Fundamental Investigation of the Water Structure-Properties Link

By Georgios M. Kontogeorgis, CERE, DTU Chemical Engineering

 Uncovering the exceptional physical, chemical and biological properties of water

Water is the most important substance in the world; it covers two thirds of the Earth and our own cells include two thirds water by volume. Hundreds of books have been written about water, yet we know so little about it. In the words of Philip Ball, consultant of Nature: “No one really understands water. It’s still a mystery” [1].

Water has over 50 exceptional properties; no other substance behaves like this. Among the most exciting ones are the maximum of density at 4 oC (ice floats, lakes freeze from top to bottom), high values of heat capacity (thus stabilizing Earth’s climate) and surface tension (small insects walk on water) and maxima and minima of many thermodynamic properties as function of temperature, e.g. the minimum hydrocarbon solubility in water at room temperature (related to the hydrophobic effect).  These properties have important implications for engineering, chemistry, biology and medicine. Yet, so far water research has been full of contradictory results [1-6].

What is the reason for these special water properties? Presumably hydrogen bonding in some form and especially the hydrogen bonding structure and its changes are the prevailing explanations. But which is the “correct” hydrogen bonding structure and how is it linked to the unusual properties of water?  Numerous theories for water structure have been presented but they are all up for debate. Figure 1 illustrates some of the problems. It is unclear whether liquid water maintains the tetrahedral structure as we know it from ice, or whether it is best described by a two-state model where most molecules are in the form of rings or chains and the literature is full of heated discussions.

Direct spectroscopic measurements could provide quantitative information on the degree of hydrogen bonding of water, but they are difficult to perform and interpret, and are not in agreement with recent statistical-thermodynamic theories and molecular simulation data. This is illustrated in figure 2.

Moreover, while I do believe that the hydrogen bonding is linked to molecular structure and properties, it is unclear exactly how. While the hydrogen bonding data shown in figure 2 do not present any maximum against the temperature, several properties of water (density, heat capacity and speed of sound) do. Could a competition between different water structures give rise to these extrema in some water properties? Better hydrogen bonding data are needed prior to addressing these questions. As “established” areas such as interpretation of “pure” water’s hydrogen bonding is clouded with so many questions, it is of little surprise that many more questions arise when looking at the interactions of water with other substances and in diverse environments as well as the associated implications. Some of these investigations have been the subject of scientific controversies.

It is necessary to bring together top scientists working with modern experimental and theoretical techniques, including those who have proposed potentially provocative theories for water structure in an attempt to establish clarity and new, seminal insights. It is our ambition to generate groundbreaking knowledge and understanding of phenomena in engineering, chemistry, biology and medicine by providing answers to fundamental questions about water’s structure and its link to properties and implications thereof.

An experimentally and theoretically consistent explanation of hydrogen bonding structure of water would be a significant breakthrough to the scientific community and enable major progress in several fields. For example, Pollack [8] has demonstrated that structured water builds up an electric potential, which can be exploited technologically.

Identifying liquid water’s hydrogen bonding structure under diverse conditions of varying complexity is of immense importance. To arrive to definite answers, we need to consider “conventional” and “provocative” theories, allow ideas to be challenged and synergies to arise, and combining a multi-level approach which includes experiments, theory and modeling and on diverse interconnected water-related areas.

 Reference list

1.     Ball, N., 2008. Water – an enduring mystery. Nature, 452, 291.

2.     Ball, N., 2003. How to keep dry in water. Nature, 423, 25.

3.     Wernet, Ph. et al., 2004. Science, 304, 995.

4.     Israelachvili, J.N., 2011. Intermolecular and surface forces. 3rd ed. Academic Press.

J.R. Errington, G.C. Boulougouris, I.G. Economou, A.Z. Panagiotopoulos and D.N. Theodorou, 1998. Molecular Simulation of Phase Equilibria for Water - Methane and Water - Ethane Mixtures, J. Phys. Chem. B, 102(44), 8865-8873.

5.     Kontogeorgis, G.M., Tsivintzelis, I., von Solms N., Grenner, A., Bogh, D, Frost, M., Knage-Rasmussen, A., Economou, I.G., 2010. Use of monomer fraction data in the parametrization of association theories. Fluid Phase Equilibria, 296(2): 219-229.

6.     Thøgersen, J., Jensen, S.K., Petersen, C., Keiding, S.R., 2008. Reorientation of hydroxide ions in water. Chem.Phys.Let., 466, 1.

7.     Liang, X.D., Maribo-Mogensen, B., Tsivintzelis, I., Kontogeorgis, G.M., 2016. A comment on water’s structure using monomer fraction data and theories. Fluid Phase Equilibria, 407, 2-6.

8.     Pollack, G.H., 2013. The fourth phase of water. Beyond Solid, Liquid, and Vapor. Ebner & Sons Publishers, Seattle WA, USA.