One More Extreme near the Critical Micelle Concentration: Optical Activity

It is reported the discovery of optical activity of micellar solutions of sodium dodecyl sulfate, cetyltrimethylammonium bromide, and poly(oxyethilene)(4)dodecyl ether as typical representatives of anionic, cationic, and nonionic surfactants. The surfactants exhibit dextrorotation. Since the optical activity of the ionic surfactants appears only above the Krafft point, it should be ascribed to micellar aggregates. Optical rotation as a function of the surfactant concentration displays a pronounced maximum near the critical micelle concentration (the subsequent decrease of optical activity corresponds to formation of spherical micelles that cannot possess optical activity by symmetry). For sodium dodecyl sulfate, the maximum correlates well with extremes of other properties of surfactant solutions (foaminess, foam stability, Gibbs' and transversal elasticity of thin foam films), which was described earlier. Naturally, films and foams of sodium dodecyl sulfate solutions also demonstrate optical activity.