Molecular Dynamics Simulations of Water Confined in Calcite Slit Pores: An NMR Spin Relaxation and Hydrogen Bond Analysis

Abstract

We study water confined in calcite (104) slit pores from 6 to 1 nm by molecular dynamics. By determining NMR parameters combined with hydrogen bond network analysis, we provide an important contribution to the understanding of the dynamics of water confined. The water dynamics was found uncorrelated upon confinement within calcite, with the translational dynamics highly dependent on the local density variations and the rotational dynamics varying with local hydrogen bond connectivity. A water layered structuring is observed, and the layer by layer analysis reveals that translational dynamics are the main contribution to spin relaxation of near surface water molecules. The T2 relaxation time for water molecules directly hydrogen bonded to the surface is short and pore size independent; however, a bulk-like spin relaxation is observed at the center of pores larger than 3 nm. The hydrogen bond network of confined water has a more continuous path topology that results in the slightly longer rotational correlation time for water located up to 2 nm from the surface. Moreover, the number of tetrahedral geometric patterns which are associated with bulk water is reduced upon confinement. The confinement effects are enhanced mainly in the 1 nm pore due to overlap of surface effects.