Exploring the Effect of Nanopore Microstructures on Crystallization and the Evolution of Molecular Assembly Structure by ¹⁹F Solid-State Nuclear Magnetic Resonance Spectroscopy

The pore microstructure of mesoporous materials has a vital influence on molecular movement and assembly as well as crystallization. Nonetheless, previous studies have predominantly concentrated on the impact of pore size and pore shape on molecular assembly and nucleation outcomes; investigations delving into the effects of more complex pore structures on molecular assembly and nucleation behaviors were absent. In this study, evolution of the molecular self-assembly process of flufenamic acid (FFA) confined in mesoporous materials with different microstructures was monitored by in situ ¹⁹F solid-state NMR spectroscopy. It was demonstrated that tortuosity, as a microstructural parameter of porous materials, has the ability to determine the molecular assembly process and nucleation behaviors of FFA. The results indicated that molecules in pores with high tortuosity tend to aggregate to an amorphous plug, while those in less tortuous nanopores are inclined to adsorb on the pore surface forming molecular layers. Besides that, this work provides the first direct proof that a mixture of two molecular layer structures exists on the FFA-silica surface through ¹⁹F solid-state NMR spectroscopy. This study explores the relationship between the microstructure of porous materials and molecular assembly, which can inform drug delivery, electronic deposition, and biomineralization.