NMR study of micellar microstructures of cationic single-chain and gemini surfactants and their mixtures with nonionic surfactant n-dodecyl-β-D-maltoside

Microstructures of the micelles of cationic surfactants, dodecyltrimethyl ammonium chloride (DTAC), alkane-bis(dodecyl dimethyl ammonium chloride) (12-4-12 and 12-8-12 geminis) in aqueous solutions, as well as the mixed micelles of their mixtures respectively with nonionic sugar-based surfactant n-dodecyl-beta-D-maltoside (DM) were studied using NMR by H-1 chemical shift, self-diffusion coefficient, spin-spin and spin-lattice relaxation time, and two-dimensional nuclear Overhauser enhancement (2D NOESY). In individual aqueous solution, 12-8-12 gemini molecules seem to be tighter packed in the micelles than 12-4-12 gemini, and DTAC molecules are looser packed than both gemini surfactants in the micelles. Calculated hydration dynamics radii indicate that DTAC is the largest in size, while 12-8-12 gemini is the smallest. It is suggested that in the mixed micelles of DM and cationic surfactant, all three cationic surfactants studied give similar relative molecular arrangement to DM, with the methylene group next to the ammonium head in the near vicinity of penultimate sugar ring. The short intermolecular distance between the two headgroups of surfactants in their mixed micelles, evidenced by the existence of intermolecular cross-peaks in 2D NOESY spectrum, implies attractive interactions between DM and these cationic surfactants. The interactions between DM and 12-4-12, and 12-8-12 geminis are stronger than that of DM and DTAC, and increase in spacer length of cationic gemini from 12-4-12 to 12-8-12 results in increased interactions with DM in their mixed micelles. (C) 2008 Elsevier B.V. All rights reserved.