This special topic on the chemical physics of interfacial and confined water contains a collection of original research papers that showcase recent theoretical and experimental advances in the field. These papers provide a timely discussion of fundamental aspects of interfacial and confined water that are important in both natural environments and engineered applications.
Interfacial and confined water are pervasive in nature and important in both natural and engineered environments. The effects of interfaces and nanoconfinement on the structure, dynamics, thermodynamics, phase transitions, and reactivity of water and ions in water are of fundamental scientific importance, and have far reaching implications in areas as diverse as atmospheric chemistry, cryopreservation, electrochemistry and energy storage, the development of new materials, and the operation of nanodevices. This collection of 35 invited papers from leaders in the field represents experimental, theoretical, and computational advances that tackle problems of fundamental importance for interfacial and confined water. These advances define the current state-of-the-art and provide directions for future study.
Below we provide a brief overview of important topics and outstanding issues that define the field of interfacial and confined water. Our choice of topics and issues is biased by our personal research interests and, as such, should be taken as representative rather than comprehensive. In a similar vein, we refrain from providing a list of references to prior work since a fair list capturing the breadth and depth of this important field would be prohibitive in length. In order to capture the true vibrancy and excitement of this rapidly evolving and expansive area of scientific inquiry we encourage the reader to peruse the 35 papers contained in this special topic issue.
The defining characteristic of water organization is the preference for tetrahedral order that arises from hydrogen bonds. Hydrogen bond patterns fluctuate in the liquid state and at interfaces. When water meets a boundary, an interface, it faces a quandary: how to reorganize the hydrogen bond network to meet the constraints associated to the decreased symmetry and new interactions at the interface. The breaking of symmetry at an interface or in confinement can elicit the formation of novel structures without counterpart in bulk water. This breaking of symmetry also leads to distinct properties of water at interfaces and confined water. The differences can be as pronounced as the existence of completely different phases in nanoconfined water, and the attraction of ions of same charge and dramatic changes in acidity of the hydronium at the liquid-vapor interface. The effect of the reorganization of the hydrogen bond network in the dynamics of water, the properties of water as a solvent, the reactivity of water and solutes, however, are not fully understood.
Hydrogen bonds terminate abruptly at the liquid-vapor interface, the most studied of water interfaces. The dialogue between experiments and modeling has resulted in significant advances in this area in the last decades. Several questions, however, are still not fully settled. For example, what drives some ions to the interface, what is the length scale of the interactions between interfacial ions, whether the interface is rich in protons or hydroxide, and what is the reactivity of these interfacial ions. It is not known to which extent most solutes are fully or partially solvated at the interface and the implications this may have on their availability for chemical reactions. The interpretation of the rich vibrational spectroscopy of water at the liquid-vapor interface, the details of the intermolecular hydrogen bonding, is still highly controversial. The elucidation of the order, fluctuation and strength of hydrogen bonds of liquid water in contact with hydrophobic surfaces, and to which extent they mirror the behavior of water at the vapor interface, is also an area of current fruitful research.
The properties of the ice-liquid interface are of high relevance to environmental chemistry, cryopreservation, and for the prediction of ice crystallization rates. Nevertheless, it has been much less studied than the liquid-vapor interface. Even some of the most fundamental properties of the ice-liquid interface are not fully known. The liquid-ice surface free energy—key for the prediction of homogeneous ice nucleation rates—has been experimentally determined only at the melting temperature, and with very high uncertainty. Its temperature dependence has not yet been measured in experiments. Contrary to the liquid-vapor interface, it is not known whether ions or other molecules accumulate at the ice-liquid interface nor how do they affect the dynamics of this boundary and the rates of crystal growth. The importance of these properties for the prediction of the rates of crystallization of water, and the surge of interest in the study of interfacial properties of ice through simulations, suggest that this will become flourishing area of research in the next years.
Ice-gas interfaces are key for processes as diverse as snow chemistry and the formation of gas clathrate hydrates. When ice is exposed to vapor, gases, and most solid surfaces, it forms a disordered, premelted layer. There is significant interest on understanding the properties of this disordered layer as a solvent, as a medium for chemical reactions and assembly, and as a locus for nucleation of hydrates. Some outstanding questions are whether the disordered layer on ice resembles liquid water, amorphous ice or a disordered form of ice, to which extent is this disordered layer homogeneous in depth, and how do solutes affect its depth and dynamics. The dynamics of interfacial water and the mechanisms and time scales of molecular transport across the liquid-vapor, liquid-ice, and ice-vapor interface strongly impact the reactivity and uptake of volatile molecules by atmospheric water particles.
Water-solid interfaces are important for many areas of science and technology including corrosion, electrochemistry, electrolysis, geochemistry, and catalysis. From a fundamental perspective, these interfaces are interesting due to the complex ways in which the hydrogen bonding arrangement within the water network adapts to the presence of the symmetry-breaking two-dimensional interface, and they also provide insight on how water adapts to hydrophilic and hydrophobic solutes. Much progress has been made in recent years using cryogenic ultrahigh vacuum surface science techniques in combination with theory and molecular modeling. In many cases the structure of the adsorbed water monolayer at interfaces is counterintuitive because of a delicate balance of water-water and water-substrate interactions. The existence of several competing structures separated by small barriers can impart high susceptibility to water on some substrates. Furthermore, the stability of these structures for thicker films or in the presence of liquid water is yet unknown. The situation is even more challenging for the study of water at extended liquid and soft solid interfaces, where the dynamics of water couples to the one of the substrate. A central challenge arises from the difficulty to study these buried interfaces experimentally.
Confinement of water in nanopores, nanoslits, or nanodroplets is ubiquitous in nature and materials. Water molecules within one to three molecular diameters of an interface are affected by the existence of a boundary. Therefore, it is not surprising that interfaces can dominate the behavior of nanoconfined water. Water confined in nanoslits and in carbon nanotubes can form a wealth ordered structures. While some of the phases formed in nanoslits are quasi-two-dimensional analogs of three-dimensional phases of water, molecular simulations have predicted—and in several cases experiments have later confirmed—that confined water displays a rich landscape of novel structures that have no counterpart in bulk water: phases built entirely from regular and irregular pentagons, layered liquids, quasicrystalline order, and multiwalled ice nanotubes.
Extreme confinement impacts not only the structures that water makes but can also radically affect the nature of the phase transitions. For example, liquid-crystal critical points are forbidden in bulk systems, but have been suggested by simulations of nanoconfined water. Liquid-vapor phase equilibrium in extremely nanoconfined water does not occur through spatial coexistence of two phases in the nanoscopic volume but through oscillations between liquid and vapor as a function of time. The oscillatory nature of phase coexistence in extreme confinement should not be restricted to the liquid-vapor equilibrium, and may have applications for the control of transport in nanodevices.
Bulk water presents distinct thermodynamic, structural and dynamical anomalies, which are the most pronounced in supercooled water. The origin of these anomalies and the thermodynamics of deeply supercooled water are still highly debated. Nanoconfinement is extensively used to access liquid water in a cold state without the looming risk of crystallization, because confinement destabilizes the crystal phase. There is no agreement, however, on the significance of results obtained for extremely confined water to explain the thermodynamics of bulk water. In terms of the dynamics, it is well accepted that confinement results in distinct relaxation modes for water.
In summary, the structure, thermodynamics, and dynamics of interfacial and confined water are rich, varied, and often counterintuitive. While recent research has answered a number of fundamental questions, it has generated many more and often more interesting ones. These quandaries arising from the presence of boundaries exemplify our present understanding of this vibrant and dynamic field and help define directions for future studies. To fully capture the vibrancy and excitement of interfacial and confined water research we encourage the reader to peruse the 35 papers contained in this special topic issue.
We gratefully acknowledge several federal agencies for support. B.D.K. was funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, and V.M. was funded by the Army Research Laboratory through Cooperative Agreement No. W911NF-12-2-0023, and by the National Science Foundation through Award Nos. CHE-1125235, CHE-1309601, and CHE-1305427.
