1. What has motivated you to conduct this work? 

It has been suggested, based on x-ray absorption spectroscopy (XAS) experiments on liquid water (Wernet et al, Science 2004) that each water molecule, on average, has only one hydrogen bond donor and in turn accepts only one hydrogen bond. The larger implication of the XAS result is that the conventional view of water organizing as a four-fold tetrahedral coordinated random network is not true, and instead water organizes as hydrogen-bonded chains or large rings embedded in a weakly hydrogen-bonded disordered network. This is a radical departure from what is known about liquid water, which is thought to belong to the class of tetrahedral liquids such as silica and germanium. This alternative structural view potentially impacts previous interpretations of experimental and theoretical work on water, ice, tetrahedral and associated liquids, and educators who teach students about hydrogen-bonding of the world's most important liquid and chemical bonding in general. Given the importance of water as a solvent, there are also broad implications for biological molecules, the design of novel materials, and experimental probes that yield fundamental signatures of water as evidence of life on other planets. Given the broader scientific and educational context, radically alternative structural interpretations of liquid water need to be challenged. 

 

2. Could you explain the significance of your article to the non-specialist? 

A vast array of experimental data on water provides a global view of the liquid that implicates its tetrahedral hydrogen-bonding network as the unifying molecular connection to its observed structural, thermodynamic, and dielectric property trends with temperature. Anyone who advocates an alternative structural picture for liquid water must consider this other, non-structural, data. Although we firmly did not think it possible that chain networks could be consistent with these known liquid water trends with temperature, there is no existing evidence to directly refute such a possibility. Therefore we decided to examine the consequences of chain networks using three different modified water models that exhibit a local hydrogen-bonding environment of two hydrogen-bonds (2HB) and therefore networks of chains. Using these very differently parameterized models we evaluate their bulk densities, enthalpies of vaporization, heat capacities, isothermal compressibilities, thermal expansion coefficients, and dielectric constants, over the temperature range of 235K-323K. We also evaluate the entropy of the 2HB models at room temperature and whether such models nucleate ice Ih. All show poor agreement with experimentally measured thermodynamic and dielectric properties over the same temperature range, and behave similarly in most respects to normal liquids. This is to be contrasted by many modern simulation models of water that reproduce experimentally determined thermodynamic, dynamic, and dielectric trends with temperature. These models yield liquid structure that shows significant tetrahedral order and increased hydrogen-bonding than advocated by Wernet et al. Thus it appears that water structure based on hydrogen-bonded chains is inconsistent with liquid water as we know it through a multitude of experiments. 

 

3. Where do you see this work developing in the future? 

Water continues to elude us in the sense that we don't fully understand why or if this unusual and ubiquitous liquid is different from other normal fluids. However, our best organizing principle for understanding liquid water at present is that it is unique because it is a strongly associating fluid that organizes as a (defective) tetrahedral network due to its directional hydrogen-bonding. There is a broad range of experimental and theoretical studies that will continue to explore how or whether this underlying structural and energetic picture connects to water's thermodynamic and transport anomalies. Thus the structure of water, throughout the phase diagram, will remain a hot topic.

 

4. Are there any particular challenges facing future research in this area? 

An alternative structure for liquid water based on chain networks should certainly have been anticipated to be controversial, but seemed to go directly at the goal of overturning conventional wisdom. Similar scientific excitement must have existed in the early days of the discovery of "polywater", but eventually that alternative view proved to be false because the characterized water contained chemical impurities that ultimately explained polywater's unusual, and un-water-like, properties. It would seem that the challenge of knowing whether a new structural view of water is correct is to first try to reconcile whether it "fits" into a larger experimental context of other structural, thermodynamic, dynamical, and dielectric data collected over decades by many able scientists. Theory and simulation can more directly address "what is" the global view of the liquid and its phases, aiding the investigation of "what is not" liquid water.