Structure and transport of aqueous electrolytes: from simple halides to radionuclide ions

Molecular simulations are used to compare the structure and dynamics of conventional and radioactive aqueous electrolytes: chloride solutions with sodium, potassium, cesium, calcium, and strontium. The study of Cs(+) and Sr(2+) is important because these radioactive ions can be extremely harmful and are often confused by living organisms for K(+) and Ca(2+), respectively. Na(+), Ca(2+), and Sr(2+) are strongly bonded to their hydration shell because of their large charge density. We find that the water molecules in the first hydration shell around Na(+) form hydrogen bonds between each other, whereas molecules in the first hydration shell around Ca(2+) and Sr(2+) predominantly form hydrogen bonds with water molecules in the second shell. In contrast to these three ions, K(+) and Cs(+) have low charge densities so that they are weakly bonded to their hydration shell. Overall, the structural differences between Ca(2+) and Sr(2+) are small, but the difference between their coordination numbers relative to their surface areas could potentially be used to separate these ions. Moreover, the different decays of the velocity-autocorrelation functions corresponding to these ions indicates that the difference in mass could be used to separate these cations. In this work, we also propose a new definition of the pairing time that is easy to calculate and of physical significance regardless of the problem at hand.