A method for generating extended X-ray absorption fine-structure (EXAFS) spectra directly from molecular dynamics trajectories is described. Conventional molecular dynamics trajectories are run, and configurations from the trajectory are saved at periodic intervals. Clusters containing the EXAFS atom at the center are extracted from the configurations and used as input to the multiple scattering code FEFF6, which produces an EXAFS spectrum for each cluster. The separate spectra for the individual clusters are then averaged together to get a configurationally averaged EXAFS spectrum that can be compared directly with experiment. The method is tested on strontium chloride solutions and compared with recent experimental results on strontium nitrate solutions at both standard and supercritical conditions.

No abstract

Recent de®elopments in micro-

We present the first direct measurement of ion hydration in supercritical water using X-ray absorption fine structure (XAFS). Radial structure functions were determined for strontium ions in supercritical water at 385 "C and 269-339 bar at a concentration of strontium of 0.2 M. For supercritical water, at a temperature of 385 "C and density of 0.54 g/cm3, the number of waters of hydration was a factor of 0.52 of the number in liquid water under ambient conditions. The radius of the first solvation shell changes very little at these elevated temperatures. This large local depletion of water around the ion would affect the short-range interactions with counterions and may increase the ion reactivity. We also report XAFS results for krypton in supercritical water and show that, in contrast to strontium ions, the local solvent environment is more gaslike in the first few solvation shells of the krypton atom. IntroductionSupercritical water (SCW) is an interesting solvent for chemical reactions and hazardous waste At temperatures above 375 "C, the solubility behaviors of two important classes of compounds reverse: most organic species have high solubility whereas the solubility of inorganic salts is limited. The high solubility of organics and the aggressive oxidizing environment are attractive for organic reactions and waste-destruction reactions. The low salt solubility presents both obstacles to and opportunities in developing SCW oxidation technology. Salt precipitation may lead to equipment fouling and erosion. However, the ability to control solubility with temperature and pressure may provide a way of separating organic from inorganic wastes and may provide a way of selectively separating different salts. A better understanding of the mechanism of salt solubility in SCW is required to fully utilize this unique solvent. In this Letter, we report X-ray absorption fine structure (XAFS) measurements of the extent of ion hydration in SCW. From these results, one can build a picture of the radial distribution of water around the ion, which is important information needed in the development and testing of models of these systems.In the supercritical state, all of the properties of the fluid, including the solvent strength, are "tunable" through adjustments in the fluid density. It is this variable nature of the fluid that makes SCW (Z', = 374 "C, Pc = 220 bar) interesting from a fundamental point of view. For liquid-phase systems, there is only a very narrow latitude for changing the properties of the solvent. In the supercritical fluid or near-critical liquid state, the density of the solution can be changed over a considerable range, offering the opportunity to fully test models of these systems over a wide range of thermodynamic conditions.To a large extent, the dielectric constant of water controls the solubility of ionic species. In the supercritical region, the dielectric constant, E , spans the range from about 5 to 20,6s7 a value that is greatly reduced from the liquid phase ( E 80).Because of the inability of...

X-ray absorption fine structure (XAFS) measurements and analyses are presented for Rb+ in supercritical water solutions. The structure of the first hydration shell at ambient conditions is compared to that in the supercritical region at a temperature of 424 °C and pressures from 382 to 633 bar. For all reported studies, RbBr at a concentration of 0.5 molal was used. XAFS results show that there is a well-defined hydration shell around the cation even at 424 °C but at these high temperatures the extent of hydration of the Rb cation is reduced by about 40%. A slight contraction of this first shell distance by about 0.10 Å is also observed under supercritical conditions. The reduction in the number of water-ion bonds is analogous to the reduction in the amount of water–water hydrogen bonding that has been observed by others under supercritical conditions. The reduction in waters-of-hydration under supercritical conditions may also be in part due to formation of contact-ion pairs.