ADSORPTION OF HYDRATED HALIDE-IONS ON CHARGED ELECTRODES - MOLECULAR-DYNAMICS SIMULATION

Constant temperature molecular dynamics has been used to simulate the adsorption of hydrated halide ions X- = F-, Cl-, Br- and I-, and lithium ion Li+ on 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 ends was +/- 0.11 e/(nm)2, corresponding to an electric field of 2 X 10(7) V/cm. The lateral dimensions of the simulation cell were 1.862 nm X 1.862 nm (x X y) in each case. All of the water molecules and ions interacted with the end walls via a weak 9-3 potential. The Stillinger ST2 water model and parameters optimized for alkali halides interacting with the model ST2 water molecule were used in the calculations. Common particles 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 = 2.41 1 kJ/mol (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, and 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 calculated 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. Comparison of the dynamics of the common ion Li+ for different anions revealed the subtle influence of a transcell interaction in the 2.362 nm thick film. In the given field, the smallest ions Li+ and F- remained fully solvated at all times. Chloride behaved quite differently. Part of the time this ion was far enough away from the electrode to be fully hydrated and part of the time it was in physical contact (i.e., physisorbed) on the electrode with no water molecules interposed between it and the electrode. Bromide favored contact adsorption over full hydration most of the time. Iodide was observed to be contact adsorbed almost all of the time. These simulations provide new insights on the behavior of strongly hydrated ions at surfaces and how the transition from noncontact to ''contact'' adsorption occurs.