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Adsorption of Hydrated Halide Ions on Charged Electrodes. Molecular Dynamics Simulation

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Constant temperature molecular dynamics has been used to simulate the adsorption of hydrated halide ion X- F-, Cl-, Br- and I-, and lithium ion Li on a flat uniformly charged surfaces. The simulations were done with either 214 water molecules and two ions Li and X- in a box 2. 362 nm deep or with 430 water molecules and the two ions in a box 4.320 nm deep. The boxes were periodically replicated in the xy directions. The magnitude of the surface charge on the box end was or - 0.11 enm2, corresponding to an electric field of 2 x 10exp 7 Vcm. The lateral dimensions of the simulation cell were 1.862 nm x 1.862 nm x times y in each case. All of the water molecules and ions interacted with the end walls via a weak 9 - 3 potential. The ST2 water model and parameters optimized for alkali halides interacting with the model ST2 water molecule were used in the calculations. Common practices of truncating the interactions at a finite distance 0.82 nm and switching off Coulomb interactions at small distances were followed. The temperature was set at T 2.411 kJmole 290 K. Some of the properties calculated were distribution density profiles for ions and water across the gap important for comparisons with Gouy-Chapman theory, adsorbed ion - water pair correlation functions, the number of water molecules in the first and second hydration shells of the ions as a function of time. The time spent by a water molecule in the hydration shell was calculation to be approximately ten times longer for lithium than any other ion. The correlation between distance from the electrode and hydration number was studied and generally found to be pronounced for the larger anions.

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  • Physical Chemistry

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