Influence of additives and cation chain length on the kinetic stability of supersaturated catanionic systems

The stability of mixed surfactant solutions of sodium dodecylsulfate (SDS) with cetyltrimethylammonium bromide (CTAB) and with dodecyltrimethylammonium bromide (DTAB) was studied as a function of time. These specific mixtures are shown to have a solubility temperature below that of pure surfactant solutions in the anionic-rich region. The stability of such supersaturated solutions was studied with and without different additives. Surfactant mixtures without additives are shown to destabilize with time varying between 3 and 28 days, depending on the surfactant ratio. Generally, the stability of solution increases by increasing the percentage of the anionic surfactant. The variation of the chain length of the cationic surfactant produces a large effect on the stability of such mixed surfactant systems. The presence of simple electrolytes decreases, while the addition of middle-chain alcohols increases its stability. Bluish solutions corresponding to a vesicular region were observed at ratios close to equimolarity in samples without salt, and in the anionic-rich region upon the addition of middle-chain alcohols. Fluorescence and dynamic light scattering measurements showed that the destabilization of the solutions is not due to the formation of bigger aggregates, but rather due to a shift of the equilibrium between micelles and monomers, leading to the liberation of monomers, which precipitate. The lifetime of vesicles and micelles can therefore be controlled by varying the composition of the surfactant solutions and by additives. Controlling the precipitation phenomena is of importance for a large number of industrial processes, such as oil/solute recovery processes after extractions or chemical reactions.