Effect of solvent activity on solute association: the formation of aqueous nickel(II) chloride complexes studied by UV-Vis and EXAFS spectroscopy

Water activity is generally considered to affect ionic association in aqueous electrolyte solutions; however, it is usually ignored when the association reactions or constants are discussed. In this work, the effect of water activity on the association reaction Ni2+(aq)+nCl−(aq)⇌NiCl2−nn (aq) was investigated by EXAFS and UV–Vis spectroscopy measurements on NiCl2 aqueous solutions at room temperature with constant Cl−/Ni2+ ratio (~66) and various water activities; the latter were adjusted by adding MgCl2 and Mg(ClO4)2. Both the EXAFS and the UV–Vis spectra measurements indicated that the extent of Ni–Cl association increases in decreasing water activity environments, independent on the Cl−/Ni2+ ratio. Thus, the effect of water activity on the ionic association should not be ignored and more emphasis should be paid on it, especially, when the water activity is very low.Ning Zhang, Dewen Zeng, Joël Brugger, Quanbao Zhou, Yung Ngotha