ELECTRON-SPIN RESONANCE LINE-WIDTH METHOD FOR MEASURING DIFFUSION - CRITIQUE

The concentration (C) dependent part of the ESR line width (ΔH) of a paramagnetic solute in a diamagnetic solvent arises from intermolecular interactions between paramagnetic molecules, and these interactions are strongly modulated by the translational motions of these molecules. Thus (d ΔH/dC) depends upon the diffusion constant (D), and measurements of (dΔH/dC) can be used to determine D. Although this method of measuring D may not be the most favored one for bulk media, it could be one of the few methods applicable to the study of diffusion within bilayers. We have evaluated this procedure for measuring D by studying (dΔH/dC) in a number of bulk solvents over a very wide range of (T/η) where T is the temperature and η is the solvent viscosity. At low viscosity, the spin-exchange interactions dominate, and dΔH/dC is linear in D or in (T/η). At moderate viscosities dΔH/dC varies little with (T/η) because the dipolar interaction which varies as D-1 competes with the spin-exchange interaction. In fact, (dΔH/dC) remains relatively constant over quite a wide range of (T/η), but at very low T/η the (dΔH/dC) values evidence an upsweep with decreasing (T/η). The low (T/η) results are not yet understood, but it is clear that (dΔH/dC) gives useful diffusion data at low viscosities (high T/η) only. © 1979 American Chemical Society.