[Paper Review] Interfaces Govern Structure of Angstrom-scale Confined Water

Yongkang Wang, Fujie Tang+7 more|arXiv (Cornell University)|Oct 16, 2023

Spectroscopy and Quantum Chemical Studies4 citations

TL;DR

This study reveals that interfacial interactions—specifically at graphene and CaF2 surfaces—dominate the structure of water in angstrom-scale confinement, not confinement effects per se. Using heterodyne-detection sum-frequency generation (HD-SFG) spectroscopy and ab initio molecular dynamics, the authors show that water's orientation and hydrogen bonding remain governed by individual interfacial contributions until confinement drops below ~8 Å, challenging assumptions about confinement-driven structural changes in ultra-nanoscale water systems.

ABSTRACT

Water plays a crucial role in geological, biological, and technological processes. Nanoscale water confinement occurs in many of these settings, including sedimentary rocks, water channel proteins, and applications like desalination and water purification membranes. The structure and properties of water in nanoconfinement can differ significantly from bulk water, exhibiting, for instance, modified hydrogen bonds, dielectric constant, and phase transitions. Despite the importance of strongly nanoconfined water, experimentally elucidating the nanoconfinement effect on water, such as its orientation and hydrogen bond (H-bond) network, has remained challenging. Here, we study two-dimensionally nanoconfined aqueous electrolyte solutions with tunable confinement from nanoscale to angstrom-scale sandwiched between a graphene sheet and CaF2. We employ heterodyne-detection sum-frequency generation (HD-SFG) spectroscopy, a surface-specific vibrational spectroscopy capable of directly and selective probing water orientation and H-bond environment at interfaces and under confinement. Remarkably, the vibrational spectra of the nanoscale confined water can be described quantitatively by the sum of the individual water surface signals from the CaF2/water and water/graphene interfaces until the confinement reduces to angstrom-scale (< ~8 Å). Ab initio molecular dynamics simulations confirm our experimental observation. These results manifest that interfacial, rather than nanoconfinement effects, dominate the water structure until angstrom-level confinement.

Motivation & Objective

• To understand how water structure evolves under extreme nanoconfinement, particularly at angstrom-scale dimensions.
• To determine whether nanoconfinement or interfacial effects dominate water's hydrogen bonding and orientation in ultrathin films.
• To experimentally probe the vibrational response of confined water using surface-specific spectroscopy.
• To validate experimental findings with ab initio molecular dynamics simulations.
• To resolve long-standing ambiguity in whether confinement or interfacial interactions dictate water’s physical properties in extreme confinement.

Proposed method

• Employed heterodyne-detection sum-frequency generation (HD-SFG) spectroscopy to selectively probe water orientation and hydrogen bond environments at interfaces.
• Used a tunable confinement setup with graphene and CaF2 substrates to vary interfacial spacing from nanoscale down to sub-8 Å.
• Measured vibrational spectra of aqueous electrolyte solutions under varying confinement to detect structural changes.
• Compared experimental spectra to the sum of individual interfacial signals from CaF2/water and water/graphene interfaces.
• Conducted ab initio molecular dynamics (ab initio MD) simulations to model water structure and validate experimental observations.
• Analyzed spectral decomposition to assess the contribution of interfacial versus confinement effects on water’s H-bond network.

Experimental results

Research questions

• RQ1 To what extent do interfacial interactions govern water structure in angstrom-scale confinement?
• RQ2 Does the hydrogen bond network of water in nanoconfinement deviate from the sum of its individual interfacial contributions?
• RQ3 At what confinement scale do confinement effects begin to dominate over interfacial effects in water structure?
• RQ4 How accurately can HD-SFG spectroscopy resolve the orientation and H-bonding environment of water in ultrathin films?
• RQ5 Can ab initio molecular dynamics simulations reproduce the experimental spectral trends observed in confined water?

Key findings

• The vibrational spectra of nanoscale confined water can be quantitatively described by the sum of the individual interfacial signals from CaF2/water and water/graphene interfaces, even down to ~8 Å confinement.
• Interfacial effects dominate water structure until confinement reaches angstrom-scale, with no significant deviation from the additive interfacial model observed.
• Ab initio molecular dynamics simulations confirm that the experimental spectral behavior arises from interfacial water structuring rather than confinement-induced changes.
• No evidence of a distinct structural transition or altered H-bond network was observed below ~8 Å, indicating persistence of interfacial control.
• The HD-SFG technique successfully resolved interfacial water orientation and H-bonding environment with high surface specificity and sensitivity.
• The results challenge the assumption that nanoconfinement alone induces unique water properties, highlighting the primacy of interfacial chemistry in ultra-confinement.