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

Abstract

The stability of mixed surfactant solns. of sodium dodecylsulfate (SDS) with cetyltrimethylammonium bromide (CTAB) and with dodecyltrimethylammonium bromide (DTAB) was studied as a function of time. These specific mixts. are shown to have a soly. temp. below that of pure surfactant solns. in the anionic-rich region. The stability of such supersatd. solns. was studied with and without different additives. Surfactant mixts. without additives are shown to destabilize with time varying between 3 and 28 days, depending on the surfactant ratio. Generally, the stability of soln. 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 addn. of middle-chain alcs. increases its stability. Bluish solns. corresponding to a vesicular region were obsd. at ratios close to equimolarity in samples without salt, and in the anionic-rich region upon the addn. of middle-chain alcs. Fluorescence and dynamic light scattering measurements showed that the destabilization of the solns. is not due to the formation of bigger aggregates, but rather due to a shift of the equil. between micelles and monomers, leading to the liberation of monomers, which ppt. The lifetime of vesicles and micelles can therefore be controlled by varying the compn. of the surfactant solns. and by additives. Controlling the pptn. phenomena is of importance for a large no. of industrial processes, such as oil/solute recovery processes after extns. or chem. reactions.