Electrical double-layer formation at the nanoscale: Molecular modeling and applications

The formation of the electrical double layer (EDL) is an important phenomenon with many applications in environmental science and engineering, such as aggregation of colloidal particles and uptake of ions including electrosorption. Therefore, the objective of this research is to study the applications of EDL in separation processes using nanostructured materials and to provide a fundamental understanding of the EDL formation at the nanoscale by molecular modeling. In the first part, electrosorption of ions by nanostructured materials is investigated to understand the controlling variables in the electrosorption process for ion removal. Theoretical modeling, which combines the classical Gouy-Chapman (G-C) model and the pore-size distribution of the material, is proposed to predict the removal efficiency. Modeling results suggest that EDL overlapping in the micropores is significant and cannot be ignored. In the second part, cyclic voltammetry experiments are employed to study the EDL overlapping in micropores. It is found that because of the overlapping effect, ions are excluded from micropores at the point of zero charge. To increase the capacitance, a material with uniform nanochannel structure is synthesized. The material yields high capacitance, and no EDL overlapping is observed. Experimental results also show that the material exhibits selectivity because of its unique pore structure. Although the G-C model is partially successful in explaining the electrosorption results, it has many limitations in describing the formation of EDL in nanopores. Thus, in the third part, Monte Carlo simulations of an electrolyte system containing water molecules and ions are employed to study the EDL formation at the nanoscale. Simulation results suggest that the nanopores are covered by water molecules on their surfaces. As a result, the maximum counterion concentration is located at the center of the pore, rather than at the charged surface as predicted by the G-C model. Therefore, the classical G-C model is questionable at the nanoscale, not only quantitatively, but also qualitatively because the importance of water molecules is ignored. The findings of this study have implications in areas beyond environmental science and engineering since EDL formation is very important to many interfacial phenomena occurring in physical, chemical, and biological systems.