Mixed micelles of octane-1,8 bis(dodecyl dimethyl ammonium chloride) and n-dodecyl-β-d-maltoside by 1H NMR study

Nuclear magnetic resonance (NMR) spectroscopy is a versatile technique at a molecular level for the study of surfactants. Mixed micellization of nonionic sugar-based surfactant n-dodecyl-β-d-maltoside (DM) and cationic surfactant octane-1,8 bis(dodecyl dimethyl ammonium chloride) gemini (12-8-12) was studied using NMR by self-diffusion coefficient, spin–spin and spin–lattice relaxation time, and two-dimensional nuclear Overhauser enhancement (2D NOESY). NMR results reveal the relative arrangement of the surfactant molecules in the mixed micelles. It is shown that the methylene group next to the ammonium head is in the near vicinity of penultimate sugar ring, and the hydrophobic chain of DM molecule and the two hydrophobic chains of 12-8-12 gemini are side by side in the interior of the micellar core. At fixed total surfactant concentrations of DM and 12-8-12 gemini, calculated self-diffusion coefficient suggests that the size of the mixed micelles is largely dependent on the DM concentration, higher the DM concentration, larger the mixed micelles in size. From 2D NOESY maps of mixed micelles, it is found that at fixed DM concentration, surfactant interaction between DM and 12-8-12 gemini in their mixed solutions increases gradually with increase of 12-8-12 gemini concentration from DM/12-8-12 ratio of 1:1–1:3. Interestingly, with further increase of 12-8-12 gemini concentration from DM/12-8-12 ratio of 1:4–1:8, the inter-molecular interaction starts to decrease, which is attributed to the more and more dominant intra-molecular proton–proton dipolar coupling of two surfactant molecules.