Amphiphilic block copolymers differences between surfactants

A special class ofblock copolymers with blocks of very different polarity is known as amphiphilic (Figure 10.1). In general, the word amphiphile is used to describe molecules that stabilize the oil-water interface (e.g., surfactants). To a certain extent, amphiphilic block copolymers allow the generalization of amphi-philicity. This means that molecules can be designed that stabilize not only the oil-water interface but any interface between different materials with different cohesion energies or surface tensions (e.g., water-gas, oil-gas, polymer-metal, or polymer-polymerinterfaces). This approach is straightforward, since the wide variability of the chemical structure of polymers allows fine and specific adjustment of both polymer parts to any particular stabilization problem. 

In Chapter 1, Murgia, Palazzo, and coworkers investigated the physicochemical behaviors of a binary IL bmimBF and water, and the ternary NaAOT, water and bmimBF mixtures essentially through the evaluation of the self-diffusion coefficients of the various chemical species in solution by PGSTE-NMR experiments. The diffusion of water molecules and bmimBF ions were found to be within different domains, which suggested that the systems were nanostructured with formation of micelles having positive curvature and a bicontinuous micellar solution for the former and the later systems, respectively. The remarkable differences between the two systems are attributed to the specific counterion effect between the aforementioned ILs and the anionic surfactant. In Chapter 2, Bermudez and coworkers focused on the characterization of small (conventional surfactants) and polymeric amphiphiles (block copolymers) in different types of ILs (imidazolium, ammonium. 

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]. 

More than two surfactants can be put together to form tri,- tetra- or polymeric surfactants. Trimeric or even tetrameric surfactants show properties often superior to monomeric surfactants. Besides, they are intermediate between conventional surfactants and polymeric surfactants. In a normal polymeric surfactant each monomer unit is amphiphilic. Another type of polymeric surfactant, called block copolymer [522], consists of at least two parts. One part is made of monomer type A, the other part is made of monomer B. If A is polar and B nonpolar, the blockcopolymer will be strongly surface active and show many properties of a conventional surfactant. If there are two different blocks we talk about a diblock copolymer. In the following part of this chapter we concentrate on conventional surfactants. 

The amphiphilic block and graft copolymers have the unique molecular structure, which consists of at least two parts with different chemical natures (amphi of both kinds philic having an affinity for) character. In fact parallels can be drawn between typical surfactants and amphiphilic copolymers having both hydrophilic and hydrophobic blocks. Such amphiphilic copolymers find numerous applications as... 

The term amphiphile implies an affinity to two different media. Familiar amphiphilic molecules incorporate two incompatible components that give rise to this behavior. Similarly, in AB and ABA block copolymers there are two incompatible blocks of different solubility. However, ABC triblock copolymers incorporate three chemically different blocks. When the three blocks are mutually incompatible and of different solubilities the ABC surfactants can exhibit affinity to three different media rather than two. The consequences of this higher functionality have not been explored in detail. For example, little attention has been given to their behavior at interfaces. Linear ABC triblock copolymers or the corresponding star copolymers may be able to form two-dimensional mesophases [56], This can occur at the interface between two fluids I and II such that the B block is selectively solubilized in I while A and C are only soluble in II. In this situation, the A and C blocks are constrained to the surface and bound to each other. A two-dimensional amphiphile is obtained when the A and C blocks are incompatible. A dense monolayer of this type should undergo microphase separation leading to the formation of circular and striped mesophases. Note that cylindrical and lamellar mesophases are indistinguishable in this case. A mixed monolayer comprised of BC, BA, and ABC block copolymers will mimic the behavior of amphiphiles in the presence of two two-dimensional and incompatible fluids. When the ABC copolymers are a minority component, they should straddle the boundary line between the two-dimensional A and C phases. 

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