Anion-dependence of fast relaxation component in Na-, K-halide solutions at low concentrations measured by high-resolution microwave dielectric spectroscopy

High-resolution microwave dielectric spectra of NaX, KX (X: F, Cl, Br, I) aqueous solutions of c = 0.05 and 0.1 M measured in the frequency range 0.2-26 GHz at 10 C are analyzed. The dielectric relaxation (DR) spectrum of each solution, which deviates slightly from the bulk-water spectrum, is mathematically divided into the bulk-water spectrum and the spectrum of solute particles covered with a water layer using a mixture theory by assuming the existence of continuous bulk-water phase. The solute spectra above 3 GHz are fitted with a linear series of pure water component (γ dispersion with DR frequency f_w), fast Debye component-1 with DR frequency f₁ (>f_w), and slow Debye component-2 with DR frequency f₂ (<f_w). Component-2 is only found for the fluorides. The sum of dispersion amplitudes of γ and components -1 and -2 for NaX and KX are found to be almost irrespective of X and equal to the pure water level, indicating that components -1 and -2 are from the water modified by ions, thus denoted as "hypermobile water" and "constrained water" (not rigidly bound to ion), respectively. Below 3 GHz, sub-GHz dispersion component is detected and assigned as a relaxation response of counterion cloud. The resulting limiting-molar conductivities of NaX and KX are in good agreement with the literature data measured at much lower frequencies. The estimated number of hypermobile water molecules is found to increase from 9 to 31 for NaX and from 9 to 37 for KX with increasing anion size. Thus, except for the fluorides, it is reported that the modified water by salt ions exhibits only Debye component-1 other than γ dispersion, indicating the existence of a water-ions collective hypermobile mode in each solution.