Stable salt-water cluster structures reflect the delicate competition between ion-water and water-water interactions

How salts affect water structure is an important topic in many research fields. Salt-water clusters can be used as model systems to extract interaction information that is difficult to obtain directly from bulk solutions. In the present study, integrated tempering sampling molecular dynamics (MD) are combined with quantum mechanics (QM) calculations to overcome the sampling problem in cluster structure searches. We used LiI(H2O)n and CsI(H2O)n as representatives to investigate the microsolvation of ion pairs. It was found that Li(+)-I(-) and Cs(+)-I(-) ion pairs interact with water molecules in very different ways, and the corresponding salt-water clusters have distinctly different structures. LiI strongly affects water-water interactions, and the LiI(H2O)n (n ≥ 5) clusters build around a Li(+)(H2O)4 motif. CsI only slightly perturbs the water cluster structure, and CsI(H2O)n favors the clathrate-like structure when n = 18 or 20. Consistent with the law of "matching water affinities", Li(+) and I(-) are more easily separated by solvent molecules than the Cs(+)-I(-) ion pair.