A stopped-flow technique combined with pulsed-field-gradient spin-echo nuclear magnetic resonance (NMR) measurements was used to study the kinetics of exchange, size, and shapes in micellar systems of cationic surfactant dimers of the alkanediyl-α-ω-bis(dodecyldimethylammonium bromide) type, with alkanediyl being 1,2-ethylene, 1,3-propylene, and 1,4-butylene. By measuring the slow relaxation time for micelles, τ 2 , the micelle lifetime as a function of spacer length was obtained and was further confirmed by micelle exchange measurements by NMR diffusometry. The micelle lifetimes for the gemini surfactants were found to be in orders of magnitude longer than for the corresponding conventional surfactants. All three cationic surfactant dimers showed an increase in micelle size in one direction, i.e., became prolates, as the concentration was increased. The growth of the micelles was most pronounced for the gemini surfactants with the shortest linker unit, i.e., ethylene.Paper no. S1407 in JSD 7, 247-255 (July 2004).KEY WORDS: Dimeric, gemini surfactants, kinetics of micellization, lifetime of micelles, NMR, self-diffusion, slow relaxation time, stopped flow.The association of many classes of surface-active molecules into micellar aggregates is a well-known phenomenon. Micelles are in dynamic equilibrium, constantly disintegrating and reforming. Two relaxation processes (1-25) are involved in micellar kinetics. The first one is a fast relaxation process referred to as τ 1 (generally on the order of microseconds), which is associated with the quick exchange of monomers between micelles and the surrounding bulk phase. This process can be considered a collision between surfactant monomers and micelles. The second relaxation time, τ 2 (on the order of milliseconds), is attributed to the micelle formation and dissolution process (i.e., the lifetime of the micelle). It has been shown that in certain surfactants, such as nonionic surfactants and mixed surfactant systems, τ 2 can be as long as minutes! For example, the τ 2 of a 0.80 mM solution of the nonionic surfactant Synperonic A7 (C 13 -C 15 E 7 ) is 150 s (2). Figure 1 shows the two characteristic relaxation times, τ 1 and τ 2 , associated with micellar solutions. Micelle formation and disintegration are analogous to the equilibrium between water and water vapor at a given temperature and pressure. For a closed system containing water and water vapor in equilibrium, the number of water molecules per unit area per second evaporating from the surface is equal to the number of water molecules condensing at the surface. Thus, the total number of molecules in the vapor phase or in the liquid phase does not change with time, so the rate of condensation is equal to the rate of evaporation. The same principle holds for a micellar solution. Under equilibrium conditions, the rate of micelle formation is equal to the rate of disintegration into surfactant monomers.The lifetime of micelles has been shown to significantly influence technological processes involving a rapid increas...
