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

A novel “[MSO4](2–b)– ion pair”-directed mechanism is presented to describe the role of nonlattice cations (Mb+) in the nucleation of calcium sulfate hemihydrate (CaSO4·0.5H2O, HH) in glycerol aqueous solution. By forming [MSO4](2–b)– ion pairs, univalent M+ ions (Li+, Na+, NH4+, and K+) and bivalent M2+ ions (Cu2+, Zn2+, Mn2+, and Mg2+) significantly increased the concentration of lattice ions (Ca2+ and SO42–), resulting in a higher supersaturation of HH and shortening the induction time of HH nucleation. Meanwhile, [MSO4](2–b)– ion pairs replaced free SO42– as dominant reactants for HH nucleation. They carried SO42– and decoupled them at the nucleation sites for nucleus formation. The efficiency of Mb+ ions on promoting HH nucleation is governed by its pairing and decoupling capability with SO42–. The difference between Mb+ ion and SO42– in limiting diffusion coefficient (ΔDMb+0) could characterize these two capabilities and was a robust indicator of the efficiency of Mb+ ion. This work provides a new u...