| Abstrakt: |
Taylor dispersion and differential refractometry are used to measure diffusion coefficients (D) at 25 °C for dilute solutions of the stable free radicals: 2,2,6,6-tetramethyl-4-piperidinol (TEMPO) (in n-hexane); 1,1-diphenyl-2-picrylhydrazyl (in ethanol); and nitrosodisulfonate (in aqueous KOH solutions). For comparison, diffusion coefficients are measured for the respective nonradical analogues: 2,2,6,6-tetramethylpiperidine; 1,1-diphenyl-2-picrylhydrazine; and hydroxylaminedisulfonate. The limiting values of D for each free radical and its analogue differ by 5% or less. In contrast to this result, recent transient-grating measurements suggest that free radicals diffuse up to 4 times more slowly than their nonradical analogues. The accuracy of the grating data is checked by using Taylor dispersion to measure D values for 10 different analogues. Values of D from the grating measurements are found to be systematically too large by 25% on average, with a maximum error of 90%. |
