DYNAMICS OF MICELLAR OLIGOMERIC AND MONOMERIC SODIUM 10-UNDECENOATE

Multifield carbon-13 NMR longitudinal relaxation time measurements were used to probe the dynamics and structure of monomeric and oligomeric sodium 10-undecenoate in micellar solutions. Longitudinal relaxation data were fit to a two-state model for the spectral density function which utilizes as adjustable parameters a fast correlation time and an order parameter for each carbon atom, and an overall slow correlation time for the entire aggregate. This model was needed to explain the relaxation phenomena because of the anisotropic reorientation of the amphiphiles. The monomeric fast correlation profile indicated a motional gradient increasing toward the micellar core. Comparison of the fast correlation time profiles for monomer and oligomer revealed that the motional gradient found in the monomer was not present in the oligomer. Slow correlation times extracted from the two-state model indicate that the radii of the oligomeric micelles, approximated from the Debye-Stokes-Einstein equation, are slightly larger than the monomeric micelles: 10.6 versus 9.4 Å, respectively. Radii calculated using self-diffusion coefficients also are larger for the oligomeric micelles: 13.4 Å for oligomeric micelles and 10.6 Å for the monomeric micelles. The values determined for the micellar radii are less than that for the extended monomeric chain, 14.7 Å. © 1990.