymers macrosurfactants. The self-assembly arises from the need of the copolymer chains to minimise energetically unfavourable solvophobic interactions. Therefore, micelle formation is dictated by two opposite forces, the attractive force between the insoluble blocks, which leads to aggregation, and the repulsive one between the soluble blocks preventing unlimited growth of the micelle. At the same time, the interaction of the soluble blocks and the solvent is responsible for the stabilisation of the micelles [1, 10]. 

Amphiphiles (surface-active compounds or surfactants) are characterized by a molecular structure with both hydrophilic and hydrophobic domains. They tend to adsorb to surfaces and interfaces, with a concomitant lowering of surface tension. At the critical micellar concentration (CMC), the limit of surface tension reduction is reached and a spontaneous self-assembly takes place with the formation of aggregates (micelles). The size and structure of the micelles depend on the type and concentration of surfactant(s) present. 

---