Ion-Water and Ion-Ion Structure in Supercritical Water
The interactions of cations with anions underlie processes in nearly all aqueous systems. In liquid systems, cation/anion interactions are the basis for ion exchange separations and many biochemical functions. In the supercritical region (T>365°C), ion pairing dominates certain types of transport in the earth's crust and it is an ever-increasing component of industrial processes from power plant water chemistry to chemical synthesis. In the gas phase, ion-ion interactions play an important role in atmospheric chemistry in areas of nucleation and catalysis. John L. Fulton and Yongsheng Chen, chemists at PNNL, recently captured the first detailed structure of the Ca2+/Cl- interaction using x-ray absorption spectroscopy (XAFS) at the Advanced Photon Source whose intense x-ray source has enabled them to study these light elements.
Most salts will fully dissociate in ambient water to form separately hydrated negative and positive ions. However, at high temperatures, the negative and positive ions associate in a structure known as a contact-ion pair. There are hundreds of papers dealing with thermodynamic and the macroscopic properties of these species in supercritical water but there are virtually no experimental studies of the details of their molecular structure. Using XAFS we have captured the complete structure of Ca2+ associating with two Cl- deriving precise bond distances, bond disorder and the degree of hydration around the cation and the anion. These XAFS measurements are now providing the basis for direct comparison to molecular simulation using a "first-principles" methods developed at PNNL.
Details can be found in Fulton JL, Y Chen, SM Heald, and M Balasubramanian. 2004. "High-Pressure, High-Temperature X-Ray Absorption Fine Structure Transmission Cell for the Study of Aqueous Ions with Low Absorption-Edge Energies." Review of Scientific Instruments 75(12):5228-5231. DOI:10.1063/1.1813131.