Self-diffusion of ions in an ethylene oxide oligomer using the gelsectioning technique

The gel-sectioning technique has been used to measure the radiotracer diffusion coefficients of Na⁺ and SCN^? ions in solutions of sodium thiocyanate in tetra(ethylene glycol) dimethyl ether (‘tetraglyme’), a liquid system which can be used as a model polymer electrolyte. Measurements were made at 298 K over the molar concentration range tetraglyme-NaSCN between 2.5 and 50. At all concentrations the diffusion coefficients of both ions were virtually identical and decreased markedly at high salt concentrations. Comparison of the diffusion results with conductivity measurements on the same samples gives evidence for the aggregation of ions at low salt concentrations (tetraglyme-NaSCN > 20). At higher salt concentrations aggregation does not need to be invoked to explain the results. The implications for other polyether electrolytes are discussed.