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) ⇔ NiCl^2-n_n(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.