Water, solid–water interfaces, and “interfacial water”

Water, the most abundant substance on Earth, has unusual properties. As a solid, water ice most commonly takes a hexagonal structure with a lateral lattice spacing of 4.52 Å, an interlayer spacing of 3.64 Å, and an oxygen–oxygen distance of 2.75 Å.^{Reference Lonsdale1} Under ambient conditions, water retains a disordered tetrahedral network (Figure 1) that is stabilized by hydrogen bonds (with an energy of ∼21 kJ/mol, 0.22 eV, or ∼9 k _BT per bond).^{Reference Franks2,Reference Symons3} This interaction is largely responsible for the properties of liquid water, including the average O–O distance of 2.8 Å and its relatively high liquid density (1 g/cm³ at 4°C, with a corresponding number density of ρ_w = 1 H₂O/30.3 ų). The distance over which water retains a memory of perturbations (i.e., its correlation length) is ∼4 Å.^{Reference Xie, Ludwig, Morales, Hare and Sorensen4} Water also is a “universal solvent” and is a necessary ingredient for all living things. Because water is a polar molecule, it solvates charged species through its strong dipolar interaction, and the hydration energies of ions can be estimated based on the ion’s charge and effective size. This tendency of water to form “solvation shells” around ions is a key control over the nature and rates of chemical reactions in aqueous solutions.^{Reference Richens5}

Figure 1.

Schematic illustration of the transition from tetrahedrally ordered bulk water (with a number density of 33 H₂O/nm³) to interfacial water ordered near the solid–water interface (with an areal density of ∼10 H₂O/nm²).

A primary characteristic of any solid surface is that atoms at the surface are generally undercoordinated with respect to those in the bulk.^{Reference Zangwill6} The greater availability of atoms at the interface for reaction (i.e., without the need for solid-state diffusion of reactants) makes interfaces more chemically reactive than solid materials. Most interfaces that are stable in ambient environments are in the form of oxides, minerals, organics, or other materials having a relatively low surface energy. They consist of functional groups whose properties are controlled by the interaction of the undercoordinated surface lattice atoms with water and dissolved species. A common example is the acid-base chemistry found at oxide surfaces, leading to a pH-dependent surface charge as a result of its equilibration with water leading to a mixture of −OH₂⁺, −OH, and −O^– surface functional groups versus pH.^{Reference Stumm7}

From these simple considerations, one can anticipate many of the properties of water in the vicinity of an interface (i.e., “interfacial water,” Figure 1) that are analogous to the hydration layers formed around dissolved ions. The structure of interfacial water is typically distinct from bulk water. The interaction of water with the surface functional groups (e.g., through hydrogen bonds to oxide surfaces) stabilizes the interfacial hydration structure (e.g., with a specific orientation of the water dipole moment) with respect to the hydrogen-bonding structure that occurs in bulk water. This interfacial hydration structure is also controlled by a balance between the lateral arrangement of the surface functional groups exposed at the solid surface with the natural lateral spacing of a dense water layer (i.e., its “epitaxy”). This can be characterized by comparing the size of the solid surface unit cell with the natural two-dimensional (2D) packing density of water, ρ_2D, estimated through the dimensional relation ρ_2D = ρ_w^2/3 ≈ 1 H₂O/10 Ų. Finally, the vertical organization of any hydration layer can be expected to be limited in extent to a few water correlation lengths,^{Reference Xie, Ludwig, Morales, Hare and Sorensen4} corresponding to distances of ∼1–2 nm from the surface.

Observing the solid–water interface structure with x-rays

Relatively few methods are available by which solid–water interfaces can be understood through direct observation. This is because most “surface science” probes use electrons or ions that are unable to penetrate through the liquid phase. Of the possible surface- and interface-sensitive tools, only scanning probe microscopies^{Reference Gratz and Bird8} and photon- or neutron-based probes^{Reference Robinson and Tweet9–Reference Ostroverkhov, Waychunas and Shen14} can interrogate the solid–liquid interfaces directly through in situ observations. Of these, x-ray based tools have the capability to probe the interfacial hydration structure with Ångström-scale resolution.

Scattering of x-rays from well-defined interfaces (Figure 2), referred to generally as x-ray reflectivity (XR), including both specular and nonspecular scattering, provides what is arguably the most direct and quantitative method for probing the structure (i.e., atom locations) of the solid–water interface. XR of interfaces is the direct analogue of x-ray crystallography and has been applied to understand a wide range of solid–water interfaces.^{Reference Wang, Ocko, Davenport and Isaacs15–Reference Zhou, Ganesh, Presser, Wander, Fenter, Kent, Jiang, Chialvo, McDonough, Shuford and Gogotsi29} The precise time-averaged locations of atoms at the solid–liquid interface within both the solid and water phases can be observed through direct in situ measurements of the intensity and distribution of the scattered x-rays.

Figure 2.

Schematic of the x-ray reflectivity measurement, with the x-ray beam reflecting from the surface with a total scattering angle of 2θ. For specular reflectivity measurements, the momentum transfer, Q_z, is oriented along the surface normal direction and reveals the laterally averaged electron density, ρ(z).

The power of XR to probe the solid–liquid interface derives from the simple and robust relationship between the scattering intensity, expressed as the fraction of the incident beam that is reflected, R(Q), and the unknown structure, ρ(r), expressed as the electron density of the interface as a function of position, r, with respect to the origin at the interface. For the case of specular (i.e., mirror-like scattering, |Q| = Q_z):

(1)$$R{\rm{(}}{{\rm{Q}}_{\rm{z}}}{\rm{)\ = \ (4\pi }}{r_{\rm{e}}}{\rm{/}}{A_{{\rm{uc}}}}{{\rm{Q}}_{\rm{z}}}{{\rm{)}}^{\rm{2}}}{\rm{\ |}}\int {\rm{\rho }} {\rm{(z)\ exp(i}}{{\rm{Q}}_{\rm{z}}}{\rm{z)\ dz}}{{\rm{|}}^{\rm{2}}} ,$$

where |Q| = (4π/λ)sin(2θ/2) is the momentum transfer, λ is the x-ray wavelength, 2θ is the total scattering angle, A _uc is the surface unit cell area, r _e = 2.828 × 10^–5 Å is the classical electron radius, and z is the height above the interface. As in any crystallographic approach, the ability to know the structure is limited primarily by the crystallographic “phase problem” (that is, while the reflectivity signal can be calculated directly from the electron density, as shown in Equation 1, it is not generally possible to obtain the electron density directly from the measured reflectivity signal).^{Reference Robinson and Tweet9} Most commonly, the data can be understood by quantitative comparison to calculated intensities based on parameterized models of the interfacial structure. The models must include all relevant aspects of the interfacial structure, including intrinsic factors (i.e., the interfacial water organization), changes to the substrate interfacial structure (including the surface termination, substrate structural relaxations allowed by the broken symmetry associated with the presence of the surface, and reconstructions due to spontaneous symmetry changes), the presence of any adsorbed layers (e.g., ions at charged surfaces), as well as any extrinsic factors such as surface roughness. The field of view of these measurements is defined within the surface plane by the surface lattice spacings, a and b, and is sensitive to the complete density profile along the surface normal direction for heights in the range of –∞ < z < ∞ (including the continuous water profile above the surface, and any changes to the substrate crystal below the interface).

Hydration layers at the solid–water interface

The behavior of water adjacent to various solid surfaces^{Reference Fenter and Sturchio30} has been well-studied using x-ray scattering approaches, including metal–aqueous electrochemistry,^{Reference Wang, Ocko, Davenport and Isaacs15,Reference Toney, Howard, Richer, Borges, Gordon, Melroy, Wiesler, Yee and Sorensen16} crystal growth,^{Reference Reedijk, Arsic, Hollander, de Vries and Vlieg17} biomineralization,^{Reference Arsic, Kaminski, Poodt and Vlieg18} metal oxides,^{Reference Eng, Trainor, Brown, Waychunas, Newville, Sutton and Rivers19–Reference Zhang, Fenter, Sturchio, Bedzyk, Machesky and Wesolowski22} natural minerals,^{Reference Cheng, Fenter, Nagy, Schlegel and Sturchio23–Reference Fenter and Sturchio25} and hydrophobic interfaces.^{Reference Mezger, Schöder, Reichert, Schröder, Okasinski, Honkimäki, Ralston, Bilgram, Roth and Dosch26–Reference Zhou, Ganesh, Presser, Wander, Fenter, Kent, Jiang, Chialvo, McDonough, Shuford and Gogotsi29}

In this article, we present a few representative examples that provide insights into the range of interfacial hydration structure profiles that can be observed for surfaces with distinct chemical properties and functionalities (Figure 3). These include the (001) plane of muscovite mica, K₂Al₄(Al₂Si₆)O₂₀(OH)₄, a planar surface that has a fixed charge;^{Reference Cheng, Fenter, Nagy, Schlegel and Sturchio23} the (001) plane of graphene, a 2D carbon structure that is uncharged and chemically inert;^{Reference Zhou, Ganesh, Presser, Wander, Fenter, Kent, Jiang, Chialvo, McDonough, Shuford and Gogotsi29} the (110) plane of rutile TiO₂, an oxide surface with chemically reactive surface functional groups and a pH-dependent surface charge;^{Reference Zhang, Fenter, Cheng, Sturchio, Bedzyk, Predota, Bandura, Kubicki, Lvov, Cummings, Chialvo, Ridley, Benezeth, Anovitz, Palmer, Machesky and Wesolowski21,Reference Zhang, Fenter, Sturchio, Bedzyk, Machesky and Wesolowski22} and the (104) plane of calcite, CaCO₃, the major cleavage plane of this ionic crystal.^{Reference Fenter, Kerisit, Raiteri and Gale24,Reference Fenter and Sturchio25,Reference Geissbühler, Fenter, DiMasi, Srajer, Sorensen and Sturchio31}

Figure 3.

Structural schematics and measured hydration structures observed at the solid–water interface for (a) muscovite mica (001),^{Reference Cheng, Fenter, Nagy, Schlegel and Sturchio23} (b) epitaxial graphene (001) (for both the G₀ and G_n layers (n = 1, 2, 3, …) (the inset shows water contact angles for [top] G₀- and [bottom] G_n-dominant surfaces),^{Reference Zhou, Ganesh, Presser, Wander, Fenter, Kent, Jiang, Chialvo, McDonough, Shuford and Gogotsi29} (c) calcite CaCO₃(104),^{Reference Fenter, Kerisit, Raiteri and Gale24} and (d) rutile TiO₂(110).^{Reference Zhang, Fenter, Sturchio, Bedzyk, Machesky and Wesolowski22} In all cases, the height origin is chosen as the average location of the surface functional groups.

Influence of adsorbed ions at charged solid–water interfaces

The preceding discussion is focused on the behavior of nominally pure water at various types of interfaces. When the interfaces are charged (e.g., the basal surface of muscovite mica), the formation of an electrical double layer neutralizes the surface, through either the specific adsorption of ions or the formation of a diffuse ion profile.^{Reference Stumm7} The impact of these adsorbed ions on the interfacial water structure is illustrated in Figure 4, where the interfacial electron density profiles of muscovite in contact with water containing various salts (including Na⁺ versus Cs⁺, and Zn²⁺ versus Ba²⁺)^{Reference Lee, Fenter, Nagy and Sturchio38,Reference Lee, Fenter, Park, Sturchio and Nagy39} are shown. In the case of monovalent ions (Figure 4a), there is an extended hydration structure in the presence of Na⁺, including approximately four distinct layers extending ∼1 nm into solution (the location of Na⁺ in this system is not known). This modulation is not observed in the presence of a different monovalent ion, Cs⁺ (here, the high-density peak at 2.3 Å is due to specific adsorption of Cs⁺). More dramatic changes are observed in the presence of divalent ions (Figure 4b), including multiple distinct hydration layers. Even when considering the contribution of each ion to the electron density profiles (shaded regions), it is apparent that the interfacial hydration structures for these systems are distinct, and these differences are controlled, at least in part, by the ion properties (e.g., charge, ion size, and hydration energy).

Figure 4.

Measured interfacial electron density profiles of the muscovite–water interface in contact with aqueous electrolyte solutions. The presence of different ions, for example, (a) Na versus Cs^{Reference Lee, Fenter, Nagy and Sturchio38} and (b) Zn versus Ba,^{Reference Lee, Fenter, Park, Sturchio and Nagy39} leads to substantial changes in the observed interfacial hydration structure. The shaded regions in (b) indicate the electron-density profiles of Zn, measured by using resonant anomalous x-ray reflectivity, and Ba, calculated using a space-filling constraint, respectively.^{Reference Lee, Fenter, Park, Sturchio and Nagy39}

Summary and perspective

The examples presented here illustrate how the relationships between solid–water interactions and the observed interfacial water structure can be revealed through the use of high-resolution synchrotron x-ray reflectivity measurements. These results, when complemented by high-level computational studies (e.g., density functional theory and molecular dynamics simulations), provide a robust approach to test and extend the current understanding of these systems.^{Reference Fenter, Kerisit, Raiteri and Gale24,Reference Zhou, Ganesh, Presser, Wander, Fenter, Kent, Jiang, Chialvo, McDonough, Shuford and Gogotsi29,Reference Wesolowski, Sofo, Bandura, Zhang, Mamontov, Předota, Kumar, Kubicki, Kent, Vlcek, Machesky, Fenter, Cummings, Anovitz, Skelton and Rosenqvist37,Reference Bandura, Sykes, Shapovalov, Troung, Kubicki and Evarestov40–Reference Park and Sposito42} Although the experimental results clearly indicate that water is ordered vertically and laterally near the solid–water interface, the ordered structures are not well-described as one of the known water ice structures (i.e., they are not “ice-like”).^{Reference Du, Freysz and Shen13} Instead, the observed hydration structures are dictated by the characteristics of the solid surface (e.g., the arrangement and coordination of atoms and the charge on the surface) and are observed to be sensitive to the adsorption of ions. This represents an important first step toward understanding the reactivity of solid–water interfaces, including how it influences the adsorption of ions, the heterogeneous growth of films, and the reactivity of the substrate solid (including dissolution and growth). More generally, it provides a basis for understanding the challenging question of how water structure is perturbed by confinement, as is now being studied.^{Reference Chodankar, Perret, Nygård, Bunk, Satapathy, Espinosa Marzal, Balmer, Heuberger and van der Veen43}