Ion Distribution and Hydration Structure at Solid-Liquid Interface between NaCl Crystal and Its Solution

Abstract

The interface structure between NaCl crystal and its solution has been investigated at the saturated concentration of 298 K by molecular dynamics simulations. We have found that there are many fine structures at this complex interface. Near the surface of crystal, most of Na⁺ only coordinate with water molecules, while almost all Cl^- coordinate with Na⁺ in addition to water molecules. An ion coordinating with more water molecules is farther away from the epitaxial position of lattice. As approaching to the interface, the first hydration shell of ions has the tendency of being ordered, while the orientation of dipole of water molecules in the first hydration shell becomes more disordered than that in the solution. Generally, the first hydration shell of Na⁺ is less affected by nearest Cl^-, whereas the first hydration shell of Cl^- is significantly affected by nearest Na⁺.

Figure 1

Density distribution of H (green dot-dash line), O (red dash line), Na⁺ (blue dot line), and Cl^– (magenta solid line) along the direction perpendicular to the interface, where the vertical dash line marks the epitaxial lattice position.

Figure 2

Distribution of density of Na⁺ (a) and Cl (b) near the interface with various n_w and n_c, where the vertical dash line marks the epitaxial lattice position. n_w denotes the number of water molecules in the first hydration shell, and n_c is the number of the nearest counter ions.

Figure 3

Ion-O RDFs. (a) Na⁺-O RDF in the first three layers and solution. (b) Same as (a) but for Cl^–. (c) Na⁺-O RDF for two kinds of Na⁺ in the first layer, namely, coordinating with (labeled as type I) and without (labeled as type II) counter ions. Here, “total” denotes the sum of type I and type II. (d) Same as (c) but for Cl^– in the first layer. The insets show the focus view of the first peak of ion-O RDF.

Figure 4

Angle distribution for water molecules in FHS. (a, c) Distribution of cos(γ) and cos(α) of Na⁺. (b, d) Same as (a) and (c), respectively, but for Cl^–.

Figure 5

Distribution of cos(α) (a) and cos(γ) (b) in FHS of two types of Na⁺ in the first layer. (c, d) Same as (a) and (b), respectively, but for Cl^–.

Figure 6

Distributions of cos(φ) in the FHS of the first layer (black dot and red dash line) and of solutions (green solid line) for Na⁺ (a) and Cl^– (b).

Figure 7

Snapshot of the interface system. The blue, magenta, green, and red spheres represent Na⁺, Cl^–, H, and O, respectively. The x, y, and z axes are defined along the [100], [010], and [001] directions of NaCl crystals, respectively. The solid–liquid interface is parallel to the x–y plane, and perpendicular to the z direction.

Figure 8

Schematic plot of FHS of an ion, in which the purple, red, and blue spheres represent the central ions, oxygen, and hydrogen atoms, respectively; n₁ is a unit vector from the central ion to an oxygen atom; n₂ is another unit vector along the water molecule dipole; the x, y, and z axes are defined in Figure 7. R_c is the cutoff radius of FHS.