Microstructure of N,N′-bis(cetyldimethyl)-α,ω-propanediammonium dibromide micelle and its dynamics in solution studied by ¹H NMR

The characteristics of N,N′-bis(cetyldimethyl)-α,ω-alkane (propane and butane) diammonium dibromide (16-3-16 and 16-4-16) aqueous solutions were studied by one- and two-dimensional ¹H nuclear magnetic resonance (NMR). The measurements of self-diffusion coefficient and inter-proton distance at 318 K suggest that 16-3-16 spherical micelles are formed in the dilute solution at a concentration of 0.26 mmol/l and the polar head groups of the surfactant are in a saw-toothed form staying at the surface of the micelle to overcome the strong electrostatic repulsion force. Relaxation measurements obviously show that the spacer chain is rigid in the surface layer of the hydrophobic micellar core, and the side alkyl chains of 16-3-16 are packed more tightly than those of 16-4-16 in the micellar core. The line-shape analysis of the methyl protons at the end of the side alkyl chain of 16-3-16 and 16-4-16 suggests that two possible momentary morphologies of their side alkyl chains are situated in the micelle, respectively.