A PULSED FIELD GRADIENT SPIN-ECHO NUCLEAR-MAGNETIC-RESONANCE STUDY OF SELF-DIFFUSION IN ISOTROPIC MICELLAR SOLUTIONS CONTAINING POTASSIUM N-ALKANOATES AND TETRAMETHYLSILANE

Using the pulsed field gradient spin-echo proton NMR technique, self-diffusion coefficients have been determined for isotropic solutions of potassium n-octanoate and potassium n-dodecanoate/tetramethylsilane in deuterium oxide at 22-degrees-C. The results suggest unrestricted translational diffusion associated with dynamic equilibria among monomers and micelles leading to changes in the size of the aggregates with an increase in surfactant concentration. From the diffusion data, rough estimates have been made of the mean aggregation number (nBAR) at different surfactant concentrations. Small, perhaps irregular micelles, with nBAR values of 3-19 for potassium n-octanoate and larger, more spherical, micelles with up to at least ca. 55 monomers per micelle for the more hydrophobic potassium n-dodecanoate/TMS in (H2O)-H-2 system were estimated. Evidence is presented for the solubilization of TMS as a pool in the micelle core thereby inducing an increase in the micelle size and nBAR along with a lower self-diffusion coefficient. Some indication of the size of the TMS pool, which increases with potassium n-dodecanoate concentration above the c.m.c., is afforded from the micelle self-diffusion data. Geometric and dynamic factors are considered which lead to the interpretation of the experimental diffusion parameters in terms of the most appropriate model of micellization.