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 T 2 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.

Fresh Molecular Look at Calcite–Brine Nanoconfined Interfaces

“…Water molecules belonging to layers 1 and 2 ( Figures 1a-d) interact directly with the calcite surface 5 , and therefore, they constitute the surface or adsorption layer. Inside layer 3 are water molecules disconnected with calcite surface, but their density is affected by the ordering of surface layer and the ionic steric effect.…”

“…This trend indeed confirms that the water self-diffusion coefficient is strongly correlated to the pore surface to volume ratio, 50 i.e, the smaller the ratio the faster the water mobility. In addition, in confined systems, the translational diffusion of water is highly dependent on the local density and ordering 5 . For the confined API brine, we have shown in the previous section that the inclusion of ions enhances local and non-local ordering, highlighted here by the distortions on the hydrogen bond network (Figures 4a-c).…”

“…Previous studies explored the effect of spatial confinement on the hydrogen bond network and the dynamics based solely on pure water [5][6][7][8] . According to Dore 7 , water in a confined geometry shows significant modification on its hydrogen bond structure in comparison to the bulk phase.…”

Self-diffusion coefficient of bulk and confined water: a critical review of classical molecular simulation studies

“…The calculation of the parallel coefficient was performed according to the method of Liu et al [230], using a flexible SPC/Fw water model. Mutisya et al [229] found values smaller than the bulk self-diffusion for pores ranging from 1.0 to 6.0 nm wide, with confinement effects enhanced for the narrowest pore due to overlap of surface effects.…”

“…The major component of carbonate rocks is calcite, an important mineral for CO2 sequestration, oil exploration, and other geological processes. Mutisya et al [229] found that the water dynamics are affected by the interaction between water and calcite surface reducing the self-diffusion coefficient and inducing water layering. The calculation of the parallel coefficient was performed according to the method of Liu et al [230], using a flexible SPC/Fw water model.…”