Formation of Gem-Like Dispersions of Soft Crystallites in Water by Vesicles of a Cationic Surfactant

A first discovery of iridescent liquid-like aqueous vesicle dispersions formed from the cationic surfactant didodecyldimethylammonium bromide (DDAB) is reported. Iridescence arises for some solid crystallites, thin films, and colloidal crystals, but has not yet been observed in systems that in effect remain liquid-like. Visual observations and spectroturbidimetry (ST) at wavelengths of 350-700 nm were used to determine vesicle sizes and microstructure formation in dispersions for DDAB weight fractions w between 0.020 and 0.030. The DDAB vesicle dispersions exhibited iridescent colors for w = 0.023-0.027, due to the formation of "soft" crystallites formed by self-assembled vesicles. Effective vesicle radii from 30 to 60 nm were inferred from the ST measurements. The volume fractions of the vesicles phi v and their effective volume fractions phi v*, which account for the electrostatic double layers around a vesicle, were also estimated. The high values of phi v* for the iridescent dispersions indicate that they contain neighboring vesicles with highly overlapping electrostatic double layers, even though their values of phi v remain relatively low. Hence, strong electrostatic repulsive interactions arise between the vesicles, which probably drive the formation of "soft" crystallites, and thus the observed iridescence. Nevertheless, these "soft" crystallites, which could be easily broken up but were quick to reform, remain suspended. Consequently, these vesicle dispersions still flowed as a bulk dispersion with a high viscosity; the dispersion as a whole remained liquid-like or as a "liquid gem", in contrast to what occurs to the other colloidal crystals made of rigid colloids. Beside their beautiful appearances, these DDAB vesicle dispersions also act as effective stabilizers of dense silica suspensions against sedimentation even at relatively low values of w.