High pressure water is a fundamental constituent of the Earth crust and upper mantle. Based on space probe observations and planetological models it has been proposed that water may even exist in the form of buried salted oceans under a deep ice crust on certain giant planet satellites and in particular on Europa [1]. There, a pressure of several kilobars (kbar) may be reached as compared with the ~1 kbar of the oceanic troughs on Earth.

These findings stimulate fundamental questions on whether, and to what extent, these harsh environments could support life. It is known that barophilic bacteria may survive pressures in the kbar range, but protein denaturation will take place at some point. In other scientific contexts, water, in the high pressure undercooled liquid range, has been predicted to display a liquid-liquid phase transition line ending in a critical point [2], as a result of the disruption of the hydrogen bonded network upon increasing pressure. These transformations will also affect the behaviour of water as a solvent which is relevant to biochemical sciences. Understanding the behaviour of water at high pressure is therefore a fundamental achievement with an interdisciplinary scientific relevance.

In addition to the possibility of investigating the structure of pure water under pressure [3], the ESRF facility allows us to conceive experiments where we investigate the behaviour of water around "impurities" present in a sample in the 1% concentration range, which is a fundamental step for the understanding of the water behaviour as a solvent at high pressure. This is possible using the so called "large volume" high pressure technique associated with the Paris-Edinburgh press [4] shown in Figure 107 and available at BM29.

 

 

Fig. 107: The BM29 Paris-Edinburgh press.

 

With this equipment it is possible to create extreme conditions up to about 100 kbar and up to 1000-2000 K in a volume of about 1 cubic mm that can be filled with an aqueous solution sample. An X-ray absorption spectroscopy (XAS) experiment can be tuned to (and above) the threshold for the transitions of the 1s electron of a selected ion, into continuum states. Therefore the X-ray absorption signal will be only be determined by the environments of those atoms selected by the absorption edge being investigated. Such sensitivity cannot be achieved by any other experimental technique.

 

 

Fig. 108: Ion-water oxygen radial distributions as a function of pressure at ambient pressure, 14 kbar, and 28 kbar.

 

We have performed an experiment of this kind on a RbBr 0.92 molal aqueous solution measuring high quality X-ray absorption spectra at the Br and Rb K-edges as a function of pressure, following the pure water melting line. We have obtained the first experimental results for the pressure dependence of the radial distributions of the water oxygen atoms around the two ionic species. These functions, shown in Figure 108, indicate that the O distribution associated with the hydration shell of the negative ion is compressed remarkably with the application of pressure. In comparison, the O distribution around Rb ions appears to be less affected thus indicating the occurrence of a considerable molecular reorientation around the ions in the investigated pressure range. These results provide evidence that the water's structural transformation with pressure heavily affects its behaviour as a solvent. These findings, which are relevant to the planetological chemistry of high-pressure water, may also have implications on the ability of apolar solutes to develop a hydrophobic interaction under high pressure.

References
[1] J.S. Kargel, J.Z. Kaye, J.W. Head, G.M. Marion, R. Sassen, J.K. Crowley, O.P. Ballesteros, S.A. Grant, and D.L. Hogenboom, Icarus 148, 226 -265 (2000).
[2] O. Mishima and H. E. Stanley, Nature 396, 329-335 (1998).
[3] J.H. Eggert, G. Weck, and P. Loubeyre, J. Phys.: Condens. Matter 14,11385 -11394 (2002).
[4] J.M. Besson, R.J. Nelmes, G. Hamel, J.S. Loveday, G. Weill, and S. Hull, Physica B 180-181, 907 (1992).

Principal Publications and Authors
A. Filipponi (a), S. De Panfilis (b), C. Oliva (c), M.A. Ricci (c), P. D'Angelo (d), D.T. Bowron (e), Phys. Rev. Lett. 91, 165505 (2003).
(a) INFM and Dipartimento di Fisica, Università dell' Aquila (Italy)
(b) ESRF
(c) Dipartimento di Fisica "E. Amaldi", Università degli Studi Roma Tre and INFM, Roma (Italy)
(d) Dipartimento di Chimica, Università di Roma "La Sapienza", Roma (Italy)
(e) ISIS Facility, Rutherford Appleton Laboratory, Didcot (UK)