Electrolytes and their dissociated ions are thought to form positive or negative hydration layers around them. In this study, we have developed a method to determine the volume and the dielectric relaxation property (relaxation frequency fc, dispersion intensity δ) of the water hydrating ions in salt solutions. The method consists of four steps: (1) By use of a high-resolution microwave dielectric spectroscopy technique, the dielectric spectra of sample salt solution and bulk water are measured in pair. (2) The dielectric spectrum of solutes (ions) with water layers for a given volume fraction φ is then calculated from each pair of dielectric spectra of a sample salt solution and reference water according to the Hanai mixture theory. (3) Each spectrum of solutes with water layers at a given 0 is decomposed into a few Debye relaxation functions and the bulk water component. (4) The volume fraction φ is operationally decreased from 0.5, and steps (2) and (3) are repeated at each φ until the bulk water component vanished. Then the volume fraction of the hydrated solutes (ions) in solution is determined. The method was applied to NaF and NaCl solutions. As a result the different spectral intensity was nearly proportional to the salt concentration below 0.2 M in the frequency range of 3-26 GHz. The hydration number Nh, and the dielectric relaxation property of the hydration layer for each salt solution was successfully determined as (fcl, δ1, N h)= (18.7, 44.9, 27.9) for NaCl and (fcl, δ1, fc2, δ2, Nh) = (26.0, 6.70, 5.64, 19.2) for NaF.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry