Structures and dynamic hydration of CaSO4 clusters in supersaturated solutions: A molecular dynamics simulation study

Abstract

The characteristics of CaSO4 clusters before the formation of crystalline calcium sulfate phases in supersaturated solutions remain uncertain. We have used extensive molecular dynamics simulations to thoroughly investigate structures and dynamic hydration of [(CaSO4)n]0 clusters in supersaturated solutions. There are three typical CaSO4 clusters, dissolved ion associated species, stable aggregates and CaSO4 precursors. For the dissolved ion associated species [(CaSO4)n]0 (n ≤ 7), the aggrgegation of these clusters accompanies with dehydration and cooperative hydration. The neutral [(CaSO4)n]0 (7 < n < 70) clusters are stable aggregates, and the size is about 30– 50 Å. the aggregation of these clusters also accompanies with dehydration of Ca2+, but the cooperative hydration between Ca2+ and SO42− does not change obviously. There are two kinds of dynamic hydration around Ca2+ in [(CaSO4)n]0 structures. Water molecules shared by Ca2+ and SO42− can be retained during the aggregation processes. The further aggregation of stable aggregates accompanying with ion coordination recombination will result in the formation of hydrous or anhydrous CaSO4 precursors ([(CaSO4)n]0 (n ≥ 70)) under different conditions, such as the supersaturation degree and temperature