The investigation of ion association characteristics in lanthanum sulfate solution by the density functional theory and molecular dynamics simulations

The ion association behavior in aqueous lanthanum sulfate solutions was investigated using density functional theory (DFT). The structures and properties of [La(SO4)m & sdot;(H2O)n](3-2m) clusters, where m =1 to 3 and n = 1 to 9, were examined at the PBE0/6-311+G(d, p) level. The results show that Lanthanum sulfate hydrated clusters exist in the aqueous solution's microscopic state of contact ion pairs (CIP). [La(SO4)(H2O)n]+ and [La(SO4)2 & sdot;(H2O)n]-, and [La(SO4)3 & sdot;(H2O)n]3- clusters approximately reach the saturation of the first water shell at n = 7 and 6 and 3. [La(SO4)2 & sdot;(H2O)6]- and [La(SO4)3 & sdot;(H2O)3]3- clusters have lower binding energy than [LaSO4 & sdot;(H2O)n]+. This indicates that lanthanum sulfate tends to aggregate in an aqueous solution. Compared to the gas-phase cluster structures, the distance of R(La-O)H2O expands in the PCM solvent model, while R(La-O)SO4 contracts. The hydration energy of LaSO4 & sdot;(H2O)7, La(SO4)2 & sdot;(H2O)6, and La(SO4)3 & sdot;(H2O)3 were -76.5, -54.1 and -332.0 kcal/ mol, respectively. The molecular dynamics simulation results show that La is more inclined to coordinate with sulfate's oxygen than water's oxygen, and the coordination number of water around La3+ is 6.075. These results are consistent with the calculated results by DFT.