Surfactant Alkyl Chain Mobility and Order in Micelles and Micro-Emulsions

Abstract

The NMR spin-lattice relaxation times (T1) of 13C were measured in two model surfactant systems, i. e., isotropic solutions of sodium octanoate in water (L1) and of sodium octanoate and water in octanoic acid (L2). From the experimental data at three magnetic fields (1.4, 2.3 and 8.5 T), information was deduced on the hydrocarbon chain mobility, order and on slow overall motions. This was achieved by applying the relaxation model of H. Wennerstrom et al.1. The measurements show clearly a frequency dependent relaxation over the entire concentration range in both systems. The fast local motion in the L1 phase is comparable to that of the free monomer in water. In the L2 phase, slightly slower local motions are obtained. As a general trend, the fast local motion is slowest at the headgroup and increases along the chain. The order parameter decreases along the chain towards the nonpolar end. The order parameter profile is quite invariable in the micellar solution but is found to change when water is added to the L2 solution phase. The slow correlation time is fairly constant in L1 but changes upon water addition in L2. On the whole, the results emphasize the liquid character of the surfactant aggregates, i. e., characterized by low order and rapid molecular motions.