NMR and neutron scattering studies on spherical and rod-like micelles of dodecyltrimethylammonium bromide in aqueous sodium salicylate solutions

Abstract

The effect of sodium salicylate (NaSal) additives on micellar structure and dynamics in aqueous solution of dodecyltrimethylammonium bromide (DTAB) has been studied by means of 1H-NMR and quasielastic neutron scattering (QENS) measurements. The molar ratio R (= C NaSal/ C DTAB) dependence on the peak shape of the NMR spectra revealed that a sphere-to-rod-like micellar transition took place around R=0.5. The chemical shift for the 1H atoms of the alkyl group of DTA + ion gave the information for the binding site of Sal − ion in micelles. The R dependence on the chemical shift for the aromatic group of Sal − ion clarified the selective binding feature of Sal − ion in micelles. The dynamic structure factor, S( Q, E), of the micellar molecules exhibited apparent broadening of the elastic scattering peak which was fitted well to a single Lorentzian function. The self-diffusion coefficient D was obtained from the Q-dependence of the HWHM of the Lorentzian function. The slope of D vs. R curve changed around the transition ratio ( R=0.5), reflecting the dynamic slowing down of the local relaxational motion of the micellar molecules in rod-like micelles.