Editorial pubs.acs.org/CR
WaterThe Most Anomalous Liquid
W
However, the interpretation of the experimental results in terms of the structure and the picture of the liquid is strongly debated. Both Saykally and co-workers8 and Hämäläinen and co-workers9 examined the temperature dependence of the intensity of the pre-edge peak by XAS and suggested only minor distortions from the tetrahedral structure. Heske and co-workers10 suggested that one XES peak in the lone-pair region is due to emission from dissociated fragments and one from intact molecules. Theoretical simulations of X-ray spectra are exploring a multitude of approaches with no consensus on which approach is the most reliable. From a formal perspective of XAS within density functional theory (DFT), time-dependent DFT and the BetheSalpeter equation provide formally stringent approaches, but cannot be applied to large models due to computational cost. Similarly, quantum chemical approaches based on coupledcluster methods give high accuracy, but require further development to be applicable to realistic models. The Bethe− Salpeter equation reproduces the pre-edge structure in calculations of XAS using small models, but it places the postedge in the wrong position. Conversely, the more approximate transition potential method applied to larger models places the post-edge in the correct position but does not find significant preedge intensity. A calibration using common benchmark structures and further developments of techniques are sorely needed! Scattering techniques using neutrons and X-rays in the wideand small-angle regions can now be reliably performed with significantly enhanced statistics. Recent wide-angle X-ray diffraction to very high q-transfers (>18 Å−1) provides a benchmark oxygen−oxygen pair-distribution function first peak at a position and height very different from earlier standards to which most theoretical force-fields have been calibrated.11 Here it is important to establish what should be the benchmark for simulators to compare with. At small angles, X-ray diffraction provides information on density inhomogeneities due to fluctuations in the liquid with the intercept at q = 0 corresponding to the isothermal compressibility. Whether or not this can be interpreted in terms of structural and density inhomogeneities in real water and what spatial extent such inhomogeneities could correspond to is under debate. An important fact is that no simulation model, force-field or ab initio based, has been shown to reproduce the temperature dependence of the thermodynamical response functions described above. There is thus room for further developments. Ultrafast, nonlinear vibrational spectroscopy techniques are extremely important in obtaining information on the H-bond dynamics in bulk water, ionic solutions, hydrophobic solvation, and confinement. A commonly used technique to simplify the interpretation is to use isotope substitution with dilute HDO in either H2O or D2O. Very recently, however, it has been demonstrated that OH is preferentially exposed at the surface of HDO liquid, i.e. that OD H-bonds are stronger than OH H-
ater is the key compound for our existence on this planet and is involved in nearly all biological, geological, and chemical processes. Access to clean water will furthermore become one of the greatest challenges to mankind in the face of global climate change. Although water is the most common liquid, it is also one of the most unusual, with many peculiar properties, such as an increased density upon melting, decreased viscosity under pressure, density maximum at 4 °C, high surface tension, and many more. These anomalous properties of water become more pronounced in the supercooled region below the melting point. In particular, if the thermal expansion coefficient, isothermal compressibility, and heat capacity are extrapolated below the temperature (−38 °C) of homogeneous ice nucleation, they appear to diverge at a temperature of −45 °C. However, this regime is experimentally difficult to investigate due to the extremely rapid ice nucleation at this low temperature. Several scenarios have been proposed to account for the anomalies of water in the supercooled regime related to either a stability limit conjecture1 or to the presence of a second critical point associated with the coexistence curve separating two states of the liquid: low density and high density water.2 Alternatively, a singularity-free scenario based on a percolation picture has been suggested.3 Other two-state models of water have also been proposed based on the coexistence of low density−low entropy and high density−high entropy forms of water.4,5 Although the anomalies are extreme in the supercooled region, they are also present at ambient conditions, where most of water’s physical, chemical, and biological processes occur. If we want to understand the properties of water, it is essential that we can create a unifying picture of the structure of water at ambient conditions all the way up to the boiling point, as well as deep into the supercooled regime and in its various crystalline and glassy forms. Since the hydrogen bonds (H-bonds) in water fluctuate, it is essential not only to obtain a static picture but also to describe the dynamics, not only in terms of local excitations involving breaking and forming H-bonds, but also in the form of collective excitations giving density fluctuations. One of the most essential questions to address for a microscopic understanding of water is What are the structure and dynamics of the hydrogen bonding network in water that give rise to all these unique properties? This question has been discussed intensively for over 100 years and has not yet been resolved. In order to gain new unique information regarding the structure of the H-bond network in water, it is essential to develop new techniques, both experimental and theoretical. This requires a close synergy between theory and experiment, as well as cross-disciplinary understanding and thinking, which characterize the reviews in the present collection and which are prerequisites for the concerted effort toward a unifying picture of water. Recently, new experimental methods, namely X-ray spectroscopy involving electronic transitions between core and valence orbitals, e.g. X-ray absorption spectroscopy (XAS)6 and X-ray emission spectroscopy (XES),7 have been applied to liquid water and ice, resulting in a serious challenge to the traditional nearly tetrahedral picture of the H-bond network in liquid water. © 2016 American Chemical Society
Special Issue: Water - The Most Anomalous Liquid Published: July 13, 2016 7459
DOI: 10.1021/acs.chemrev.6b00363 Chem. Rev. 2016, 116, 7459−7462
Chemical Reviews
Editorial
bonds.12 A similar result has recently been obtained in participator decay in XES,13 which makes it imperative to reconsider conclusions based on isotope substituted HDO in either H2O or D2O. What information can be gained in light of these new experiments? As if pure water were not complicated enough, the presence of ions, solutes, and interfaces introduces additional conceptual difficulties, since they modulate water’s properties in ways that are even less well understood. Achieving a complete understanding of these many-component, many-body effects such as hydrophobicity or ion-specific adsorption at interfaces represents a severe challenge, making it essential to develop new and better methods of study. The availability of the LCLS free-electron X-ray laser at SLAC has opened up new experimental possibilities. Recent experiments of X-ray diffraction14 and XES15 of liquid water droplets at temperatures as low as 227 K provide new possibilities to discriminate between models of the liquid, but the new experiments still require interpretation. Furthermore, the recent development of accelerator-based neutron sources with either high flux or time resolution will open new avenues to improve the knowledge of the structure and dynamics of water. A concerted effort of experimentalists and theoreticians, as demonstrated in the present volume, is required to achieve the goal of a unified description of Nature’s most important liquid and how it achieves solvationboth ionic and hydrophobicand how this knowledge can ultimately be used to, for example, purify water in a world subject to global climate change. To assess the state-of-the-art and guide the research direction toward a unified and complete picture of water, the editors of this thematic issue brought together about one hundred members from across the water community to make an inventory of existing knowledge on what we agree, on what we disagree, and on what new research needs to be done, and to publish an overview of the findings. The meeting took place as a Nordita Program which ran over four weeks from 13 October to 7 November 2014. Nordita is the Nordic Institute for Theoretical Physics, which is now hosted by Stockholm University. The present program was named “Water: The Most Anomalous Liquid”, and it comprised three weeks of workshops focusing on a range of topics and a conference highlighting the main issues. The workshop weeks were organized into four working groups consisting of 5−10 people each, with participants giving presentations in the morning and discussing and writing in the afternoon. Their efforts became the articles in this thematic issue. The first week focused on the thermodynamics of water and simulations of water across its complex phase diagram. Here Gallo et al. address the challenging task of describing the thermodynamics of water in connection with the several models that have been proposed. The challenge originates from the experimental difficulties to explore the region of the phase space, the so-called “no-man’s land”, where response functions seem to diverge. Thus, this review not only encompasses the thermodynamical models, with a focus on the two-state picture of water as fluctuating between two forms of local structures, but also discusses the difficulties that must be overcome to finally determine outstanding issues. This includes the nucleation of ice and ways to inhibit ice formation, such as water in confinement and salt solutions with a look also to the dynamics under different conditions. Furthermore, the interesting region of the phase diagram at negative pressure, where water can be said to be stretched, is explored, since additional clues to the origin of water’s unique properties can potentially be found there.
Simulations of water and the ices provide ever more reliable information through the ongoing development of force-fields and ab initio techniques as well as simulation methods. Paesani et al. describe recent developments of force-fields and techniques to generate highly accurate models. The narrative builds on a manybody expansion of the interaction and discusses how the different contributions can be obtained and reproduced. They give a comprehensive summary of presently available force-fields and focus on two exciting new approaches, the MB-pol16−18 and SCME19 with Gaussian Approximation Potentials.20 In the final review from the first week of the program, Ceriotti et al. describe and discuss nuclear quantum effects in water with a special focus on the principle of competing quantum effects with which the interplay of water’s quantum effects and their manifestation in experimental observables can be understood. Recent years have seen significant methods development when it comes to including nuclear quantum effects in simulations of liquid water and solutions. These developments are described and discussed in the review. Ab initio simulations of water were also part of the first week of the program but did not result in a review in the present volume. For completeness, we instead refer the interested reader to the recent review by Gillan et al.21 for a discussion of density functional theory (DFT) and simulations of water. In order to probe water experimentally, it is essential to obtain a fundamental understanding of the techniques involved and their theoretical description that assist in the interpretation of the structure and dynamics. This was the focus of the second working week, and here we have four reviews covering different areas that have attracted some recent attention in probing water. Fransson et al. connect together all the electronic structure methods using X-rays in terms of photoelectron spectroscopy, Xray absorption spectroscopy, X-ray emission spectroscopy, resonant inelastic X-ray scattering, and X-ray Raman scattering and our current knowledge about the information content in these techniques when applied to water and ice. Amann-Winkel et al. describe the progress in X-ray and neutron scattering of water and amorphous ices at various temperatures and pressures to derive the pair-correlation functions. The importance of dynamical studies using quasi-elastic and inelastic scattering is elucidated and also compared to other techniques. Perakis et al. present recent static and time-resolved vibrational spectroscopic studies of liquid water from ambient conditions to the supercooled state, as well as of crystalline and amorphous ice forms. Various techniques are described, such as conventional infrared and Raman spectroscopy, femtosecond pump−probe, photon-echo, optical Kerr effect, sum-frequency generation, and two-dimensional infrared spectroscopic studies. Additionally, novel approaches are discussed, such as two-dimensional sum frequency generation, three-dimensional infrared, and 2D Raman-THz spectroscopy. Cerveny et al. discuss water in confined geometries with a focus on measurements in the deeply supercooled regime where bulk water undergoes rapid ice formation. There are many different interpretations of the experimental data, and the authors describe and discuss the three main directions proposed in the literature. Water’s anomalous behavior manifests further in its interaction with ions, other molecules, and interfaces. Consequently, the scope of research on aqueous solutions is almost without limit. In the third working week, we examined four representative topics to explore this complexity. Water’s dipolar nature makes it an excellent solvent for many ionic compounds, in which water−ion interactions often out-compete the ion−ion interactions. Van der 7460
DOI: 10.1021/acs.chemrev.6b00363 Chem. Rev. 2016, 116, 7459−7462
Chemical Reviews
Editorial
Vegt et al. explain the intricate techniques that are required to pick out the many possible types of solution structure, including water-mediated or water-shared ion pairs, larger ion clusters, and ions binding to interfaces. Agmon et al. examine the nature of two special types of ion intrinsic to water, namely the hydronium and hydroxide ion. These familiar but ephemeral species are by no means as simple as a water molecule plus or minus a proton, and their structure and dynamics, like that of water, defy a simple explanation. Much is still to be done in reconciling the findings from many different methods, and significant challenges remain in understanding the behavior of these ions at interfaces and in confined environments such as ion channels. The complexity of water is taken to a higher level by Bellissent-Funel et al. for the case of biomolecular hydration. They examine how a host of techniques are improving our ability to understand the delicate interplay of interactions whereby water and proteins influence each other’s stability and dynamics. Finally, Björneholm et al. consider the nature of water at the many kinds of interfaces found on a global scale at the boundaries between earth, oceans, and the atmosphere. The difficulty of selectively probing interfacial water is compounded by the sensitivity of water’s properties to different types of surfaces, the variability of the surfaces, as well as water’s innate anomalies. The present collection of reviews represents a strong community effort, with close to 90 authors getting together to summarize the field. As such, we believe it a unique effort, and we as editors appreciate very much the hard work that has gone into these reviews, involving different perspectives and views on each subtopic. Sadly, in the final preparations of the present thematic issue, one of our contributors, Charusita Chakravarty, passed away. We dedicate this thematic issue to her memory.
Lars G. M. Pettersson was born in Norrköping, Sweden, in 1951 and received a B.Sc. in physics (1976) and a Ph.D. in theoretical physics (1984) from Stockholm University, Sweden, where he focused on highly accurate quantum chemical calculations as well as developing approximate modeling techniques. He did postdoctoral studies (1984−1986) in California at IBM, San Jose, and NASA Ames Research Center. He returned to Stockholm University, where he is currently professor in theoretical chemical physics. His research focus lies in quantum chemical modeling of processes at surfaces, and theoretical treatment and application of inner-shell spectroscopies, but the main effort is currently devoted to understanding the structure and dynamics of water and aqueous solutions.
Richard H. Henchman received his B.Sc. Honours degree from the University of Sydney in 1996 and his Ph.D. from the University of Southampton in 2000, followed by postdoctoral research at the University of California, San Diego and Howard Hughes Medical Institute from 2000−2004. From there he moved to a Lecturer position at the University of Manchester, later being promoted to Senior Lecturer. He develops theory to determine the structure and entropy of multimolecular systems from computer simulation. This has helped him discern a number of aspects concerning the structure and dynamics of water and aqueous solutions.
Lars Gunnar Moody Pettersson Stockholm University, AlbaNova University Center
Richard Humfry Henchman The University of Manchester
Anders Nilsson* Stockholm University, AlbaNova University Center
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. Biographies
Anders Nilsson received a Ph.D. in physics at Uppsala University, Sweden (1989), in the laboratory created by Kai Siegbahn. He is currently professor in Chemical Physics at Stockholm University. He received the Lindbomska Award at the Swedish Royal Academy of Science, the Royal Oscar Award at Uppsala University in 1994, the Shirley Award in Berkeley, 1998, and the Alexander von Humboldt Senior Scientist Award in 2010, and was awarded an honorary doctorate at Technical University of Denmark in 2015. His research interests include synchrotron radiation and X-ray laser spectroscopy and scattering, chemical bonding and reactions on surfaces, ultrafast science heterogeneous catalysis, electrocatalysis in fuel cells, photocatalysis for 7461
DOI: 10.1021/acs.chemrev.6b00363 Chem. Rev. 2016, 116, 7459−7462
Chemical Reviews
Editorial
Flexible Monomers. III: Liquid Phase Properties. J. Chem. Theory Comput. 2014, 10, 2906−2910. (19) Wikfeldt, K. T.; Batista, E. R.; Vila, F. D.; Jonsson, H. A transferable H2O Interaction Potential Based on a Single Center Multipole Expansion: SCME. Phys. Chem. Chem. Phys. 2013, 15, 16542−16556. (20) Bartok, A. P.; Payne, M. C.; Kondor, R.; Csanyi, G. Gaussian Approximation Potentials: The Accuracy of Quantum Mechanics, Without the Electrons. Phys. Rev. Lett. 2010, 104, 136403. (21) Gillan, M. J.; Alfè, D.; Michaelides, A. Perspective: How Good is DFT for Water? J. Chem. Phys. 2016, 144, 130901.
converting sunlight to fuels, and structure of water and aqueous solutions.
REFERENCES (1) Speedy, R. J. Stability-limit conjecture. An Interpretation of the Properties of Water. J. Phys. Chem. 1982, 86, 982. (2) Poole, P. H.; Sciortino, F.; Essmann, U.; Stanley, H. E. PhaseBehavior of Metastable Water. Nature 1992, 360, 324−328. (3) Sastry, S.; Debenedetti, P. G.; Sciortino, F.; Stanley, H. E. Singularity-Free Interpretation of the Thermodynamics of Supercooled Water. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1996, 53, 6144−6154. (4) Tanaka, H. Simple Physical Model of Liquid Water. J. Chem. Phys. 2000, 112, 799−809. (5) Holten, V.; Anisimov, M. A. Entropy-Driven Liquid−Liquid Separation in Supercooled Water. Sci. Rep. 2012, 2, 713. (6) Wernet, P.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Näslund, L. Å.; Hirsch, T. K.; Ojamäe, L.; Glatzel, P.; et al. The Structure of the First Coordination Shell in Liquid Water. Science 2004, 304, 995−999. (7) Tokushima, T.; Harada, Y.; Takahashi, O.; Senba, Y.; Ohashi, H.; Pettersson, L. G. M.; Nilsson, A.; Shin, S. High Resolution X-ray Emission Spectroscopy of Liquid Water: The Observation of Two Structural Motifs. Chem. Phys. Lett. 2008, 460, 387−400. (8) Smith, J. D.; Cappa, C. D.; Wilson, K. R.; Messer, B. M.; Cohen, R. C.; Saykally, R. J. Energetics of Hydrogen Bond Rearrangements in Liquid Water. Science 2004, 306, 851. (9) Pylkkänen, T.; Sakko, A.; Hakala, M.; Hämäläinen, K.; Monaco, G.; Huotari, S. Temperature Dependence of the Near-Edge Spectrum of Water. J. Phys. Chem. B 2011, 115, 14544−14550. (10) Fuchs, O.; Zharnikov, M.; Weinhardt, L.; Blum, M.; Weigand, M.; Zubavichus, Y.; Bär, M.; Maier, F.; Denlinger, J. D.; Heske, C.; et al. Isotope and Temperature Effects in Liquid Water Probed by X-ray Absorption and Resonant X-ray Emission Spectroscopy. Phys. Rev. Lett. 2008, 100, 027801. (11) Skinner, L. B.; Huang, C.; Schlesinger, D.; Pettersson, L. G. M.; Nilsson, A.; Benmore, C. J. Benchmark Oxygen-Oxygen PairDistribution Function of Ambient Water from X-ray Diffraction Measurements with a Wide Q-Range. J. Chem. Phys. 2013, 138, 074506. (12) Liu, J.; Andino, R. S.; Miller, C. M.; Chen, X.; Wilkins, D. M.; Ceriotti, M.; Manolopoulos, D. E. A Surface-Specific Isotope Effect in Mixtures of Light and Heavy Water. J. Phys. Chem. C 2013, 117, 2944− 2951. (13) Harada, Y.; Tokushima, T.; Horikawa, Y.; Takahashi, O.; Niwa, H.; Kobayashi, M.; Oshima, M.; Senba, Y.; Ohashi, H.; Wikfeldt, K. T.; et al. Selective Probing of the OH or OD Stretch Vibration in Liquid Water Using Resonant Inelastic Soft X-Ray Scattering. Phys. Rev. Lett. 2013, 111, 193001. (14) Sellberg, J. A.; Huang, C.; McQueen, T. A.; Loh, N. D.; Laksmono, H.; Schlesinger, D.; Sierra, R. G.; Nordlund, D.; Hampton, C. Y.; Starodub, D.; et al. Ultrafast X-ray Probing of Water Structure below the Homogeneous Ice Nucleation Temperature. Nature 2014, 510, 381−384. (15) Schreck, S.; Beye, M.; Sellberg, J. A.; McQueen, T. A.; Laksmono, H.; DePonte, D.; Kennedy, B.; Eckert, S.; Schlesinger, D.; Nordlund, D.; et al. Reabsorption of Soft X-ray Emission at High X-ray Free-Electron Laser Fluences. Phys. Rev. Lett. 2014, 113, 153002. (16) Babin, V.; Leforestier, C.; Paesani, F. Development of a First Principles Water Potential with Flexible Monomers: Dimer Potential Energy Surface, VRT Spectrum, and Second Virial Coefficient. J. Chem. Theory Comput. 2013, 9, 5395−5403. (17) Babin, V.; Medders, G. R.; Paesani, F. V.; Babin, G. R.; Medders, F. Paesani, “Development of a First Principles Water Potential with Flexible Monomers. II: Trimer Potential Energy Surface, Third Virial Coefficient, and Small Clusters. J. Chem. Theory Comput. 2014, 10, 1599−1607. (18) Babin, V.; Medders, G. R.; Paesani, F. V.; Babin, G. R.; Medders, F. Paesani, “Development of a First Principles Water Potential with 7462
DOI: 10.1021/acs.chemrev.6b00363 Chem. Rev. 2016, 116, 7459−7462
