A total scattering study of prenucleation structures in saturated aqueous magnesium sulfate - observation of extended clusters

Through a combination of X-ray and neutron total scattering and Empirical Potential Structure Refinement (EPSR) we explore the prenucleation structures of saturated aqueous magnesium sulfate. The atomistic model we present reveals a system characterised by isolated octahedral aquo magnesium species Mg(H2O)(6), magnesium sulfate pairs (Mg(H2O)(5)SO4) and extended clusters built from corner-sharing MgO6 and SO4 polyhedra. Many of these features are directly observed in the crystal structures of the known solid form hydrates, including isolated polyhedra, corner sharing chains and rings, and it is only for the extended 3D polyhedral networks of the lower hydrates (mono- & di-) that no proto structures are observed in 2M solution. Looking at the average first solvation shell of the sulfate anion we see a complex and flexible environment that commonly includes water molecules brought into proximity by a coordinated hydrated magnesium. What emerges is a high probability that 10 water molecules will be observed in a combined tetrahedral/octahedral arrangement with a further 7 taking up more dispersed positions giving an average coordination of 17. The tendency for ions to aggregate into clusters allows areas of bulk water to exist that exhibit subtle differences in structure to that of pure water.

Automated summary

Through X-ray and neutron total scattering, as well as Empirical Potential Structure Refinement (EPSR), the prenucleation structures of saturated aqueous magnesium sulfate are explored. The provided atomistic model reveals isolated octahedral aquo magnesium species, magnesium sulfate pairs, and extended clusters built from corner-sharing MgO6 and SO4 polyhedra. The average first solvation shell of the sulfate anion exhibits a complex and flexible environment, with water molecules brought into proximity by a coordinated hydrated magnesium. Ions' tendency to aggregate into clusters allows for differences in structure between bulk water and pure water.