Isotopic Fractionation of Potassium by Diffusion in Methanol

We present the results of experiments involving the isotopic fractionation by diffusion of K⁺ cations in methanol at 298 K along with supporting molecular dynamic simulations. The experiments, using glass Rayleigh fractionators filled with a methanol solution of KCl, constrain the ratio of the individual diffusivities of ⁴¹K and ³⁹K (D /D ) in methanol, and so the isotopic fractionation of K⁺ due to diffusion in methanol, to 0.9995 ± 0.0001. This isotopic fractionation is 25% of the fractionation resulting from K⁺ diffusion in water. This is consistent with published molecular dynamics simulations indicating greater hydrodynamic coupling between K⁺ ions and solvating methanol molecules compared to K⁺ ions and solvating water molecules.