Size-dependent diffusion in cycloalkanes

Abstract

The translational diffusion constants, D, of biphenyl, trans-stilbene, 1,4-diphenyl-1,3-butadiene, 1,1,4,4-tetraphenyl-1,3-butadiene, 1,6-diphenyl-1,3,5-hexatriene, tetraphenylethylene, 9,10-diphenylanthracene, bibenzyl, triptycene, perylene and 2,3-benzanthracene (tetracene) have been measured in combinations of the cycloalkanes cyclohexane, methylcyclohexane, n-butylcyclohexane, cis-decalin and trans-decalin using capillary flow techniques. Tetracene and chrycene have been studied in a series of n-alkanes. Deviations from the Stokes–Einstein (SE) relation (D = k _B T/6πηr) were found. For a given solute, the hydrodynamic radius r decreases as both the viscosity η and the solvent/solute size ratio increase; the data were fitted to D/T = A/η ^p with p<1 (p = 1 for the SE relation). The p values in the cycloalkanes increase as the solute size increases, are compared to the values in the n-alkanes and are discussed in terms of the properties of the two types of solvent. The experimental D values also are compared to the predictions of the Wilke–Chang equation and a free volume model which includes both the masses and sizes of the solution components.