Effects of molecular association on mutual diffusion: A study of hydrogen bonding in dilute solutions

Diffusivities of pseudoplanar molecules at trace concentration in methanol have been measured at 298.2 K using Taylor's dispersion method. The data of the polar and nonpolar aromatic solutes are compared, and the effects due to solute-solvent interactions on diffusion, together with the solvation nu mbers, are determined. In this study, the effects are combined with the rec ently developed solute hydrogen-bond scales to unravel hydrogen bonding bet ween solute and solvent. It is found that the degrees of association of the solutes with methanol decrease in the sequence hydroquinone>aromatic acids >phenols>aromatic amines>aprotic aromatic compounds. Except for o-nitrophen ol, which is capable of intramolecular hydrogen bonding, all aromatic acids , phenols, and amines studied behave more as hydrogen-bond donor than accep tor in methanol. The present work also indicates that motions of associated molecules can be understood in terms of the molecular behavior of nonassoc iated solutes and the hydrogen-bond acidity/basicity of polar solutes. (C) 1999 American Institute of Physics. [S0021-9606(99)52306-4].