A general topological network criterion for exploring the structure of icy nanoribbons and monolayers

Abstract

We develop intuitive metrics for quantifying complex nucleating systems under confinement. These are shown to arise naturally from the analysis of the topological ring network, and are amenable for use as order parameters for such systems. Drawing inspiration from qualitative visual inspection, we introduce a general topological criterion for elucidating the ordered structures of confined water, using a graph theoretic approach. Our criterion is based on primitive rings, and reinterprets the hydrogen-bond-network in terms of these primitives. This approach has no a priori assumptions, except the hydrogen bond definition, and may be used as an exploratory tool for the automated discovery of new ordered phases. We demonstrate the versatility of our criterion by applying it to analyse well-known monolayer ices. Our methodology is then extended to identify the building blocks of one-dimensional n-sided prismatic nanoribbon ices.Confinement is known to cause deviations from bulk system properties [1][2][3][4], imparting unique complexities to the structure determination of icy confined water. The structure and phase behaviour of quintessential confined systems, like that of two-dimensional monolayer and one-dimensional nanoribbons are of enormous theoretical[4-6] and experimental interest [7,8]. However, well-defined simulation methodologies and methods of analysis for bulk systems like the mean squared displacement (MSD), often used as a broad indicator of phase transitions, may be unreliable for constrained ice systems. The reasons are two-fold: confined water often exhibits continuous freezing transitions [9][10][11], and the mobility of water layers close to the confining sheet is lower than that of intermediate layers [12,13]. Parameters engineered to characterize phase transitions in simpler Lennard-Jones systems, inspired by the MSD, are not universally valid. For example, the Lindemann parameter, which has been used for melting studies [14,15], fails for confined systems with attractive pores due to instabilities in the MSD [16]. Structure determination techniques, which are accurate for bulk water, are often intractable for nucleating water systems under confinement, as the hydrogen-bonding network (HBN) is significantly influenced by the confining wall. Compared to bulk water, water under nanoscale confinement shows considerable polymorphic diversity [17].Visual inspection of the HBN, paired with the angle distribution, has been used in the literature for the analysis of confined water systems. Such techniques are often used for qualitative analyses [5,10,11,18,19] but are unable to describe the structures quantitively. Voronoi tessellation [20] has been previously used to quantify the structures of two-dimensional confined ice systems. The bond orientational parameter has also been used for distinguishing the degree of square and hexagonal order in ice [21]. However, the bond orientational parameter has implicit a priori * Corresponding Author arXiv:1909.09827v1 [physics.chem-ph] 21 Sep 2019 PREPRINT -SEPTE...

Exploring the Anomalous Phase Behavior of High-Pressure Ices in Diamond Confinement

“…To investigate the relative proportions of the dominant square and triangular ices, during this solid-to-solid phase transition, we employ the coverage area percentage, an area-based metric of quasi-two-dimensional ice classification. The theoretical coverage percentage for any n -membered ring phase is 100%, corresponding to a state wherein the n -membered rings entirely cover the confining sheet area.…”

“…We have used OVITO for generating the visuals in this work. Primitive rings were obtained and further analyzed according to our topological network criterion for quasi-two-dimensional systems, using a FOSS molecular dynamics analysis engine, d-SEAMS …”

Finite Systems under Pressure: Assessing Volume Definition Models from Parallel-Tempering Monte Carlo Simulations

“…In spite of these, the volume values predicted by the M6 method are found to be very close to those of the M5 method for pressures up to 2 GPa. We should highlight that solid–solid transformations are the hardest to be determined, and thus more efficient methods for structure identification and classification, such as machine-learning computational tools employing, for example, order parameter metrics, − should be implemented and/or developed.…”

Salt-Water System under Diamond Confinement

“…The planarity of primitive rings formed by the HBN in the pure water and salt-water monolayer phases enables us to apply an area-based metric to quantify the extent of disruption by encapsulated ions. The coverage area metric (CA n ) is a qualitative percentage estimate of the area occupied by n -membered primitive rings in the HBN . For instance, a value of CA 4 = 100% indicates that the 4-membered primitive rings formed by the HBN completely fill the slit area.…”

“…The coverage area metric (CA n ) is a qualitative percentage estimate of the area occupied by nmembered primitive rings in the HBN. 70 For instance, a value of CA 4 = 100% indicates that the 4-membered primitive rings formed by the HBN completely fill the slit area. Table 3 lists the CA n for the salt-water monolayer and fMSI.…”