Small angle neutron scattering measurements in iridescent surfactant solutions

Quaternary solutions of the surfactant Tetradecyldimethylaminoxid (C14H29N(CH3)2O, TDMAO) in D2O, equimolar concentrations of Decane and some cosurfactant Hexanol show at low concentration the phenomenon of iridescence. The colour of the backscattered light changes from red to blue with increasing surfactant concentration. Small-angle neutron diffraction (SANS) patterns were recorded for different concentrations between 27 and 584 mM TDMAO. It is shown that the iridescent colours are due to Bragg reflections of visible light from lamellar layers. By simulation based on a one-dimensional crystal model the thickness of the bilayers and the lattice parameter of the liquid crystalline structure could be determined. At higher concentrations the samples showed a pronounded anisotropic SANS-pattern, the origin of which is related to the procedure of filling the solution into the flat, disklike quartz cells of path length 1 mm. (orig.)