Rapidly fluctuating ion complexes in aqueous solutions of 2:1 valent salts

Ultrasonic absorption spectra at frequencies between 300 kHz and 3 GHz are reported for aqueous solutions of MgCl2, CaCl2, SrCl2, NiCl2, and CuCl2. All spectra reveal a Debye-type relaxation term with small amplitude. The relaxation time values derived from the spectra display a small range only (0.2 ns⩽τ⩽0.6 ns), corresponding with the τ values for solutions of Mg(NO3)2 and Ca(NO3)2 in water. The relaxation term is suggested to be due to the second step in the Eigen–Tamm scheme of stepwise association and dissociation of ions, namely the equilibrium between the complex of encounter and the outer sphere complex. The relaxation amplitudes reflect special properties of the ions.