Diffusivity in a non-ionic surfactant/water mixture of critical composition

Abstract

Results of measurements of the temperature dependence of the mutual diffusion coefficient D of a tetraethyleneglycoloctylether (abbreviated C8E4) /water mixture of critical composition are reported (lower critical point, visually determined critical temperature Tc(vis)=40.587 °C; critical composition, mass fraction yc(C8E4)=0.071). The critical micelle concentration of C8E4 is smaller than the critical concentration, so that micelles are expected to be present in the mixture of critical composition. The reduced diffusion coefficients D*(=6πηξ/(kBT)⋅D) calculated from static and dynamic light scattering and viscosity data scale when plotted as function of the scaling variable x (=qξ) (η, shear viscosity; ξ, correlation length; D, diffusion coefficient; q, absolute value of scattering vector; kB, Boltzmann constant; T, temperature). They can be represented by the approximant of the dynamic scaling function proposed by Burstyn et al. [Phys. Rev. A 28, 1567 (1983)] assuming that the measured diffusion coefficient can be identified with the singular contribution (i.e., D=Dc). The influence of micelles on the critical properties of the system does not show up in the D* versus x plot.