Hydration of Bromide Ion in Supercritical Water: An X-ray Absorption Fine Structure and Molecular Dynamics Study

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Abstract

X-ray absorption fine structure (XAFS) measurements and molecular dynamics simulations(MD) are used to explore the extent of Br- ion hydration in supercritical water solutions. The structure of the first hydration shell under ambient conditions is compared to that in the supercritical region spanning a temperature range from 25 to 475 °C and pressures from 1 to 650 bar. The RbBr salt concentration was varied from 0.02 to 1.5 molal. The wide range of conditions studied allowed a detailed examination of the effect of temperature, density, and concentration on the extent of ion hydration under supercritical conditions. The present results provide important new insights into factors affecting ion hydration in supercritical water. Changing the density of the supercritical solution by a factor of 1.5 causes only minor changes in the extent of ion hydration at 425 °C, whereas a pronounced dehydration occurs as the temperature is increased from 25 to 475 °C. Specifically, the number of water molecules in the first hydration shell is reduced from 7.1(±1.5) under ambient conditions to 2.8(±0.4) under the supercritical conditions of 425 °C and 413 bar. Over a concentration range of almost two orders-of-magnitude, there is little change in the extent of hydration. MD simulations of this system are used to generate XAFS spectra that are directly compared to the experimental results. Analysis of the MD-simulated XAFS spectra verified the data reduction technique used for the high temperature conditions. There is qualitative agreement between the simulation and experiment with respect to the number of nearest neighbor waters, the nearest-neighbor distances, the degree of disorder in the first shell, and the trends of these parameters with increasing temperature. It is, however, evident that refinements in the water−bromine intermolecular potentials are required to fully capture the observed behavior under supercritical conditions.

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