Diffusion of spherical solutes: A fractional molecular-hydrodynamic study of solvent dependence

- Hydrodynamic and molecular effects of solvent coexist in diffusion of non-macromolecular solutes.
- Solvent dependence of diffusion can be expressed by a fractional molecular-hydrodynamic (FMH) relation.
- FMH relation consists of molecular and hydrodynamic viscosity functions with fractional power.
- Viscosity function is relatively more important for large than for small solutes.
- Relative importance of the molecular function is opposite to that of the viscosity function.

Diffusivities of tetramethyltin, tetraethyltin, tetraethyllead, and 2,2-dichloropropane in methanol as well as tetraethyltin and tetraethyllead in ethanol were measured and combined with other literature data of spherical solutes in various solvents to study the effects of solvent on diffusivity. A new fractional molecular-hydrodynamic relation is found to well represent the solvent dependence for the diffusivities of carbon tetrachloride, tetramethyltin, tetraethyltin, tetrapropyltin, and tetrabutyltin at diverse temperatures. The relation consists of two components: one is a function of the hydrodynamic viscosity and the other associated with the molecular properties of solvent. Each has a fractional exponent dependent on solute size. The results here indicate that the relative importance of the viscosity component consistently increases with solute size, but that the trend for the molecular component is reversed. Comparison with other diffusion relations reveals that the newly developed model is more applicable and accurate for expressing the effects of solvent on diffusivity.

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