Structural and configurational properties of nanoconfined monolayer ice from first principles

Abstract

Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. Here we investigate the properties of ice confined to a quasi-2D monolayer by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use density-functional theory simulations with a non-local van der Waals density functional. An ab initio random structure search reveals all the energetically competitive monolayer configurations to belong to only two of the previously-identified families, characterized by a square or honeycomb hydrogen-bonding network, respectively. We discuss the modified ice rules needed for each network, and propose a simple point dipole 2D lattice model that successfully explains the energetics of the square configurations. All identified stable phases for both networks are found to be non-polar (but with a topologically non-trivial texture for the square) and, hence, non-ferroelectric, in contrast to previous predictions from a five-site empirical force-field model. Our results are in good agreement with very recently reported experimental observations.

Evidence for stable square ice from quantum Monte Carlo

“…In the 2D ice phase diagram, the question of the stability of square ice is particularly interesting because it is arguably the only experimentally observed 2D ice, and disagreements exist between FFs and DFT, and also within DFT itself [1,2,6,7]. We have already seen that the square phase is less stable than the pentagonal and hexagonal phases at low pressures.…”

“…Meanwhile, DFT results of Corsetti et al [6] showed that the square ice structure is more stable than the hexagonal phase at all pressures. These disparate findings raise serious questions about the reliability of the adopted computational approaches (both DFT and FF) applied so far to 2D ice.…”

“…They also found that square ice is only stable in the GPa pressure regime and that even at these pressures it is only favored enthalpicly by <10 meV/H 2 O (1/4 kcal/mol). Meanwhile, DFT results of Corsetti et al [6] showed that the square ice structure is more stable than the hexagonal phase at all pressures. These disparate findings raise serious questions about the reliability of the adopted computational approaches (both DFT and FF) applied so far to 2D ice.…”

“…However, these experiments have been questioned [2], with it even being suggested that it is sodium chloride contamination and not ice that is responsible for the square symmetry observed. So far, it is not clear under what conditions (if any) square ice is stable.Theoretical investigations of the stability of confined 2D ice at high lateral pressures can, in principle, help in disentangling this issue and in complementing experimental findings [3][4][5][6][7][8][9][10]. From a theoretical perspective, the prediction of square 2D ice can be traced back to Nagle's 1970's "unit model" of ice [3].…”

“…However, later atomistic force field (FF) simulations found that 2D ice prefers a buckled rhombic structure [4,5]. More recently, density functional theory (DFT) based investigations [6][7][8][9] have been performed. However, these have produced qualitatively different results depending on the precise details of the calculations and, in particular, on the choice of exchange-correlation (XC) functional.…”