Effect of chain length compatibility on micellar stability in sodium dodecyl sulfate/alkyltrimethylammonium bromide solutions

The effect of chain length compatibility on the micellar stability was investigated for mixtures of sodium dodecyl sulfate (SDS) and alkyltrimethylammonium bromides (C(n)TAB, n = 8, 10, 12, 14, 16) at a 20/1 molar ratio. It was observed that the surface properties of surfactant solutions were influenced by the chain length compatibility of the surfactant molecules. The slow micellar relaxation time (tau(2)), which directly correlates to the micellar stability, was determined using the pressure-jump technique. The addition of C,TAB to SDS solutions results in a significant increase in the relaxation time compared to SDS alone showing a maximum for the SDS/C(12)TAB system. The high micellar stability results in minimum surface tension, maximum surface viscosity, maximum micellar lifetime, maximum foam stability, and minimum foamability. The higher micellar stability as observed for SDS/C(12)TAB is attributed to both the chain length compatibility and the Coulombic interaction of the surfactant head groups, resulting in an optimum packing of molecules in micelles and at the air/water interface.