Thermal Stability of Two-Dimensional Crystals with Extended OH Hydrogen-Bonded Chains

Technical Summary: Thermal Stability of Two-Dimensional Crystals with Extended OH Hydrogen-Bonded Chains

Problem Statement
The development of proton-exchange membranes (PEMs) capable of operating under elevated temperatures and anhydrous conditions remains a critical challenge. Current PEMs, often reliant on phosphoric acid, suffer from performance deterioration at high temperatures and low humidity. A primary bottleneck is the lack of molecular systems that can form high-temperature-stable hydrogen-bonded chains (specifically O–H···O–H···O–H chains) necessary for efficient proton transport. While biological systems utilize such chains for proton conduction, designing synthetic molecular systems that maintain these extended networks on a substrate at high temperatures requires a balance between strong substrate adsorption and the formation of continuous hydrogen-bonded networks.

Methodology
The author employed molecular dynamics (MD) simulations to assess the feasibility of fabricating stable 2D crystals from planar molecules adsorbed on a hexagonal boron nitride (h-BN) sheet. The study utilized a united-atom approximation, where CH and CH₃ groups were treated as single interaction sites.

• Force Fields: The simulations used a force field derived from the AMBER General Force Field (v2.1). Intramolecular interactions were modeled using harmonic potentials for bond stretching and valence angle deformations, and periodic potentials for dihedral angles. Non-bonded interactions were described by Lennard-Jones (LJ) potentials and Coulombic interactions.
• Substrate Interaction: The interaction between the molecules and the h-BN substrate was modeled using an (m, l) Lennard-Jones potential with exponents $l=10$ and $m=4.25$, representing a fixed attractive plane.
• Simulation Protocol: The study minimized potential energy to find ground-state configurations for monolayers of various molecules. To assess thermal stability, the systems were coupled to a Langevin thermostat. Simulations tracked the dimensionless heat capacity ($c$), the average number of hydrogen bonds per molecule ($p_{hb}$), and the fraction of molecules adsorbed flat on the substrate ($p_s$) as temperature increased. Both finite crystallites (with free edges) and infinite 2D crystals (with periodic boundary conditions) were modeled to observe melting transitions.

Molecules Investigated
The study focused on planar molecules containing benzene rings (to ensure strong substrate adsorption) and hydroxyl (OH) or peptide groups (to form hydrogen bonds). The specific molecules analyzed were:

1. Phenol (C₆H₅OH)
2. Hydroquinone (C₆H₄(OH)₂)
3. 4-Phenylphenol (C₆H₅–C₆H₄OH)
4. 4-(4-Phenylphenyl)phenol (C₆H₅–C₆H₄–C₆H₄OH)
5. Paracetamol (CH₃C(O)NHC₆H₄OH)
6. 4-Hydroxybenzanilide (C₆H₅C(O)NHC₆H₄OH)
7. 4,4-Dihydroxybenzanilide (C₆H₄OHC(O)NHC₆H₄OH)

Key Results
The simulations demonstrated that these molecules can form stable two-dimensional crystals characterized by linear chains of hydrogen bonds.

• Structural Formation: Molecules such as paracetamol, hydroxybenzanilides, and phenols formed periodic structures with parallel molecular chains. In the homochiral paracetamol structure, linear chains of hydroxyl groups (Type 1) and peptide groups (Type 12) were observed. In contrast, racemic structures often formed mixed zigzag chains, which the author notes are unsuitable for proton conduction as they lack the continuous Type 1 OH chains.
• Thermal Stability: The 2D crystals exhibited high thermal stability, with melting occurring over a continuous temperature range rather than at a single sharp point. The onset melting temperatures ($T_1$ for finite crystallites and $T_3$ for infinite crystals) were significantly higher than those of bulk counterparts in some cases, or comparable to them.
  • Phenol: Stable up to ~320 K (infinite crystal).
  • Hydroquinone: Stable up to ~460 K.
  • 4-Phenylphenol: Stable up to ~400 K.
  • 4-(4-Phenylphenyl)phenol: Stable up to ~540 K.
  • Paracetamol: Stable up to ~440 K (infinite crystal).
  • 4-Hydroxybenzanilide: Stable up to ~580 K.
  • 4,4-Dihydroxybenzanilide: Stable up to ~650 K.
• Correlation with Bulk Data: The simulated melting onset temperatures for the 2D crystals showed excellent agreement with the experimental melting temperatures of the corresponding bulk 3D crystals (e.g., Paracetamol bulk $T_0 \approx 445$ K vs. 2D crystal stability up to 440 K).
• Mechanism of Stability: The benzene rings were found to be crucial for strong van der Waals interaction with the flat h-BN substrate, preventing desorption, while the specific arrangement of OH and peptide groups allowed for the formation of extended hydrogen-bonded networks without steric hindrance.

Significance and Claims
The paper concludes that planar molecules with benzene rings and hydroxyl/peptide groups can form 2D crystals on h-BN sheets that maintain linear hydrogen-bonded chains at substantially elevated temperatures. The author highlights that the onset melting temperatures for these 2D crystals range from 47°C to 377°C, far exceeding the melting point of phosphoric acid (42°C), a common electrolyte in PEMs.

The primary significance claimed is that these structures offer a potential pathway for developing novel, anhydrous proton-exchange membranes capable of operating at high temperatures. Specifically, the author suggests that multilayer structures composed of h-BN sheets and molecules of hydroquinone, paracetamol, or 4-hydroxybenzanilide represent promising platforms for such applications. The study posits that the combination of strong substrate adsorption and extended hydrogen-bonded networks provides the necessary thermal stability for proton conduction in extreme conditions, addressing a key limitation in current PEM technology.