Breaking the Air-Water Paradigm: Ion Behavior at Hydrophobic Solid-Water Interfaces

Abstract

Hydrophobic solid-water interfaces underpin processes in nanofluidics, electrochemistry, and energy technologies. Microscopic insights into these systems are often inferred from our understanding of the air-water interface, which is assumed to exhibit similar behavior. Here, we challenge this paradigm by combining heterodyne-detected vibrational sum-frequency generation spectroscopy with machine-learning molecular dynamics simulations at first-principles accuracy to investigate the graphene-NaCl(aq) interface as a prototypical hydrophobic solid-water system. Spectroscopic results suggest that ions have a minimal effect on the structure of the interfacial water, while simulations reveal that Na$^{+}$ and Cl$^{-}$ accumulate densely at the surface. Together, these findings reveal a new adsorption mechanism that departs from the established air-water interface paradigm, where interfacial ion adsorption is typically associated with, and often detected through, pronounced alteration of the interfacial water alignment and orientation. This difference arises because ions cannot penetrate the solid boundary and reside at a similar depth as the interfacial water molecules. As a consequence, large ion populations can be accommodated within the extended two-dimensional hydrogen-bond network at the interface, causing only minor local distortions but significant changes to its longer-range connectivity. These results reveal a distinct mechanism of electrolyte organization at aqueous-carbon interfaces, relevant to energy applications, where performance is highly sensitive to the local organization of interfacial water.

Figures (5)

• Figure 1: Experimental HD-VSFG spectra of electrolyte solutions at air-liquid and solid-liquid interfaces. HD-VSFG spectra at room temperature for (a) 1.0 M HCl at the air–water interface, (b) 1.5 M NaCl at the air–water interface, and (c) 1.0 M NaCl at the graphene–water interface. In each panel, the corresponding pure-water spectrum for the same interface is shown as a reference. (a) and (b) are adapted with permission from Litman et al.litman_surface_2024. The arrows indicate the direction of spectral changes with respect to the pure water spectrum, while gray rectangles mark regions where changes are negligible. The accompanying illustrations schematically indicate the presence or absence of ions at each interface.
• Figure 2: Molecular structure of the graphene–NaCl(aq) interface at varying concentrations.(a) Schematic illustration of the system studied, along with the density profiles of the water oxygen atoms, Cl$^{-}$ ions, and Na$^{+}$ ions at the graphene–NaCl(aq) interface for a 2 M NaCl solution. (b) Total number of interfacial water molecules, classified according to whether they solvate no ions, Na$^{+}$, or Cl$^{-}$. The accompanying bar plots show the percentage contribution of each type. Interfacial water molecules were defined as those located between the graphene surface and the first minimum of the water oxygen density profile (see Figure S1), located at approximately 4.5 Å from the surface.
• Figure 3: Theoretical VSFG spectra of aqueous NaCl solutions at varying concentrations, with decomposition by O--H bond type.(a) Theoretical $\mathrm{Im}(\chi^{(2)})$ spectra for the graphene–NaCl(aq) interface at NaCl concentrations of 0, 0.5, 1, and 2 M. (b--d) Decomposition of the spectra into contributions from (b) water molecules not solvating ions, (c) Na$^{+}$-solvated waters, and (d) Cl$^{-}$-solvated waters, obtained using the ssVVCF methodology ssvcf_2015. The arrows indicate the direction of spectral changes with respect to pure water upon increasing salt concentration, while gray rectangles mark frequency regions where changes are negligible. In panels (b--d), the spectra are normalized by the number of interfacial water molecules of each type.
• Figure 4: Microscopic analyses of the graphene–NaCl(aq) interface, with molecular-level illustrations of how Na$^{+}$ and Cl$^{-}$ ions modify interfacial water structures for a 2 M NaCl solution. (a) Probability distribution of O--H bond orientations in water molecules not solvating ions as a function of their depth, defined as the distance between the water oxygen and the graphene surface, and the angle relative to the surface normal. An angle of $0^\circ$ indicates that the O--H bond points toward the bulk solution. The accompanying profile shows the O--H angle distribution integrated over all depths, highlighting the most probable orientations. (b) Difference in the probability distribution shown in (a) relative to that of pure water at the graphene interface. Positive values indicate features that appear compared to pure water, while negative values indicate features that disappear compared to pure water. (c) Molecular-level depiction of water molecules solvating other water molecules at the graphene interface, including the definition of the O--H bond angle, $\alpha$. The horizontal dashed line indicates the position of the interfacial layer based on oxygen atom positions. The left water molecule illustrates the dominant in-plane orientation at the interface, whereas the right one represents the principal out-of-plane configuration. (d–f) Same as (a–c) but for water molecules solvating Na$^{+}$. (g–i) Same as (a–c) but for water molecules solvating Cl$^{-}$. For water molecules surrounding Cl$^{-}$, hydrogen bonding to the ion is indicated by the purple dotted line in (i). Note the relative depths of Na$^{+}$ and Cl$^{-}$ ions with respect to the horizontal dashed line. Results for additional concentrations are provided in Section S2.
• Figure 5: Ion-induced reorganization of the interfacial hydrogen-bond network. (a) Snapshots of the interfacial water structure at the graphene–water interface for pure water and a 2 M NaCl solution. Sodium ions are shown in purple, and chloride ions are in green. (b) Distribution of hydrogen-bond topologies at different NaCl concentrations. Fractions of DDAA, DDA, DAA, and DA motifs are reported, where D and A denote hydrogen-bond donors and acceptors, respectively. The accompanying snapshots illustrate representative examples of the hydrogen-bonding motifs discussed. (c) Average number of hydrogen bonds per interfacial water molecule, decomposed into total, intralayer (within the topmost layer), and interlayer (between the first and second layers). The horizontal dashed blue line marks the bulk-water value.