Abstract

Dynamic simulations are used to investigate ion cluster formation in unsaturated aqueous NaCl at 25 C. Statistical, structural, and dynamic properties are reported. An effort is made to identify general behaviors that are expected to hold beyond the limitations of the force field. Above ~1 M, clusters with more than ten ions begin to form after ~1020 ns of simulation time, but no evidence of irreversible ion aggregation is observed. Cluster survival times are estimated, showing that the kinetics becomes increasingly complex as salt is added, leading to multiple decay rates. Cluster dipole moment distributions show characteristic peaks that reflect the cluster preferred conformations in solution. These are modulated by electrostatic and liquid-structure forces, and are described in detail for clusters of up to five ions. For a given size and charge, the cluster morphology is independent of salt concentration. Below ~2 M, clusters affect the structure of water in their first hydration shells, so dipole moments parallel to the cluster macrodipoles are induced. These effects show a weak dependence with concentration below ~2 M, but vanish in the 23 M range. A possible connection with the structural transition recently suggested by NMR data in concentrated electrolytes is discussed. The effects of electrostatics on cluster speciation and morphology are discussed based on results from a set of simulations carried out with the ionic charges removed.



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