The effects of molecular association on mutual diffusion

Abstract

Limiting mutual diffusion coefficients (D₁₂) of a series of pseudoplannar polar compounds in various organic solvents at 298.2K and in ethanol at different temperatures ranging from 283.2K to 328.2K were measured by using the chromatographic peak-broadening method. The data were compared with those of the nonassociated pseudoplanar molecules of similar size and shape. Solutes capable of strong solute-solvent interactions in this work were found to diffuse slower than the nonassociated counterparts. The effects of molecular association on diffusion as well as the solvation numbers were determined for the polar solutes. It was found that the effects of hydrogen bonding and the solvation numbers are approximately equal for solutes containing the same polar group that forms hydrogen bonds with the solvent molecules. The effects as well as the solvation numbers are nonetheless different for different functional groups, with -OH > -NH₂ > -SH > -C(O)- in general. For both the associated and nonassociated solutes studied, there exists a linear relationship between the reciprocal of the diffusion coefficients and the molecular volume (V) of the solutes in a given solvent at constant temperature. It appears that the diffusivities are also insensitive to the mass of the nonassociated and associated solute molecules. The temperature dependence of the diffusion coefficients was also studied, and a linear correlation between ln D₁₂ and 1/T was found for all the aromatic solutes studied. From the plots of ln D₁₂ against 1/T, activation energies of different functional solutes were calculated. The activation energies for the solutes studied have the following order: phenols > aromatic amines > phenones > aromatic ethers > nonassociated solutes > 2-naphthalenethiol.