Water is, perhaps, the most important material known to humankind—fascinating even in its pure state for the range of anomalous properties it displays. There has been an increasing realization that understanding the behavior of water at interfaces—from those of small solutes to biomolecules and polymers to inorganic materials and metals—holds the key to understanding disparate phenomena, from self-assembly, biofouling, and catalysis to corrosion. In this issue of MRS Bulletin, we highlight recent advances in understanding the molecular behavior of water near a range of interfaces of interest to the broader materials community.

The solid–water interface is ubiquitous in natural and synthetic systems as the primary site for chemical reactions under near-ambient conditions. Examples include the interactions of contaminants with mineral–water interfaces in natural environments, electrochemical reactions at the electrode-electrolyte interface relevant to energy storage (e.g., ion adsorption/electrical double layer formation, ion insertion), and oxidation of structural materials (e.g., rust). Yet many of these phenomena remain largely mysterious at a mechanistic level. The x-ray reflectivity technique, using highly penetrating hard x-rays, directly probes the solid–water interfaces through in situ studies. This approach has provided new insights into the molecular-scale structures and processes that occur at these “wet” interfaces. In this article, we review recent advances in the understanding of these systems, focusing specifically on the organization of interfacial “hydration layers” and the important role of adsorbed ions at charged solid–liquid interfaces.

Interfacial water is believed to determine practical outcomes in systems of interest to biology, materials science, geology, and many other disciplines. In this article, recent progress in understanding interfacial water achieved using molecular simulations is reviewed. After the reliability of recent approaches is discussed, three possible research directions are described. These future developments promise to have a large impact on both fundamental science and applications of societal importance.

Nanoparticle (NP) surface properties define how engineered nanomaterials interact with proteins. In aqueous systems, these interactions are driven by the binding of water to NPs and proteins. Understanding the true nature of this NP–water interface and its involvement in the properties of nanomaterials is a fundamental challenge in nanotechnology. Here, we review recent studies on the involvement of water molecules in the interaction of NPs with proteins. We first address the thermodynamic aspects of the NP–protein interaction and the means by which solvation shells can alter the nature of this phenomenon. We then discuss how the chemical nature of the NP surface affects the adsorption of water molecules and how this adsorption can either favor or inhibit protein–NP interactions.

Clay minerals are layered magnesium or aluminum silicates, which are abundant in the earth’s crust. Used since ancient times for the fabrication of bricks or terracotta, they now find application in the pharmaceutical and plastics industries. They play an essential role in oil and gas recovery, in water availability, and in preventing the dissemination of pollutants. In all of these contexts, the relevant properties of clay minerals are intimately linked to their microscopic structure, which results in a rich behavior with respect to water, solutes, and other fluids. This article provides a brief overview of the structure, dynamics, thermodynamics, and reactivity of water in clays, highlighting the role of the various types of water–mineral interfaces. Based on recent experimental and simulation studies, we discuss several features of these interfacial materials arising from their interactions with water on the molecular scale, including swelling, wetting, hydrodynamics in clay nanopores, reactivity of clay edge sites, ion exchange, and sorption.

The study of water’s functional role on hydrophilic surfaces of biological and soft materials benefits from experimental measurements that operate at finer length and time scales than contact-angle-measurement-derived surface wettability. We show that local translational diffusivity permits the empirical determination of local hydrophilicity by means of Overhauser dynamic nuclear polarization (ODNP) amplified 1H NMR relaxometry. Large unilamellar vesicles (LUVs) in dilute bulk water solution serve as good model hydrophilic surfaces, and their surface water diffusion was shown to be partially or entirely decoupled from the bulk water viscosity, with the extent of decoupling dependent on the surface activity of the particular viscogen chosen. The effective hydrophilicity of a hydrated LUV surface in solution was further shown to be tunable by specific ions or osmolytes dissolved in solution, and the chemical makeup of a surface can give rise to a heterogeneous hydration dynamics landscape, as demonstrated on protein surfaces. ODNP-derived surface water diffusivity is suggested to be a unique tool for the site-specific mapping of the interaction landscape of a wide range of functional materials operating in aqueous solution, from wet adhesives to fuel cell membranes.

Particles ranging in size from a few nanometers to tens of micrometers have a strong tendency to adsorb at interfaces between two immiscible fluids (e.g., water and oil or air). The driving force for this strong interfacial attachment is a reduction in interfacial area, and thus, interfacial energy. To design and engineer the structure and properties of materials constructed by such colloidal systems, it is imperative to understand the behavior of particles at fluid interfaces at the single-particle level and to establish the relationship between the microscopic behavior of interfacial particles and the bulk properties of particle-laden interfaces. In this article, we present background information on the behavior of particles at fluid–fluid interfaces and highlight recent advances in understanding the effects of particle shape and surface wettability on the behavior of particles at the interfaces. We also discuss recent advances in using interfacial attachment to direct the assembly of nanomaterials to create hierarchical structures with designed properties.

Thin-film electrocaloric and pyroelectric sources for electrothermal energy interconversion have recently emerged as viable means for primary and auxiliary solid-state cooling and power generation. Two significant advances have facilitated this development: (1) the formation of high-quality polymeric and ceramic thin films with figures of merit that project system-level performance as a large percentage of Carnot efficiency and (2) the ability of these newer materials to support larger electric fields, thereby permitting operation at higher voltages. This makes the power electronic architectures more favorable for thermal to electric energy interconversion. Current research targets to adequately address commercial device needs including reduction of parasitic losses, increases in mechanical robustness, and the ability to form nearly freestanding elements with thicknesses in the range of 1–10 μm. This article describes the current state-of-the-art materials, thermodynamic cycles, and device losses and points toward potential lines of research that would lead to substantially better figures of merit for electrothermal energy interconversion.