Inner sphere complex

Type of ion surface binding

Inner sphere complex is a type of surface complex that refers to the surface chemistry changing a water surface interface to one without water molecules bridging a ligand to the metal ion. Formation of inner sphere complexes occurs when ions bind directly to the surface with no intervening water molecules. These types of surface complexes are restricted to ions that have a high affinity for surface sites and include specifically adsorbed ions that can bind to the surface through covalent bonding. Inner sphere complexes describe active surface sites that are involved in nucleation, crystal growth, redox processes, soil chemistry, alongside other reactions taking place between a cation and surface. This affinity to surface sites can be attributed to covalent bonding. When compared to outer sphere complexes that have water molecules separating ions from ligands, inner sphere complexes have surface hydroxyl groups that function as donor ligands, increasing the coordinated metal ion's electron density. This is an example of competitive complex formation, in which ligands will compete for space on an activation site of a metal ion. Surface structures are able to reduce and oxidize ligands, whereas transport phenomena do not. Therefore, surface structure serves an important role in surface reactivity, with the coordination environment at the solid water interface changing intensity or rate of a reaction. The Gibb's Free Energy is spontaneous only when S is positive or zero. Another method of wetting is adhesional wetting, where the liquid makes contact with the solid surface for the first time. However, this initial wetting decreases the liquid gas interface that can be modeled by the Dupré equation

An example of sorption of ligands occurs in metallic oxides and silicate surfaces. In a mineral surface, the metal ion acts as a Lewis acid, and the ligands act as the Lewis base. This can also be observed in cases where hydroxyl groups are also present. pH is a consideration within these reactions, but the symmetrical, molecular adsorption of water is considered unstable and possesses a high activation energy. As a result, the rate determining step relies on the breakage of a critical oxo bond that may increase inductive effects through changes in electron density. This causes nucleophilic attacks and further dissolution to occur. In particular, the sorbent materials found in nature are often metal oxide inner sphere complexes. In nature, this is particularly important for iron and manganese cycling, as both are effected by the redox potential of their environments for weathering to occur. For nonpolar ligands, van der Waals forces instead play a larger role in sorption interactions. Hydrogen bonding does also occur, but is not a part of the adsorption process itself. Due to these factors, the soil quality influences the retention and depletion of nutrients, pollutants, and other ligands that perform sorption with the soil. In engineering applications, the clay minerals can promote sodium ion adsorption in petroleum extraction, alongside the creation of environmental liners through the development of a stern layer. This is theorized to occur due to metal metal precipitation, such as in the case of iron arsenic. However, pH can greatly affect the surface binding effectiveness in this case as well.