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.T_c– 30 °C (T_c 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 T_c–30–T_c–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.T_c–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 (T_c–4 °C–T_c), attractive interactions become dominant. Proton n.m.r. linewidths and viscosities have also been measured.