Molecular Dynamics Study on Swelling and Exfoliation Properties of Montmorillonite Nanosheets for Application as Proton Exchange Membranes

Existing proton exchange membranes have the disadvantages of low proton conductivity at high temperature, high cost, and poor barrier properties. Montmorillonite (MMT) materials have excellent thermal and chemical stability, and two-dimensional nanostructures formed by exfoliated MMT show excellent proton conductivity. Therefore, they are expected to become new proton exchange membrane materials. However, the existing exfoliation methods still lack a unified theoretical guidance. In this work, we systematically investigate the detailed swelling and exfoliation properties of MMT with different intercalation cations via extensive molecular dynamics simulations. We find that the swelling properties are determined by the hydration capacity of interlayer cations, and the exfoliation mainly occurs via a lateral sliding process between adjacent layers. The exfoliation energy barrier strongly depends on the interaction strength between interlayer cations and MMT nanosheets, swelling degree, and surface charge density. Lower interaction strength and a higher swelling degree together with larger surface charge density can result in an easy exfoliation process. Our work unveiled the detailed mechanism of the exfoliation process of MMT nanosheets and may contribute to the search for efficient MMT exfoliation methods.

Automated summary

The existing proton exchange membranes have drawbacks such as low proton conductivity at high temperature, high cost, and poor barrier properties. Montmorillonite (MMT) materials, with excellent thermal and chemical stability, and exfoliated MMT with two-dimensional nanostructures shows promising proton conductivity, thus being expected to become new proton exchange membrane materials. However, there is still a lack of unified theoretical guidance for exfoliation methods. In this study, detailed swelling and exfoliation properties of MMT with different intercalation cations are systematically investigated through extensive molecular dynamics simulations. The findings reveal that the swelling properties depend on the hydration capacity of interlayer cations, and exfoliation mainly occurs through lateral sliding between adjacent layers. The exfoliation energy barrier depends strongly on the interaction strength, swelling degree, and surface charge density. Lower interaction strength, higher swelling degree, and larger surface charge density facilitate an easier exfoliation process. This work provides insights into the exfoliation mechanism of MMT nanosheets and may contribute to the development of efficient exfoliation methods.