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

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 firs...