Water at interfaces

Water is perhaps the most important chemical substance known. Without it, the very existence of life would be questionable. Yet its detailed structure and behaviour in the condensed phase and at the interfaces between the condensed phase and its environment remain somewhat controversial:

• The gas-phase water cluster community has provided detailed experimental and theoretical pictures of the behaviour of small water clusters as exemplified by the work of Saykally and co-workers (e.g. H. A. Harker, M. R. Viant, F. N. Keutsch, E. A. Michael, R. P. McLaughlin and R. J. Saykally, Water Pentamer: Characterization of the Torsional-Puckering Manifold by Terahertz VRT Spectroscopy, J. Phys. Chem. A., 2005, 109, 6483–6497), yet questions remain as to the whether existing interaction potentials for water can adequately describe its phase and interface behaviour fully.

• X-Ray absorption studies on liquid water have been interpreted in terms of rings and chains of water molecules (see Ph. Wernet, D. Nordlund, U. Bergmann, M. Cavalleri, M. Odelius, H. Ogasawara, L. Å. Näslund, T. K. Hirsch, L. Ojamäe, P. Glatzel, L. G. M. Pettersson and A. Nilsson, The Structure of the First Coordination Shell in Liquid Water, Science, 2004, 304(5673), 995–999) that other techniques, both theoretical and empirical, have failed to observe.

• The structure of water layers at metal surfaces has been a topic of considerable debate since, based on first-principles quantum chemical calculations, Feibelman proposed (see Science, 2002, 295(5552), 99–102) that an arrangement of adsorbed water molecules, hydroxyls and hydrogen atoms explains structural observations of 1 ML water on ruthenium, and later, both core-level spectroscopy (see J. Weissenrieder, A. Mikkelsen, J. N. Andersen, P. J. Feibelman and G. Held, Phys. Rev. Lett., 2004, 93, 196102, and P. J. Feibelman, Chem. Phys. Lett., 2005, 410, 120) and infrared absorption data (see C. Clay, S. Haq and A. Hodgson, Chem. Phys. Lett., 2004, 388, 89) offered support for this idea.

Indeed, as ever more sophisticated and novel experimental and theoretical tools are applied to the study of bulk liquid water, ice and their interfaces, it is becoming increasingly clear that this disparate information can heat the debate on the phase and interface behaviour of water rather than cooling it.

In the theoretical modelling of water, the important physical and chemical role played by water at the atomic level is becoming more widely appreciated, advancing our understanding beyond the simple dielectric continuum models of the past. The need for an atomistic level of description presents new challenges for ab initio calculations and computational molecular modelling of aqueous systems. Furthermore, the quality of additive and non-additive intermolecular potential models for water, often derived from such ab initio modelling, is advancing to the stage where it is possible to predict the properties and behaviour of liquid and solid water from a many-body ‘gas-phase' expansion. The same may soon be true for water at interfaces and for solutes in aqueous solution.

Water remains an exciting challenge for the experimentalist too. Important information on potential-energy surfaces has recently come from the production of kinetically (as opposed to thermodynamically) stable water clusters within superfluid helium droplets, the spectroscopy of hetero-clusters containing water, and the study of molecular collisions, including orientationally aligned molecules. Studies of clusters and ultrathin films on surfaces are leading us to revise our views of the electrochemical interface and to a greater understanding of the structure and phase behaviour of water. Neutron-scattering developments are giving new insight into details of structure in both pure water and complex solutions, especially those of biological importance. Surface-specific spectroscopies and scanning probe techniques are revolutionising our microscopic understanding of the interfaces water presents to the gas phase, to solid surfaces and to immiscible liquids.

It was with this excitement in mind that some three years ago several colleagues and I proposed a Faraday Discussion aimed at achieving a unification of views towards the goal of understanding the microscopic structure and behaviour of condensed phases of water at interfaces. Faraday Discussion 141 at Heriot-Watt University is the realisation of that goal. This Themed Issue of PCCP has been published to coincide with Faraday Discussion 141.

Experimental and theoretical scientists in diverse sub-disciplines that traditionally do not interact (the gas-phase clusters community, the surface science community and the condensed (liquid) phase community) have taken the opportunity offered by this Themed Issue of PCCP to present their latest work and it would be unfair of me to highlight any particular contribution. My thanks go to all of those whose work is represented in this Themed Issue of PCCP and I commend this issue to the reader, in parallel with the discussion volume that will ultimately follow Faraday Discussion 141, as a revealing and erudite snapshot of our current understanding of water and its interfaces.

In closing I thank my colleagues on the scientific organising committee of Faraday Discussion 141: Professor Colin Bain (University of Durham, UK), Professor Victoria Buch (Hebrew University of Jerusalem, Israel), Professor John Finney (University College London, UK), Professor Jean-Pierre Hansen (University of Cambridge, UK), Dr Georg Held (University of Reading, UK), Professor Andrea Russell (University of Southampton, UK) and Dr Richard Wheatley (University of Nottingham, UK) for their extensive help and support in the development and operation of the discussion. My thanks too go to the staff of PCCP for their understanding and efficiency once production of this Themed Issue was agreed.

Martin R. S. McCoustra, Heriot-Watt University, UK

Papers in this issue
1	V. Buch et al., DOI: 10.1039/b809839p
2	S. B. Rempe et al., DOI: 10.1039/b810017a
3	P. J. Feibelman, DOI: 10.1039/b808482n
4	O. Vendrell and H.-D. Meyer, DOI: 10.1039/b807317a
5	F. Bresme, E. Chacón et al., DOI: 10.1039/b807437m
6	K. Szalewicz, A. van der Avoird et al., DOI: 10.1039/b809435g
7	M. Havenith et al., DOI: 10.1039/b807458p
8	F. Despa, DOI: 10.1039/b805699b
9	L. F. Phillips et al., DOI: 10.1039/b810081k
10	P. Jedlovszky et al., DOI: 10.1039/b807299j
11	M. Meuwly et al., DOI: 10.1039/b807492e
12	D. J. Tobias, J. C. Hemminger et al., DOI: 10.1039/b807041e
13	P. Ayotte et al., DOI: 10.1039/b806654j
14	L. J. Criscenti, H. C. Allen et al., DOI: 10.1039/b807090n
15	B. Rotenberg et al., DOI: 10.1039/b807288d
16	H. Kang et al., DOI: 10.1039/b807730b
17	A. H. Fuchs et al., DOI: 10.1039/b807471b
18	P. O. Momoh and M. S. El-Shall, DOI: 10.1039/b809440n
19	M. Fárník et al., DOI: 10.1039/b806865h
20	A. M. Djerdjev and J. K. Beattie, DOI: 10.1039/b807623e
21	P. M. Rodger et al., DOI: 10.1039/b807455k
22	J.-M. Zanotti et al., DOI: 10.1039/b808217k
23	E. Bonaccurso et al., DOI: 10.1039/b806236f
24	J. Guilment et al., DOI: 10.1039/b803479f
25	F. Dong, J. A. Wegener and N. A. Baker, DOI: 10.1039/b807384h
26	T. Head-Gordon et al., DOI: 10.1039/b806995f
27	M. Sliwinska-Bartkowiak, K. E. Gubbins et al., DOI: 10.1039/b808246d
28	H.-f. Wang et al., DOI: 10.1039/b806362a
29	M. Meyer et al., DOI: 10.1039/b807314g
30	J. Klein et al., DOI: 10.1039/b807459n
31	J. Lützenkirchen, T. Preočanin and N. Kallay, DOI: 10.1039/b807395c
32	W. A. Brown et al., DOI: 10.1039/b807220e
33	D. Russo et al., DOI: 10.1039/b807551b
34	D. Horinek, P. Jungwirth et al., DOI: 10.1039/b806432f
35	S. H. Kim et al., DOI: 10.1039/b810309g
36	K. Uosaki et al., DOI: 10.1039/b807297n
37	L. B. F. Juurlink et al., DOI: 10.1039/b808219g
38	I. Bakóet al., DOI: 10.1039/b808326f

---