Structures and energy landscapes of hydrated sulfate clusters

The sulfate ion is the most kosmotropic member of the Hofmeister series, but the chemical origins of this effect are unclear. We present a global optimization and energy landscape mapping study of microhydrated sulfate ions, SO₄²(H₂O)_n, in the size range 3 ≤ n ≤ 50. The clusters are modeled using a rigid-body empirical potential and optimized using basin-hopping Monte Carlo in conjunction with a move set including cycle inversions to explore hydrogen bond topologies. For clusters containing a few water molecules (n ≤ 6) we are able to reproduce ab initio global minima, either as global minima of the empirical potential, or as low-energy isomers. This result justifies applications to larger systems. Experimental studies have shown that dangling hydroxyl groups are present on the surfaces of pure water clusters, but absent in hydrated sulfate clusters up to n ≈ 43. Our global optimization results agree with this observation, with dangling hydroxyl groups absent from the low-lying minima of small clusters, but competitive in larger clusters.