Coexistence of spheres and rods in micellar solution of dodecyldimethylamine oxide

Micelles of dimethyldodecylamine oxide (DMDAO) are known to exhibit sphere-to-rod transitions as a function of pH and ionic strength. Long micelles are stabilized at pH corresponding to half-protonation, because hydrogen bonding between nonionic and protonated monomers yields an effectively double-tailed monomer whose geometry favors cylindrical growth. Dynamic light scattering (DLS) was used to follow particle size distribution as a function of pH, ionic strength (I), and surfactant concentration. The key finding was the coexistence of spherical micelles with rodlike ones at 4 < pH < 6 and I > 0.2 M. These observations have been verified by varying the algorithm used for the Laplace transformation of the autocorrelation function and also with different DLS systems. The effect of surfactant concentration was used to confirm the absence of any influence of micelle-micelle interaction on the dynamics of diffusion. A molecular level self-consistent field analysis of finite size rodlike micelles confirms the idea that the endcaps are swollen with respect to the cylindrical part. The theoretical results support the coexistence of rods and spherical micelles, i.e., the existence of gaps in the size distribution of wormlike micelles. The cause of coexistence has been explained in terms of the instability of dumbbell-like micelles with domains of negative curvature (neck). The endcap energy is shown to be given (in first order) by the grand potential of spherical micelles that coexist with the wormlike micelles.