SOLVENT STRUCTURE, DYNAMICS, AND ION MOBILITY IN AQUEOUS-SOLUTIONS AT25-DEGREES-C

We calculate the mobilities u(i) of the metal cations Li+, Na+, K+, Rb +, Cs+, and Ca2+ and the halides F-, Cl-, Br-, and I- at infinite dilu tion by molecular dynamics simulation using the SPC/E model for water at 25 degrees C and a reaction field for the long-range interactions. The ion mobilities show the same trends as the experimental results wi th distinct maxima for cations and anions. The mobilities (defined by u(i) = D-i/kT) of the corresponding uncharged species are also determi ned by simulation and are in qualitative agreement with Stokes' law. T he mobilities of Li+, Na+, K+, Rb+ and F- increase oil discharge, wher eas Cl, Br, and I have smaller mobilities than the corresponding anion s. The mobility of the fictitious I+ ion, which differs from I- only i n its charge, lies between that of I- and I in the order u(I) < u(I)() < u(I)-. The residence time of water in the first solvation shell of small cations (Li+ and Na+) and Ca2+ decreases when the ions are disc harged, while the opposite is observed on neutralizing I-, suggesting the formation of a solvent cage around the large uncharged I which par tially breaks up on charging, increasing the mobility of the correspon ding ion. The cage breakup is greater for I- than for I+ which correla tes with the asymmetry in the entropies of solvation of I- and I+, in SPC/E water on charge reversal, providing an explanation for the trend s 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 correspondin g solvation entropy, suggesting different types of hydration, i.e., el ectrostatic ion solvation (hydrophilic) and cage formation (hydrophobi c) 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.; Rasai ah, 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 d ynamics of individual cations and anions are distinct at short times b ut characterized by the solvent at long times. Solvent dynamics, struc ture, and caging modulated by the charge and size of the ions are stro ngly implicated in determining their mobilities.