Diffusion in viscous solvents. Part 3.—Interdiffusion coefficients for planar and spherical solutes in 2-methylpentane 2,4-diol, and their relationship to diffusion coefficients derived from luminescence measurements

Abstract

Interdiffusion coefficients have been measured for dilute solutions of benzene, naphthalene, phenanthrene, pyrene and carbon tetrabromide in 2-methylpentane 2,4-diol (MPD), between 269 and 308 K, by shearing interferometry, supplemented by some measurements using the chromatographic broadening method. The results, when combined with measurements of viscosity coefficients, confirm that planar molecules diffuse more rapidly than spherical molecules of similar molar volume and that, as the temperature falls, a significant difference in the effect of temperature on the visocisty and diffusion coefficients leads to progressively larger deviations from the Stokes–Einstein relationship. Comparison with diffusion coefficients derived from fluorescence quenching and similar data shows that, in comparable systems, the results are generally in reasonable agreement except when nanosecond time-scales are involved.