Anisotropy of the Proton Kinetic Energy as a Tool for Capturing Structural Transition in Water Confined in a Graphene Nanoslit Pore

Abstract

The proton dynamics of a 2D water monolayer confined inside a graphene slit pore is studied in Cartesian and molecular frames of reference using molecular dynamics simulations. The vibrational density of states of the proton was calculated versus temperature and was further used to deduce the mean kinetic energy of the hydrogen atoms, Ke(H), in both frames of reference. The directional components of Ke(H) are in good agreement with experimental observations for bulk as well as nanoconfined water. Nonetheless, while in the molecular frame of reference the effect of temperature on the anisotropy ratios of Ke(H) (the ratio between its directional components) are practically invariant between the 2D and 3D cases, those in the Cartesian frame of reference reveal a rather notable reduction across 200 K, indicating the occurrence of an order−disorder transition. This result is further supported by the calculated entropy and enthalpy of the confined water molecules. Overall, it is shown that Ke(H) anisotropy ratios may serve as a valuable order parameter for detecting structural transformations in hydrogen bonds containing molecular systems.

Molecular-Level Insights into Unique Behavior of Water Molecules Confined in the Heterojunction between One- and Two-Dimensional Nanochannels

“…In the past decades, nanoconfined water has always received intensive attention since it plays a significant role in many applications such as drug delivery, DNA sequencing and therapeutic, , nanofluidic, water purification, , and energy storage in supercapacitors and batteries. , Compared to the bulk water, nanoconfinement with different shapes and dimensions can lead to anomalous changes in the structures, dynamics, and other physical properties of water, such as distinct ice phase, , enhancement of the diffusion rate and different modes of diffusion, − and the transformation of the hydrogen bond (HB) network. ,, In spite of great progress in experimental characterizations, it still has to fight against enormous challenges for experimental observations to provide a comprehensive understanding of the unique properties of nanoconfined water. ,− As a powerful complementary analysis tool, theoretical simulations, especially molecular dynamics (MD), can offer a molecular-level insight into the structures and dynamics of water confined in multi-dimensional nanoscale environment. − …”