Variable Charge and Electrical Double Layer of Mineral–Water Interfaces: Silver Halides versus Metal (Hydr)Oxides

Classically, silver (Ag) halides have been used to understand thermodynamic principles of the charging process and the corresponding development of the electrical double layer (EDL). A mechanistic approach to the processes on the molecular level has not yet been carried out using advanced surface complexation modeling (SCM) as applied to metal (hydr)­oxide interfaces. Ag halides and metal (hydr)­oxides behave quite differently in some respect. The location of charge in the interface of Ag halides is not a priori obvious. For AgI(s), SCM indicates the separation of interfacial charge in which the smaller silver ions are apparently farther away from the surface than iodide. This charge separation can be understood from the surface structure of the relevant crystal faces. Charge separation with positive charge above the surface is due to monodentate surface complex formation of Ag⁺ ions binding to I sites located at the surface. Negative surface charge is due to the desorption of Ag⁺ ions out of the lattice. These processes can be described with the charge distribution (CD) model. The MO/DFT optimized geometry of the complex is used to estimate the value of the CD. SCM reveals the EDL structure of AgI(s), having two Stern layers in series. The inner Stern layer has a very low capacitance (<i>C</i>₁ = 0.15 ± 0.01 F/m²) in comparison to that of metal (hydr)­oxides, and this can be attributed to the strong orientation of the (primary) water molecules on the local electrostatic field of the Ag⁺ and I^– ions of the surface (relative dielectric constant ε_r ≈ 6). Depending on the extent of water ordering, mineral surfaces may in principle develop a second Stern layer. The corresponding capacitance (<i>C</i>₂) will depend on the degree of water ordering that may decrease in the series AgI (<i>C</i>₂ = 0.57 F/m²), goethite (<i>C</i>₂ = 0.74 F/m²), and rutile (<i>C</i>₂ = ∞), as discussed. The charging principles of AgI minerals iodargyrite and miersite may also be applied to minerals with the same surface structure (e.g., sphalerite and würtzite (ZnS))