Surfactants And Micelles

Surfactants And Micelles

Substances that drastically lower the surface tension of water even at low concentrations are called surface−active compounds or surfactants. As the name implies, they are strongly positively adsorbed at the air−water interface. The earliest surfactants were the soaps, such as sodium salts of long−chain fatty acids (n = 14 to 18): CH₃(CH₂)n COO−Na+. The anion has polar end group and a long hydrocarbon tail, so the it tends to orient in the surface layer with the hydrophilic carboxyl in the water and the hydrophobic tail in air. Synthetic surfactants also include anionic types, such as sodium dodecyl sulphate (SDS), CH₃(CH₂)₁₁OSO₃−Na+. Many household detergents are of this type. Cationic surfactants, for example, the Triton series,

All these surfactant molecules are amphipathic, i.e., they have both polar and nonpolar moieties.

At concentration below about 10^-3, most of ionic surface in aqueous solution display conductance properties similar to other strong electrolytes a slow decline in molar conductance A with √c. The surface tension y declines steeply with concentration in this range. Between 10^-3, and 10^-1 however, sharp breaks occur in the A and y curves vs. c. The concentration at which the break occurs is called the critical micelle concentration. At this concentration, individual surface molecules aggregate to form micelles containing well defined (but not absolutely fixed) numbers of molecules. Usually 50 to 100 molecules take part in a micelle.

The formation of micelles is another instance of the hydrophobic interaction. Hundreds or more molecules generally, constitute a micelle. When hydrocarbon chains, or other nonpolar groups, of amphipathic molecules pack together, they can no longer modify the structure of water in their neighborhood. Thus a micelle arrangement lowers free energy by decreasing colloidal, about 0.1 μ m. The micelle presents a hydrophilic surface to the solution and has therefore little tendency to adsorb at the interface.