Hydrogen bond breaking in aqueous solutions near the critical point

The nature of water–anion bonding is examined using X-ray absorption fine structure spectroscopy on a 1m ZnBr2/6m NaBr aqueous solution, to near critical conditions. Analyses show that upon heating the solution from 25°C to 500°C, a 63% reduction of waters occurs in the solvation shell of ZnBr42−, which is the predominant complex at all pressure–temperature conditions investigated. A similar reduction in the hydration shell of waters in the Br− aqua ion was found. Our results indicate that the water–anion and water–water bond breaking mechanisms occurring at high temperatures are essentially the same. This is consistent with the hydration waters being weakly hydrogen bonded to halide anions in electrolyte solutions.