Insight into the structures and properties of polyamide lithium extraction nanofiltration membranes via molecular dynamics simulation

Abstract

A high-permeance and high-selective Mg²⁺/Li⁺ separation membrane, synthesized through interfacial polymerization using a quaternized spiro-piperazine monomer (QSPIP) and trimesoyl chloride (TMC), named QSPIP–TMC, was studied via molecular dynamics simulations and quantum chemical calculations. We successfully constructed dry and hydrated QSPIP–TMC films, alongside PIP–TMC films as a control group. Theoretical results indicated that the voluminous QSPIP monomer leads to the formation of expanded cavity structures, enhancing water diffusion coefficients through the membrane. The restrained diffusion behavior of the QSPIP monomer toward the interface facilitates the fabrication of more uniform nanofiltration membranes, boosting membrane stability and separating behavior. Additionally, the Li⁺ diffusion coefficient within the QSPIP–TMC membrane is larger than that within PIP–TMC, suggesting that QSPIP–TMC has a slightly superior lithium-ion extraction capacity. These theoretical findings were highly consistent with the experimental observations. This work offers a solid theoretical foundation for designing new, highly efficient lithium–magnesium separation nanofiltration membranes.