Extreme Nanoconfinement Reshapes the Self-Dissociation of Water

TL;DR

This study demonstrates that extreme nanoconfinement dramatically lowers the self-dissociation barrier of water. By combining ab initio BOMD with enhanced-sampling machine-learning potential MD and global Voronoi-based collective variables, the authors reveal a lower barrier in monolayer water ($ΔF_{ ext{1L}} o 11.8$ kcal/mol) relative to bulk ($ΔF_{ ext{bulk}} o 14.9$ kcal/mol) and identify a dissociation pathway facilitated by constrained HB networks and a 55$^\circ$ interstitial motif. Analysis of maximally localized Wannier functions and vibrational densities of states shows increased electron delocalization and amplified low-frequency HB dynamics in 1L water, together with enhanced in-plane dielectric fluctuations ($ε_{\parallel}$). Nuclear quantum effects and higher density further promote autoionization, suggesting monolayer water can behave as a superdielectric and potentially enter a superionic-like regime, with wide implications for geochemistry, biology, and nanofluidics.

Abstract

Water's ability to self-dissociate into H$_3$O$^+$ and OH$^-$ ions is central to acid-base chemistry and bioenergetics. Recent experimental advances have enabled the confinement of water down to the nanometre scale, even to the single-molecule limit, yet how this process is altered at the extreme nanoconfinement remains unclear. Using \emph{ab-initio} calculations and enhanced-sampling machine-learning potential molecular dynamics, we show that monolayer-confined water exhibits a markedly lower barrier to auto-dissociation than bulk water. Confinement restructures both intramolecular bonding and the intermolecular hydrogen-bond network, while enforcing quasi-2D dipolar correlations that amplify dielectric fluctuations. Our results imply that two-dimensional confined water could act as a \emph{superdielectric} medium and may exhibit \emph{superionic} behavior, as observed in recent experiments. These findings reveal confinement as a powerful route to enhanced proton activity, shedding light on geochemical niches, biomolecular environments, and nanofluidic systems where water's chemistry is fundamentally reshaped.

Figures (5)

• Figure 1: (a) Two-dimensional free energy surface (FES) of water self-dissociation for bulk (left panel) and 1L (right panel) with the CVs of $S_a$ and $S_t’$ (see exact definitions in Methods). The representative states are indicated by arrows. The inset is the illustration of the simulation models. (b) The one-dimensional FES along $S_a$ and (c) $S_t’$. (d) The snapshots for representative states for water self-dissociation in 1L (Top row) and bulk (Bottom row) water. Here "H$_2$O" represents pure water with no ion. The gray, red, white balls are C, O and H atom, respectively. "$\text{O} \times \text{H}$" is the state with the breaking of the OH covalent bonds, "TS" is the transition state that the H$_2$O has just dissociated into H$_3$O$^+$ and OH$^-$ forming a contact ion pair, "H$_3$O$^+$···OH$^-$" is the finally state the water molecule is fully dissociated where H$_3$O$^+$ and OH$^-$ are separated. The atoms for H$_3$O$^+$ and OH$^-$ are highlighted by large atoms.
• Figure 2: The free energy level for representative states for bulk water and 1L water.
• Figure 3: (a)-(c) Distributions of the centers of maximally localized Wannier functions (MLWFs) with respect to the position of oxygen for lone and bonding electron pairs for bulk water, 1L water and ice Ih. The distributions are also decomposited into $n$ acceptors (A) or donors (D). The results of A$_2$ and D$_1$ are highlighted by solid and thick lines. The inset is the representative snapshot of the MLWFs of water molecule. The lone and bonding pair MLWFs are colored in green and cyan transparent iso-surface, respectively. (d) The vibrational density of states (DOS) for bulk water, 1L water and ice Ih. Here, for clarity, only the bands for the intermolecular vibration and intramolecular OH stretching are shown. For full bands are shown in Fig. S5. (e) An illustration of the deconvolution of the OH stretching band into five sub-bands, each corresponding to water molecules with different hydrogen-bonding states defined by their number of donors and acceptors sun2013_cpl. (f) The HB statistics of bulk and 1L water in percentage ($\%$) of corresponding water molecules that accept or donate $i=0, 1, 2$ or 3 HBs denoted by $\rm {A}_{\textit{i}}$ and $\rm {D}_{\textit{i}}$, respectively zhou2024_jctca. The background color indicates the percentage ($P({\rm {D}}_i{\rm A}_j)$). The value of $P({\rm {D}}_i{\rm A}_j)$ larger than 1$\%$ is indicated by the number for visualization of the qualitative differences and similarities.
• Figure 4: (a) The bond angular distribution of the triples $P_{\rm {OOO}}(\theta)$. The inset is the illustration of $\theta$. (b) Free energy landscape $F(r, \theta)$ as a function of $\theta$ and the O–O distance $r$ for bulk and 1L water. (c) The joint probability distributions of $P(v, {\cos}\,\alpha)$. The insets are the schematic representation of the proton-transfer coordinate. Here $v = d_{\rm {O_{D}H_{D}}} - d_{\rm{O_{A}H_{D}}}$, $\alpha$ is the angle of $\angle$O$_{\rm A}$O$_{\rm D}$H$_{\rm D}$. Three regions of local minima are indicated by Roman numerals. The red dash line means the transition path between configuration II and III. (d) Schematic representation of the water auto-dissociation process, including neutral water molecules (H$_2$O), the O···H interaction, the transition state (TS), and the resulting H$_3$O$^+$···OH$^-$ species. The angle indicated by the red lines represents 55$^\circ$ motif. The configuration of ${\rm {D}}_1{\rm A}_2$ is highlighted on the figure.
• Figure 5: (a) The dissociation barrier ($\Delta F$) at different temperature. (b) The relation between $\Delta F$ and dipole moment of water molecule ($|\boldsymbol{\mu}|$). Results from other methods and experimental data are included for comparison (see Table S2 for a summary).