ESR Study of Heisenberg Spin Exchange in a Binary Liquid Solution near the Critical Point

A careful study of the Heisenberg spin‐exchange contribution ${\omega }_{\mathrm{HE}}$ to the ESR linewidths of the di‐t‐butyl nitroxide (DTBN) radical dissolved in mixtures of 2,2,4‐trimethylpentane and n‐perfluoroheptane was performed. The study includes samples of different radical concentration dissolved in the critical composition of the two solvents. This critical solvent system is known to exhibit an anomaly in the macroscopic kinematic viscosity ν near T_c. It is found that in the critical region, ${\omega }_{\mathrm{HE}}$ is not linear in T/ν. However, it was observed that ${\omega }_{\mathrm{HE}}$ is linear in T/ν′ both for noncritical compositions and critical compositions above T_c. Here ν′ is the macroscopically measured viscosity, but with the ``anomalous portion'' subtracted out. Deviations from ideal behavior of ${\omega }_{\mathrm{HE}}$ with respect to T/ν were observed and discussed. The experiments near the critical region required temperature stability and control to within ±0.01°C at the ESR sample, and a description is given of the experimental design.