Solvent Structure, Dynamics, and Ion Mobility in Aqueous Solutions at 25 °C

We calculate the mobilities ui of the metal cations Li+, Na+, K+, Rb+, Cs+, and Ca2+ and the halides F-, Cl-, Br-, and I- at infinite dilution by molecular dynamics simulation using the SPC/E model for water at 25 °C and a reaction field for the long-range interactions. The ion mobilities show the same trends as the experimental results with distinct maxima for cations and anions. The mobilities (defined by ui = Di/kT) of the corresponding uncharged species are also determined by simulation and are in qualitative agreement with Stokes' law. The mobilities of Li+, Na+, K+, Rb+ and F- increase on discharge, whereas Cl, Br, and I have smaller mobilities than the corresponding anions. The mobility of the fictitious I+ ion, which differs from I- only in its charge, lies between that of I- and I in the order uI < uI+ < uI?. The residence time of water in the first solvation shell of small cations (Li+ and Na+) and Ca2+ decreases when the ions are discharged, while the opposite is observed on neutralizing I-, suggesting the formation of a solvent cage around the large uncharged I which partially breaks up on charging, increasing the mobility of the corresponding ion. The cage breakup is greater for I- than for I+ which correlates with the asymmetry in the entropies of solvation of I- and I+, in SPC/E water on charge reversal, providing an explanation for the trends in the mobilities of I, I-, and I+. The residence times of water in the primary hydration shell around cations pass through a minimum as a function of size that correlates with the maximum in the corresponding solvation entropy, suggesting different types of hydration, i.e., electrostatic ion solvation (hydrophilic) and cage formation (hydrophobic) respectively for small and large cations. The results are in accord with recent calculations of the solvation entropy and free energy as continuous functions of the charge and size (Lynden-Bell, R. M.; Rasaiah, J. C. J. Chem. Phys. 1997, 107, 1981). Hydrophilic and hydrophobic solvation are reflected in the exchange dynamics of the water in the hydration shells around charged and uncharged solutes. The solvation dynamics of individual cations and anions are distinct at short times but characterized by the solvent at long times. Solvent dynamics, structure, and caging modulated by the charge and size of the ions are strongly implicated in determining their mobilities.