Nonlattice Cation-SO42– Ion Pairs in Calcium Sulfate Hemihydrate Nucleation

A novel “[MSO 4 ] (2– b )– ion pair”-directed mechanism is presented to describe the role of nonlattice cations (M b + ) in the nucleation of calcium sulfate hemihydrate (CaSO 4 ·0.5H 2 O, HH) in glycerol aqueous solution. By forming [MSO 4 ] (2– b )– ion pairs, univalent M + ions (Li +, Na +, NH 4 +, and K + ) and bivalent M 2+ ions (Cu 2+, Zn 2+, Mn 2+, and Mg 2+) significantly increased the concentration of lattice ions (Ca 2+ and SO 4 2– ), resulting in a higher supersaturation of HH and shortening the induction time of HH nucleation. Meanwhile, [MSO 4 ] (2– b )– ion pairs replaced free SO 4 2– as dominant reactants for HH nucleation. They carried SO 4 2– and decoupled them at the nucleation sites for nucleus formation. The efficiency of M b + ions on promoting HH nucleation is governed by its pairing and decoupling capability with SO 4 2– . The difference between M b + ion and SO 4 2– in limiting diffusion coefficient (Δ D M b + 0 ) could characterize these two capabilities and was a robust indicator of the efficiency of M b + ion. This work provides a new understanding of cation-assisted crystallization and helps identify regulators for nucleation control.