Mössbauer Study of Diffusion in Liquids: DispersedFe2+in Glycerol and Aqueous-Glycerol Solutions

Abstract

M\"ossbauer parameters, diffusion constants, and coefficients of viscosity for ${\mathrm{Fe}}^{2+}$ ions, dissolved from Fe${\mathrm{Cl}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 4${\mathrm{H}}_{2}$O, in nominally dry glycerol, glycerol-5-wt% water, and glycerol-10-wt% water have been measured. The broadening of the M\"ossbauer line is observed to have a complicated temperature dependence, which we explain by assuming two mechanisms of diffusion. At low temperatures we postulate that diffusion is via a jumping process described by an Arrhenius relation, while at high temperatures, diffusion is via a gaslike motion obeying the Einstein-Stokes and Litovitz relations. A quantitative fit to the observed line broadening is excellent. A very rapid falloff in the quadrupole splitting and the logarithm of the recoil-free fraction is observed in the temperature region where diffusive broadening is measurable; this falloff is proportional to the observed broadening, with two distinct proportionality constants for the low- and high-temperature regions. These observations are explained by a phenomenological theory due to Jensen and by postulating that the average electric field gradient at the iron nucleus reduces in proportion with the iron diffusivity.