MOLECULAR PROCESSES IN ELECTROLYTE-SOLUTIONS AT MICROWAVE-FREQUENCIES

The microwave dispersion and absorption spectra are discussed for various protic and aprotic electrolyte solutions 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 solutions of the hydrogen-bonding solvents water, methanol and higher alcohols, formamide, N-methylformamide, and the dipolar aprotic solvents acetonitrile, propylene carbonate, dimethylsulfoxide, and N,N-dimethylformamide. The hydrogen-bonding mode at tau-congruent-to-1 ps is not affected by the addition 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 concentration dependence of the corresponding relaxation time permits separation into rotational and kinetic modes of ion-pair formation and decomposition.