ZWITTERIONIC SURFACTANT SOLUTIONS .1. A LIGHT-SCATTERING INVESTIGATION

Dynamic and static light scattering studies are reported on a series of alkyl dimethyl betaines C(m)DMB (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 of 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-degrees-C the persistence lengths of C18DMB micelles in 0.01 and 0.1 M NaCl aqueous solutions are approximately 1400 +/-200 angstrom and 1600 +/- 250 angstrom, respectively. The persistence length of C18DMB micelles in 0.1 M NaCl solution at 51.5-degrees-C is 1650 +/- 250 angstrom. 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 C-16DMB 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 C-16DMB micelles are larger than in pure water and become semiflexible. After accounting for polydispersity, the persistence length is calculated to be roughly 600 +/- 100 angstrom at 25-degrees-C. Therefore, micelles of C-16DMB are less stiff than those of C18DMB. In contrast to C18DMB and C-16DMB systems, C-12DMB micelles remain small and spherical over a wide range of surfactant concentration, in agreement with previous studies.