Molecular processes in electrolyte solutions at microwave frequencies

Abstract

The microwave dispersion and absorption spectra are discussed for various protic and aprotic electrolyte solns. and their solvents over large frequency ranges, in general 0.9-90 GHz. Wider frequency ranges are covered for water (0.9-409 GHz) and for methanol, N-methylformamide, and N,N-dimethylformamide (all 0.9-293 GHz). The role of insufficient frequency coverage is critically discussed. Permittivities and relaxation times of the underlying relaxation processes are compared for electrolyte solns. of the hydrogen-bonding solvents water, methanol and higher alcs., formamide, N-methylformamide, and the dipolar aprotic solvents acetonitrile, propylene carbonate, DMSO, and N,N-dimethylformamide. The hydrogen-bonding mode at τ ≃1 ps is not affected by the addn. of alkali metal and tetrabutylammonium salts, in contrast to the bulk and the internal relaxation of the H-bonded chains; NH4+ in methanol produces peculiar effects. For 1:1-electrolytes ion-pair formation is detectable in all solvents of static permittivity below 50; the concn. dependence of the corresponding relaxation time permits sepn. into rotational and kinetic modes of ion-pair formation and decompn.