Diffusion in viscous solvents. Part 2.—Planar and spherical molecules in propane-1,2-diol at 15, 25 and 35°C

Abstract

Interdiffusion coefficients have been measured for carbon tetrachloride, carbon tetrabromide, methyl pivalate, chlorobenzene, benzene and phenol in propane-1,2-diol for solute concentrations up to 0.7 mol dm^–3, using the optical method described by Mitchell and Tyrrell. The first four solutes were studied at 15, 25 and 35°C. For phenol and benzene, examined previously by this technique, sufficient new data were collected to provide a complete set of diffusion coefficients at these three temperatures. The results show that the “spherical” molecules (CCl₄, CBr₄, methyl pivalate) diffuse at about the rate expected from the Stokes–Einstein relationship (D=kT/4πηr) and a molecular radius calculated from molar volume data. They confirm that the planar molecules (benzene, chlorobenzene) diffuse about twice as fast as expected. This contrasts with the data on intradiffusion coefficients in carbon tetrachloride, benzene and chlorobenzene, and with some pair diffusion coefficients derived from analysis of transient effects in fluorescence quenching. In both sets of data the Stokes–Einstein relationship is “obeyed” quite closely.

The diffusion behaviour of phenol in propane-1,2-diol at all three temperatures can be adequately explained in terms of solute-solvent interactions, as suggested earlier on the basis of less complete evidence.