Intermolecular interactions in aqueous solution under conditions of elevated temperature and pressure are related to the structure of the solvent and in particular to the strength of the hydrogen bonding. Therefore, studying the structural and electronic evolution of solute particles with changing conditions of temperature and pressure is a way of obtaining information about the solvent itself [1].

The aim of our studies is to understand the relation between physical properties and structural organisation of liquids under conditions of elevated temperature and pressure, up to and beyond their critical point. These studies are possible thanks to a combination of the extremely bright and well-focused X-ray beams at the ESRF with specially-developed high-temperature and high-pressure cells. Recently we have investigated the molecular structure of the hydration of ions and molecules in supercritical water and aqueous solutions.Several techniques were used: X-ray Absorption spectroscopy (XAS at the FAME BM30B and ID26 beamlines) and X-ray Raman Inelastic Scattering to determine local structures of solvation around solute and solvent molecules, and Small Angle X-ray Scattering (SAXS) to determine the structure of the mesoscopic inhomogeneous distribution of water molecules in the compressible regime.

The molecule aqueous arsenious acid, As(OH)3, is a very good candidate as a probe: it possesses a nonzero dipole moment that may evolve with the solvent permittivity e and three OH groups likely to establish hydrogen bonds with solvent water molecules. The geometrical and electronic structure of the arsenious acid molecule As(OH)3 in aqueous solutions was investigated by XAS. A new optical cell (Figure 105) developed for fluorescence spectroscopic studies permitted conditions from ambient to supercritical (0.3 m, P = 250 and 600 bars, T < 500°C) to be employed.



Fig. 105: 3D drawing of the high pressure (HP)-high temperature (HT) setup: (1) He inlet through the HP vessel plug; (2) HP vessel plug; (3) thermocouples and electrical connections; (4) HP vessel (150mm); (5) viton seal; (6) water cooling circulation; (7) thin beryllium windows.



EXAFS analysis shows the constancy of the As-O distance, which remains about 1.77 Å in all conditions. These new results confirm that the As-O bonding has a strong covalent character, which remains under the supercritical temperature conditions. In the XANES region, new first principle calculations using the FDMNES code [2] reveal an enlargement of the intramolecular O-As-O angles when temperature increases (Figure 106), coupled with a small reduction of the atomic partial charge magnitude. The changes in the solvent structure, such as the weakening of the hydrogen bonded network concomitant with the decrease of the solvent dielectric constant, are believed to explain the structural and electronic modifications in the As(OH)3 molecule with increasing temperature. Indeed, a weakening of the hydrogen bonding between the As hydroxide complexes and water molecules, releases the molecule from the hydrogen-bonded network and allows it to open its structure to a more tetrahedral configuration (consistent with the sp3 hybridisation of the As electronic orbitals). At the same time, the decrease of the bulk water dielectric constant results in the reduction of the partial atomic charges. The combination of EXAFS analysis with realistic XANES simulations is demonstrated to be a powerful tool, sensitive to both the 3D geometrical and electronic structures of aqueous complexes and on the solvent itself, in high temperature/high pressure fluids.



Fig. 106: a) Xanes spectra of arsenious acid solutions (0.3m) at 250 bars (30°C, 100°C and 425°C). The vertical arrow denotes a resonance at 8 eV above the edge which is damped with temperature. b) Simulated spectra (distances As-O=1.77 Å, As-O-H=120°) for O-As-O angles varying from 92° to 104° with a 2° step. The 8 eV resonance is also damped.



[1] V. Simonet, Y. Calzavara, J.-L. Hazemann, R. Argoud, O. Geaymond and D. Raoux, J. Chem. Phys. 116, 2997 (2002) and J. Chem. Phys. 117, 2771 (2002).
[2] Y. Joly, Phys. Rev. B 63, 125120 (2001).

Principal Publications and Authors
D. Testemale (a, b), J.L. Hazemann (a), G. Pokrovski (c), Y. Joly (a), J. Roux (d), J. Chem. Phys. 121, 8973 (2004).
(a) Laboratoire de Cristallographie, CNRS, Grenoble (France)
(c) Laboratoire de Géochimie, CNRS, Toulouse (France)
(d) Institut de Physique du Globe, CNRS, Paris (France)