Microstructure of NiSO4 Solution: A Combined DFT and EXAFS Study

The microhydration structure of nickel sulfate aqueous solution has been determined via density functional theory (DFT) calculation and extended X-ray absorption fine structure (EXAFS) spectroscopy.The geometric optimization and energy calculation of nickel sulfate hydrated clusters of the molecular formula [NiSO4(H2O)n ]^0 (n=1-12) were determined via DFT using the B3LYP method.Several possible initial structures were considered for clusters of each size to locate the equilibrium geometry.Based on the DFT calculation,the favorable structure of Ni^2+ includes the six-coordinated form of [NiSO4(H2O)n ]^0 clusters.The results of hydration energy calculation suggest that the six-coordinated contact ion pair (CIP) is the stable configuration for small hydration clusters (n≤5),while the solvent-shared ion pair (SSIP) represents the favorable structure for medium hydration clusters (6≤n≤10).The solvent is separated by x water molecules (xSIP,x≥2 is the number of water molecule between Ni^2+ and SO4^2-) in larger hydration clusters (n≥11).The EXAFS analysis of the NiSO 4 aqueous solutions and NiSO4 ·6H2O solid established that Ni^2+ was surrounded by six water molecules tightly forming an octahedral structure in the first hydration shell,and no CIP was found from 0.70 mol/L to 2.22 mol/L (near saturation).The Ni-O distance and coordinated number were 2.040±0.020 and 6.0±1.0,respectively.These results are consistent with the DFT calculations for [NiSO4(H2O)n ]^0 clusters.DFT and EXAFS are powerful techniques that can be used to enhance the resolution of NiSO 4 solution microstructure.