Effect of sodium salicylate on the structure of micelles of different hydrocarbon chain lengths

The effect of adding sodium salicylate (NaSal) on the structural properties of micelles of alkyltrimethylammonium bromides (CmTAB) with various hydrocarbon chain lengths (C12TAB, C14TAB and C16TAB) in 100 mM aqueous solutions has been investigated using viscosity and small-angle neutron scattering (SANS) experiments. The viscosity of these surfactant solutions increases with increase in NaSal concentration and shows a maximum. It is seen that the concentration of NaSal at which the viscosity maximum occurs as well as magnitude of the viscosity at the maximum depends strongly on the surfactant chain length. SANS measurements show that the micelles in all 100 mM solutions of CmTAB are ellipsoidal for m=12, 14 and 16. The ratio of major to minor axis, however, increases with increase in the value of m. SANS experiments further show that in all three cases, the major axis of the ellipsoidal micelle increases with length with increase in NaSal concentration, leading to large elongated micelles at high NaSal concentrations. The charge on the micelle decreases with increase in NaSal concentration. The differences in the viscosity curves for the above surfactant solutions are explained in terms of the charge neutralization by Sal− ions at the micellar surface, which depends on the chain length of the surfactant.