How do interfaces alter the dynamics of supercooled water?

Hydrogen Bond Network Shaping Proton Penetration Behavior across Two-Dimensional Nanoporous Materials

In this study, we investigate aqueous proton penetration behavior across four types of two-dimensional (2D) nanoporous materials with similar pore sizes using extensive ReaxFF molecular dynamics simulations. The results reveal significant differences in proton penetration energy barriers among the four kinds of 2D materials, despite their comparable pore sizes. Our analysis indicates that these variations in energy barriers stem from differences in the hydrogen bond (HB) network formed between the 2D nanoporous materials and the aqueous environment. The HB network can be classified into two categories: those formed between the surface of the 2D nanoporous materials and the aqueous environment, and those formed between the edge atoms of the nanopores and the water molecules inside the pores. A strong HB network formed between the surface of the 2D nanoporous materials and the aqueous environment induces an orientational preference of water molecules, resulting in an aggregated water layer with high density. This high-density water region traps protons, making it difficult for them to escape and penetrate the nanopores. On the other hand, a strong HB network formed between the edge atoms of the nanopores and the water molecules inside the pores impedes the rotation and migration of water molecules, further inhibiting proton penetration behavior. To facilitate the proton penetration process, in addition to a sufficiently large pore size, a weak HB network between the 2D nanoporous material and the aqueous environment is necessary.

Surface-induced water crystallisation driven by precursors formed in negative pressure regions

Ice nucleation is a crucial process in nature and industries; however, the role of the free surface of water in this process remains unclear. To address this, we investigate the microscopic freezing process using brute-force molecular dynamics simulations. We discover that the free surface assists ice nucleation through an unexpected mechanism. The surface-induced negative pressure enhances the formation of local structures with a ring topology characteristic of Ice 0-like symmetry, promoting ice nucleation despite the symmetry differing from ordinary ice crystals. Unlike substrate-induced nucleation via water-solid interactions that occurs directly on the surface, this negative-pressure-induced mechanism promotes ice nucleation slightly inward the surface. Our findings provide a molecular-level understanding of the mechanism and pathway behind free-surface-induced ice formation, resolving the longstanding debate. The implications of our discoveries are of substantial importance in areas such as cloud formation, food technology, and other fields where ice nucleation plays a pivotal role.

In-layer inhomogeneity of molecular dynamics in quasi-liquid layers of ice

Quasi-liquid layers (QLLs) are present on the surface of ice and play a significant role in its distinctive chemical and physical properties. These layers exhibit considerable heterogeneity across different scales ranging from nanometers to millimeters. Although the formation of partially ice-like structures has been proposed, the molecular-level understanding of this heterogeneity remains unclear. Here, we examined the heterogeneity of molecular dynamics on QLLs based on molecular dynamics simulations and machine learning analysis of the simulation data. We demonstrated that the molecular dynamics of QLLs do not comprise a mixture of solid- and liquid water molecules. Rather, molecules having similar behaviors form dynamical domains that are associated with the dynamical heterogeneity of supercooled water. Nonetheless, molecules in the domains frequently switch their dynamical state. Furthermore, while there is no observable characteristic domain size, the long-range ordering strongly depends on the temperature and crystal face. Instead of a mixture of static solid- and liquid-like regions, our results indicate the presence of heterogeneous molecular dynamics in QLLs, which offers molecular-level insights into the surface properties of ice.

Controlling crystallization: what liquid structure and dynamics reveal about crystal nucleation mechanisms

Over recent years, molecular simulations have provided invaluable insights into the microscopic processes governing the initial stages of crystal nucleation and growth. A key aspect that has been observed in many different systems is the formation of precursors in the supercooled liquid that precedes the emergence of crystalline nuclei. The structural and dynamical properties of these precursors determine to a large extent the nucleation probability as well as the formation of specific polymorphs. This novel microscopic view on nucleation mechanisms has further implications for our understanding of the nucleating ability and polymorph selectivity of nucleating agents, as these appear to be strongly linked to their ability in modifying structural and dynamical characteristics of the supercooled liquid, namely liquid heterogeneity. In this perspective, we highlight recent progress in exploring the connection between liquid heterogeneity and crystallization, including the effects of templates, and the potential impact for controlling crystallization processes. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Quantifying dynamical heterogeneity length scales of interface water across model membrane phase transitions

All-atom molecular dynamics simulations of 1,2-dimyristoyl-sn-glycero-3-phosphocholine lipid membranes reveal a drastic growth in the heterogeneity length scales of interface water (IW) across fluid to ripple to gel phase transitions. It acts as an alternate probe to capture the ripple size of the membrane and follows an activated dynamical scaling with the relaxation time scale solely within the gel phase. The results quantify the mostly unknown correlations between the spatiotemporal scales of the IW and membranes at various phases under physiological and supercooled conditions.