Effect of ionic surfactants on the iridescent color in lamellar liquid crystalline phase of a nonionic surfactant

A nonionic surfactant, n-dodecyl glyceryl itaconate (DGI), self-assembles into bilayer membranes in water having a spacing distance of sub-micrometer in the presence of small amounts of ionic surfactants, and shows beautiful iridescent color. Ionic surfactants have strong effects on this iridescent system. We have interestingly found that the iridescent color changes with time after mixing DGI and ionic surfactants and the color in equilibrium state changes greatly with concentration of the ionic surfactants. The time-dependent color change results from the transformation of DGI aggregate structure after being mixed with ionic surfactant. It is first found that the iridescent color of this nonionic system can be changed from red to deep blue by altering the concentration of ionic surfactants added even though the total concentration of surfactant is almost constant. Such large blue shift of the iridescent color in equilibrium state cannot be fully explained by the ordinary undulation theory applied so far for this phenomenon. The flat lamellar sheets tend to curve by increasing the concentration of ionic surfactants to form separated onion-like and/or myelin-like structures. These separated structures of lamellar system result in the decrease of spacing distance between bilayer membranes because some vacant spaces necessarily appear among these structures.