Chogani Alireza, King Helen E, Živković Aleksandar, Plümper Oliver
Department of Earth Sciences, Utrecht University, Utrecht 3584 CB, The Netherlands.
ACS Earth Space Chem. 2024 Nov 5;8(11):2284-2293. doi: 10.1021/acsearthspacechem.4c00210. eCollection 2024 Nov 21.
Nanoporosity is a characteristic feature of geological formations that provides potential pathways for geofluids to meander and interact with minerals. Confinement of water within nanopores leads to unique phenomena. The dielectric constant of water becomes anisotropic and adopts tensorial properties rather than remaining a scalar value. In such nanoconfinement, it has been found that the permittivity of water decreases perpendicularly and increases parallel to the interface. As geofluids are rarely pure water in nature, being a water-salt(-gas) mixture within the Earth, it becomes pivotal to examine how these additional constituents of water affect the permittivity of fluids confined within the nanopores of rocks. In this study, we present the calculation of the permittivity of saline water in calcite slit nanopores using molecular dynamics simulations under low-pressure-temperature conditions. The dielectric properties are weakly dependent on salinity for both the perpendicular and parallel dielectric permittivity components. We analyzed the atomic charge and polarization density of the fluid perpendicular to the nanochannel walls and the orientation of water molecules' dipole inside the nanochannel. From our analysis, most of these factors were generally not altered significantly in the presence of salinity. These findings are significant because they enable us to use well-studied pure water properties under nanoconfinement to determine the geochemical behavior of fluids within natural nanoporous systems.
纳米孔隙是地质构造的一个特征，它为地下流体蜿蜒流动并与矿物质相互作用提供了潜在路径。水在纳米孔隙中的受限导致了独特的现象。水的介电常数变得各向异性，并具有张量性质，而不是保持标量值。在这种纳米限域情况下，人们发现水的电容率垂直方向降低，平行于界面方向增加。由于地下流体在自然界中很少是纯水，而是地球内部的水 - 盐（ - 气）混合物，因此研究水的这些额外成分如何影响岩石纳米孔隙中受限流体的电容率变得至关重要。在本研究中，我们展示了在低压 - 低温条件下使用分子动力学模拟计算方解石狭缝纳米孔隙中盐水电容率的过程。对于垂直和平行介电常数分量，介电性质对盐度的依赖性较弱。我们分析了垂直于纳米通道壁的流体的原子电荷和极化密度以及纳米通道内水分子偶极的取向。从我们的分析来看，在有盐度存在的情况下，这些因素大多通常不会发生显著变化。这些发现具有重要意义，因为它们使我们能够利用在纳米限域下经过充分研究的纯水性质来确定天然纳米多孔系统中流体的地球化学行为。
