Abstract
A careful study of the Heisenberg spin-exchange contribution ωHE 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 Tc. It is found that in the critical region, ωHE is not linear in T/ν. However, it was observed that ωHE is linear in T/ν′ both for noncritical compositions and critical compositions above Tc. Here ν′ is the macroscopically measured viscosity, but with the ``anomalous portion'' subtracted out. Deviations from ideal behavior of ωHE 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.
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