Diffusion studies in viscous media

Abstract

The rate constants of quenching of the triplet state of naphthalene by $\alpha$-iodonaphthalene, t-butyl hydroperoxide and oxygen have been studied in several solvents as a function of viscosity and temperature. The diffusion controlled rate constants for $\alpha$-iodonaphthalene in hydroxylic media agree with those predicted by the equation k$_{calc.}$ = 8RT/2000$\eta$ ($\eta$ being viscosity). This is interpreted as evidence for the validity of the Stokes-Einstein expression, Stoke's law with 'slip' (i.e. coefficient of sliding friction zero) being used for diffusing species comparable in size to the molecules comprising the medium. In liquid paraffin/n-hexane mixtures, the ratio k$_{obs.}$:k$_{calc.}$ became progressively larger as the proportion of liquid paraffin:n-hexane was increased and in 100% liquid paraffin the ratio was 4.5. This is interpreted as being due to breakdown of the Stokes-Einstein expression in cases where the diffusing species is small compared with the molecules of the solvent. The observed rate constant for quenching by oxygen is also anomalously high, being over 100 times greater than predicted under conditions of high oxygen concentration and high solvent viscosity.