Zwitterionic surfactant solutions 1. A light scattering investigation

Dynamic and static light scattering studies are reported on a series of alkyl dimethyl betaines C_mDMB (m = 12, 16, 18) in aqueous solutions as functions of surfactant concentration, salt concentration and temperature. The characteristics of the micelles change dramatically as the alkyl chain length is increased from 12 to 18. In the case or C₁₈DMB, large micelles form at very low surfactant concentrations (about 0.2 wt%) and grow rapidly to become entangled at slightly higher concentrations (about 1 wt%). Upon addition of salt, C₁₈DMB micelles become very large and semiflexible. At 40.3°C the persistence lengths of C₁₈DMB micelles in 0.01 and 0.1 M NaCl aqueous solutions are approximately 1400 ± 200 Å and 1600 ± 250 Å, respectively. The persistence length of C₁₈DMB micelles in 0.1 M NaCl solution at 51.5°C is 1650 ± 250 Å. The dynamic light scattering data are consistent with estimated persistence lengths. Quantitative agreement is obtained with the theory developed by Fujime and co-workers (S. Fujime and M. Maruyama, Macromolecules, 6 (1973) 237; T. Maeda and S. Fujime, Macromolecules, 14 (1981) 809) for semiflexible, rodlike particles. Micelles of C₁₆DMB in water are very small compared to C₁₈DMB. The micelles are polydisperse and the size increases with increasing surfactant concentration. In 1.0 M NaCl aqueous solutions, the C₁₆DMB micelles are larger than in pure water and become semiflexible. After accounting for polydispersity, the persistence length is calculated to be roughly 600 ± 100 Å at 25°C. Therefore, micelles of C₁₆DMB are less stiff than those of C₁₈DMB. In contrast to C₁₈CMB and C₁₆DMB systems, C₁₂DMB micelles remain small and spherical over a wide range of surfactant concentration, in agreement with previous studies.