Water in Confinement between Nanowalls: Results for Hexagonal Boron Nitride versus Graphene Sheets from Ab Initio Molecular Dynamics

Abstract

Structural, dynamical, and dipolar properties of water molecules in nanoconfinement between either two hexagonal boron nitride (h-BN) or two graphene sheets are investigated by means of ab initio and classical molecular dynamics simulations. In particular, we have focused on the perturbation of water properties caused by the two confining h-BN and graphene sheets when they are separated by a distance in the nanodomain. The structure of water near h-BN sheets is found to be noticeably different from that for graphene sheets because of the presence of polar bonds in the BN sheets. The density profiles show that water is more structured near the h-BN sheets than near graphene. The orientational profiles of water molecules near the h-BN and graphene surfaces also reveal differences in water orientational structure near the two surfaces. Various dynamical quantities such as the rotational relaxation, diffusion, hydrogen bond dynamics, and the escape dynamics from the solvation layers near the surfaces reveal a slower relaxation for the interfacial water near h-BN sheets. The slowdown of the dynamics can be attributed to the interactions of water with polar B–N bonds. The partial charges on the B and N atoms are found to make the water–surface interactions more favorable for the BN sheets than for the nonpolar graphene surfaces. The force-field-based classical simulation results are found to be qualitatively similar to those of ab initio simulations for the structural behavior, although some differences are found in the dynamical properties. The ab initio simulations reveal a faster rotational dynamics of the interfacial water molecules than that of bulk, whereas an opposite behavior is predicted by the empirical force fields used in the current study.