Slowing Down of the Mutual Diffusion Process Approaching the Liquid/Liquid Coexistence Curve of a Nonionic Surfactant/Deuterium Oxide System: Study of the System C12E5/D2O

In the “large aggregates” region of the phase diagram of the system C 12 E 5 /D 2 O the mutual diffusion coefficient of mixtures with critical and noncritical composition is determined by dynamic light scattering (compositions: 0.23 × 10 -2 ≤ y ≤ 10.01 × 10 -2; y c = 1.08 × 10 -2; y, mass fraction of C 12 E 5 ). In all experiments the diffusion coefficient decreases with increasing temperatures, approaching the liquid/liquid coexistence curve at fixed values of y . This is a system independent property observed in “simple” as well as in “complex” binary mixtures with a miscibility gap. The change of size and shape of C 12 E 5 micelles with composition and temperature in aqueous solutions reported for the “large aggregates” region appears to be the result of concentration fluctuations existing in a wide temperature and composition region below the binodal curve. Approaching T p ( T c ) the diffusive property of the mixtures is dominated by the dynamics of concentration fluctuations.