Trimethylammonium head group, effect

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. 

Solvation of the head group has two effects on the adsorbed surfactant molecules one is the penetration depth of the ionic head group into the aqueous phase and the other surface roughness. For example, neutron reflection studies have found a difference in the penetration depth of the ionic head groups of SDS and hexadecyl-trimethylammonium bromide (HDTAB) at the air/water interface [50]. The measured separation between the position of the ionic head group and that of the mean position of the aqueous interface is (7.5 0.1) A for SDS and (8.0 0.1) A for HDTAB. These results are found to be independent of the structure of the adsorbed layer. 

The simplest, and at the same time the most effective way to achieve dispersion in water or aqueous media is the addition of amphiphihc molecules carbon nanotubes without any functional groups are hydrophobic. This characteristic may be employed to enclose them in micelles. The surfactant molecules arrange around the single tubes with their hydrophilic head directed outward and their hydrophobic tail oriented toward the nanotube in the center. In this manner, each individual tube is surrounded by an envelop that enables a dispersion, for example, in water. Yet complete solubilization requires considerable amounts of the surfactant. Solutions obtained this way may in fact contain up to 80% of detergent and only 20% of carbon nanotubes. Surfactants suitable to enclose nanotubes in micelles include, for example, sodium dodecylsulfate (SDS), Triton X-100, or octadecyl-trimethylammonium bromide (OTAB). 

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