Intermicellar interactions and micelle size distribution in aqueous solutions of polyoxyethylene surfactants

Abstract

Mutual diffusion coefficients (D) and self-diffusion coefficients (D+) in D2O solution of n-dodecylpolyoxyethylene surfactants [C12H25(OC2H4)mOH, m= 5 and 8] have been measured by dynamic light scattering and pulsed-gradient F.t.n.m.r., respectively. On the basis of the ratio D/D+, micelle size distribution and intermicellar interactions have been discussed. The results have been compared with those for the C12E6 system reported previously. Below ca.Tc– 30 °C (Tc is the lower critical solution temperature) size distribution is narrow and intermicellar interactions are weak; this is consistent with the existence of spherical micelles. In the temperature range Tc–30–Tc–15 °C, the excluded volume effects are dominant and increase in magnitude with increasing temperature. At the same time, the size distribution becomes broad. These results originate from elongation of micelles. Above ca.Tc–15 °C, the results can be explained by the decrease in the excluded volume effects and/or rapid increase in attractive interactions with increasing temperature. At still higher temperatures (Tc–4 °C–Tc), attractive interactions become dominant. Proton n.m.r. linewidths and viscosities have also been measured.