SELF-DIFFUSION PROCESSES IN SEMIDILUTE SOLUTIONS OF NONIONIC SURFACTANT (C16E7) STUDIED BY LIGHT-SCATTERING AND PULSED-GRADIENT SPIN-ECHO METHODS

Light scattering and pulsed-gradient spin echo (PGSE) have been measured on semidilute solutions of a nonionic surfactant, heptakis(oxyethylene glycol) n-hexadecyl ether (C16E7). Light scattering results at higher temperatures can be explained by the scaling theory for semidilute solutions of flexible polymers which exhibit phase separations. The self-diffusion coefficient (D) of surfactant molecules obtained by the PGSE method decreases with increasing concentration (c) in the lower concentration range. The slope of the log D-log c plot is discussed on the basis of the theories for polymers and ''living polymers''. In the higher concentration range (> 10% by weight), on the other hand, the log D-log c plot gives a positive slope (congruent-to 2/3). In this range, the activation energy for diffusion processes has been found to be anomalously large (approximately 160 kJ.mol-1). These results can be explained by a simple model which takes into account the intermicellar exchange of surfactant molecules. Below about 1% by weight, effects of micelle size distribution on the attenuation of the echo amplitude were observed, suggesting that the lifetime of micelles is longer or of the same order as the diffusion time.