Critical micelle concentration nonionic head groups

The hydrogen bonding between the head group of the surfactant and water is very sensitive to temperature for this reason d/u /dT is expected to be larger at low temperatures. Because the hydrophobicity of the surfactant increases with temperature, dC wl/dT < 0. In reality, the critical micelle concentration passes through a minimum with increasing temperature [20]. The minimum is, however, located around 55°C for nonionic surfactants. As a result, the interfacial tension is expected to pass through a minimum. 

Submicroscopic, colloidal aggregates can influence chemical reactivity. Aqueous micelles are the most widely studied of these aggregates, and these micelles form spontaneously when the concentration of a surfactant (sometimes known as a detergent) exceeds the critical micelle concentration, cmc (1-3). Surfactants have apolar residues and ionic or polar head groups, and in water at surfactant concentrations not much greater than the cmc, micelles are approximately spherical and the polar or ionic head groups are at the surface in contact with water. The head groups may be cationic, (e.g., trimethylammonium), anionic, (e.g., sulfate), zwitterionic (as in carboxylate or sulfonate betaines), or nonionic. The present discussion covers the behavior of ionic and zwitterionic micelles and their effects on chemical reactivity. 

In the case of ordered mesoporous oxides, the templating relies on supramolecular arrays micellar systems formed by surfactants or block copolymers. Surfactants consist of a hydrophihc part, for example, ionic, nonionic, zwitterionic or polymeric groups, often called the head, and a hydrophobic part, the tail, for example, alkyl or polymeric chains. This amphiphiUc character enables surfactant molecules to associate in supramolecular micellar arrays. Single amphiphile molecules tend to associate into aggregates in aqueous solution due to hydrophobic effects. Above a given critical concentration of amphiphiles, called the critical micelle concentration (CMC), formation of an assembly, such as a spherical micelle, is favored. These micellar nanometric aggregates may be structured with different shapes (spherical or cylindrical micelles, layered structures, etc. Fig. 9.8 Reference 70). The formation of micelles. 

Micelles form only above a certain concentration of amphiphiles called the critical micelle concentration (CMC) and above the Krafft temperature. Nonionic amphipathic molecules may cluster together in clumps of 1000 or more, but ionic species tend to be disrupted by the electrostatic repulsions between head groups and are normally limited to groups of fewer than about 100. The interior of a micelle is like a droplet of oil, and experiments show that the hydrophobic tails are mobile, but slightly more restricted than in the bulk. 

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. 

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