Aggregation Behaviors in Aqueous Solutions of Amphiphiles Studied by Light Scattering and Pulsed-Gradient Spin Echo Methods

Abstract

Static and dynamic light scattering and pulsed-gradient spin-echo (PGSE) have been measured on aqueous solutions of nonionic surfactants in order to elucidate aggregation behaviors of amphiphiles in a wide concentration and temperature range. In "dilute" region (-1 wt%), temperature dependences of intermicellar interactions and micelle size distribution is discussed on the basis of the ratio of the mutual and self-diffusion coefficients. It is shown that micelles grow with increasing temperature above about Tc-30 K (T: the critical solution temperature)and effects of critical concentration fluctuations become important only above about Tc-4 K. In semidilute region (1-30 wt%), light scattering results have been analyzed by using the scaling theory for entangled solutions of flexible polymers which exhibit phase separations. The results suggest entanglement of elongated micelles above about Tc-15 K. The PGSE data in this region can be explained by a simple model which takes into account the intermicellar exchange of surfactant molecules. Similar measurements have been made on mixed anionic/cationic surfactant solutions. Light scattering results indicate that micelles grow with the decrease in the total surfactant concentration. A method is proposed for determining the composition of mixed micelles from PGSE data. It is shown that the micellar growth comes from the change in the micellar composition.