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Diffusivities of nonassociated aromatic compounds at infinite dilution in methanol and in cyclohexane were measured by the Taylor dispersion method. At constant temperatures, the diffusivity recipr...

Diffusion coefficients of pseudoplanar aromatic compounds at infinite dilution in acetone have been measured at different temperatures by the Taylor dispersion technique. The data of the polar solutes that can form hydrogen bonds with acetone are compared with those of the nonpolar ones incapable of hydrogen bonding to quantify the effects of hydrogen-bonded association on diffusion. The effects are further found to correlate strongly with the overall hydrogen-bonded acidity of the polar solutes containing proton-donating groups. For the nonpolar solutes in this study, the diffusivities at different temperatures can be expressed very well by the recently developed molecular-modified fractional Stokes-Einstein relation with only two constants. An innovative model for solute diffusion in liquid solutions, which is constructed by combining the molecular-hydrodynamic relation for nonpolar solutes with the overall hydrogen-bonded acidity scale for polar solutes, is introduced for representing the diffusivities of different types of disc-shaped molecules at various temperatures. An equation developed from this model is demonstrated to be capable of calculating a total of 191 diffusion data of both the hydrogen-bonded and the nonassociated aromatic solutes in acetone from 268.2 to 328.2 K to a standard deviation of 2.7%.

Limiting mutual diffusivities of o- and m-isomers of methylaniline, nitroaniline, nitrophenol, and aminophenol were measured in acetone at 298.2 K by the Taylor dispersion method. The data reveal that all of the o-substituted solutes capable of intramolecular hydrogen bonding diffuse faster than their m-counterparts without such bonding. By taking into account of the small corrections for the differences in molecular shape and steric hindrance between the o- and m-isomers that can form solute-solvent complexes, the net effects of intramolecular hydrogen bonding were uncovered to render the o-isomers greater in diffusivity by 3-15% as compared to their m-isomers in this study. For aromatic amines and phenols diffusing in acetone, the overall effects of intermolecular hydrogen bonding on diffusivity were ascertained by comparing the available diffusivity data of the associated aromatic solutes with those of the nonassociated ones. The intermolecular effects that cause solutes to diffuse slower were found to vary from approximately 12-39% in the present work. The results indicate that both of the opposite intra- and intermolecular effects are significant. In addition, the quantified effects were analyzed to show that they are closely related to the nature and position of the functional groups contained in the aromatic solutes, including those that are unable to form hydrogen bonds with acetone. A relation that can correlate the diffusivities of the hydrogen-bonded aromatic amines and phenols in acetone with the overall acidities of the compounds is also presented.

Limiting mutual diffusion coefficients of aromatic compounds containing two polar groups in ethanol have been measured at 298.2 K by using the chromatographic peak-broadening method. The data are compared with those of nonpolar pseudoplanar solutes, and the effects due to hydrogen bonding on diffusion together with the solvation numbers are evaluated. It is found that the effects and the solvation numbers are generally large and dependent on the nature and position of the polar groups attached. For all hetero-disubstituted aromatic isomers studied, the diffusivity trends are p < m < o, which are different from the previous results for the nonpolar isomers. The findings are discussed in terms of the chemical structure of the solutes as well as the intra- and intermolecular interactions.