Iridescent phases in aminoxide surfactant solutions

Abstract

Surfactant systems that show iridescent colors are reported. The systems consist of dimethylaminoxide surfactants (RN(CH₃)O), n-hexanol and water. Lamellar phases are formed at room temperature which can be diluted with water saturated with n-hexanol to iridescent phases with brilliant colors ranging from blue to red. The mole ratio of hexanol molecules to surfactant molecules in the bilayers is 1.40, 1.38, 1.62, and 1.67 for R=C₁₂H₂₅, C₁₄H₂₉, C₁₆H₃₃ and Oleyl (C₁₆H₃₁) respectively. The areas of the lamellae per surfactant molecule and hexanol are 67 Ų, 62 Ų and 83 Ų. Analogous lamellar phases are found with the cosurfactants n-pentanol, n-hexanol, n-heptanol, and n-nonanol. n-alcoholes with n < 5 or n > 10 are not able to form such iridescent phases at room temperature. The double chain surfactant dodecyl-octyl-methylaminoxide forms iridescent phases in the binary system. The colors of these phases are intensified with small amounts of C₁₄H₂₅(CH₃)₂PO. In the single phase regions the colors of all systems do not depend on the temperature. The observed phase inversion with small amounts of electrolyte is shown to be determined by the ionic strength. The electrostatic repulsion is necessary, but is too small to explain the stability of the systems. The bilayers are strongly fluctuating and are stabilized by steric repulsion.