Mineral Interfacial Charge at the Water/Mineral Interface
The emergence of surface-modified materials, many of which depend on the interface between the substrate and aqueous solution, is a steadily growing trend in the field of materials science. Of particular interest are the mineral and metal-based solids that line the surfaces of soils and other bodies in the form of oxides or hydroxides. Of particular importance are the properties at the water/mineral interface, where the electric charge of the ionic or neutral components of this interface can determine the physical and chemical characteristics of the solution and substrate. As a result, the knowledge of mineral interfacial charge is vital in understanding the specics of soil science, water surface chemistry, and resource extraction.
To define the electric charge of the water/mineral interface, the charge must first be deconstructed and the components at the interphase broken down. The following table lays out the major and minor components of the interface, the charge that component carries, and the contributions each component makes to the total charge of the water/mineral interface.
Component Charge Per Unit Charge Contribution
Dissolved Cations +1 ±0.05
Adsorbed Cations +1 ±0.30
Dissolved Anions -1 ±0.05
Adsorbed Anions -1 ±0.40
Permeable Surface Vacancies 0 ±0.2
Soluble Surface Adsorbates 0 ±0.3
Total 0 ±0.74
The table above provides an overview of the major components, and the charges each component can provide when aligned at the water/mineral interface. Of all the components outlined, dissolved cations and anions contribute the most significantly to the total charge at the interface. Dissolved cations provide a charge of +1, while dissolved anions provide a charge of -1. The contribution of each dissolved species is approximately ±0.05. Adsorbed cations and anions provide a slightly higher contribution to the total charge, at approximately ±0.30 and ±0.40 respectively.
The charge of the water/mineral interface is also determined by the presence of pores, permeable surface vacancies, and soluble surface adsorbates. These components contribute an average of ±0.2, ±0.3, and ±0.3 respectively. The presence of these components imparts or subtracts charge from the interface, meaning that the nature of some of these components can lead to a change in the total charge of the interface. For example, a surface vacancy that carries a positive charge can increase the total charge of the interface, while a surface vacancy that carries a negative charge can decrease the total charge of the interface. To accurately measure the total charge of the interface, each component must be identified and the charge accurately determined.
Once all components have been identified and their charge determined, the total charge at the water/mineral interface can be calculated by adding all the components together. The total charge of the water/mineral interface can range from ±0.74, depending on the nature of the components at the interface. This calculation is important in understanding the chemical and physical properties of the interface and the interactions between the aqueous solution and the substrate. As a result, the knowledge of the mineral interfacial charge is vital in understanding the behavior of surfacial materials. Moreover, this understanding of the water/mineral interface can help to develop new materials, interpret the actions of pollutants, and increase the efficiency of resource extraction.
