NMR investigation of the exchange kinetics of quaternary ammonium dimeric Surfactants C14-s-C14•2Br

The exchange kinetics of cationic gemini surfactants of the alkanediyl-alpha-omega-bis(tetradecyldimethylammonium bromide) type, with alkanediyl being 1,2-ethylene, 1,3-propylene, and 1,4-butylene, were investigated by H-1 NMR, 2D COSY, and 2D EXSY experiments. In contrast to the conventional surfactants, a second set of well-resolved resonance peaks appeared in the 1H NMR spectra of these surfactants when their concentrations reached their critical concentrations. These two sets of resonance peaks originate from their monomers and micelles, which are proved by the correlation in the 2D COSY experiments and the cross polarization in the 2D NOESY spectra. Therefore, exchanges between monomers in the bulk solution and in the micelles or other aggregates of this series of surfactants occur slowly on the NMR time scale. The exchange rate constants were obtained by both NMR line shape analysis and 2D EXSY experiments, which are very consistent with each other. The exchange rate constants for the gemini surfactants were found to be orders of magnitude less than those for the conventional single surfactants, and for geminis 14-s-14, the shorter the spacer, the slower the exchange dynamic. It still has been found that the fast exchange between monomers in the bulk solution and in the micelles for gemini surfactant 12-2-12 at 25 degrees C occurs slowly at 5 degrees C on the NMR time scale.